1.
Brackets
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A bracket is a tall punctuation mark typically used in matched pairs within text, to set apart or interject other text. The matched pair may be described as opening and closing, or left, forms include round, square, curly, and angle brackets, and various other pairs of symbols. Chevrons were the earliest type of bracket to appear in written English, desiderius Erasmus coined the term lunula to refer to the rounded parentheses, recalling the shape of the crescent moon. Some of the names are regional or contextual. Sometimes referred to as angle brackets, in cases as HTML markup. Occasionally known as broken brackets or brokets, ⸤ ⸥, ｢ ｣ – corner brackets ⟦ ⟧ – double square brackets, white square brackets Guillemets, ‹ › and « », are sometimes referred to as chevrons or angle brackets. The characters ‹ › and « », known as guillemets or angular quote brackets, are actually quotation mark glyphs used in several European languages, which one of each pair is the opening quote mark and which is the closing quote varies between languages. In English, typographers generally prefer to not set brackets in italics, however, in other languages like German, if brackets enclose text in italics, they are usually set in italics too. Parentheses /pəˈrɛnθᵻsiːz/ contain material that serves to clarify or is aside from the main point, a milder effect may be obtained by using a pair of commas as the delimiter, though if the sentence contains commas for other purposes, visual confusion may result. In American usage, parentheses are considered separate from other brackets. Parentheses may be used in writing to add supplementary information. They can also indicate shorthand for either singular or plural for nouns and it can also be used for gender neutral language, especially in languages with grammatical gender, e. g. he agreed with his physician. Parenthetical phrases have been used extensively in informal writing and stream of consciousness literature, examples include the southern American author William Faulkner as well as poet E. E. Cummings. Parentheses have historically been used where the dash is used in alternatives, such as parenthesis) is used to indicate an interval from a to c that is inclusive of a. That is, [5, 12) would be the set of all numbers between 5 and 12, including 5 but not 12. The numbers may come as close as they like to 12, including 11.999 and so forth, in some European countries, the notation [5, 12[ is also used for this. The endpoint adjoining the bracket is known as closed, whereas the endpoint adjoining the parenthesis is known as open, if both types of brackets are the same, the entire interval may be referred to as closed or open as appropriate. Whenever +∞ or −∞ is used as an endpoint, it is considered open
2.
Jmol
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Jmol is computer software for molecular modelling chemical structures in 3-dimensions. Jmol returns a 3D representation of a molecule that may be used as a teaching tool and it is written in the programming language Java, so it can run on the operating systems Windows, macOS, Linux, and Unix, if Java is installed. It is free and open-source software released under a GNU Lesser General Public License version 2.0, a standalone application and a software development kit exist that can be integrated into other Java applications, such as Bioclipse and Taverna. A popular feature is an applet that can be integrated into web pages to display molecules in a variety of ways, for example, molecules can be displayed as ball-and-stick models, space-filling models, ribbon diagrams, etc. Jmol supports a range of chemical file formats, including Protein Data Bank, Crystallographic Information File, MDL Molfile. There is also a JavaScript-only version, JSmol, that can be used on computers with no Java, the Jmol applet, among other abilities, offers an alternative to the Chime plug-in, which is no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS9. Jmol requires Java installation and operates on a variety of platforms. For example, Jmol is fully functional in Mozilla Firefox, Internet Explorer, Opera, Google Chrome, fast and Scriptable Molecular Graphics in Web Browsers without Java3D
3.
ChemSpider
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ChemSpider is a database of chemicals. ChemSpider is owned by the Royal Society of Chemistry, the database contains information on more than 50 million molecules from over 500 data sources including, Each chemical is given a unique identifier, which forms part of a corresponding URL. This is an approach to develop an online chemistry database. The search can be used to widen or restrict already found results, structure searching on mobile devices can be done using free apps for iOS and for the Android. The ChemSpider database has been used in combination with text mining as the basis of document markup. The result is a system between chemistry documents and information look-up via ChemSpider into over 150 data sources. ChemSpider was acquired by the Royal Society of Chemistry in May,2009, prior to the acquisition by RSC, ChemSpider was controlled by a private corporation, ChemZoo Inc. The system was first launched in March 2007 in a release form. ChemSpider has expanded the generic support of a database to include support of the Wikipedia chemical structure collection via their WiChempedia implementation. A number of services are available online. SyntheticPages is an interactive database of synthetic chemistry procedures operated by the Royal Society of Chemistry. Users submit synthetic procedures which they have conducted themselves for publication on the site and these procedures may be original works, but they are more often based on literature reactions. Citations to the published procedure are made where appropriate. They are checked by an editor before posting. The pages do not undergo formal peer-review like a journal article. The comments are moderated by scientific editors. The intention is to collect practical experience of how to conduct useful chemical synthesis in the lab, while experimental methods published in an ordinary academic journal are listed formally and concisely, the procedures in ChemSpider SyntheticPages are given with more practical detail. Comments by submitters are included as well, other publications with comparable amounts of detail include Organic Syntheses and Inorganic Syntheses
4.
PubChem
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PubChem is a database of chemical molecules and their activities against biological assays. The system is maintained by the National Center for Biotechnology Information, a component of the National Library of Medicine, PubChem can be accessed for free through a web user interface. Millions of compound structures and descriptive datasets can be downloaded via FTP. PubChem contains substance descriptions and small molecules with fewer than 1000 atoms and 1000 bonds, more than 80 database vendors contribute to the growing PubChem database. PubChem consists of three dynamically growing primary databases, as of 28 January 2016, Compounds,82.6 million entries, contains pure and characterized chemical compounds. Substances,198 million entries, contains also mixtures, extracts, complexes, bioAssay, bioactivity results from 1.1 million high-throughput screening programs with several million values. PubChem contains its own online molecule editor with SMILES/SMARTS and InChI support that allows the import and export of all common chemical file formats to search for structures and fragments. In the text search form the database fields can be searched by adding the name in square brackets to the search term. A numeric range is represented by two separated by a colon. The search terms and field names are case-insensitive, parentheses and the logical operators AND, OR, and NOT can be used. AND is assumed if no operator is used, example,0,5000,50,10 -5,5 PubChem was released in 2004. The American Chemical Society has raised concerns about the publicly supported PubChem database and they have a strong interest in the issue since the Chemical Abstracts Service generates a large percentage of the societys revenue. To advocate their position against the PubChem database, ACS has actively lobbied the US Congress, soon after PubChems creation, the American Chemical Society lobbied U. S. Congress to restrict the operation of PubChem, which they asserted competes with their Chemical Abstracts Service
5.
International Chemical Identifier
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Initially developed by IUPAC and NIST from 2000 to 2005, the format and algorithms are non-proprietary. The continuing development of the standard has supported since 2010 by the not-for-profit InChI Trust. The current version is 1.04 and was released in September 2011, prior to 1.04, the software was freely available under the open source LGPL license, but it now uses a custom license called IUPAC-InChI Trust License. Not all layers have to be provided, for instance, the layer can be omitted if that type of information is not relevant to the particular application. InChIs can thus be seen as akin to a general and extremely formalized version of IUPAC names and they can express more information than the simpler SMILES notation and differ in that every structure has a unique InChI string, which is important in database applications. Information about the 3-dimensional coordinates of atoms is not represented in InChI, the InChI algorithm converts input structural information into a unique InChI identifier in a three-step process, normalization, canonicalization, and serialization. The InChIKey, sometimes referred to as a hashed InChI, is a fixed length condensed digital representation of the InChI that is not human-understandable. The InChIKey specification was released in September 2007 in order to facilitate web searches for chemical compounds and it should be noted that, unlike the InChI, the InChIKey is not unique, though collisions can be calculated to be very rare, they happen. In January 2009 the final 1.02 version of the InChI software was released and this provided a means to generate so called standard InChI, which does not allow for user selectable options in dealing with the stereochemistry and tautomeric layers of the InChI string. The standard InChIKey is then the hashed version of the standard InChI string, the standard InChI will simplify comparison of InChI strings and keys generated by different groups, and subsequently accessed via diverse sources such as databases and web resources. Every InChI starts with the string InChI= followed by the version number and this is followed by the letter S for standard InChIs. The remaining information is structured as a sequence of layers and sub-layers, the layers and sub-layers are separated by the delimiter / and start with a characteristic prefix letter. The six layers with important sublayers are, Main layer Chemical formula and this is the only sublayer that must occur in every InChI. The atoms in the formula are numbered in sequence, this sublayer describes which atoms are connected by bonds to which other ones. Describes how many hydrogen atoms are connected to each of the other atoms, the condensed,27 character standard InChIKey is a hashed version of the full standard InChI, designed to allow for easy web searches of chemical compounds. Most chemical structures on the Web up to 2007 have been represented as GIF files, the full InChI turned out to be too lengthy for easy searching, and therefore the InChIKey was developed. With all databases currently having below 50 million structures, such duplication appears unlikely at present, a recent study more extensively studies the collision rate finding that the experimental collision rate is in agreement with the theoretical expectations. Example, Morphine has the structure shown on the right, as the InChI cannot be reconstructed from the InChIKey, an InChIKey always needs to be linked to the original InChI to get back to the original structure
6.
Simplified molecular-input line-entry system
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The simplified molecular-input line-entry system is a specification in form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules, the original SMILES specification was initiated in the 1980s. It has since modified and extended. In 2007, a standard called OpenSMILES was developed in the open-source chemistry community. Other linear notations include the Wiswesser Line Notation, ROSDAL and SLN, the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. The Environmental Protection Agency funded the project to develop SMILES. It has since modified and extended by others, most notably by Daylight Chemical Information Systems. In 2007, a standard called OpenSMILES was developed by the Blue Obelisk open-source chemistry community. Other linear notations include the Wiswesser Line Notation, ROSDAL and SLN, in July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is generally considered to have the advantage of being slightly more human-readable than InChI, the term SMILES refers to a line notation for encoding molecular structures and specific instances should strictly be called SMILES strings. However, the term SMILES is also used to refer to both a single SMILES string and a number of SMILES strings, the exact meaning is usually apparent from the context. The terms canonical and isomeric can lead to confusion when applied to SMILES. The terms describe different attributes of SMILES strings and are not mutually exclusive, typically, a number of equally valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol, algorithms have been developed to generate the same SMILES string for a given molecule, of the many possible strings, these algorithms choose only one of them. This SMILES is unique for each structure, although dependent on the algorithm used to generate it. These algorithms first convert the SMILES to a representation of the molecular structure. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database, there is currently no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, and these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES
7.
Chemical formula
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These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, the simplest types of chemical formulas are called empirical formulas, which use letters and numbers indicating the numerical proportions of atoms of each type. Molecular formulas indicate the numbers of each type of atom in a molecule. For example, the formula for glucose is CH2O, while its molecular formula is C6H12O6. This is possible if the relevant bonding is easy to show in one dimension, an example is the condensed molecular/chemical formula for ethanol, which is CH3-CH2-OH or CH3CH2OH. For reasons of structural complexity, there is no condensed chemical formula that specifies glucose, chemical formulas may be used in chemical equations to describe chemical reactions and other chemical transformations, such as the dissolving of ionic compounds into solution. A chemical formula identifies each constituent element by its chemical symbol, in empirical formulas, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, as ratios to the key element. For molecular compounds, these numbers can all be expressed as whole numbers. For example, the formula of ethanol may be written C2H6O because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of compounds, however, cannot be written with entirely whole-number empirical formulas. An example is boron carbide, whose formula of CBn is a variable non-whole number ratio with n ranging from over 4 to more than 6.5. When the chemical compound of the consists of simple molecules. These types of formulas are known as molecular formulas and condensed formulas. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the formula for glucose is C6H12O6 rather than the glucose empirical formula. However, except for very simple substances, molecular chemical formulas lack needed structural information, for simple molecules, a condensed formula is a type of chemical formula that may fully imply a correct structural formula. For example, ethanol may be represented by the chemical formula CH3CH2OH
8.
Aromatic hydrocarbon
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An aromatic hydrocarbon or arene is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle. In contrast, aliphatic hydrocarbons lack this delocalization, the term aromatic was assigned before the physical mechanism determining aromaticity was discovered, the term was coined as such simply because many of the compounds have a sweet or pleasant odour. The configuration of six carbon atoms in compounds is known as a benzene ring, after the simplest possible such hydrocarbon. Aromatic hydrocarbons can be monocyclic or polycyclic, some non-benzene-based compounds called heteroarenes, which follow Hückels rule, are also called aromatic compounds. In these compounds, at least one atom is replaced by one of the heteroatoms oxygen, nitrogen. Benzene, C6H6, is the simplest aromatic hydrocarbon, and it was the first one named as such, the nature of its bonding was first recognized by August Kekulé in the 19th century. Each carbon atom in the cycle has four electrons to share. One goes to the atom, and one each to the two neighbouring carbons. The structure is alternatively illustrated as a circle around the inside of the ring to show six electrons floating around in delocalized molecular orbitals the size of the ring itself. This depiction represents the equivalent nature of the six carbon–carbon bonds all of bond order 1.5, the electrons are visualized as floating above and below the ring with the electromagnetic fields they generate acting to keep the ring flat. The proper use of the symbol is debated, it is used to describe any cyclic π system in some publications, jensen argues that, in line with Robinsons original proposal, the use of the circle symbol should be limited to monocyclic 6 π-electron systems. In this way the symbol for a six-center six-electron bond can be compared to the Y symbol for a three-center two-electron bond. A reaction that forms a compound from an unsaturated or partially unsaturated cyclic precursor is simply called an aromatization. Many laboratory methods exist for the synthesis of arenes from non-arene precursors. Many methods rely on cycloaddition reactions, alkyne trimerization describes the cyclization of three alkynes, in the Dötz reaction an alkyne, carbon monoxide and a chromium carbene complex are the reactants. Diels–Alder reactions of alkynes with pyrone or cyclopentadienone with expulsion of carbon dioxide or carbon monoxide also form arene compounds, in Bergman cyclization the reactants are an enyne plus a hydrogen donor. Arenes are reactants in many organic reactions, in aromatic substitution one substituent on the arene ring, usually hydrogen, is replaced by another substituent. The two main types are electrophilic aromatic substitution when the reagent is an electrophile and nucleophilic aromatic substitution when the reagent is a nucleophile
9.
Cyclodecapentaene
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Cyclodecapentaene or annulene is an annulene with molecular formula C10H10. This organic compound is a conjugated 10 pi electron cyclic system and it is not aromatic, however, because various types of ring strain destabilize an all-planar geometry. The all-cis isomer, a fully convex decagon, would have bond angles of 144°, instead, the all-cis isomer can adopt a planar boat-like conformation to relieve the angle strain. This is still unstable compared to the next planar trans, cis, trans, yet even this isomer is also unstable, suffering from steric repulsion between the two internal hydrogen atoms. The nonplanar trans, cis, cis, cis, cis isomer is the most stable of all the possible isomers, the annulene compound can be obtained by photolysis of cis-9, 10-dihydronaphthalene as a mixture of isomers. Because of their lack in stability even at low temperatures the reaction products revert to the original dihydronaphthalene, aromaticity can be induced in compounds having a annulene-type core by fixation of the planar geometries. There exist two methods to bring it about, one way is to replace two hydrogen atoms by a methylene bridge gives the planar bicyclic 1, 6-methanoannulene. This compound is aromatic as indicated by from lack in bond length alternation seen in its X-ray structure, another way to restore planarity, and therefore aromaticity, in annulene rings is incorporation of a methine bridge to a tricyclicannulene core structure. When deprotonated to form the anion this type of compound is even more stabilized, the central carbanion makes the molecule even more planar and the number of resonance structures that can be drawn is extended to 7 included two resonance forms with a complete benzene ring.4. Azulene is also a 10 π-electron system in which aromaticity is maintained by direct transannular bonding to form a fused 7–5 bicyclic molecule
10.
Naphthalene
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Naphthalene is an organic compound with formula C 10H8. It is the simplest polycyclic aromatic hydrocarbon, and is a crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalenes structure consists of a pair of benzene rings. It is best known as the ingredient of traditional mothballs. In the early 1820s, two separate reports described a white solid with a pungent odor derived from the distillation of coal tar, in 1821, John Kidd cited these two disclosures and then described many of this substances properties and the means of its production. He proposed the name naphthaline, as it had been derived from a kind of naphtha, naphthalenes chemical formula was determined by Michael Faraday in 1826. The structure of two fused benzene rings was proposed by Emil Erlenmeyer in 1866, and confirmed by Carl Gräbe three years later, a naphthalene molecule can be viewed as the fusion of a pair of benzene rings. As such, naphthalene is classified as a polycyclic aromatic hydrocarbon. There are two sets of equivalent hydrogen atoms, the positions are numbered 1,4,5, and 8, and the beta positions,2,3,6. Unlike benzene, the bonds in naphthalene are not of the same length. The bonds C1−C2, C3−C4, C5−C6 and C7−C8 are about 1.37 Å in length and this difference, established by X-ray diffraction, is consistent with the valence bond model in naphthalene and in particular, with the theorem of cross-conjugation. This theorem would describe naphthalene as an aromatic benzene unit bonded to a diene, as such, naphthalene possesses several resonance structures. Two isomers are possible for mono-substituted naphthalenes, corresponding to substitution at an alpha or beta position, bicyclodecapentaene is a structural isomer with a fused 4–8 ring system. In electrophilic aromatic substitution reactions, naphthalene reacts more readily than benzene, for example, chlorination and bromination of naphthalene proceeds without a catalyst to give 1-chloronaphthalene and 1-bromonaphthalene, respectively. In terms of regiochemistry, electrophiles attack occurs at the alpha position, for beta substitution, the intermediate has only six resonance structures, and only two of these are aromatic. Sulfonation, however, gives a mixture of the alpha product 1-naphthalenesulfonic acid, the 1-isomer forms predominantly at 25 °C, and the 2-isomer at 160 °C. Sulfonation to give the 1- and 2-sulfonic acid occurs readily, H 2SO4 + C 10H8 → C 10H 7−SO 3H + H 2O Further sulfonation occurs to give di-, tri- and these 1, 8-dilithio derivatives are precursors to a host of peri-naphthalene derivatives. With alkali metals, naphthalene forms the dark blue-green radical anion salts such as sodium naphthalenide, the naphthalenide salts are strong reducing agents
11.
Methylene bridge
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It is the repeating unit in the skeleton of the unbranched alkanes. A methylene bridge can also act as a bidentate ligand joining two metals in a compound, such as titanium and aluminum in Tebbes reagent. A methylene bridge is called a methylene group or simply methylene. Compounds possessing a methylene bridge located between two electron withdrawing groups are sometimes called active methylene compounds. Treatment of these with strong bases can form enolates or carbanions, examples include the Knoevenagel condensation and the malonic ester synthesis. Some examples of compounds are, Methyl group Methylene group Methyne
12.
Aromaticity
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Aromatic molecules are very stable, and do not break apart easily to react with other substances. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, since the most common aromatic compounds are derivatives of benzene, the word “aromatic” occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the bases in RNA and DNA. An aromatic functional group or other substituent is called an aryl group, the earliest use of the term aromatic was in an article by August Wilhelm Hofmann in 1855. Hofmann used the term for a class of compounds, many of which have odors. In terms of the nature of the molecule, aromaticity describes a conjugated system often made of alternating single and double bonds in a ring. This configuration allows for the electrons in the pi system to be delocalized around the ring, increasing the molecules stability. The molecule cannot be represented by one structure, but rather a hybrid of different structures. These molecules cannot be found in one of these representations, with the longer single bonds in one location. Rather, the molecule exhibits bond lengths in between those of single and double bonds and this commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds, was developed by August Kekulé. The model for benzene consists of two forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds. Benzene is a stable molecule than would be expected without accounting for charge delocalization. As is standard for resonance diagrams, the use of an arrow indicates that two structures are not distinct entities but merely hypothetical possibilities. Neither is a representation of the actual compound, which is best represented by a hybrid of these structures. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal—all six carbon–carbon bonds have the same length, intermediate between that of a single and that of a double bond. In a cyclic molecule with three alternating double bonds, cyclohexatriene, the length of the single bond would be 1.54 Å. However, in a molecule of benzene, the length of each of the bonds is 1.40 Å, a better representation is that of the circular π-bond, in which the electron density is evenly distributed through a π-bond above and below the ring
13.
Bond length
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In molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded atoms in a molecule. It is a property of a bond between atoms of fixed types, relatively independent of the rest of the molecule. Bond length is related to order, when more electrons participate in bond formation the bond is shorter. Bond length is inversely related to bond strength and the bond dissociation energy, all other factors being equal. In a bond between two atoms, half the bond distance is equal to the covalent radius. Bond lengths are measured in the phase by means of X-ray diffraction. A bond between a pair of atoms may vary between different molecules. For example, the carbon to hydrogen bonds in methane are different from those in methyl chloride and it is however possible to make generalizations when the general structure is the same. A table with experimental single bonds for carbon to other elements is given below, bond lengths are given in picometers. By approximation the bond distance between two different atoms is the sum of the covalent radii. As a general trend, bond distances decrease across the row in the periodic table and this trend is identical to that of the atomic radius. The bond length between two atoms in a molecule depends not only on the atoms but also on such factors as the orbital hybridization, the carbon–carbon bond length in diamond is 154 pm, which is also the largest bond length that exists for ordinary carbon covalent bonds. Since one atomic unit of length is 52.9177 pm, unusually long bond lengths do exist. In one compound, tricyclobutabenzene, a length of 160 pm is reported. The current record holder is another cyclobutabenzene with length 174 pm based on X-ray crystallography, in this type of compound the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°. The existence of a very long C–C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylene dianions, although this concerns a 2-electron-4-center bond and this type of bonding has also been observed in neutral phenalene dimers. The bond lengths of these so-called pancake bonds are up to 305 pm, shorter than average C–C bond distances are also possible, alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond. In benzene all bonds have the length,139 pm
14.
X-ray crystallography
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By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the positions of the atoms in the crystal can be determined, as well as their chemical bonds, their disorder. The method also revealed the structure and function of biological molecules, including vitamins, drugs, proteins. X-ray crystallography is still the method for characterizing the atomic structure of new materials. In a single-crystal X-ray diffraction measurement, a crystal is mounted on a goniometer, the goniometer is used to position the crystal at selected orientations. The crystal is illuminated with a finely focused monochromatic beam of X-rays, poor resolution or even errors may result if the crystals are too small, or not uniform enough in their internal makeup. X-ray crystallography is related to other methods for determining atomic structures. Similar diffraction patterns can be produced by scattering electrons or neutrons, for all above mentioned X-ray diffraction methods, the scattering is elastic, the scattered X-rays have the same wavelength as the incoming X-ray. By contrast, inelastic X-ray scattering methods are useful in studying excitations of the sample, Crystals, though long admired for their regularity and symmetry, were not investigated scientifically until the 17th century. Johannes Kepler hypothesized in his work Strena seu de Nive Sexangula that the symmetry of snowflake crystals was due to a regular packing of spherical water particles. The Danish scientist Nicolas Steno pioneered experimental investigations of crystal symmetry, hence, William Hallowes Miller in 1839 was able to give each face a unique label of three small integers, the Miller indices which remain in use today for identifying crystal faces. In the 19th century, a catalog of the possible symmetries of a crystal was worked out by Johan Hessel, Auguste Bravais, Evgraf Fedorov, Arthur Schönflies. Wilhelm Röntgen discovered X-rays in 1895, just as the studies of crystal symmetry were being concluded, physicists were initially uncertain of the nature of X-rays, but soon suspected that they were waves of electromagnetic radiation, in other words, another form of light. Single-slit experiments in the laboratory of Arnold Sommerfeld suggested that X-rays had a wavelength of about 1 angstrom, however, X-rays are composed of photons, and thus are not only waves of electromagnetic radiation but also exhibit particle-like properties. Albert Einstein introduced the concept in 1905, but it was not broadly accepted until 1922. Therefore, these properties of X-rays, such as their ionization of gases. Nevertheless, Braggs view was not broadly accepted and the observation of X-ray diffraction by Max von Laue in 1912 confirmed for most scientists that X-rays were a form of electromagnetic radiation, Crystals are regular arrays of atoms, and X-rays can be considered waves of electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atoms electrons and this phenomenon is known as elastic scattering, and the electron is known as the scatterer
15.
Single bond
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In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms, therefore, a single bond is a type of covalent bond. When shared, each of the two electrons involved is no longer in the possession of the orbital in which it originated. Rather, both of the two spend time in either of the orbitals which overlap in the bonding process. As a Lewis structure, a bond is denoted as AːA or A-A. In the first rendition, each dot represents an electron, and in the second rendition. A covalent bond can also be a bond or a triple bond. A single bond is weaker than either a double bond or a triple bond and this difference in strength can be explained by examining the component bonds of which each of these types of covalent bonds consists. Usually, a bond is a sigma bond. An exception is the bond in diboron, which is a pi bond, in contrast, the double bond consists of one sigma bond and one pi bond, and a triple bond consists of one sigma bond and two pi bonds. The number of component bonds is what determines the strength disparity, the single bond has the capacity for rotation, a property not possessed by the double bond or the triple bond. The structure of pi bonds does not allow for rotation, so the double bond, the sigma bond is not so restrictive, and the single bond is able to rotate using the sigma bond as the axis of rotation. Another property comparison can be made in bond length, Single bonds are the longest of the three types of covalent bonds as interatomic attraction is greater in the two other types, double and triple. The increase in component bonds is the reason for this increase as more electrons are shared between the bonded atoms. Single bonds are seen in diatomic molecules. Examples of this use of bonds include H2, F2. Single bonds are seen in molecules made up of more than two atoms. Examples of this use of bonds include, Both bonds in H2O All 4 bonds in CH4 Single bonding even appears in molecules as complex as hydrocarbons larger than methane
16.
Double bond
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A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that is two carbon atoms, can be found in alkenes. Many types of double bonds exist between two different elements, for example, in a carbonyl group with a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds, imines and sulfoxides, in skeletal formula the double bond is drawn as two parallel lines between the two connected atoms, typographically, the equals sign is used for this. Double bonds were first introduced in chemical notation by prominent Russian chemist Alexander Butlerov, double bonds involving carbon are stronger than single bonds and are also shorter. Double bonds are also electron-rich, which makes them reactive, the type of bonding can be explained in terms of orbital hybridization. In ethylene each carbon atom has three sp2 orbitals and one p-orbital, the three sp2 orbitals lie in a plane with ~120° angles. The p-orbital is perpendicular to this plane, when the carbon atoms approach each other, two of the sp2 orbitals overlap to form a sigma bond. At the same time, the two p-orbitals approach and together form a pi-bond. For maximum overlap, the p-orbitals have to parallel, and, therefore. This property gives rise to cis-trans isomerism, double bonds are shorter than single bonds because p-orbital overlap is maximized. With 133 pm, the ethylene C=C bond length is shorter than the C−C length in ethane with 154 pm. The double bond is stronger,636 kJ mol−1 versus 368 kJ mol−1. In an alternative representation, the double bond results from two overlapping sp3 orbitals as in a bent bond, in molecules, with alternating double bonds and single bonds, p-orbital overlap can exist over multiple atoms in a chain, giving rise to a conjugated system. Conjugation can be found in such as dienes and enones. In cyclic molecules, conjugation can lead to aromaticity, in cumulenes, two double bonds are adjacent. Double bonds are common for period 2 elements carbon, nitrogen, and oxygen, metals, too, can engage in multiple bonding in a metal ligand multiple bond. Double bonded compounds, alkene homologs, R2E=ER2 are now known for all of the heavier group 14 elements, unlike the alkenes these compounds are not planar but adopt twisted and/or trans bent structures
17.
Resonance (chemistry)
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In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. A molecule or ion with such delocalized electrons is represented by several contributing structures, each contributing structure can be represented by a Lewis structure, with only an integer number of covalent bonds between each pair of atoms within the structure. Several Lewis structures are used collectively to describe the molecular structure. Electron delocalization lowers the energy of the substance and thus makes it more stable than any of the contributing structures. The difference between the energy of the actual structure and that of the contributing structure with the lowest potential energy is called the resonance energy or delocalization energy. An isomer is a molecule with the chemical formula but with different arrangements of atoms in space. Resonance contributors of a molecule, on the contrary, can differ by the arrangements of electrons. Therefore, the resonance hybrid cannot be represented by a combination of isomers, benzene undergoes substitution reactions, rather than addition reactions as typical for alkenes. He proposed that the bond in benzene is intermediate of a single and double bond. The mechanism of resonance was introduced into quantum mechanics by Werner Heisenberg in 1926 in a discussion of the states of the helium atom. He compared the structure of the atom with the classical system of resonating coupled harmonic oscillators. Linus Pauling used this mechanism to explain the partial valence of molecules in 1928, the alternative term mesomerism popular in German and French publications with the same meaning was introduced by C. K. Ingold in 1938, but did not catch on in the English literature. The current concept of effect has taken on a related. The double headed arrow was introduced by the German chemist Fritz Arndt who preferred the German phrase zwischenstufe or intermediate stage, the real structure is an intermediate of these structures represented by a resonance hybrid. The contributing structures are not isomers and they differ only in the position of electrons, not in the position of nuclei. Each Lewis formula must have the number of valence electrons. Bonds that have different bond orders in different contributing structures do not have typical bond lengths, the real structure has a lower total potential energy than each of the contributing structures would have. This means that it is more stable than each separate contributing structure would be and it is a common misconception that resonance structures are actual transient states of the molecule, with the molecule oscillating between them or existing as an equilibrium between them
18.
Benzene
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Benzene is an important organic chemical compound with the chemical formula C6H6. The benzene molecule is composed of 6 carbon atoms joined in a ring with 1 hydrogen atom attached to each, because it contains only carbon and hydrogen atoms, benzene is classed as a hydrocarbon. Benzene is a constituent of crude oil and is one of the elementary petrochemicals. Because of the cyclic continuous pi bond between the atoms, benzene is classed as an aromatic hydrocarbon, the second -annulene. Benzene is a colorless and highly flammable liquid with a sweet smell and it is used primarily as a precursor to the manufacture of chemicals with more complex structure, such as ethylbenzene and cumene, of which billions of kilograms are produced. Because benzene has a high number, it is an important component of gasoline. Because benzene is a carcinogen, most non-industrial applications have been limited. The word benzene derives historically from gum benzoin, a resin known to European pharmacists. An acidic material was derived from benzoin by sublimation, and named flowers of benzoin, the hydrocarbon derived from benzoic acid thus acquired the name benzin, benzol, or benzene. Michael Faraday first isolated and identified benzene in 1825 from the oily residue derived from the production of illuminating gas, in 1833, Eilhard Mitscherlich produced it by distilling benzoic acid and lime. He gave the compound the name benzin, in 1845, Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later, Mansfield began the first industrial-scale production of benzene, gradually, the sense developed among chemists that a number of substances were chemically related to benzene, comprising a diverse chemical family. In 1855, Hofmann used the word aromatic to designate this family relationship, in 1997, benzene was detected in deep space. The empirical formula for benzene was known, but its highly polyunsaturated structure. In 1865, the German chemist Friedrich August Kekulé published a paper in French suggesting that the structure contained a ring of six carbon atoms with alternating single and double bonds, the next year he published a much longer paper in German on the same subject. Kekulés symmetrical ring could explain these facts, as well as benzenes 1,1 carbon-hydrogen ratio. Here Kekulé spoke of the creation of the theory and he said that he had discovered the ring shape of the benzene molecule after having a reverie or day-dream of a snake seizing its own tail. This vision, he said, came to him years of studying the nature of carbon-carbon bonds
19.
Proton nuclear magnetic resonance
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In samples where natural hydrogen is used, practically all the hydrogen consists of the isotope 1H. A full 1H atom is called protium, simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere. However, a solvent without hydrogen, such as carbon tetrachloride, CCl4 or carbon disulphide, CS2, historically, deuterated solvents were supplied with a small amount of tetramethylsilane as an internal standard for calibrating the chemical shifts of each analyte proton. TMS is a molecule, with all protons being chemically equivalent, giving one single signal. It is volatile, making sample recovery easy as well, modern spectrometers are able to reference spectra based on the residual proton in the solvent. Deuterated solvents are now commonly supplied without TMS, deuterated solvents permit the use of deuterium frequency-field lock to offset the effect of the natural drift of the NMRs magnetic field B0. In order to provide deuterium lock, the NMR constantly monitors the deuterium signal resonance frequency from the solvent and makes changes to the B0 to keep the resonance frequency constant. Additionally, the signal may be used to accurately define 0 ppm as the resonant frequency of the lock solvent. Proton NMR spectra of most organic compounds are characterized by chemical shifts in the range +14 to -4 ppm, the integration curve for each proton reflects the abundance of the individual protons. The spectrum of ethyl chloride consists of a triplet at 1.5 ppm, the spectrum of benzene consists of a single peak at 7.2 ppm due to the diamagnetic ring current. Together with Carbon-13 NMR, proton NMR is a tool for molecular structure characterization. Chemical shift values, symbolized by δ, are not precise, deviations are in ±0.2 ppm range, sometimes more. The exact value of chemical shift depends on structure and the solvent, temperature, magnetic field in which the spectrum is being recorded. Hydrogen nuclei are sensitive to the hybridization of the atom to which the atom is attached. Nuclei tend to be deshielded by groups which withdraw electron density, deshielded nuclei resonate at higher δ values, whereas shielded nuclei resonate at lower δ values. Examples of electron withdrawing substituents are -OH, -OCOR, -OR, -NO2 and these cause a downfield shift of approximately 2–4 ppm for H atoms on Cα and of less than 1–2 ppm for H atoms on Cβ. Cα is an aliphatic C atom directly bonded to the substituent in question, carbonyl groups, olefinic fragments and aromatic rings contribute sp2 hybridized carbon atoms to an aliphatic chain. This causes a shift of 1–2 ppm at Cα
20.
Diamagnetic ring current
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An aromatic ring current is an effect observed in aromatic molecules such as benzene and naphthalene. If a magnetic field is directed perpendicular to the plane of the aromatic system, the ring current creates its own magnetic field. Outside the ring, this field is in the direction as the externally applied magnetic field, inside the ring. As a result, the net magnetic field outside the ring is greater than the applied field alone. Aromatic ring currents are relevant to NMR spectroscopy, as they influence the chemical shifts of 1H nuclei in aromatic molecules. The effect helps distinguish these nuclear environments and is therefore of great use in structure determination. In contrast any proton inside the aromatic ring experiences shielding because both fields are in opposite direction and this effect can be observed in cyclooctadecanonaene with 6 inner protons at −3 ppm. The situation is reversed in antiaromatic compounds, in the dianion of annulene the inner protons are strongly deshielded at 20.8 ppm and 29.5 ppm with the outer protons significantly shielded at −1.1 ppm. Hence a diamagnetic ring current or diatropic ring current is associated with aromaticity whereas a paratropic ring current signals antiaromaticity, a similar effect is observed in three-dimensional fullerenes, in this case it is called a sphere current. Numerous attempts have been made to quantify aromaticity with respect to the ring current. Large negative values are aromatic, for example, benzene, values close to zero are non-aromatic, for example, borazine and cyclohexane. And large positive values are antiaromatic, for example, cyclobutadiene, thus the lithium atom in cyclopentadienyl lithium has a chemical shift of −8.6 ppm and its Cp2Li− complex a shift of −13.1. Both methods suffer from the disadvantage that values depend on ring size, the nucleus-independent chemical shift is a computational method that calculates the absolute magnetic shielding at the center of a ring. The values are reported with a sign to make them compatible with the chemical shift conventions of NMR spectroscopy. In this method, negative NICS values indicate aromaticity and positive values antiaromaticity, an aromatic compound has HOMA value 1 whereas a non-aromatic compound has value 0. For all-carbon systems, the HOMA value is defined as, H O M A =1 −257.7 n ∑ i n 2, where 257.7 the normalization value, n is the number of carbon–carbon bonds, and d are bond lengths in angstrom
21.
Chemical shift
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In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard in a magnetic field. Often the position and number of shifts are diagnostic of the structure of a molecule. Chemical shifts are used to describe signals in other forms of spectroscopy such as photoemission spectroscopy. Some atomic nuclei possess a magnetic moment, which rise to different energy levels. The total magnetic field experienced by a nucleus includes local magnetic fields induced by currents of electrons in the molecular orbitals, the electron distribution of the same type of nucleus usually varies according to the local geometry, and with it the local magnetic field at each nucleus. This is reflected in the energy levels. The variations of magnetic resonance frequencies of the same kind of nucleus. The size of the shift is given with respect to a reference frequency or reference sample. Since the numerator is usually expressed in hertz, and the denominator in megahertz, the detected frequencies for 1H, 13C, and 29Si nuclei are usually referenced against TMS or DSS, which by the definition above have a chemical shift of zero if chosen as the reference. Other standard materials are used for setting the chemical shift for other nuclei, although the absolute resonance frequency depends on the applied magnetic field, the chemical shift is independent of external magnetic field strength. On the other hand, the resolution of NMR will increase with applied magnetic field, the electrons around a nucleus will circulate in a magnetic field and create a secondary induced magnetic field. This field opposes the field as stipulated by Lenzs law and atoms with higher induced fields are therefore called shielded. The chemical milieu of an atom can influence its electron density through the polar effect, electron-donating alkyl groups, for example, lead to increased shielding while electron-withdrawing substituents such as nitro groups lead to deshielding of the nucleus. Not only substituents cause local induced fields, bonding electrons can also lead to shielding and deshielding effects. A striking example of this are the pi bonds in benzene, circular current through the hyperconjugated system causes a shielding effect at the molecules center and a deshielding effect at its edges. Trends in chemical shift are explained based on the degree of shielding or deshielding, nuclei are found to resonate in a wide range to the left of the internal standard. In real molecules protons are surrounded by a cloud of charge due to adjacent bonds, in an applied magnetic field electrons circulate and produce an induced field which opposes the applied field. The effective field at the nucleus will be B = B0 − Bi, the nucleus is said to be experiencing a diamagnetic shielding
22.
Homoaromaticity
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Homoaromaticity, in organic chemistry, refers to a special case of aromaticity in which conjugation is interrupted by a single sp3 hybridized carbon atom. This formal discontinuity is apparently bridged by p-orbital overlap, maintaining a cycle of π electrons that is responsible for this preserved chemical stability. The concept of homoaromaticity was pioneered by Saul Winstein in 1959, to date, homoaromatic compounds are known to exist as cationic and anionic species, and some studies support the existence of neutral homoaromatic molecules, though these are less common. The homotropylium cation is perhaps the best studied example of a homoaromatic compound, the term homoaromaticity derives from the structural similarity between homoaromatic compounds and the analogous homo-conjugated alkenes previously observed in the literature. The IUPAC Gold Book requires that Bis-, Tris-, etc. prefixes be used to describe compounds in which two, three, etc. sp3 centers separately interrupt conjugation of the aromatic system. The concept of homoaromaticity has its origins in the debate over the non-classical carbonium ions that occurred in the 1950s, Saul Winstein, a famous proponent of the non-classical ion model, first described homoaromaticity while studying the 3-bicyclohexyl cation. In a series of experiments, Winstein et al. observed that the solvolysis reaction occurred empirically faster when the tosyl leaving group was in the equatorial position. The group ascribed this difference in rates to the anchimeric assistance invoked by the cis isomer. This result thus supported a non-classical structure for the cation, Winstein subsequently observed that this non-classical model of the 3-bicyclohexyl cation is analogous to the previously well-studied aromatic cyclopropenyl cation. Like the cyclopropenyl cation, positive charge is delocalized over three equivalent carbons containing two π electrons and this electronic configuration thus satisfies Huckels rule for aromaticity. The group thus proposed the name tris-homocyclopropenyl—the tris-homo counterpart to the cyclopropenyl cation, the criterion for aromaticity has evolved as new developments and inisights continue to contribute to our understanding of these remarkably stable organic molecules. The required characteristics of these molecules has remained the subject of some controversy. Classically, aromatic compounds were defined as planar molecules that possess a cyclically delocalized system of π electrons, most importantly, these conjugated ring systems are known to exhibit enormous thermochemical stability relative to predictions based on localized resonance structures. Consequently, the criterion for homoaromatic delocalization remains similarly ambiguous and somewhat controversial, after initial reports of a homoaromatic structure for the tris-homocyclopropenyl cation were published by Winstein, many groups began to report observations of similar compounds. Much of the evidence for homoaromaticity comes from observations of unusual NMR properties associated with this molecule. From this observation, Pettit, et al. concluded that the structure of the cyclooctatrienyl cation must be incorrect. Upon further consideration, Pettit was inclined to represent the compound as the homotropylium ion and this structure shows how delocalization is cyclic and involves 6 π electrons, consistent with Huckels rule for aromaticity. The magnetic field of the NMR could thus induce a current in the ion
23.
International Standard Book Number
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The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker
24.
Journal of the American Chemical Society
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The Journal of the American Chemical Society is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the Journal of Analytical and Applied Chemistry and the American Chemical Journal and it publishes original research papers in all fields of chemistry. Since 2002, the journal is edited by Peter J. Stang, in 2014, the journal moved to a hybrid open access publishing model. The Journal of the American Chemical Society is abstracted and indexed in Chemical Abstracts Service, Scopus, EBSCOhost, Thomson-Gale, ProQuest, PubMed, Web of Science, and SwetsWise. According to the Journal Citation Reports, it had a factor of 12.113 for 2014
25.
Hydrocarbon
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In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon, and thus are group 14 hydrides. Hydrocarbons from which one atom has been removed are functional groups. Aromatic hydrocarbons, alkanes, alkenes, cycloalkanes and alkyne-based compounds are different types of hydrocarbons, the classifications for hydrocarbons, defined by IUPAC nomenclature of organic chemistry are as follows, Saturated hydrocarbons are the simplest of the hydrocarbon species. They are composed entirely of single bonds and are saturated with hydrogen, the formula for acyclic saturated hydrocarbons is CnH2n+2. The most general form of saturated hydrocarbons is CnH2n+2, where r is the number of rings and those with exactly one ring are the cycloalkanes. Saturated hydrocarbons are the basis of petroleum fuels and are found as linear or branched species. Substitution reaction is their characteristics property, hydrocarbons with the same molecular formula but different structural formulae are called structural isomers. As given in the example of 3-methylhexane and its higher homologues, chiral saturated hydrocarbons constitute the side chains of biomolecules such as chlorophyll and tocopherol. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms and those with double bond are called alkenes. Those with one double bond have the formula CnH2n and those containing triple bonds are called alkyne. Those with one triple bond have the formula CnH2n−2, aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring. Hydrocarbons can be gases, liquids, waxes or low melting solids or polymers, in terms of shells, carbon consists of an incomplete outer shell, which comprises 4 electrons, and thus has 4 electrons available for covalent or dative bonding. Some hydrocarbons also are abundant in the solar system, lakes of liquid methane and ethane have been found on Titan, Saturns largest moon, confirmed by the Cassini-Huygens Mission. Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon compounds, hydrocarbons are a primary energy source for current civilizations. The predominant use of hydrocarbons is as a fuel source. In their solid form, hydrocarbons take the form of asphalt, mixtures of volatile hydrocarbons are now used in preference to the chlorofluorocarbons as a propellant for aerosol sprays, due to chlorofluorocarbons impact on the ozone layer. Methane and ethane are gaseous at ambient temperatures and cannot be liquefied by pressure alone. Propane is however easily liquefied, and exists in propane bottles mostly as a liquid, butane is so easily liquefied that it provides a safe, volatile fuel for small pocket lighters
