Thioacetals are the sulfur analogues of acetals. There are two classes: dithioacetals. Monothioacetals are less common, have the functional group RCH. Dithioacetals have the formula RC2H and RCH; the symmetric dithioacetals are common. They are prepared by condensation of dithiols with aldehydes; these reactions proceed via the intermediacy of hemithioacetals: Thiol addition to give hemithioacetal: RSH + R'CH → R'CH Thiol addition with loss of water to give dithioacetal: RSH + R'CHSR → R'CH2 + H2OSuch reactions employ either a Lewis acid or Brønsted acid as catalyst. Dithioacetals generated from aldehydes and either 1,2-ethanedithiol or 1,3-propanedithiol are common among this class of molecules for use in organic synthesis; the carbonyl carbon of an aldehyde is electrophilic and therefore susceptible to attack by nucleophiles, whereas the analogous central carbon of a dithioacetal is not electrophilic. As a result, dithioacetals can serve as protective groups for aldehydes. Far from being unreactive, in a reaction unlike that of aldehydes, that carbon can be deprotonated to render it nucleophilic: R'CHS2C2H4 + R2NLi → R'CLiS2C2H4 + R2NHThe inversion of polarity between R'Cδ+=Oδ− and R'CLi2 is referred to as umpolung.
The reaction is performed using the 1,3-dithiane. The lithiated intermediate can be used for various nucleophilic bond-forming reactions, the dithioketal hydrolyzed back to its carbonyl form; this overall process, the Corey–Seebach reaction, gives the synthetic equivalent of an acyl anion. Mozingo reduction Thioketal
Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only small amounts of catalyst are required to alter the reaction rate in principle. In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst reacts to form a temporary intermediate, which regenerates the original catalyst in a cyclic process. A substance which provides a mechanism with a higher activation energy does not decrease the rate because the reaction can still occur by the non-catalyzed route. An added substance which does reduce the reaction rate is not considered a catalyst but a reaction inhibitor. Catalysts may be classified as either heterogeneous. A homogeneous catalyst is one whose molecules are dispersed in the same phase as the reactant's molecules.
A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant's, which are gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are considered as a third category. In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations; the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have a lower activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. However, the detailed mechanics of catalysis is complex. Catalysts may bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause dissociation of reagents to reactive forms, such as chemisorbed hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions are typical chemical reactions. The catalyst participates in this slowest step, rates are limited by amount of catalyst and its "activity". In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid–liquid system or sublimate in a solid–gas system; the production of most industrially important chemicals involves catalysis. Most biochemically significant processes are catalysed.
Research into catalysis is a major field in applied science and involves many areas of chemistry, notably organometallic chemistry and materials science. Catalysis is relevant to many aspects of environmental science, e.g. the catalytic converter in automobiles and the dynamics of the ozone hole. Catalytic reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated, as opposed to stoichiometric reactions in which all reactants are consumed and more side products are formed. Many transition metals and transition metal complexes are used in catalysis as well. Catalysts called. A catalyst works by providing an alternative reaction pathway to the reaction product; the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates oxygen, as shown below. 2 H2O2 → 2 H2O + O2This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow.
In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available. This reaction is affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms. Upon the addition of a small amount of manganese dioxide, the hydrogen peroxide reacts rapidly; this effect is seen by the effervescence of oxygen. The manganese dioxide is not consumed in the reaction, thus may be recovered unchanged, re-used indefinitely. Accordingly, manganese dioxide catalyses this reaction. Catalytic activity is denoted by the symbol z and measured in mol/s, a unit, called katal and defined the SI unit for catalytic activity since 1999. Catalytic activity is not a kind of reaction rate, but a property of the catalyst under certain conditions, in relation to a specific chemical reaction. Catalytic activity of one katal of a catalyst means one mole of that catalyst will catalyse 1 mole of the reactant to product in one second. A catalyst may and will have different catalytic activity for di
Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic linkages, on hydrolysis give the constituent monosaccharides or oligosaccharides. They range in structure from linear to branched. Examples include storage polysaccharides such as starch and glycogen, structural polysaccharides such as cellulose and chitin. Polysaccharides are quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure, these macromolecules can have distinct properties from their monosaccharide building blocks, they may be amorphous or insoluble in water. When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present they are called heteropolysaccharides or heteroglycans. Natural saccharides are of simple carbohydrates called monosaccharides with general formula n where n is three or more.
Examples of monosaccharides are glucose and glyceraldehyde. Polysaccharides, have a general formula of Cxy where x is a large number between 200 and 2500; when the repeating units in the polymer backbone are six-carbon monosaccharides, as is the case, the general formula simplifies to n, where 40≤n≤3000. As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas oligosaccharides contain three to ten monosaccharide units. Polysaccharides are an important class of biological polymers, their function in living organisms is either structure- or storage-related. Starch is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally similar glucose polymer is the more densely branched glycogen, sometimes called "animal starch". Glycogen's properties allow it to be metabolized more which suits the active lives of moving animals. Cellulose and chitin are examples of structural polysaccharides.
Cellulose is used in the cell walls of plants and other organisms, is said to be the most abundant organic molecule on Earth. It has many uses such as a significant role in the paper and textile industries, is used as a feedstock for the production of rayon, cellulose acetate and nitrocellulose. Chitin has nitrogen-containing side branches, increasing its strength, it is found in the cell walls of some fungi. It has multiple uses, including surgical threads. Polysaccharides include callose or laminarin, xylan, mannan and galactomannan. Nutrition polysaccharides are common sources of energy. Many organisms can break down starches into glucose; these carbohydrate types can be metabolized by some protists. Ruminants and termites, for example, use microorganisms to process cellulose. Though these complex polysaccharides are not digestible, they provide important dietary elements for humans. Called dietary fiber, these carbohydrates enhance digestion among other benefits; the main action of dietary fiber is to change the nature of the contents of the gastrointestinal tract, to change how other nutrients and chemicals are absorbed.
Soluble fiber binds to bile acids in the small intestine, making them less to enter the body. Soluble fiber attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging physiological activities. Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown. Not yet formally proposed as an essential macronutrient, dietary fiber is regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake. Starch is a glucose polymer, it is made up of a mixture of amylopectin. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units. Starches are insoluble in water, they can be digested by breaking the alpha-linkages. Both humans and other animals have amylases, so they can digest starches.
Potato, rice and maize are major sources of starch in the human diet. The formations of starches are the ways. Glycogen serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue. Glycogen is made by the liver and the muscles, but can be made by glycogenesis within the brain and stomach. Glycogen is analogous to starch, a glucose polymer in plants, is sometimes referred to as animal starch, having a similar structure to amylopectin but more extensively branched and compact than starch. Glycogen is a polymer of α glycosidic bonds linked, with α-linked branches. Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, plays an important role in the glucose cycle. Glycogen forms an energy reserve that can be mobilized to meet a sudden need for glucose, but one, less compact and more available a
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science. IUPAC is registered in Zürich and the administrative office, known as the "IUPAC Secretariat", is in Research Triangle Park, North Carolina, United States; this administrative office is headed by IUPAC's executive director Lynn Soby. IUPAC was established in 1919 as the successor of the International Congress of Applied Chemistry for the advancement of chemistry, its members, the National Adhering Organizations, can be national chemistry societies, national academies of sciences, or other bodies representing chemists. There are fifty-four National Adhering Organizations and three Associate National Adhering Organizations. IUPAC's Inter-divisional Committee on Nomenclature and Symbols is the recognized world authority in developing standards for the naming of the chemical elements and compounds.
Since its creation, IUPAC has been run by many different committees with different responsibilities. These committees run different projects which include standardizing nomenclature, finding ways to bring chemistry to the world, publishing works. IUPAC is best known for its works standardizing nomenclature in chemistry and other fields of science, but IUPAC has publications in many fields including chemistry and physics; some important work IUPAC has done in these fields includes standardizing nucleotide base sequence code names. IUPAC is known for standardizing the atomic weights of the elements through one of its oldest standing committees, the Commission on Isotopic Abundances and Atomic Weights; the need for an international standard for chemistry was first addressed in 1860 by a committee headed by German scientist Friedrich August Kekulé von Stradonitz. This committee was the first international conference to create an international naming system for organic compounds; the ideas that were formulated in that conference evolved into the official IUPAC nomenclature of organic chemistry.
IUPAC stands as a legacy of this meeting, making it one of the most important historical international collaborations of chemistry societies. Since this time, IUPAC has been the official organization held with the responsibility of updating and maintaining official organic nomenclature. IUPAC as such was established in 1919. One notable country excluded from this early IUPAC is Germany. Germany's exclusion was a result of prejudice towards Germans by the Allied powers after World War I. Germany was admitted into IUPAC during 1929. However, Nazi Germany was removed from IUPAC during World War II. During World War II, IUPAC was affiliated with the Allied powers, but had little involvement during the war effort itself. After the war and West Germany were readmitted to IUPAC. Since World War II, IUPAC has been focused on standardizing nomenclature and methods in science without interruption. In 2016, IUPAC denounced the use of chlorine as a chemical weapon; the organization pointed out their concerns in a letter to Ahmet Üzümcü, the director of the Organisation for the Prohibition of Chemical Weapons, in regards to the practice of utilizing chlorine for weapon usage in Syria among other locations.
The letter stated, "Our organizations deplore the use of chlorine in this manner. The indiscriminate attacks carried out by a member state of the Chemical Weapons Convention, is of concern to chemical scientists and engineers around the globe and we stand ready to support your mission of implementing the CWC." According to the CWC, "the use, distribution, development or storage of any chemical weapons is forbidden by any of the 192 state party signatories." IUPAC is governed by several committees. The committees are as follows: Bureau, CHEMRAWN Committee, Committee on Chemistry Education, Committee on Chemistry and Industry, Committee on Printed and Electronic Publications, Evaluation Committee, Executive Committee, Finance Committee, Interdivisional Committee on Terminology and Symbols, Project Committee, Pure and Applied Chemistry Editorial Advisory Board; each committee is made up of members of different National Adhering Organizations from different countries. The steering committee hierarchy for IUPAC is as follows: All committees have an allotted budget to which they must adhere.
Any committee may start a project. If a project's spending becomes too much for a committee to continue funding, it must take the issue to the Project Committee; the project committee either decides on an external funding plan. The Bureau and Executive Committee oversee operations of the other committees. IUPAC committee has a long history of naming organic and inorganic compounds. IUPAC nomenclature is developed so that any compound can be named under one set of standardized rules to avoid duplicate names; the first publication on IUPAC nomenclature of organic compounds was A Guide to IUPAC Nomenclature of Organic Compounds in 1900, which contained information from the International Congress of Applied Chemistry. IUPAC organic nomenclature has three basic parts: the substituents, carbon chain length and chemical ending; the substituents are any functional groups attached to the main carbon chain. The main carbon chain is the longest possible continuous chain; the chemical ending denotes. For example, the ending ane denotes a single bonded carbon chain, as in "hexane".
Another example of IUPAC organic no
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium CH+3, methanium CH+5 and vinyl C2H+3 cations. Carbocations that bear more than one positively charged carbon atom are encountered; until the early 1970s, all carbocations were called carbonium ions. In present-day chemistry, a carbocation is any ion with a positively charged carbon atom, classified in two main categories according to the coordination number of the charged carbon: three in the carbenium ions and five in the carbonium ions; this nomenclature was proposed by G. A. Olah. Carbonium ions, as defined by Olah, are characterized by a three-center two-electron delocalized bonding scheme and are synonymous with so-called'nonclassical carbocations', which are carbocations that contain bridging C–C or C–H σ-bonds. However, others have more narrowly defined the term'carbonium ion' as formally protonated or alkylated alkanes, to the exclusion of nonclassical carbocations like the 2-norbornyl cation.
According to the IUPAC, a carbocation is any cation containing an number of electrons in which a significant portion of the positive charge resides on a carbon atom. Prior to the observation of five-coordinate carbocations by Olah and coworkers and carbonium ion were used interchangeably. Olah proposed a redefinition of carbonium ion as a carbocation featuring any type of three-center two-electron bonding, while a carbenium ion was newly coined to refer to a carbocation containing only two-center two-electron bonds with a three-coordinate positive carbon. Subsequently, others have used the term carbonium ion more narrowly to refer to species that are derived from electrophilic attack of H+ or R+ on an alkane, in analogy to other main group onium species, while a carbocation that contains any type of three-centered bonding is referred to as a nonclassical carbocation. In this usage, 2-norbornyl cation is not a carbonium ion, because it is formally derived from protonation of an alkene rather than an alkane, although it is a nonclassical carbocation due to its bridged structure.
The IUPAC acknowledges the three divergent definitions of carbonium ion and urges care in the usage of this term. For the remainder of this article, the term carbonium ion will be used in this latter restricted sense, while nonclassical carbocation will be used to refer to any carbocation with C–C and/or C–H σ-bonds delocalized by bridging. Since the late 1990s, most textbooks have stopped using the term carbonium ion for the classical three-coordinate carbocation. However, some university-level textbooks continue to use the term carbocation as if it were synonymous with carbenium ion, or discuss carbocations with only a fleeting reference to the older terminology of carbonium ions or carbenium and carbonium ions. One textbook retains the older name of carbonium ion for carbenium ion to this day, uses the phrase hypervalent carbonium ion for CH+5. A carbocation with an two-coordinate sp-hybridized positive carbon is known as a vinyl cation, while a two-coordinate sp2-hybridized cation resulting from the formal removal of a hydride ion from an arene is termed an aryl cation.
These carbocations are unstable and are infrequently encountered. Hence, they are omitted from introductory and intermediate level textbooks; the IUPAC definition stipulates. The history of carbocations dates back to 1891 when G. Merling reported that he added bromine to tropylidene and heated the product to obtain a crystalline, water-soluble material, C7H7Br, he did not suggest a structure for it. This ion is predicted to be aromatic by Hückel's rule. In 1902, Norris and Kehrman independently discovered that colorless triphenylmethanol gives deep-yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride formed orange complexes with aluminium and tin chlorides. In 1902, Adolf von Baeyer recognized the salt-like character of the compounds formed, he dubbed the relationship between color and salt formation halochromy, of which malachite green is a prime example. Carbocations are reactive intermediates in many organic reactions; this idea, first proposed by Julius Stieglitz in 1899, was further developed by Hans Meerwein in his 1922 study of the Wagner–Meerwein rearrangement.
Carbocations were found to be involved in the SN1 reaction, the E1 reaction, in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them; the first NMR spectrum of a stable carbocation in solution was published by Doering et al. in 1958. It was the heptamethylbenzenium ion, made by treating hexamethylbenzene with methyl chloride and aluminium chloride; the stable 7-norbornadienyl cation was prepared by Story et al. in 1960 by reacting norbornadienyl chloride with silver tetrafluoroborate in sulfur dioxide at −80 °C. The NMR spectrum established. In 1962, Olah directly observed the tert-butyl carbocation by nuclear magnetic resonance as a stable species on dissolving tert-butyl fluoride in magic acid; the NMR of the norbornyl cation was first reported by Schleyer et al. and it was shown to undergo proton-scrambling over a barrier by Saunders et al.
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis. In many preparations of delicate organic compounds, some specific parts of their molecules cannot survive the required reagents or chemical environments; these parts, or groups, must be protected. For example, lithium aluminium hydride is a reactive but useful reagent capable of reducing esters to alcohols, it will always react with carbonyl groups, this cannot be discouraged by any means. When a reduction of an ester is required in the presence of a carbonyl, the attack of the hydride on the carbonyl has to be prevented. For example, the carbonyl is converted into an acetal; the acetal is called a protecting group for the carbonyl. After the step involving the hydride is complete, the acetal is removed, giving back the original carbonyl; this step is called deprotection.
Protecting groups are more used in small-scale laboratory work and initial development than in industrial production processes because their use adds additional steps and material costs to the process. However, the availability of a cheap chiral building block can overcome these additional costs. Protection of alcohols: Acetyl -- Removed by base. Benzoyl – Removed by acid or base, more stable than Ac group. Benzyl – Removed by hydrogenolysis. Bn group is used in sugar and nucleoside chemistry. Β-Methoxyethoxymethyl ether – Removed by acid. Dimethoxytrityl, – Removed by weak acid. DMT group is used for protection of 5'-hydroxy group in nucleosides in oligonucleotide synthesis. Methoxymethyl ether – Removed by acid. Methoxytrityl – Removed by acid and hydrogenolysis. P-Methoxybenzyl ether – Removed by acid, hydrogenolysis, or oxidation. Methylthiomethyl ether – Removed by acid. Pivaloyl – Removed by acid, base or reductant agents, it is more stable than other acyl protecting groups. Tetrahydropyranyl – Removed by acid.
Tetrahydrofuran – Removed by acid. Trityl – Removed by acid and hydrogenolysis. Silyl ether – Removed by acid or fluoride ion.. TBDMS and TOM groups are used for protection of 2'-hydroxy function in nucleosides in oligonucleotide synthesis. Methyl ethers – Cleavage is by TMSI in dichloromethane or acetonitrile or chloroform. An alternative method to cleave methyl ethers is BBr3 in DCM Ethoxyethyl ethers – Cleavage more trivial than simple ethers e.g. 1N hydrochloric acid Protection of amines: Carbobenzyloxy group – Removed by hydrogenolysis p-Methoxybenzyl carbonyl group – Removed by hydrogenolysis, more labile than Cbz tert-Butyloxycarbonyl group – Removed by concentrated strong acid, or by heating to >80 °C. 9-Fluorenylmethyloxycarbonyl group – Removed by base, such as piperidine Acetyl group is common in oligonucleotide synthesis for protection of N4 in cytosine and N6 in adenine nucleic bases and is removed by treatment with a base, most with aqueous or gaseous ammonia or methylamine. Ac is too stable to be removed from aliphatic amides.
Benzoyl group is common in oligonucleotide synthesis for protection of N4 in cytosine and N6 in adenine nucleic bases and is removed by treatment with a base, most with aqueous or gaseous ammonia or methylamine. Bz is too stable to be removed from aliphatic amides. Benzyl group -- Removed by hydrogenolysis Carbamate group -- Removed by mild heating. P-Methoxybenzyl – Removed by hydrogenolysis, more labile than benzyl 3,4-Dimethoxybenzyl – Removed by hydrogenolysis, more labile than p-methoxybenzyl p-methoxyphenyl group – Removed by ammonium cerium nitrate Tosyl group – Removed by concentrated acid & strong reducing agents Troc group – Removed by Zn insertion in the presence of acetic acid Other Sulfonamides groups – Removed by samarium iodide, tributyltin hydride Protection of carbonyl groups: Acetals and Ketals – Removed by acid; the cleavage of acyclic acetals is easier than of cyclic acetals. Acylals – Removed by Lewis acids. Dithianes – Removed by metal salts or oxidizing agents. Protection of carboxylic acids: Methyl esters -- Removed by base.
Benzyl esters – Removed by hydrogenolysis. Tert-Butyl esters – Removed by acid and some reductants. Esters of 2,6-disubstituted phenols – Removed at room temperature by DBU-catalyzed methanolysis under high-pressure conditions. Silyl esters – Removed by acid and organometallic reagents. Orthoesters – Removed by mild aqueous acid to form ester, removed according to ester properties. Oxazoline – Removed by strong hot acid or alkali, but not e.g. LiAlH4, organolithium reagents or Grignard reagents 2-cyanoethyl – removed by mild base; the group is used in oligonucleotide synthesis. Methyl – removed by strong nucleophiles e.c. thiophenole/TEA. Propargyl alcohols in the Favorskii reactio
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies between different classes of hydrates, some of which were so labeled before their chemical structure was understood. In organic chemistry, a hydrate is a compound formed by the addition of water or its elements to another molecule. For example: ethanol, CH3–CH2–OH, is the product of the hydration reaction of ethene, CH2=CH2, formed by the addition of H to one C and OH to the other C, so can be considered as the hydrate of ethene. A molecule of water may be eliminated, for example by the action of sulfuric acid. Another example is chloral hydrate, CCl3–CH2, which can be formed by reaction of water with chloral, CCl3–CH=O. Many organic molecules, as well as inorganic molecules, form crystals that incorporate water into the crystalline structure without chemical alteration of the organic molecule; the sugar trehalose, for example, exists as a dihydrate. Protein crystals have as much as 50% water content.
Molecules are labeled as hydrates for historical reasons not covered above. Glucose, C6H12O6, was thought of as C66 and described as a carbohydrate. Methanol is sold as "methyl hydrate", implying the incorrect formula CH3OH2, while the correct formula is CH3–OH. Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the crystal" that are either bound to a metal center or that have crystallized with the metal complex; such hydrates are said to contain water of crystallization or water of hydration. If the water is heavy water, where the hydrogen involved is the isotope deuterium the term deuterate may be used in place of hydrate. A colorful example is cobalt chloride, which turns from blue to red upon hydration, can therefore be used as a water indicator; the notation "hydrated compound⋅nH2O", where n is the number of water molecules per formula unit of the salt, is used to show that a salt is hydrated. The n is a low integer, though it is possible for fractional values to occur.
For example, in a monohydrate n is one, in a hexahydrate n is 6. Numerical prefixes of Greek origin are: A hydrate which has lost water is referred to as an anhydride. A substance that does not contain any water is referred to as anhydrous; some anhydrous compounds are hydrated so that they are said to be hygroscopic and are used as drying agents or desiccants. Clathrate hydrates are water ice with gas molecules trapped within. An important example is methane hydrate. Nonpolar molecules such as methane can form clathrate hydrates with water under high pressure. Although there is no hydrogen bonding between water and guest molecules when methane is the guest molecule of the clathrate, guest-host hydrogen bonding forms when the guest is a larger organic molecule such as tetrahydrofuran. In such cases the guest-host hydrogen bonds result in the formation of L-type Bjerrum defects in the clathrate lattice; the stability of hydrates is determined by the nature of the compounds, their temperature, the relative humidity