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.
Chemical Reviews is peer-reviewed scientific journal published twice per month by the American Chemical Society. It publishes review articles on all aspects of chemistry, it was established in 1924 by William Albert Noyes. As of 1 January 2015 the editor-in-chief is Sharon Hammes-Schiffer; the journal is abstracted and indexed in Chemical Abstracts Service, CAB International, EBSCOhost, ProQuest, PubMed and the Science Citation Index. According to the Journal Citation Reports, the journal has a 2017 impact factor of 52.613. Official website
A 1,2-rearrangement or 1,2-migration or 1,2-shift or Whitmore 1,2-shift is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. In the example below the substituent R moves from carbon atom C2 to C3; the rearrangement is intramolecular and the starting compound and reaction product are structural isomers. The 1,2-rearrangement belongs to a broad class of chemical reactions called rearrangement reactions. A rearrangement involving a hydrogen atom is called a 1,2-hydride shift. If the substituent being rearranged is an alkyl group, it is named according to the alkyl group's anion: i.e. 1,2-methanide shift, 1,2-ethanide shift, etc. A 1,2-rearrangement is initialised by the formation of a reactive intermediate such as: a carbocation by heterolysis in a nucleophilic rearrangement or anionotropic rearrangement a carbanion in a electrophilic rearrangement or cationotropic rearrangement a free radical by homolysis a nitrene.
The driving force for the actual migration of a substituent in step two of the rearrangement is the formation of a more stable intermediate. For instance a tertiary carbocation is more stable than a secondary carbocation and therefore the SN1 reaction of neopentyl bromide with ethanol yields tert-pentyl ethyl ether. Carbocation rearrangements are more common than radical counterparts; this observation can be explained on the basis of Hückel's rule. A cyclic carbocationic transition state stabilized because it holds 2 electrons. In an anionic transition state on the other hand 4 electrons are present thus antiaromatic and destabilized. A radical transition state is neither destabilized; the most important carbocation 1,2-shift is the Wagner–Meerwein rearrangement. A carbanionic 1,2-shift is involved in the benzilic acid rearrangement; the first radical 1,2-rearrangement reported by Heinrich Otto Wieland in 1911 was the conversion of bisperoxide 1 to the tetraphenylethane 2. The reaction proceeds through the triphenylmethoxyl radical A, a rearrangement to diphenylphenoxymethyl C and its dimerization.
It is unclear to this day whether in this rearrangement the cyclohexadienyl radical intermediate B is a transition state or a reactive intermediate as it has thus far eluded detection by ESR spectroscopy. An example of a less common radical 1,2-shift can be found in the gas phase pyrolysis of certain polycyclic aromatic compounds; the energy required in an aryl radical for the 1,2-shift can be high but much less than that required for a proton abstraction to an aryne. In alkene radicals proton abstraction to an alkyne is preferred; the following mechanisms involve a 1,2-rearrangement: 1,2-Wittig rearrangement Alpha-ketol rearrangement Beckmann rearrangement Benzilic acid rearrangement Brook rearrangement Criegee rearrangement Curtius rearrangement Dowd–Beckwith ring expansion reaction Favorskii rearrangement Friedel–Crafts reaction Fritsch–Buttenberg–Wiechell rearrangement Halogen dance rearrangement Hofmann rearrangement Lossen rearrangement Pinacol rearrangement Seyferth–Gilbert homologation SN1 reaction Stevens rearrangement Wagner–Meerwein rearrangement Westphalen–Lettré rearrangement Wolff rearrangement 1,3-rearrangements take place over 3 carbon atoms.
Examples: the Fries rearrangement a 1,3-alkyl shift of verbenone to chrysanthenone
Sulfuric acid known as vitriol, is a mineral acid composed of the elements sulfur and hydrogen, with molecular formula H2SO4. It is a colorless and syrupy liquid, soluble in water, in a reaction, exothermic, its corrosiveness can be ascribed to its strong acidic nature, and, if at a high concentration, its dehydrating and oxidizing properties. It is hygroscopic absorbing water vapor from the air. Upon contact, sulfuric acid can cause severe chemical burns and secondary thermal burns. Sulfuric acid is a important commodity chemical, a nation's sulfuric acid production is a good indicator of its industrial strength, it is produced with different methods, such as contact process, wet sulfuric acid process, lead chamber process and some other methods. Sulfuric acid is a key substance in the chemical industry, it is most used in fertilizer manufacture, but is important in mineral processing, oil refining, wastewater processing, chemical synthesis. It has a wide range of end applications including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries, in various cleaning agents.
Although nearly 100% sulfuric acid can be made, the subsequent loss of SO3 at the boiling point brings the concentration to 98.3% acid. The 98.3% grade is more stable in storage, is the usual form of what is described as "concentrated sulfuric acid". Other concentrations are used for different purposes; some common concentrations are: "Chamber acid" and "tower acid" were the two concentrations of sulfuric acid produced by the lead chamber process, chamber acid being the acid produced in the lead chamber itself and tower acid being the acid recovered from the bottom of the Glover tower. They are now obsolete as commercial concentrations of sulfuric acid, although they may be prepared in the laboratory from concentrated sulfuric acid if needed. In particular, "10M" sulfuric acid is prepared by adding 98% sulfuric acid to an equal volume of water, with good stirring: the temperature of the mixture can rise to 80 °C or higher. Sulfuric acid reacts with its anhydride, SO3, to form H2S2O7, called pyrosulfuric acid, fuming sulfuric acid, Disulfuric acid or oleum or, less Nordhausen acid.
Concentrations of oleum are either expressed in terms of % SO3 or as % H2SO4. Pure H2S2O7 is a solid with melting point of 36 °C. Pure sulfuric acid has a vapor pressure of <0.001 mmHg at 25 °C and 1 mmHg at 145.8 °C, 98% sulfuric acid has a <1 mmHg vapor pressure at 40 °C. Pure sulfuric acid is a viscous clear liquid, like oil, this explains the old name of the acid. Commercial sulfuric acid is sold in several different purity grades. Technical grade H2SO4 is impure and colored, but is suitable for making fertilizer. Pure grades, such as United States Pharmacopeia grade, are used for making pharmaceuticals and dyestuffs. Analytical grades are available. Nine hydrates are known, but four of them were confirmed to be tetrahydrate and octahydrate. Anhydrous H2SO4 is a polar liquid, having a dielectric constant of around 100, it has a high electrical conductivity, caused by dissociation through protonating itself, a process known as autoprotolysis. 2 H2SO4 ⇌ H3SO+4 + HSO−4The equilibrium constant for the autoprotolysis is Kap = = 2.7×10−4The comparable equilibrium constant for water, Kw is 10−14, a factor of 1010 smaller.
In spite of the viscosity of the acid, the effective conductivities of the H3SO+4 and HSO−4 ions are high due to an intramolecular proton-switch mechanism, making sulfuric acid a good conductor of electricity. It is an excellent solvent for many reactions; because the hydration reaction of sulfuric acid is exothermic, dilution should always be performed by adding the acid to the water rather than the water to the acid. Because the reaction is in an equilibrium that favors the rapid protonation of water, addition of acid to the water ensures that the acid is the limiting reagent; this reaction is best thought of as the formation of hydronium ions: H2SO4 + H2O → H3O+ + HSO−4 Ka1 = 2.4×106 HSO−4 + H2O → H3O+ + SO2−4 Ka2 = 1.0×10−2 HSO−4 is the bisulfate anion and SO2−4 is the sulfate anion. Ka1 and Ka2 are the acid dissociation constants; because the hydration of sulfuric acid is thermodynamically favorable and the affinity of it for water is sufficiently strong, sulfuric acid is an excellent dehydrating agent.
Concentrated sulfuric acid has a powerful dehydrating property, removing water from other chemical compounds including sugar and other carbohydrates and producing carbon and steam. In the laboratory, this is demonstrated by mixing table sugar into sulfuric acid; the sugar changes from white to dark brown and to black as carbon is formed. A rigid column of black, porous carbon will emerge as well; the carbon will smell of caramel due to the heat generated. C 12 H 22 O 11 ⏞ sucrose → H 2 SO 4 12 C + 11 H 2
The pinacol–pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. The 1,2-rearrangement takes place under acidic conditions; the name of the rearrangement reaction comes from the rearrangement of pinacol to pinacolone. This reaction was first described by Wilhelm Rudolph Fittig in 1860 of the famed Fittig reaction involving coupling of 2 aryl halides in presence of sodium metal in dry ethereal solution. In the course of this organic reaction, protonation of one of the –OH groups occurs and a carbocation is formed. If both the –OH groups are not alike the one which yields a more stable carbocation participates in the reaction. Subsequently, an alkyl group from the adjacent carbon migrates to the carbocation center; the driving force for this rearrangement step is believed to be the relative stability of the resultant oxonium ion, which has complete octet configuration at all centers. The migration of alkyl groups in this reaction occurs in accordance with their usual migratory aptitude, i.e.hydride > phenyl > tertiary carbanion > secondary carbanion > methyl carbanion.
The conclusion is that the group which stabilizes the carbocation more is migrated. In cyclic systems, the reaction presents more features of interest. In these reactions, the stereochemistry of the diol plays a crucial role in deciding the major product. An alkyl group, situated trans- to the leaving –OH group alone may migrate. If otherwise, ring expansion occurs, i.e. the ring carbon itself migrates to the carbocation centre. This reveals another interesting feature of the reaction, viz. that it is concerted. There appears to be a connection between the migration origin and migration terminus throughout the reaction. Moreover, if the migrating alkyl group has a chiral center as its key atom, the configuration at this center is retained after migration takes place. Although Fittig first published about the pinacol rearrangement, it was not Fittig but Aleksandr Butlerov who identified the reaction products involved. In an 1859 publication Wilhelm Rudolph Fittig described the reaction of acetone with potassium metal.
Fittig wrongly assumed a molecular formula of n for acetone, the result of a long-standing atomic weight debate settled at the Karlsruhe Congress in 1860. He wrongly believed acetone to be an alcohol which he hoped to prove by forming a metal alkoxide salt; the reaction product he obtained instead he called paraceton which he believed to be an acetone dimer. In his second publication in 1860 he reacted paraceton with sulfuric acid. Again Fittig was unable to assign a molecular structure to the reaction product which he assumed to be another isomer or a polymer. Contemporary chemists who had adapted to the new atomic weight reality did not fare better. One of them, Charles Friedel, believed the reaction product to be the epoxide tetramethylethylene oxide in analogy with reactions of ethylene glycol. Butlerov in 1873 came up with the correct structures after he independently synthesised the compound trimethylacetic acid which Friedel had obtained earlier by oxidizing with a dichromate; some of the problems during the determination of the structure are because carbon skeletal rearrangements were unknown at that time and therefore the new concept had to be found.
Butlerov theory allowed the structure of carbon atoms in the molecule to rearrange and with this concept a structure for pinacolone could be found. Semipinacol rearrangement Tiffeneau–Demjanov rearrangement, in which the leaving
Sergey Semyonovich Namyotkin was a Russian chemist, a prominent researcher in terpene chemistry and rearrangement of camphenes. The Nametkin rearrangement is the shift of a methyl group in this scheme and called the'Nametkin' step; the shift of the ring bond is a standard Wagner-Meerwein shift. The reaction can in fact be used using chlorocamphene. Academician S. S. Nametkin's memorial office, a department of the A. V. Topchiev institute of Petrochimical Synthesis, established in 1974 on occasion of the centenary of academician Nametkin's birth. S. S. Nametkin an outstanding scientist in the field of organic chemistry and petrochemistry, professor of the Moscow State University. Main direction of activity: study and use of archives. S. Nametkin in organic chemistry. Sergey Nametkin's Cabinet Museum
Terpenes are a large and diverse class of organic compounds, produced by a variety of plants conifers, by some insects. They have a strong odor and may protect the plants that produce them by deterring herbivores and by attracting predators and parasites of herbivores. Although sometimes used interchangeably with "terpenes", terpenoids are modified terpenes as they contain additional functional groups oxygen-containing. Terpenes are hydrocarbons. Terpenes are the major components of turpentine produced from resin; the name "terpene" is derived from the word "terpentine", an obsolete form of the word "turpentine". Terpenes are major biosynthetic building blocks. Steroids, for example, are derivatives of the triterpene squalene. Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used as fragrances in perfumery and traditional medicine, such as aromatherapy. Synthetic variations and derivatives of natural terpenes and terpenoids greatly expand the variety of aromas used in perfumery and flavors used in food additives.
Vitamin A is a terpenoid. The term "terpene" was coined in 1866 by the German chemist August Kekulé. Terpenes are derived biosynthetically from units of isopentenyl pyrophosphate. Although the structures of terpenoids are rationalized as derivatives of isoprene, isoprene is not involved in the biosynthesis; the biogenetic isoprene rule or the C5 rule was described in 1953, by Leopold Ružička, who explained that terpinoids can be visualized as the result of linking isoprene units "head to tail" to form chains, which can be arranged to form rings. There are two metabolic pathways that create terpenoids: Many organisms manufacture terpenoids through the HMG-CoA reductase pathway, known as the Mevalonate pathway, which produces cholesterol. One of the intermediates in this pathway is mevalonic acid; the reactions take place in the cytosol. The pathway was discovered in the 1950s; the 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway known as non-mevalonate pathway or mevalonic acid-independent pathway, takes place in the plastids of plants and apicomplexan protozoa, as well as in many bacteria.
It was discovered in the late 1980s. Pyruvate and glyceraldehyde 3-phosphate are converted by DOXP synthase to 1-deoxy-D-xylulose 5-phosphate, by DOXP reductase to 2-C-methyl-D-erythritol 4-phosphate; the subsequent three reaction steps catalyzed by 4-diphosphocytidyl-2-C-methyl-D-erythritol synthase, 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase mediate the formation of 2-C-methyl-D-erythritol 2,4-cyclopyrophosphate. MEcPP is converted to -4-hydroxy-3-methyl-but-2-enyl pyrophosphate by HMB-PP synthase, HMB-PP is converted to isopentenyl pyrophosphate and dimethylallyl pyrophosphate by HMB-PP reductase. IPP and DMAPP are the end-products in either pathway, are the precursors of isoprene, diterpenoids, carotenoids and plastoquinone-9. Synthesis of all higher terpenoids proceeds via formation of geranyl pyrophosphate, farnesyl pyrophosphate, geranylgeranyl pyrophosphate; the MVA and MEP are mutually exclusive in most organisms. In both MVA and MEP pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.
IPP and DMAPP condense to give geranyl pyrophosphate, the precursor to monoterpenes and monoterpenoids. Geranyl pyrophosphate is converted to farnesyl pyrophosphate and geranylgeranyl pyrophosphate C15 and C20 precursors to sesquiterpenes and diterpenes. Biosynthesis is mediated by terpene synthase. Selected terpenes Terpenes may be classified by the number of isoprene units in the molecule. Hemiterpenes consist of a single isoprene unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids. Monoterpenes consist of two isoprene units and have the molecular formula C10H16. Examples of monoterpenes and monoterpenoids include geraniol, limonene, linalool or pinene. Iridoids derive from monoterpenes. Sesquiterpenes consist of three isoprene units and have the molecular formula C15H24. Examples of sesquiterpenes and sesquiterpenoids include humulene, farnesol. Diterpenes are composed of four isoprene units and have the molecular formula C20H32.
They derive from geranylgeranyl pyrophosphate. Examples of diterpenes and diterpenoids are cafestol, kahweol and taxadiene. Diterpenes form the basis for biologically important compounds such as retinol and phytol. Sesterterpenes, terpenes having 25 carbons and five isoprene units, are rare relative to the other sizes. An example of a sesterterpenoid is geranylfarnesol. Triterpenes consist of six isoprene units and have the molecular formula C30H48; the linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate. Squalene is processed biosynthetically to generate either lanosterol or cycloartenol, the structural precursors to all the steroids. Sesquarterpenes are composed of s