A carbocation (IPA pronunciation: [kɑ(r)'bəʊkæt'aɪ.ɒn]) is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore the carbon cation is often unstable and very reactive, seeking to fill its octet of valence electrons as well as regain its neutral charge. The carbocation has sp2 hybridization with a trigonal planar molecular geometry. For information on how to read IPA transcriptions of English words see here. ... Multivalent redirects here. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Atomic redirects here. ... e- redirects here. ... Example of a sodium electron shell model An electron shell, also known as a main energy level, is a group of atomic orbitals with the same value of the principal quantum number n. ... The bonding in carbon dioxide The octet rule is a simple chemical rule of thumb that states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, similar to the electronic configuration of a noble gas. ... A cation is an ion with positive charge. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... four sp³ orbitals three sp² orbitals In chemistry, hybridisation or hybridization (see also spelling differences) is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. ... ǃǁɚɵ A generic trigonal planar molecule showing ideal bond angle. ...

Definitions

A carbocation was previously often called a carbonium ion but questions arose on the exact meaning ^[1]. In present day chemistry a carbocation is any positively charge carbon atom and two special types exist: carbenium ions are trivalent and carbonium ions are pentavalent or hexavalent. University level textbooks only discuss carbocations as if they are carbenium ions ^[2], or discuss carbocations with a fleeting reference to the older phrase of carbonium ion ^[3] or carbenium and carbonium ions ^[4]. One textbook to this day clings on to the older name of carbonium ion for carbenium ion and reserves the phrase hypervalent carbenium ion for CH₅^{+ [5]}. A carbenium ion is a carbocation of the trivalent and classical type R3C+. A carbonium ion is a carbocation of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+. Carbonium ions appear as nonclassical carbocations such as the norbornyl cation These definitions have been proposed... A carbonium ion is a carbocation of the penta- or tetracoordinated nonclassical type such as an ion of the type R5C+ . The parent compound methonium or CH5+ is protonated methane and a superacid. ...

History

The history of the science of carbocations dates back to 1891 when G. Merling (Ber. 24, 3108 (1891)) reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water soluble material, C7H7Br. He did not suggest a structure for it; however Doering and Knox (JACS 76, 3203 (1954)) convincingly showed that it was tropylium bromide (cycloheptatrienylium bromide). In 1902 chemists Norris and Kehrman independently discovered that colorless triphenylmethyl alcohol gave deep yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes with aluminum and tin chlorides. Adolf von Baeyer recognized in 1902 the salt like character of the compounds formed. Sulfuric acid (British English: sulphuric acid), H2SO4, is a strong mineral acid. ... Johann Friedrich Wilhelm Adolf von Baeyer (October 31, 1835 - August 20, 1917) was a German chemist who synthesized indigo, and was the 1905 recipient of the Nobel Prize in Chemistry. ...

Ph₃C-OH + H₂SO₄ → Ph₃C⁺HSO₄^- + H₂O (Ph stands for a phenyl substituent)

He dubbed the relationship between color and salt formation halochromy of which malachite green is a prime example. In chemistry, the phenyl group or phenyl ring (often abbreviated as -Ph) is the functional group with the formula -C6H5 Picture where the six carbon atoms are arranged in a cyclic manner. ... In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... Structure Chromatic Form of Malachite green with Chlorine salt Malachite green (also aniline green, basic green 4, diamond green B, or victoria green B, IUPAC name:4-[(4-dimethylaminophenyl)-phenyl-methyl]-N,N-dimethyl-aniline) is a toxic chemical primarily designed to be a dye and is subsequently used to...

A carbocation in a chemical reaction is a reactive intermediate. This idea was first developed by Hans Meerwein in his 1922 study ^[6] of the Wagner-Meerwein rearrangement. Carbocations were also found to be involved in the SN1 reaction and E1 reaction and 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. A chemical reaction occurs when vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances [1]. The substance or substances initially involved... In chemistry a reactive intermediate is a short-lived high energy highly reactive molecule. ... Year 1922 (MCMXXII) was a common year starting on Sunday (link will display full calendar). ... A Wagner-Meerwein rearrangement is a class of carbocation 1,2-rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighbouring carbon. ... The SN1 reaction is an substitution reaction in organic chemistry. ... An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. ... A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. ... A 1,2-rearrangement or 1,2-shift or Whitmore 1,2-shift is a organic reaction where a substituent moves from one atom to another atom in a chemical compound. ...

The first NMR spectrum of a stable carbocation in solution was published by Doering et. al. (Tetrahedron 4, 178 (1958)). It was the heptamethylbenzenonium ion, made by treating hexamethylbenzene with methyl chloride and aluminum chloride. The stable 7-norbornadienyl cation was prepared by Story et. al. (JACS 82, 6199 (1960). The NMR spectrum established that it was nonclassically bridged (the first stable nonclassical ion observed).

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 norbornyl cation was first reported by Schleyer et. al. (JACS 86, 5679 (1964)) and it was shown to undergo proton scrambling over a barrier by Saunders et. al. (JACS 86, 5680 (1964)). George Andrew Olah (born May 22, 1927 as György Oláh) is a Hungarian-born American chemist. ... In organic chemistry, butyl is a four-carbon alkyl substituent with chemical formula -C4H9. ... Pacific Northwest National Laboratorys high magnetic field (800 MHz, 18. ... A superacid is an acid with an acidity greater than that of 100% sulfuric acid. ...

Properties

Order of stability of examples of
tertiary ( III ), secondary ( II ), and primary ( I ) alkyl carbocations

In organic chemistry, a carbocation is often the target of nucleophilic attack by nucleophiles like OH^- ions or halogen ions. Image File history File links Order of stability of alkyl carbocations selfmade by Polimerek 22:47, 21 Mar 2005 (UTC) File links The following pages link to this file: Carbocation ... Organic chemistry is a specific discipline within chemistry which involves the scientific study of the structure, properties, composition, reactions, and preparation (by synthesis or by other means) of chemical compounds consisting of primarily carbon and hydrogen, which may contain any number of other elements, including nitrogen, oxygen, halogens as well... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... Oh can refer to: An abbreviation for the U.S. State of Ohio. ... The halogens are a chemical series of nonmetals. ...

Carbocations are classified as primary, secondary, or tertiary depending on the number of carbon atoms bonded to the ionized carbon. Primary carbocations have one or zero carbons attached to the ionized carbon, secondary carbocations have two carbons attached to the ionized carbon, and tertiary carbocations have three carbons attached to the ionized carbon.

Stability of the carbocation increases with the number of alkyl groups bonded to the charge-bearing carbon. Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher-order carbons are stabilized by Hyperconjugation, unsubstituted (primary) carbons are not. Therefore, reactions such as the S_N1 reaction and the E1 elimination reaction normally do not occur if a primary carbocation would be formed. An exception to this occurs when there is a carbon-carbon double bond next to the ionized carbon. Such cations as allyl cation CH₂=CH-CH₂⁺ and benzyl cation C₆H₅-CH₂⁺ are more stable than most other carbocations. Molecules which can form allyl or benzyl carbocations are especially reactive. ( Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a s-bond (usually C-H or C-C) with an adjacent empty (or partially filled) p-orbital or a p-orbital to give an extended molecular orbital that increases the stability of the system. ... The SN1 reaction is an substitution reaction in organic chemistry. ... An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. ... An allyl group is an alkene hydrocarbon group with the formula H2C=CH-CH2-. It is made up of a vinyl group, CH2=CH-, attached to a methylene -CH2. ... Benzyl is the Organic chemistry term for the radical C6H5CH2-. It is obtained by removing a proton from benzene. ...

Carbocations undergo rearrangement reactions from less stable structures to equally stable or more stable ones with rate constants in excess of 1.0E9/sec. This fact complicates synthetic pathways to many compounds. For example, when 3-pentanol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce about 1/3 3-chloropentane and 2/3 2-chloropentane. A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. ... In chemical kinetics a reaction rate constant quantifies the speed of a chemical reaction. ...

Some carbocations such as the norbornyl cation exhibit more or less symmetrical three centre bonding. Cations of this sort have been referred to as non-classical ions. The energy difference between "classical" carbocations and "non-classical" isomers is often very small, and there is generally little, if any activation energy involved in the transition between "classical" and "non-classical" structures. The "non-classical" form of the 2-butyl carbocation is essentially 2-butene with a proton directly above the centre of what would be the carbon-carbon double bond. "Non-classical" carbocations were once the subject of great controversy. One of George Olah's greatest contributions to chemistry was resolving this controversy ^[7]. Non-classical ions in organic chemistry are a special type of carbonium ions displaying delocalization of sigma bonds in 3-center-2-electron bonds of bridged systems . ...

References