Carbonium ion is a positively charged unstableorganic compound。Like free radicals, it is an active intermediatepositive chargeThere are six electrons on the outermost layer.
The classical carbonium ion is a planar structure.The positively charged carbon atom isSp2 hybridThe three sp2 hybrid orbitals form σ bonds with the orbitals of the other three atoms to form a plane, the bond angle is close to 120 °, and the remaining p orbitals of the carbon atoms are perpendicular to this plane.Analysis of this substance is crucial to finding dozens of modern necessary chemical products that can be manufactured cheaply.
The history of Carbonium ion can be traced back to 1891. G. Merling said that he added bromine toCycloheptatrieneAnd then heat the crystallization product to obtain water-soluble substance CsevenHsevenBr，It produced a structure that he could not explain. However, Doering and Knox predicted that the brominated cyclohexanetriene was in line with H ü ckel's ruleAromatic groupIn 1902, Norris and Kehrman found that colorless triphenyl methanol would turn dark yellow in concentrated sulfuric acid. Similarly,Triphenyl chloromethaneWill followaluminumIn the same year, Adolf von Baeyer thought that its compounds formed a salt like structure. Adolf guessed that the relationship between color and salt structure was like malachite green. Carbon cations played the role of intermediates in many organic reactions. The concept was first published by Julius Stieglitz in 1899,Hans Meerwein's research in 1922 Wagner Meerwein rearrangement (Wagner Meerwein rearrangement) has further developed its conceptN1. E1 and recombination reaction (such as Whitmore 1,2 transfer) were found to be involved in the reaction. For a long time, American chemists were unwilling to accept a new chemical concept and did not mention the concept of carbocation in the article
The NMR spectrum of the first stable carbonium ion in solution was published by Doering et al. in 1958. It is heptamethylbenzene ion, which is added by hexamethylbenzeneAluminium chloride, methyl chloride. Story et al. prepared stable 7-norbornadienyl cation in 1960, which was dissolved in liquid by norbornadienyl chloride at - 80 ℃sulfur dioxideOfSilver tetrafluoroborate The NMR spectrum shows a non-traditional intermediate (the first non-traditional stable ion found) in 1962,Olah directly found that the tertiary butyl carbon cation is as stable as the tertiary butyl fluoride dissolved in magic acid by means of nuclear magnetic resonance. The NMR spectrum of the norbornyl cation was first published by Schleyer et al., while Saunders et al. showed the process of proton grabbing between barriers[1]
structure
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Like free radicals, carbocations are active intermediates.There is one carbonionpositive chargeThere are six electrons on the outermost layer.Positively charged carbonatomWith sptwoHybrid orbitWith 3 atoms (orAtomic cluster）Combine to form 3σ bond, in the same plane as the carbon atom.The remaining P orbitals of the carbon atoms are perpendicular to this plane.The carbonium ion is a planar structure.
It was reported in 1963 that a simple carbonium ion was observed directly, which proved its planar structure and provided an experimental basis for its existence and structure.
According to the position of the positively charged carbon atom, it can be divided into primary carbonium ion, secondary carbonium ion and tertiary carbonium ion.
The structure and stability of carbonium ions are directly affected by the groups connected to them.The general rule of their stability is as follows:
（1）benzylType A or allyl type is generally stable;
(2) Other carbon positive ions are: 3 °>2 °>1 °;
However, the stability comparison between allyl and benzyl carbocations and secondary carbocations is still controversial.
(The stability of different carbonium ions can be explained by hyperconjugation)
The more stable the carbon positive ion is, the lower the energy is, and the easier the formation is.
According to different structural characteristics, carbocations can be divided into classical carbocations and non classical carbocations[2]
Formation process
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The formation process of carbon positive ion is like this: there was aElectronegativityLarge or electron absorbing groups (such as - Br, - OH, etc.) will attract electrons on the connected carbon to make the carbon slightly positiveElectron withdrawing groupIt is possible to leave with a pair of electrons (e.g. Br -) in a suitable solution, then the remainingAlkyl groupThe carbon positive ion is formed.
example:
+
=
(+) +
stability
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Stability is usually achieved by increasing the amount of alkyl bonded to charge bearing carbon.Tertiary carbon cations are more stable (and easier to form) than secondary carbon cations because they are formed byHyperconjugation。The main carbopositive ions are very unstable.Therefore, the reaction is as followsSñ1 ReactionAnd E1Elimination reactionUsually, if primary carbon is not formed, it is happening.
However, double bonded carbon with ionized carbon can stabilize the passage of ionsresonance。These cations act asAllyl groupCation, CHtwo= CH-CHtwo, andbenzylCation, Csixħfive-CHtwo, more stable than most other carbon cations.The ability to form allyl or benzyl carbeniums is particularly reactive.Carbonium ion, which can also beHeteroatom。
Carbion may occurRearrangement reaction, an unstable structure, with the same or more stable peoplerate constantMore than 10/sec.This fact complicates the synthesis of many compounds.For example, when 3-amyl alcohol is heated in HCl aqueous solution, the initially formed 3-amyl carbocation is rearranged to the statistical mixture of 3-amyl and 2-amyl.These cations react with chloride ions to produce about 1/3 3-chloropentane and 2/3 - 2-chloropentane.[3-4]
type
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Carbonium ions are classified as primary, secondary, or tertiary carbon positive ions, depending on whether the number of carbon atoms bound to the ionized carbon is 1, 2, or 3.
DescribedOnium ionIs a kind ofAromatic groupSubstance and formula
。From the molecule, its name comes fromTropicin(named molecule itselfatropine）。The salt of the onium cation can be stable, such asOnium tetrafluoroborate。It can be made fromCycloheptetriene(trophylidene) and bromine orPhosphorus pentachloride。It is aHeptagonalFlat, circular ions;It also has 6 π electrons (4n+2, where n=1), which satisfiesHull ruleThe fragrance of.It can be used as a coordinateLigands, with metalatom。
Triphenylmethyl cation
TriphenylCation, (+) C (CsixH5)3, is particularly stable, because the positive charge can be located at the carbon atom in 10 (the position of the three phenyl groups adjacent to and para to the distribution of three carbon atoms, plus the central carbon atom).It exists in the compoundTriphenylmethyl hexafluorophosphate（C(CsixH5)3PFsix）AndTriphenyl perchlorate（C(CsixHfive)threeClOfour）。
Arenium ion
An arenium ion is shown as a cyclohexadiene cation in a reaction intermediateElectroaromatic substitution。Due to historical reasons, this complex is also called Welland intermediate, or σ complex.The arenium ion with two hydrogen atoms bonded to a carbon in a plane perpendicular to the benzene ring is no longer an aromatic substance;However, it is due to the relatively stable delocalization: positive charges pass through the π system with more than five carbon atoms delocalized.
Acylium ion
An acylum ion is a cation with formula RCO.The structure of is described as RC ∨ O or RC=O.It is synthetic and reactive equivalent acyl carbon cation, but the actual structure has oxygen and carbon connected through three bonds.These species are commonActive intermediateFor example, Friedel Kraft acylation is also involved in many other organic reactions.[2]
Main role
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Carbon positive ions exist widely in many chemical reactions. Understanding carbon positive ions is helpful to grasp the essence of many complex chemical reactions.Analysis of this substance is crucial to finding dozens of modern necessary chemical products that can be manufactured cheaply.EulerThe professor discovered the use ofSuperacidThe method of keeping the carbopositive ion stable can prepare high concentration carbopositive ion and study it carefully.His findings have been used to improve the efficiency of oil refining, produce unleaded gasoline and develop new drugs.[5]
