Carbenium ion

The carbenium ion is a kind of positive ion with the structure RR′R″C⁺, that is, a chemical species with carbon atom having three covalent bonds, and it bears a +1 formal charge. Carbenium ions are a major subset of carbocations, which is a general term for diamagnetic carbon-based cations. In parallel with carbenium ions is another subset of carbocations, the carbonium ions with the formula R₅⁺. In carbenium ions charge is localized. They are isoelectronic with monoboranes such as B(CH₃)₃.

Carbenium ions are generally highly reactive due to having an incomplete octet of electrons; however, certain carbenium ions, such as the tropylium ion, are relatively stable due to the positive charge being delocalised between the carbon atoms.（It can even exist stably in aqueous solution.）

Carbenium ions sometimes rearrange readily. For example, when pentan-3-ol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce 3-chloropentane and 2-chloropentane in a ratio of approximately 1:2.^{[full citation needed]} Migration of an alkyl group to form a new carbocationic center is also observed. This often occurs with rate constants in excess of 10¹⁰ s⁻¹ at ambient temperature and still takes place rapidly (compared to the NMR timescale) at temperatures as low as −120 °C (see Wagner-Meerwein shift). In especially favorable cases like the 2-norbornyl cation, hydrogen shifts may still take place at rates fast enough to interfere with X-ray crystallography at 86 K (−187 °C). Typically, carbocations will rearrange to give a tertiary isomer. For instance, all isomers of C₆H+11 rapidly rearrange to give the 1-methyl-1-cyclopentyl cation. This fact often complicates synthetic pathways. 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 one third 3-chloropentane and two thirds 2-chloropentane. The Friedel–Crafts alkylation suffers from this limitation; for this reason, the acylation (followed by Wolff–Kishner or Clemmensen reduction to give the alkylated product) is more frequently applied.

Carbocations are susceptible to attack by nucleophiles, like water, alcohols, carboxylates, azide, and halide ions, to form the addition product. Strongly basic nucleophiles, especially hindered ones, favor elimination over addition. Because even weak nucleophiles will react with carbocations, most can only be directly observed or isolated in non-nucleophilic media like superacids.

The stability order of carbocations, from most stable to least stable as reflected by hydride ion affinity (HIA) values, are as follows (HIA values in kcal/mol in parentheses):

Since carbenium ions can be highly reactive, a major consideration is their stability. The stability of carbenium ions correlates with the electron-donating properties of the substituents. Trialkylcarbenium ions, such as (CH₃)₃C⁺, are isolable as salts, but H₃C⁺ cannot. An analogous situation applies to triarylcarbenium ions: salts of triphenylcarbenium (C₅H₅)₃C⁺ are readily isolable (see trityl), and those with amine substituents so robust that they are used as dyes, e.g. crystal violet. Carbenium ions can also be stabilized by conjugation to double bonds giving allyl cations, which enjoy some resonance stabilization. This situation is illustrated by the isolation of protonated benzene. Lone-pair bearing heteroatoms also stabilize carbenium ions.

The stability of alkyl-substituted carbocations follows the order 3° > 2° > 1° > methyl. This trend can be inferred by the hydride ion affinity values (231, 246, 273, and 312 kcal/mol for (CH₃)₃C⁺, (CH₃)₂CH⁺, CH₃CH+2, and CH+3). The effect of alkyl substitution is a strong one:

Carbenium ions can be prepared directly from alkanes by removing a hydride anion, H⁻
, with a strong acid. (Equivalently, the corresponding carbonium ions tend to eliminate H₂.) For example, magic acid, a mixture of antimony pentafluoride (SbF
₅) and fluorosulfuric acid (FSO
₃H), turns isobutane into the trimethylcarbenium cation, (CH
₃)
₃C⁺
.