Carbocation

Structure and properties

Carbonium ions

Carbonium ions can be thought of as protonated or alkylated alkanes, bearing the general formula CR₅⁺ (R = alkyl or H). A typical example is the methanium ion, CH₅⁺, which is formed by protonation of methane using a superacid. By necessity of having five bonds on carbon but only four valence electron pairs available for bonding, they feature delocalized 3c-2e σ bonding and are thus regarded as type of non-classical carbocation. Like carbenium ions, carbonium ions are often invoked as intermediates in the upgrading of hydrocarbons in refineries. They are generally fleeting intermediates with fluxional structures that are challenging to observe and interpret spectroscopically. They can undergo decomposition by expulsion of a proton or alkyl group, or by loss of H₂ to give a carbenium ion.

Carbenium ions

At least in a formal sense, carbenium ions (CR₃⁺) are derived from the protonation (addition of H⁺) or alkylation (addition of R⁺) of a carbene or alkene. They admit a resonance depiction in which one carbon atom bears a formal positive charge and is surrounded by six valence electrons instead of the usual octet. Therefore, carbenium ions (and carbocations in general) are often reactive, seeking to fill the valence octet and regain a neutral charge.

In accord with VSEPR and Bent's rule, unless geometrically constrained to be pyramidal (e.g., 1-adamantyl cation), 3-coordinate carbon in carbenium ions are usually trigonal planar and sp² hybridized; the lowest unoccupied molecular orbital is an empty pure p orbital pointing out-of-plane. A prototypical example is the t-butyl cation, CMe+3. Although classical carbenium ions have a structure that corresponds to a non-bridging Lewis structure, donation of electron density from neighboring C–H or C–C bonds into the "empty" p orbital, known as hyperconjugation, is still an important stabilizing factor, and these bonds have a tendency to "lean" towards the carbocationic center to improve orbital overlap.

There is, in fact, an entire spectrum of bonding scenarios between a slight lean due to hyperconjugation to a fully symmetric bridging structure featuring 3c2e bonding. Consequently, there is no firm dividing line between "classical" and the so-called "non-classical" structures.

Non-classical carbenium ions

Non-classical carbenium ions feature also σ delocalization (3c2e bonds) in their bonding but have the general formula CR₃⁺ (R = alkyl or H). Thus, in principle, one can propose non-bridged, classical structures for these cations, as well as a bridged non-classical structure. Because of the subtle differences in the expected behavior of a non-classical carbenium ions compared to the alternative hypothesis of two rapidly equilibrating classical structures, a lively and often acrimonious debate took place over several decades regarding the merits of each model. For a detailed history of this dispute, see the article on the 2-norbornyl cation. Currently, there is overwhelming evidence that, at least in some cases (notably the extremely well-studied 2-norbornyl cation), the equilibrium structure of a carbenium ion is non-classical, although even minor changes in structure could result in a classical structure being favored.

History

The history of carbocations dates back to 1891 when G. Merling reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble material, C
₇H
₇Br. He did not suggest a structure for it; however, Doering and Knox convincingly showed that it was tropylium (cycloheptatrienylium) bromide. 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 similarly formed orange complexes when treated 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. The trityl carbocation (shown below) is indeed a stable carbocationic system, for example in the form of trityl hexafluorophosphate.

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 also found to be involved in the S_N1 reaction, the 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.

An NMR spectrum of a carbocation was first reported 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 that it was non-classically bridged (the first stable non-classical 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 spectrum of the norbornyl cation was reported by Schleyer et al. It was shown to rapidly undergo proton-scrambling.