Observation of Carbocations

A review of the extensive studies of carbocations in superacid media is available in G. A. Olah, G. K. Surya Prakash, and J. Sommer, Super Acids, John Wiley Sons, New York, 1985. 

R = methyl, ethyl, n-propyl, /-propyl, n-butyl, s-butyl, n-amyl, /-amyl, neopentyl, n-hexyl, neohexyl 

Another important development in permitting structural conclusions from NMR studies on carbocations resulted from the use of theoretical computations of and chemical shifts. Known as the MP2-G1AO method,it has also been applied successfully to allylic, cyclopropylmethyl, and phenonium ions. °  

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Early Unsuccessful Attempts. Until the early 1960s, simple alkyl cations were considered only as transient species.15 Their existence has been inferred from the kinetic and stereochemical studies of reactions. No reliable physical measurements, other than electron impact measurements in the gas phase (mass spectrometry), were known. The formation of gaseous organic cations under electron bombardment of alkenes, haloalkanes, and other precursors has been widely investigated in mass spectrometric studies.81 No direct observation of carbocations in solutions was achieved prior to the early 1960s. 

The presence of cyclohexenyl cation intermediates was firmly established by the observation of carbocation rearrangement during the solvolysis of 25, in which an initially generated bent vinyl cation 26 with sp2 hybridization rearranges to a more stable linear vinyl cation 27 with sp hybridization [Eq. (16)]. The ratio of the rearranged to the unrearranged ketones depends on the nature of solvents used and changed from 14 86 (27a 26a) in 60% aqueous ethanol to 46 54 in the less nucleophilic 2,2,2-trifluoroethanol. 

Until the early 1960s information about carbocations has been obtained almost exclusively from indirect evidence. A major change occurred in 1962 when Olah reported the generation and direct observation of carbocations as long-lived species in solvents of low nucleophilicity (superacidic conditions) [11,12]. Many types of carbocations have since... 

In summary, the observation of carbocations in superacids has provided a wealth of previously inaccessible information on stmcture and reactivity. The infinite lifetime of the cations precludes, of course, any stereochemical studies. On a scale of activation energies, the rearrangements studied in superacids overlap in part with those observed in more nucleophilic solvents and extend toward higher energy barriers. 

The arguments discussed to this point, both for and against the nonclassical structure, rest on indirect evidence derived from interpretation of the kinetic and stereochemical characteristics of the substitution reactions. When techniques for direct observation of carbocations became available, the norbomyl cation was subjected to intense study from this perspective. 

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

This IS a frequently used proce dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability R3C > R2CH > RCH2 Benzylic al cohols react readily Rearrangements are sometimes observed... 

These reactions involve the intermediate formation of thiols, followed by condensation to the sulfides. The observation of isomerized products in suitable cases indicates the intermediate formation of carbocations, either by protolysis of alkanes by the superacid or reversible ionisation of the thiol products (149). 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

The pinacol rearrangement is frequently observed when geminal diols react with acid. The stmcture of the products from unsymmetrical diols can be predicted on the basis of ease of carbocation formation. For example, l,l-diphenyl-2-metltyl-l,2-propanediol rearranges to... 

Compare atomic charges and electrostatic potential maps of the two carboeations. In which is the positive charge more delocalized For each carbocation, draw whatever resonance contributors are needed to account for all of your observations. Which carbocation is better stabilized by resonance ... 

The second point to explore involves carbocation stability. 2-Methyl-propene might react with H+ to form a carbocation having three alkyl substituents (a tertiary ion, 3°), or it might react to form a carbocation having one alkyl substituent (a primary ion, 1°). Since the tertiary alkyl chloride, 2-chloro-2-methylpropane, is the only product observed, formation of the tertiary cation is evidently favored over formation of the primary cation. Thermodynamic measurements show that, indeed, the stability of carbocations increases with increasing substitution so that the stability order is tertiary > secondary > primary > methyl. 

In view of the observations of the ionic dissociation of nitro-cyano compounds, it is hardly surprising that even a hydrocarbon could dissociate ionically into a stable carbocation and carbanion, provided that the medium is polar enough to prevent the recombination reaction and to ensure equilibration. 

A few observations of the isolation of carbocation-carbanion salt containing heteroatoms, such as oxygen and nitrogen, have been reported. Over 30 years ago LeGoff and LaCount (1963) reported the formation of a... 

Direct reduction of carbocations is not common but has been observed, e.g. with vanadium(II) chloride ... 

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