Solution carbocation stabilities measured by internal competition for a hydride ion

Abstract

Although many techniques are known which allow one to compare the stabilities of solution carbocations, that involving the intermolecular competition for a hydride ion is conceptually (but not experimentally) the simplest procedure. This paper describes a variant of this which is experimentally more reliable and which uses intramolecular equilibria where the two competing systems are held together by a —(CH2)n— chain, e.g.[Formula: see text]By systematically varying "n" in this example (n = 0, 1,2, or 3), it has been found that a methylene chain of two or more carbons is necessary in order to minimize steric interactions between the end "systems". It has also been found that all cycloalkyl rings studied (except cyclohexyl) stabilize a cation centre much better than an aliphatic equivalent, i.e. [Formula: see text], in agreement with solvolysis rate studies. The same situation was found when comparing this aliphatic "system" against the 2-norbornyl cation (bicyclic) or against the tricyclic 2-adamantyl cation. In fact, in these cases the equilibria are too lop-sided to obtain numerical values for the equilibrium constants concerned. Finally, three carbocations were looked at where the 2-norbornyl cation structure was pitted against the structurally very related cyclopentyl, bicyclo[2.1.1]hexyl, and bicyclo[3.2.1]octyl cations. In all cases, the 2-norbornyl cation is the more stable. 13C nmr spectroscopy was used as the analytical tool to measure (or attempt to measure) the equilibrium constants. Depending on the rate of the equilibration process, three different techniques are involved and the relative merits of these are discussed in the latter part of the paper.