Classical carbonium ions. Part I. The solvolysis of some typical secondary arenesulphonates

Abstract

Rigorous analytical studies are reported on solvolysis products of arenesulphonates derived from octan-4-ol, the 4-t-butylcyclohexanols, and the trans-2-decalols. Factors varied include temperature, solvent, the concentration and nature of the buffer salt present, and the substituent present in the aromatic ring. The results, taken with kinetic data already published and with ancilliary work, also now reported, on the related oct-2-yl, 3-t-butylcyclohexyl, and trans-1-decalyl systems, allow some conclusions about the behaviour of classical secondary carbonium ions when present in ion-pairs. Thus, such carbonium ions undergo hydride shift to isomeric carbonium ions, to an extent favoured by weakly nucleophilic solvents, high temperatures, and electronegative substituents in the counter-ion. Arenesulphonates having the functional group mainly equatorial may react (contrary to earlier reports) with as much or more rearrangement than their mainly axial epimers; they probably do so via non-chair conformers. These rearrangements occur within ion-pairs; the counter-ion controls the configuration of the substitution products formed by rearrangement (indeed rearranged covalent arenesulphonates are possible intermediates); this is true even in solvents as polar as 50 % ethanol. A nucelophilic group (m-CO₂^–) in the counter-ion does not compete effectively with solvent, probably because of the short life of the cationic intermediate. The side-reaction recently observed in an acyclic system, external return of arenesulphonate ion, is observed in cyclic systems by a convenient new technique.