Elimination–addition. Part XIII. Reactions of ω-bromoalkyl p-tolyl sulphones with bases: the role of inductive effects in elimination and substitution

Abstract

Rates of reaction of a series of sulphones, p-Me·C₆H₄·SO₂·[CH₂]_n·Br (n= 3–6) with sodium ethoxide in ethanol and with potassium t-butoxide in t-butyl alchol have been measured. Three types of reaction occur, namely substitution (ethoxide reactions only), elimination, and cyclisation. Quantitative analysis of the products allows dissection of the overall rate constants into the respective rate constants for each type of reaction in each system.

For sulphones in which n= 4–6, rates of elimination and of substitution fall as n rises in each system. Even when n= 6, however, rates for each type of reaction are markedly higher than for the corresponding alkyl bromide, and there is a relatively slow diminution in elimination rates with increase in n. It is concluded that the inductive effect of a remote substituent promotes both elimination and substitution and that there is an important field component in the overall inductive effect of the sulphonyl group.

By contrast, reactions of 3-bromopropyl p-tolyl sulphone (n= 3) with sodium ethoxide give rates of overall reaction and of substitution which are the highest in the series, but the rate of elimination is less than when n= 4.

Cyclisation occurs when n= 3 in ethoxide reactions and when n= 3 and 5 in t-butoxide reactions. In the latter system, this is the exclusive reaction when n= 3, and formation of the three-membered ring occurs about one hundred times as rapidly as that of a five-membered ring. This observation is discussed together with related instances from the literature.