Allenes. Part XXIII. Mechanistic studies on the formation of 1-bromoallenes from prop-1-yn-3-ols

Abstract

The formation of 1-bromo-3-methylpenta-1,2-diene by the action of hydrobromic acid on 3-methylpent-1-yn-3-ol in the presence of cuprous bromide has been investigated in the temperature range 0–25° and found to follow first-order kinetics. The rate constant and half-life at 0·00, 20·80, and 25·20° and the activation energy for the reaction have been determined. Evidence has been obtained to support the intermediacy of an acetylene–copper(I)π-complex from which the 1 -bromoallene is formed by a stereospecific S_Ni′ process in which the configuration is retained.