The specificity and mechanism of the Kober reaction

Abstract

When estrane derivatives are put through the Kober reaction up to four different chromogens may be obtained. There are three essential features which the molecule must possess simultaneously if a positive Kober reaction is to occur — a double bond or oxygen function in ring D, an oxygen function in ring A and an aromatic ring A. There is a fourth feature — the angular methyl group at Cl3 — which considerably enhances the second stage of the reaction. It is suggested that the reaction is initiated by dehydration at the ring D oxygen function which, with protonation of the double bond so formed and angular methyl migration leads to the formation of a stable carbonium ion in which several adjacent tertiary carbon atoms are involved in delocalization of the positive charge. It has been shown that the pink Kober chromogen is not a polyene or free radical. In at least three of the four Kober chromogens — those absorbing in the visible region — the ground state is more polar than the excited state.