E2cB mechanism

Alumina is a catalyst which shows intermediate behaviour and over which the concerted E2 mechanism is accepted [66] with slight transition either to the E2cA or E2cB mechanisms according to the structure of the reactant. Salts of strong acids and bases also show similar intermediate properties. The concerted or partly concerted mechanisms require two-site adsorption and because the mechanisms are ionic, the active centre must consist of a pair of an acidic and a basic site. Metal salts fulfil this... 

The nature of the transition state for the E2 reaction of some 3-thiophenoxypro-panones in aprotic solvents has been investigated.5 The linear free energy correlation and the calculated activation parameters Aand Aof the reactions suggested an unequal and synchronous E2cB mechanism via base-catalysed amine-elimination reaction. 

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