Methanol bimolecular dehydration

The difficulty in this mechanism is the highly unfavorable endothermic nature of the a elimination of methanol to carbene [Eq. (3.51)] in contrast with the thermodynamically favored bimolecular dehydration [Eq. (3.52)] ... 

It has been shown that only those alcohols that form detectable surface alco-holate species on alumina undergo bimolecular dehydration with ether and water as reaction products (340). Thus, ether formation is the dominant reaction direction of methanol and ethanol at low temperatures, and the tendency toward ether formation is reduced as the chain length increases and chain branching occurs (28, 340). The same trends are observed for the stability and surface concentrations of the surface alcoholate species (27, 28, 47, 340). Alcoholate formation is due to a dissociative chemisorption step of the alcohol that occurs on A1—O pair sites (47, 340, 354-358). One is, thus, led to the conclusion that incompletely coordinated Al3+ ions and O2- ions are both important sites in the ether formation from alcohols and that their participation should be detectable by specific poisoning. 

An important principle of synthesis is to avoid mixtures of isomers wherever possible minimizing separations increases recovery of products. Bimolecular dehydration is a random process. Heating a mixture of ethanol and methanol with acid will produce all possible combinations dimethyl ether, ethyl methyl ether, and diethyl ether. This mixture would be troublesome to separate. 

Figueras Roca and co-workers (346) have used preadsorbed TCNE to poison the basic sites specifically. The rate of ether formation from methanol and ethanol responded very sensitively to the poisoning with TCNE, so that the participation of basic sites in the bimolecular alcohol dehydration seems to be proved. 

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