Ethanol bimolecular dehydration

Bimolecular dehydration is generally used for the synthesis of symmetrical ethers from unhindered 1° alcohols. Industrially, diethyl ether is obtained by heating ethanol at 140 °C in the presence of H2SO4. In this reaction, ethanol is protonated in the presence of an acid, which is then attacked hy another molecule of ethanol to give diethyl ether. This is an acid-catalysed Sn2 reaction. If the temperature is too high, alkene is formed via elimination. 

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. 

In some cases, a protonated primary alcohol may be attacked by another molecule of the alcohol and undergo an Sn2 displacement. The net reaction is a bimolecular dehydration to form an ether. For example, the attack by ethanol on a protonated molecule of ethanol gives diethyl ether. 

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. 

Saito and Niiyama (241) investigated the transient behavior of ethanol dehydration catalyzed by Baj sPW O. When the ethanol feed was stopped after a steady state had been attained, ethylene continued to form for a prolonged period, whereas ether, formation decreased rapidly. This transient behavior, as well as the kinetics under stationary conditions, was well simulated with a model based on the assumption that the ethylene and ether are formed by unimolecular and bimolecular reactions in the bulk, respectively. 

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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