Steric Course of Dehydration

One of the most fruitful approaches to the elucidation of reaction mechanisms in organic chemistry is the study of the effect of structure on the reactivity and the course of the reaction. This approach is used extensively in homogeneous reactions and found to be equally rewarding in the study of the mechanism of dehydration of alcohols over alumina catalysts. Much information was obtained by changing the configuration of the alcohols. 

Much evidence supports the conclusion that the elimination of the group HX from alkyl halides by bases is a trans elimination reaction. This means that the atoms H and X leave from the opposite site of the incipient double bond. It is mostly explained by assuming that the electrons which are left by the leaving proton and which will form the double bond prefer to attack the leaving group X from the rear (50). The transition state for the elimination, if it is concerted, is most stable if H, X, and the carbon atoms 1 and 2 lie on one plane, which in most molecules is best realized in the trans position (51).  

In order to determine whether trans elimination may occur also in the removal of elements of water from alcohols, the dehydration of menthol 

When the dehydration of menthol is carried out on an acidic alumina or at a long contact time the 2-menthene can isomerize to the more stable 1- and 3-menthenes. In order to avoid the consecutive reactions which proceed by acid catalysis, the alumina can be modified either by adding pyridine to the menthol or by passing ammonia over the catalyst during dehydration. 

For the neomenthol to undergo trans elimination it would be necessary for the original chair conformation with the hydroxyl in the more stable equatorial position (55, 56) to flip to another chair conformation with hydroxyl in the axial position. 

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