Use of Model Alcohols in Mechanistic Studies

Use of Model Alcohols in Mechanistic Studies. - Much use has been made of model alcohols of various types in order to elucidate the detailed mechanism of dehydration, and in so doing, most catalysts have been compared with either alumina or thoria representing respectively E1/E2 and ElcB mechanisms. 

Kibby et al compared the behaviour of a hydroxyapatite (Ca/P = 1.58) and an alumina catalyst for the dehydration of threo- and erythro- 3- x -butan-2-ol. Both catalysts gave but-2-enes preferentially, but water can be eliminated by a syn- or anft -mechanism, the former occurring, when groups are removed from the same side of the molecule and the latter, when they are removed from opposite side. Furthermore, syn- and fl/ift -eliminations occur respectively with eclipsed and gauche conformations. In the case of homogeneous reactions, anti-elimination is preferred since the gauche form is thermodynamically more stable. 

The product distributions indicated very clearly that alumina favours anti-elimination, whereas hydroxyapatite favours syn-elimination and alumina appears to be atypical, as geometric considerations in respect of the solid surface are expected to favour the syn-elimination. 

The occurrence of anti-eUmination has been explained previously in terms 

Jambor and L. Ber4nek, Collect. Czech. Chem. Commun., 1975, 40, 1374. 

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