Separation of primary and secondary processes

The majority of stereochemical investigations refer to cyclic and rigid substrates. Unlike the corresponding simple pyrolytic eliminations (Section 2.7), the dehydration of menthol and neomenthol enhibits mainly anti rather than syn or syn-clinal stereospecificity, viz.  

Traces of l-menthene are found among the primary products and are thought to arise by abstraction of the gamma hydrogen and migration of the beta hydrogen, viz. 

The importance of coplanarity of the eliminating fragments is emphasised by the large difference in the Arrhenius activation energies for elimination from cw-4-r-butylcyclohexanol (E (cis) = 21 kcal.mole ) and rra/iJ-4-r-butylcyclo-hexanol (Epitrans) = 38 kcal.mole ) . In the trans isomer, elimination must occur from a diequatorial syn-clinal conformation, or a diaxial conformation with the bulky r-butyl substituent in an unfavourable axial conformation, or an initial epimerisation to the cis isomer, or via a boat conformation, all possibilities which involve much higher energy intermediates than the diaxial elimination from the cis isomer. The products of elimination from the four stereoisomers of 1-decalol are also consistent with anti stereospecificity  

To explain the predominance of anti stereospecificity in dehydration over alumina, participation by both acidic and basic sites of the catalyst is invoked. Furthermore, as the alumina must surround the substrate, it has been regarded as a solvating agent and an analogy to solvolytic elimination has been drawn  

The dehydration of 1,4-cyclohexane diols emphasises the critical importance of stereochemical arrangement of the hydroxy functions in a substrate . The m-diol eliminates to give the cyclohexenol as the major product but the trans isomer yields mainly 1,4-epoxycyclohexane. As elimination from trans-1,4-cyclohexanediol occurs more rapidly than from t-butyl alcohol, anchimeric assistance was suggested, the intramolecular concerted ring closure occurring through a boat conformation, viz. 

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