<P> The mechanism requires that the departing atoms or groups follow antiparallel trajectories . For open chain substrates this geometric prerequisite is met by at least one of the three staggered conformers . For some cyclic substrates such as cyclohexane, however, an antiparallel arrangement may not be attainable depending on the substituents which might set a conformational lock . Adjacent substituents on a cyclohexane ring can achieve antiperiplanarity only when they occupy trans diaxial positions . </P> <P> One consequence of this analysis is that trans - 4 - tert - butylcyclohexyl chloride cannot easily eliminate but instead undergoes substitution (see diagram below) because the most stable conformation has the bulky t - Bu group in the equatorial position, therefore the chloride group is not antiperiplanar with any vicinal hydrogen . The thermodynamically unfavored conformation has the t - Bu group in the axial position, which exhibits the high energetic 7 - atom interactions (see A value) of 4.7--4.9 kcal / mol . As a result, the t - Bu group "locks" the ring in the conformation where it is in the equatorial position and substitution reaction is observed . On the other hand, cis - 4 - tert - butylcyclohexyl chloride undergoes elimination because antiperiplanarity of Cl and H can be achieved when the t - Bu group is in the favorable equatorial position . </P>

Why are different conformations of an alkane not considered structural isomers