vic-Dibromides usually react with bases to produce more alkyne than conjugated diene. What factor(s) may be controlling this? Two exceptions to this "behavior are 1,2 -dibromo-cyclohexane and 2,3 -dichloro \(-2\), 3-dimethylbutane. How do you account for these exceptions?

Typically when a vicinal dibromide reacts with a base, it undergoes an E2 elimination reaction to produce an alkyne rather than a conjugated diene. This is because the two bromines are on adjacent carbon atoms, making it more likely for the elimination reaction to form a triple bond between those carbons. Let's consider these factors in detail:

There are several factors that contribute to the formation of alkynes in the reaction of vic-dibromides with bases: 1. Steric effect: There is often less steric hindrance for the formation of an alkyne as an elimination product, as the preferred transition state requires relatively less steric force from surrounding groups. 2. Electronic factors: The formation of a conjugated diene requires the removal of two vicinal protons in a syn or anti conformation. This increases the selectivity for the anti elimination, leading to less of the conjugated diene product. 3. Stability of intermediates: The intermediates formed during the elimination reaction to produce alkynes are typically more stable than those that would result in conjugated diene formation. Now, we will analyze the exceptions given in the exercise to understand why these vic-dibromide systems do not follow the usual trend.

To account for the exceptions - 1,2-dibromo-cyclohexane and 2,3-dichloro-2,3-dimethylbutane - we need to consider the structural and electronic factors specific to these molecules. 1,2-dibromo-cyclohexane: In this case, the formation of an alkyne would result in a highly strained cyclic alkyne, which is much less stable compared to a cyclic conjugated diene. The cyclic alkyne intermediate has difficulty adopting the conformation necessary for an elimination reaction. Hence, the reaction would proceed to form the more stable conjugated diene. 2,3-dichloro-2,3-dimethylbutane: In this situation, the steric hindrance caused by the two methyl groups flanking the vicinal chlorines prevents the formation of an alkyne. The larger chlorine atoms and t-butyl group increase steric interactions making it improbable for the necessary transition state to form and pushing the reaction towards the formation of a conjugated diene. Overall, for both exceptions, it's the increased stability of the conjugated diene and the destabilizing effects on the potential alkyne that cause the reaction to follow a different trajectory from the typical vic-dibromide reactions.