Question

Recall that Zaitsev’s rule says that more substituted alkenes are more stable. Therefore, propene is less stable than trans-but-2-ene, which is less stable than 2-methylbut-2- ene, which is less stable than 2,3-dimethylbut-2-ene. In Chapter 7, we showed that we could demonstrate this stability ordering by using heats of hydrogenation.

a) Does the same apply for other groups, besides alkyl groups? For example, compare the stability of propene, trans-but-2-ene and trans-1-chloroprop-1-ene to compare the effects of adding a chlorine instead of methyl. Is the effect the same or not? If not, how is it different?

Answer 1

H C Propene CH3 13ans but -2-ene CH3 H3 2- methylut - 2-ene と= CH3 H3 2,3 dimethyl but-2-ene CH3 C#3 H .chloropiop 1-ene hans- 1- CH3

The structure of the alkenes mentioned in the question is given above. According to Zaitsev's rule, an elimination reaction will have the double bond mainly toward the most highly substituted carbon. Zaitsev,s rule predicts that the alkene predominantly formed will be the one with the largest possible number of alkyl groups on the C=C carbons. It is found that alkene stability increases with alkyl substitution explained as due to hyperconjugation. Hence according to or following the Zaysev's rule trans-but-2-ene will be more stable than propene. But trans-1-chloroprop-1-ene will be less stable than trans-but-2-ene because according to this rule adding alkyl group increases the stability.