SolutionsCis-Trans Isomerism: Relative Stability
1. True or false?
Discuss whether the following statement is true or false: “cis-But-2-ene is less stable than its trans isomer by 1 kcal/mol and, for that reason, it quickly converts into the trans isomer”.
False. cis-But-2-ene is indeed 1 kcal/mol less stable than its trans isomer, but THEY DO NOT INTERCONVERT: to do so, the molecule has to go through a transition state in which the π bond is broken, and that costs an activation energy of about 60 kcal/mol, which cannot be surmounted at room temperature.
2. Heats of hydrogenation
The heats of hydrogenation of but-1-ene, cis-but-2-ene and trans-but-2-ene are −30.3, −28.6 and −27.6 kcal/mol. Order them by stability and explain the order.
trans-But-2-ene > cis-but-2-ene > but-1-ene. All three give the same product (butane), so the one that releases least heat is the most stable.
More substituted alkenes are more stable (hyperconjugation of the neighbouring C–H bonds with the double bond) and, among the disubstituted ones, the trans isomer is more stable because in the cis isomer the groups clash.
3. Does it have cis-trans isomers?
Which of these compounds show cis-trans isomerism? a) pent-1-ene; b) pent-2-ene; c) 2-methylbut-2-ene; d) 1-chloropropene; e) cyclohexene.
Only pent-2-ene and 1-chloropropene. For isomerism each carbon of the double bond must carry two different substituents; in pent-1-ene and 2-methylbut-2-ene one carbon has two identical groups, and in cyclohexene the six-membered ring only allows a cis double bond.