Which of the following alkenes would you expect to have the lowest heat of hydrogenation?

• A. ${\mathrm{CH}}_3\mathrm{CH}=\mathrm{CH}{\mathrm{CH}}_3$ (trans-2-butene)
• B. ${\mathrm{CH}}_2={\mathrm{CH}}_2$ (ethylene)
• C. ${\mathrm{CH}}_3\mathrm{CH}=\mathrm{CH}{\mathrm{CH}}_3$ (cis-2-butene)
• D. ${\mathrm{CH}}_2={\mathrm{CH}}_{\mathrm{CH}}{\mathrm{CH}}_3$ (1-butene)

Answer: (A)

Verified step by step guidance

1. Understand that the heat of hydrogenation is the amount of energy released when an alkene is hydrogenated to an alkane. It is inversely related to the stability of the alkene: more stable alkenes have lower heats of hydrogenation.
2. Recall that alkene stability is influenced by factors such as alkyl substitution and stereochemistry. More substituted alkenes (like disubstituted or trisubstituted) are generally more stable than less substituted ones (like monosubstituted or unsubstituted).
3. Consider the difference between cis- and trans- isomers: trans-alkenes are usually more stable than their cis counterparts due to less steric strain between substituents across the double bond.
4. Compare the given alkenes: ethylene (no alkyl substituents), 1-butene (monosubstituted), cis-2-butene (disubstituted, cis), and trans-2-butene (disubstituted, trans). The trans-2-butene is expected to be the most stable due to its substitution pattern and stereochemistry.
5. Conclude that the alkene with the lowest heat of hydrogenation is the most stable one, which is trans-2-butene, because its greater stability means less energy is released upon hydrogenation.