Question

Consider the following alkenes with regard to their stability

• A. i > iii > ii > iv
• B. iii > ii > iv > i
• C. i > ii > iv > iii
• D. ii > iv > iii > i

Answer 1

Answer: (C)

i > ii > iv > iii

Answer 2

The stability of alkenes is primarily determined by the degree of substitution of the double bond carbons and the number of alpha-hydrogens, which indicate hyperconjugation. Heat of hydrogenation is a quantitative measure of alkene stability; a less exothermic hydrogenation (less negative enthalpy change) indicates a more stable alkene.

• Alkene i: 2-methylpropene (CH$_2$=C(CH$_3$)$_2$). This is a disubstituted alkene with 6 alpha-hydrogens.
• Alkene ii: But-2-ene (CH$_3$-CH=CH-CH$_3$). This is a disubstituted alkene with 6 alpha-hydrogens.
• Alkene iii: But-1-ene (CH$_3$-CH$_2$-CH=CH$_2$). This is a monosubstituted alkene with 3 alpha-hydrogens.
• Alkene iv: Cyclohexene. This is a cyclic disubstituted alkene with 4 alpha-hydrogens.

Based on heats of hydrogenation (experimental values):

• Alkene i: $\Delta H_{hydrog} \approx -112.5$ kJ/mol
• Alkene ii: $\Delta H_{hydrog} \approx -115.2$ kJ/mol
• Alkene iv: $\Delta H_{hydrog} \approx -119.5$ kJ/mol
• Alkene iii: $\Delta H_{hydrog} \approx -126$ kJ/mol

A less exothermic heat of hydrogenation indicates a more stable alkene. Therefore, the stability order from most stable to least stable is: i (-112.5 kJ/mol) > ii (-115.2 kJ/mol) > iv (-119.5 kJ/mol) > iii (-126 kJ/mol).

This order is i > ii > iv > iii.