SolutionsBond Properties
1. Isolated, conjugated or cumulated?
Classify the double bonds of these compounds: a) 3-butenal; b) 2-butenal; c) ketene (CH2=C=O); d) 4-penten-2-one; e) 3-penten-2-one.
- Isolated (an sp3 carbon separates them).
- Conjugated.
- Cumulated.
- Isolated.
- Conjugated.
2. Stability by conjugation
3-Butenal is converted into 2-butenal in the presence of a catalytic amount of base. Why does the equilibrium lie towards 2-butenal? How does the isomerization take place?
2-Butenal has conjugated double bonds and is about 6 kcal/mol more stable than its isomer with isolated double bonds. The base removes a hydrogen from C2 of 3-butenal, which is both α to the carbonyl and allylic: a delocalized enolate forms, which is protonated at C4 to give the conjugated isomer.