SolutionsDienes and Polyenes
1. Which dienes are conjugated?
Which of these compounds are conjugated dienes? a) 1,3-pentadiene; b) 1,4-pentadiene; c) 2-methyl-1,3-butadiene (isoprene); d) 1,2-butadiene; e) 1,3-cyclopentadiene.
1,3-Pentadiene, isoprene and 1,3-cyclopentadiene are conjugated. 1,4-Pentadiene is an isolated diene and 1,2-butadiene an allene (cumulated diene).
2. Stability
The heat of hydrogenation of 1,3-pentadiene (−54.1 kcal/mol) is lower than that of 1,4-pentadiene (−60.8 kcal/mol). What does this difference show?
Both give pentane, so the conjugated diene is about 6.7 kcal/mol more stable than the isolated one. Part of that stability comes from conjugation (delocalization of the π electrons over the four carbons) and part from 1,3-pentadiene having a disubstituted double bond.
3. Characteristic reactivity
Name three reactions of conjugated dienes that isolated alkenes do not undergo.
- Electrophilic 1,4-addition (conjugate addition).
- The Diels-Alder cycloaddition with a dienophile.
- Electrocyclic (pericyclic) reactions and 1,4-polymerization.