Question

Name the following dienes and rank them from most stable to least stable. (Hint: Alkyl groups stabilize dienes in the same way they stabilize alkenes; Section 5.9.)

Short answer

The smaller the heat of hydrogenation (the positive value of heat of hydrogenation) the more stable the compound. Therefore, the relative stabilities of dienes are:

Conjugated diene˃ isolated diene˃ cumulated diene

Step-by-step solution

Some definitions

Conjugated dienes =Conjugated dienes are those wheretwo double bonds separated by a single bond.

Isolated dienes =Isolated dienes are those wheretwo double bonds are separated by more than one single bond.

Cumulated dienes =Cumulated dienes are those where two double bonds connected to the same atom.

Final answer

2,5-dimethyl-2,4-hexadiene is more stable because firstly it is conjugated diene and it has more electron donating groups with double bond carbon so it is most stable.

After that 2,4-hexadiene is stable because firstly it is conjugated diene and it has more electron donating groups than 1,3-pentadiene with double bond carbon but it has less electron donating groups than 2,5-dimethyl-2,4-hexadiene so it is more stable than 1,3-pentadiene but less stable than 2,5-dimethyl-2,4-hexadiene.

1,4-pentadiene is isolated diene.