Chapter 20: Dienes, Conjugated Systems, and Pericyclic Reactions
Table of Contents:
Stability of Conjugated Dienes
Conjugated dienes are extra stable when compared to the unconjugated dienes.
Electrophilic Addition to Conjugated Dienes
1,2- and 1,4-Addition to a Conjugated Diene
Kinetic product (under low temperature) is always the one before resonance (1,2)
Thermodynamic product (under high temperature) is always the one with the most substituted double bond.
When we are given a conjugated diene with two different double bonds, we need to figure out which double bond would get protonated. To do this, we protonate both double bonds, one at a time, and show the resonance structures for both as well. Then, we check for the stability of carbocation intermediates in both pathways. The double bond that gave the most stable carbocation is the one that had to react.
Let's try an example.
Addition of one mole of HBr to 2,4-hexadiene gives a mixture of 4-bromo- 2-hexene and 2-bromo-3-hexene. No 5-bromo-2-hexene is formed. Account for the formation of the first two bromoalkenes and for the fact that the third bromoalkene is not formed.
The same reaction can happen with Br2.