Ch 10: Dienes

Chapter 10: Conjugation in Alkadienes and Allylic Systems

Dienes

Systems with more than one C=C can be referred to as "polyenes"
The simplest types of polyenes are those in which there are two double bonds, the "dienes".
The relative arrangement of the double bonds defines the characteristic reactions of the systems.
There are three possible scenarios, which are described below (from most to least stable)

Conjugated	The double bond units occur consecutively giving a continuous π system since the adjacent "p" orbitals can all overlap with each other. The result is that conjugated diene reactivity differs to that of simple alkenes. This extra bonding interaction between the adjacent π systems makes the conjugated dienes the most stable type of diene. Conjugated dienes are about 15kJ/mol or 3.6 kcal/mol more stable than simple alkenes.
Isolated	The double bond units occur separately. The π systems are isolated from each other by sp³ hybridised centers. The result is that isolated dienes have reactivity that is characteristic of simple alkenes.
Cumulated	The double bond units share a common sp hybridised C atom. The result is that cumulated dienes have reactivity more like simple alkynes. Note the relative spatial positions of the two CH₂ units at the ends of the cumulated system.