Return to Contents Chapter 10: Conjugation in Alkadienes and Allylic Systems Ch 10 contents
Dienes
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.
a conjugated diene pi system
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.
an isolated diene pi system
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.
a cumulated diene pi system

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  Dr. Ian Hunt, Department of Chemistry, University of Calgary