Return to Contents Chapter 10: Conjugation in Alkadienes and Allylic Systems Ch 10 contents

Conjugation

The word "conjugation" is derived from a Latin word that means "to link together".  In organic chemistry terms, it is used to describe the situation that occurs when π systems (e.g. double bonds) are "linked together".

The fundamental requirement for the existence of a conjugated system is revealed if one considers the p orbitals involved in the bonding within the π system. You can investigate these differences by studying the following examples, paying particular attention to the "p" orbitals in the π system. Use the JSMOL models to look at the hybridisations of the atoms in the systems.
 
System
JSMOL
 Line diagram
π system
Type
ethene
ethene
ethene pi system
isolated
propene
propene
propene pi system
isolated
1,2-propadiene  (allene)
allene
Allene pi system
cumulated
1,3-butadiene
1,3butadiene
1,3-butadiene pi system
conjugated
1,3-pentadiene
1,3-pentadiene
1,3-pentadiene pi system
conjugated
1,4-pentadiene
1,4-pentadiene
1,4-pentadiene pi system
isolated
1,3-cyclopentadiene
cyclopentadiene
1,3-cyclopentadiene pi system
conjugated
1,3-cyclohexadiene
1,3-cyclohexadiene
1,3-cyclohexadiene pi system
conjugated
1,4-cyclohexadiene
1,4-cyclohexadiene
1,4-cyclohexadiene pi system
isolated
benzene
benzene
benzene pi system
conjugated

The result of conjugation is that there are extra π bonding interactions between the adjacent π systems. This extra bonding results in an overall stabilisation of the system. This increased stability due to conjugation is refered to as the delocalisation energy or the resonance energy or conjugation energy.  This will be explored in more detail in Chapter 11.


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