Return to Contents
 Chapter 18: Enols and Enolates 
Ch 18 contents
Conjugated Systems.... Implications
carbonyl resonance
The reactivity of simple aldehydes and ketones can be rationalised by looking at the resonance contributors (see left).
nucleophilic addition of aldehydes and ketones You should recall that they are attacked by nucleophiles at the C=O due to the electrophilic character of the carbonyl C.
(methyl vinyl ketone)
In the JMOL images to the left, are examples of a conjugated aldehyde and a conjugated ketone and a conjugated diene for comparison.
In each compound note the geometric relationship between the 2 double bonds they all lie in the same plane.
This means the the π-orbitals of the two double bond systems are aligned and there is an interaction between them... RESONANCE.... this "modifies" the reactivity to some degree.
electrostatic potential of 3-butenone
Electrostatic potential of 3-butenone
Look at the electrostatic potential diagram of the ketone (left). Note the red region show the high electron density at the O of the C=O and the lower electron density of the conjugated C=C, compare this with the electrostatic potential diagram of the diene to the right.... here the C=C are electron rich.
electrostatic potential of 1,3-butadiene
Electrostatic potential of 1,3-butadiene
resonance in a conjugated carbonyl In a conjugated carbonyl system, there is an extra resonance structure that also shows electrophilic character at the terminal alkene C. Compare these resonance contributors with the electrostatic potential of 3-butenone shown above.... they tell us the same thing.
Therefore, αβ-unsaturated aldehydes and ketones can potentially react with nucleophiles at two sites : directly at the carbonyl C or the end of the conjugated system:
there are two possible addition modes for conjugated aldehydes and ketones
We can draw the curly arrows for the reaction of the nucleophiles to form these products from the resonance contributors:
curly arrows for direct addition to the carbonyl In direct addition, the Nu attacks "directly" at the carbonyl C=O (see left), this reaction is usually faster but the product is less stable (i.e. it is the kinetic product).
In conjugate addition, the Nu attacks the end of the conjugated system at the end of the C=C (see right)
curly arrows for conjugate addition to the carbonyl
Note that the product of 1,4-addition is an enol that will tautomerise to the more stable carbonyl compound, making it the thermodynamic product. tautomerisation of 1,4-addition product

This situation is similar to that we saw previously for dienes where we observed  1,2- or direct addition and 1,4- or conjugate addition.

Like these reactions of dienes, the addition to the unsaturated aldehyde or ketone can be under kinetic or thermodynamic control.

previous page
next page
organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary