Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O 
 Ch 17 contents

Reactivity
Overall a simple nucleophilic addition can be represented with curly arrows as follows:

Nucleophilic addition of aldehydes and ketones

The reactivity of aldehydes and ketones can be easily rationalized by considering the important resonance contributor which has charge separation with a +ve C and -ve O.  resoance in a ketone
Question:
Why is this resonance structure more important than the one with a -ve C and +ve O ? O is more electronegative than C so the O should be -ve
In general the reactivity order towards nucleophiles is : aldehydes > ketones (see below)
The substituents have two contributing factors on the reactivity at the carbonyl C:
  1. Size of the substituents attached to the C=O. Larger groups will tend to sterically hinder the approach of the Nu.
  2. The electronic effect of the substituent. Alkyl groups are weakly electron donating so they make the C in the carbonyl less electrophilic and therefore less reactive towards nucleophiles.
 relative reactivity order towards nucleophiles

These trends are supported by the trends in the equilibrium data for the formation of hydrate (see later)
 
 

Carbonyl
K / M-1
% Hydrate

Carbonyl
methanal
41
99.96
Hydrate
ethanal
1.8 x 10-2
50
2,2-dimethylpropanal
4.1 x 10-3
19
propanone
2.5 x 10-5
0.14
(K = [hydrate]/[C=O][H2O])
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