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

Aldehydes and Ketones

Nomenclature:
Aldehydes: functional group suffix = -al (review)
Ketones: functional group suffix = -one (review)
Functional group prefix = oxo-
(Note that an aldehyde is higher priority than a ketone)
 

methanal
ethanal
propanone
5-oxohexanal

Physical Properties:

The implications of these effects are: Structure:
methanal
ethene
propanone

Reactivity:

electrostatic potential of methanal The image shows the electrostatic potential for methanal (formaldehyde) 
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. 
  • The O atom in the C=O is a region of high electron density (red) due to the lone pairs.
  • The carbonyl oxygen atoms are Lewis bases (site of protonation)
  • There is low electron density (blue) on the C atom of the C=O group.
electrostatic potential for propanone The image shows the electrostatic potential for propanone (acetone). 
The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. 
  • The O atom in the C=O is a region of high electron density (red) due to the lone pairs.
  • The carbonyl oxygen atoms are Lewis bases (site of protonation)
  • There is low electron density (blue) on the C atom of the C=O group
Therefore the general mode of reaction of aldehydes and ketones is attack of a nucleophile at the electrophilic carbonyl C:

reaction of aldehydes and ketones
IMPORTANT : The reactivity of aldehydes and ketones can be easily rationalised by considering the important resonance contributor which has a +ve C and -ve O. 
Note that this matches the ideas shown in the electrostatic potential diagrams shown above.
resoance in a ketone

 
 

In general the reactivity order towards nucleophiles is as shown to the right : methanal > aldehydes > ketones
Methanal is a unique aldehyde in the sense that it is the only aldehyde that has 2 H atoms attached to the carbonyl group and ALL other aldehydes have only 1 H atom attached to the carbonyl group.
To rationalise the reactivity order, we look at what is changing and that is the nature of the substituents attached to the carbonyl group.
The substituents have two contributing factors: 

  1. The electronic effect of the substituent. Alkyl groups are weakly electron donating so they make the C in the carbonyl less electrophilic. Therefore methanal is more reaactive than other aldehydes (extra R group) and ketones are less reactive than aldehydes for the same reason.
  2. The size of the alkyl substitutents attached to the C=O. Larger groups will sterically hinder the approach of the Nu and reduce the relative reactivity.
relative reactivity order towards nucleophiles

The images below show spacing filling models so that you can investigate the accessiblity to the electrophilic C across a series of aldehydes and ketones where the size of the alkyl groups attached to the carbonyl C change from -H to t-butyl (-C(CH3)3 ).
 

methanal
ethanal
propanone
di-t-butyl ketone




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