Aldehydes and Ketones

Aldehydes and ketones are both characterized by the presence of a carbonyl group (C=O), and their reactivity can generally be understood by recognizing that the carbonyl carbon contains a partial positive charge (d+) and the carbonyl oxygen contains a partial negative charge (d-). Aldehydes are typically more reactive than ketones due to the following factors.

  1. Aldehydes are less hindered than ketones (a hydrogen atom is smaller than any other organic group).
  2. The carbonyl carbon in aldehydes generally has more partial positive charge than in ketones due to the electron-donating nature of alkyl groups. Aldehydes only have one e- donor group while ketones have two.

Synthesis of Aldehydes

Oxidation
Reduction
or

or
or

Synthesis of Ketones

Ozonolysis +
Oxidation    
Friedel-Crafts
Acylations		 +
Coupling R'2CuLi +
Addition to
Alkynes		 R-CºC-R'   +   H2O

Nucleophilic Addition

Nucleophilic addition reactions generally follow one of two possible mechanisms. The first is direct attack of the carbonyl carbon (d+) by a nucleophile. This is the predominant mechanism for strong nucleophiles.

The second mechanism predominates in strongly acidic environments. In this mechanism, the carbonyl oxygen is protonated, generating a positively charged compound. This intermediate can be drawn as two resonance structures, where the positive charge is localized on either the oxygen or the carbonyl carbon. In the second step, the carbonyl carbon (which carries a partial positive charge) is attacked by a nucleophile. Since the intermediate is already positively charged, the attacking nucleophile does not have to be particularly strong for this reaction to proceed.

Specific Examples

Addition
of Water		  
(gem-diol)
Addition
of Hydrides		    
Addition
of Alcohols
or
(hemiacetal)
(acetal)
Addition
of Amines		 +
(1° amine)
(imine)
Wolff-Kishner
Reduction
(Formation
of ylide)
(ylide)
Wittig
Reaction		 +
(ylide)
Reformatsky
Reaction		 +
(a-bromoester)
(b-hydroxyester)
(Dehydration
of
hydroxyester)		    
(a,b-unsaturated
ester)
Oxidation
(carboxylic acid)