The Relative Reactivities of Carbonyl Compounds

Fresh perspiration is odorless. The smells we associate with perspiration result from a chain of events initiated by bacteria that are always present on our skin. These bacteria produce lactic acid, which creates an acidic environment that allows other bacteria to break down the components of perspiration, producing compounds with the unappealing odors we associate with armpits and sweaty feet. One such compound is butanedione. 

Why are numbers not used to designate the position of the functional group in propanone and butanedione  

We have seen that the carbonyl group is polar because oxygen is more electronegative than carbon, so oxygen has a greater share of the double bond s electrons (Section 16.5). As a result, the carbonyl carbon is electron deficient (it is an electrophile), and it reacts with nucleophiles. The electron deficiency of the carbonyl carbon is indicated by the blue region in the electrostatic potential maps. 

Steric factors are also important in the transition state, which is tetrahedral (so it has bond angles of 109.5°). This causes the alkyl groups to be closer to one another than they are in the carbonyl compound, where the bond angles are 120°. As a result of the greater steric crowding in their transition states, ketones have less stable transition states than aldehydes have. In summary, alkyl groups stabilize the carbonyl confound and destabilize the transition state. Both factors increase AG, causing ketones to be less reactive than aldehydes. 

Steric crowding also causes ketones with large alkyl groups bonded to the carbonyl carbon to be less reactive than those with small alkyl groups. 

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