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Chapter 17: Aldehydes
and Ketones. Nucleophilic Addition to C=O
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Nucleophilic Addition
Nucleophilic addition reactions
are an important class of reactions that allow the interconversion of C=O
into a
range of important functional groups.
What does the term "nucleophilic
addition" imply ?
A nucleophile, Nu-,
is an electron rich species that will react with an electron poor species (here
the C=O)
An addition implies that two systems combine to a single entity.
There are three fundamental events in
a nucleophilic addition reaction:
- formation of the new s bond between
the nucleophile, Nu, to the electrophilic
C of the C=O group
- breaking of the p bond to the
O resulting in the formation of an intermediate alkoxide
- protonation of the intermediate
alkoxide to give an alcohol derivative
Depending on the reactivity of the nucleophile,
there are two possible general scenarios:
- Strong nucleophiles (anionic)
add directly to the C=O to form the intermediate alkoxide. The
alkoxideis then protonated on work-up with dilute acid.
Examples of such nucleophilic systems are : RMgX, RLi, RC≡CM,
LiAlH4, NaBH4
- Weaker nucleophiles (neutral)
require that the C=O be activated prior to attack of the Nu.
This can be done using a acid catalyst which protonates on the Lewis basic O
and makes the system more electrophilic.
Examples of such nucleophilic systems are : H2O, ROH,
R-NH2
| The protonation of a carbonyl
gives a structure that can be redrawn in another resonance form that reveals
the electrophilic character of the C since it is a carbocation. |
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The reactions on the following pages
are catalogued based on the nature of the nucleophilic atom involved.
In most cases it is useful to indentify the mechanism in each case in terms
of the two general schemes presented above.
