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Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O
Nucleophilic Addition Reactions - Overview |
Relative Reactivity |
Reduction of Aldehydes and Ketones |
Using Carbon Nucleophiles |
Using Nitrogen Nucleophiles |
Oxygen Nucleophiles |
Oxidation Reactions |
Aldehydes and Ketones. Nucleophilic Addition to C=O
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 ?
Nu-, is an electron rich species
that will react with an electron poor species (here the C=O)
There are three fundamental
events in a nucleophilic addition reaction:
An addition implies that two systems combine to a single entity.
Depending on the reactivity
of the nucleophile, there are two possible general scenarios:
- 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
- Strong nucleophiles
(anionic) add directly to the C=O to form the intermediate alkoxide.
The alkoxides are then protonated on work-up with dilute acid.
Examples of such nucleophilic systems are : RMgX, RLi, RCºCM,
Examples of such nucleophilic systems are : H2O, ROH,
- Weaker nucleophiles
(neutral) require that the C=O be activated prior to attack of the
This can be done using a acid catalyst which protonates on the Lewis basic O
and makes the system more electrophilic.
|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.
The reactions on the following
pages are catalogued based on the nature of the nucleophilic atom involved.
In most cases it is useful to identify the mechanism in each case in terms of
the two general schemes presented above.
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