Carbon and Hydrogen Leaving Groups

Previously we learned that the carbonyl group of an aldehyde or a ketone does not undergo the substitution reactions of Chapter 19 because hydride ion and carban-ions are strong bases and poor leaving groups. There are, however, some special situations in which these species do leave. Some of these exceptions are described in this section. 

A carbanion can act as a leaving group if it is stabilized somehow. We have already seen several examples of this. For example, CX3 , a methyl carbanion stabilized by the inductive effect of three electronegative halogen atoms, leaves in the haloform reaction (see Section 20.2). 

O The Cannizzaro reaction begins with the attack of the hydroxide ion nucleophile at the electrophilic carbon of the aldehyde, as described in Chapter 18. 

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