As we have seen, the C=O bond is polarized, with a partial positive charge on the carbonyl carbon
and a partial negative charge on the oxygen. This makes the carbonyl carbon an electrophile, ripe
for nucleophilic attack.
When the nucleophile attacks, it forms a covalent bond to the carbon, breaking the π bond in the
carbonyl. The electrons from the π bond are pushed onto the oxygen atom. Oxygen happily accepts
extra electrons due to its electronegativity. Breaking the π bond forms a tetrahedral intermediate.
Any time a carbonyl is opened, one should ask: Can I reform the carbonyl? If no good leaving group is
present (as is the case with aldehydes and ketones), the carbonyl will not reform. Generally, O– will
accept a proton from the solvent to form a hydroxyl group, resulting in an alcohol. However, if a
good leaving group is present (as is the case with carboxylic acids and their derivatives), the carbonyl
double bond can reform, pushing off the leaving group. Figure 6.5 shows the reaction mechanism of
nucleophilic addition for an aldehyde.
Figure 6.5. Nucleophilic Addition Reaction Mechanism
The nucleophile attacks the carbonyl carbon, opening the carbonyl. The carbonyl cannot reform
because there is no good leaving group; thus, the O− is protonated to generate an alcohol.HYDRATION
In the presence of water, aldehydes and ketones react to form geminal diols (1,1-diols), as shown in
Figure 6.6. In this case, the nucleophilic oxygen in water attacks the electrophilic carbonyl carbon.
This hydration reaction normally proceeds slowly, but we can increase the rate by adding a small
amount of catalytic acid or base.