19.4 Nucleophilic Addition Reactions of Aldehydes and Ketones

Nucleophilic Addition Reactions

Mechanism of nucleophilic addition reactions

Nucleophilic addition is the central reaction type for aldehydes and ketones. The carbonyl carbon is electrophilic because the C=O bond is polarized: oxygen pulls electron density toward itself, leaving a partial positive charge ($\delta^+$) on carbon. That partial positive charge makes the carbonyl carbon a target for nucleophiles like amines, alcohols, and hydride ions.

The reaction follows a two-step mechanism:

1. Nucleophilic attack: The nucleophile donates a pair of electrons to the electrophilic carbonyl carbon, forming a new bond. As this happens, the $\pi$ electrons of the C=O double bond shift onto the oxygen atom. The geometry at carbon changes from trigonal planar ($sp^2$) to tetrahedral ($sp^3$), producing an alkoxide intermediate with a negative charge on oxygen.

2. Protonation: The negatively charged oxygen picks up a proton from an acid, water, or another proton source in the reaction mixture. This neutralizes the alkoxide and gives the final addition product.

The strength of the nucleophile (its nucleophilicity) directly affects how fast and how completely this reaction proceeds. Strong nucleophiles like $\text{CN}^-$, Grignard reagents ($\text{RMgBr}$), and $\text{H}^-$ (from $\text{LiAlH}_4$ or $\text{NaBH}_4$) react readily, while weak nucleophiles like water often need acid or base catalysis.

Aldehydes vs ketones in reactivity

Aldehydes are generally more reactive than ketones toward nucleophilic addition. Two factors explain this:

• Steric factors: Aldehydes have one hydrogen and one substituent on the carbonyl carbon, while ketones have two bulkier alkyl or aryl groups. Those two groups crowd the carbonyl carbon in ketones, making it harder for a nucleophile to approach.
• Electronic factors: Alkyl groups are weakly electron-donating through induction and hyperconjugation. In ketones, two alkyl groups push electron density toward the carbonyl carbon, reducing its partial positive charge and making it less electrophilic. Aldehydes, with only one alkyl group (or none, in the case of formaldehyde), have a more electrophilic carbonyl carbon.

Both effects work in the same direction, so aldehydes consistently react faster than comparable ketones. For example, formaldehyde ($\text{HCHO}$) is the most reactive common aldehyde, while a sterically hindered ketone like di-tert-butyl ketone is among the least reactive carbonyl compounds.

Substituent Effects on Reactivity

Substituent effects on carbonyl reactivity

Substituents attached to or near the carbonyl group tune its electrophilicity through electronic effects:

• Electron-withdrawing groups (EWGs) such as $\text{-NO}_2$, $\text{-CN}$, and halogens pull electron density away from the carbonyl carbon, increasing its partial positive charge. This makes the carbonyl more reactive toward nucleophiles. For instance, chloroacetaldehyde ($\text{ClCH}_2\text{CHO}$) is more reactive than acetaldehyde ($\text{CH}_3\text{CHO}$).
• Electron-donating groups (EDGs) such as alkyl, $\text{-OH}$, and $\text{-NH}_2$ push electron density toward the carbonyl carbon, decreasing its electrophilicity and making the carbonyl less reactive.

Aromatic aldehydes deserve special attention. Benzaldehyde is less reactive than a comparable aliphatic aldehyde (like propanal) because the aromatic ring donates electron density into the carbonyl through resonance, reducing the electrophilicity of the carbonyl carbon.

Substituents on the aromatic ring further modify reactivity:

• Electron-withdrawing ring substituents (e.g., $\text{-NO}_2$, $\text{-Cl}$) make the carbonyl carbon more electrophilic and increase reactivity. A para-nitrobenzaldehyde reacts faster than benzaldehyde itself.
• Electron-donating ring substituents (e.g., $\text{-OH}$, $\text{-OCH}_3$) make the carbonyl carbon less electrophilic and decrease reactivity.

Common Nucleophilic Addition Reactions

Hydration and related reactions

Several important reactions follow the nucleophilic addition pattern, each using a different nucleophile:

• Hydration: Water adds across the C=O bond to form a geminal diol (two $\text{-OH}$ groups on the same carbon). This reaction is reversible and usually favors the carbonyl form, except for highly electrophilic aldehydes like formaldehyde and chloral ($\text{Cl}_3\text{CCHO}$), where the hydrate is stable.
• Cyanohydrin formation: Cyanide ion ($\text{CN}^-$) attacks the carbonyl carbon, and subsequent protonation gives a cyanohydrin, a compound with both $\text{-OH}$ and $\text{-CN}$ on the same carbon. This reaction is synthetically useful because the nitrile group can later be converted to a carboxylic acid or amine.
• Imine formation: A primary amine ($\text{RNH}_2$) adds to the carbonyl, then loses water in a condensation step to form an imine (also called a Schiff base), which contains a $\text{C=N}$ double bond. This reaction is acid-catalyzed and is important both in the lab and in biological chemistry (for example, in the active sites of enzymes that use pyridoxal phosphate as a cofactor).