09 Aldehydes and Ketones

Structure and polarity

Aldehydes and ketones both contain a carbonyl group, $\mathrm{C=O}$. They differ in what is attached to the carbonyl carbon:

• In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen: $\mathrm{R{-}CHO}$.

• In a ketone, it is bonded to two carbon groups: $\mathrm{R{-}CO{-}R'}$.

The carbonyl carbon is $\mathrm{sp^2}$-hybridized and trigonal planar. Oxygen attracts the shared electrons more strongly than carbon, so the bond is polarized: oxygen is partially negative, $\delta^-$, and carbon is partially positive, $\delta^+$. The same polarization can be represented by resonance:

This polarization explains the main pattern of reactivity: oxygen is a site for electrophiles, while the electron-poor carbonyl carbon is an electrophile that attracts nucleophiles.

Naming note: An aldehyde attached to a ring is commonly named using the suffix -carbaldehyde.

How nucleophilic addition works

A nucleophile attacks the electrophilic carbonyl carbon. As the new bond forms, the $\mathrm{C=O}$ $\pi$ electrons shift onto oxygen, producing an alkoxide. The carbonyl carbon changes from trigonal planar to tetrahedral. Protonation of the alkoxide commonly gives an alcohol:

This process is addition, not substitution: aldehydes and ketones usually have no suitable leaving group on the carbonyl carbon. The nucleophile adds, and the carbon–oxygen double bond becomes a single bond.

In acidic conditions, the carbonyl oxygen may be protonated first. This increases the electrophilicity of the carbonyl carbon, so a weaker nucleophile such as water can attack. Proton transfers then lead to the addition product; the exact steps depend on the reagent and conditions.

Takeaway: Addition changes the carbonyl carbon’s geometry and forms a new bond to the nucleophile without displacing a leaving group.

Comparing aldehyde and ketone reactivity

Aldehydes generally undergo nucleophilic addition more readily than ketones. An aldehyde has one hydrogen attached to its carbonyl carbon, whereas a ketone has a second carbon substituent. That extra substituent has two effects:

• It crowds the carbonyl carbon, making nucleophilic approach more difficult.

• It donates electron density toward the carbonyl carbon, making that carbon less electrophilic.

Together, these effects make ketones generally less reactive than aldehydes toward nucleophilic addition.

Takeaway: When comparing these two functional groups, aldehydes are generally more accessible and more electrophilic than ketones.

Hydride reduction and alcohol products

Hydride reduction is a nucleophilic addition. Hydride, $\mathrm{H^-}$, attacks the carbonyl carbon, and the $\mathrm{C=O}$ $\pi$ electrons move onto oxygen to form an alkoxide. A subsequent proton transfer produces an alcohol.

The product depends on the starting carbonyl compound:

• Aldehyde to primary alcohol: $\mathrm{R{-}CHO \rightarrow R{-}CH_2OH}$

• Ketone to secondary alcohol: $\mathrm{R{-}CO{-}R' \rightarrow R{-}CH(OH){-}R'}$

Sodium borohydride, $\mathrm{NaBH_4}$, is a common, comparatively mild reagent for reducing aldehydes and ketones. Lithium aluminum hydride, $\mathrm{LiAlH_4}$, also reduces them and is more reactive. Because $\mathrm{LiAlH_4}$ is incompatible with water, protonation occurs during a separate work-up step. For example, reduction of propanone gives propan-2-ol.

The carbonyl group is planar, so a nucleophile can attack from either face. If ketone reduction creates a new stereogenic center and both faces are equally accessible in an achiral environment, both enantiomers may form.

Takeaway: Hydride addition followed by protonation converts aldehydes to primary alcohols and ketones to secondary alcohols.