Reactivity of Aldehydes and Ketones: MDCAT Chemistry notes
Reactivity of Aldehydes and Ketones for MDCAT: polarity of C=O, steric and electronic effects, and why formaldehyde is the most reactive carbonyl.
Why the carbonyl group reacts
In C=O, oxygen is more electronegative than carbon. The $\pi$ electrons are loosely held and are pulled strongly towards oxygen, so the $\pi$ bond is the bond most distorted towards the oxygen atom. This leaves the carbon partially positive:
$$\mathrm{C^{\delta+}{=}O^{\delta-}}$$
The carbon is therefore electrophilic and is attacked by nucleophiles, while the oxygen can be protonated by acids. This is why aldehydes and ketones typically undergo nucleophilic addition, unlike alkenes, which undergo electrophilic addition. Because the carbonyl group is planar (sp$^2$ carbon, 120$^\circ$ angles), the nucleophile can approach from either face.
Addition can be catalysed in two ways:
- Base catalysis: the base generates a stronger nucleophile (e.g. $\mathrm{CN^-}$ from HCN) which attacks the carbon.
- Acid catalysis: $\mathrm{H^+}$ adds to the carbonyl oxygen, making the carbon even more positive so that a weak nucleophile can attack.
What makes one carbonyl more reactive than another
Two factors decide how readily the carbonyl carbon is attacked.
1. Electronic (inductive) effect
Alkyl groups are electron-releasing (+I). Each alkyl group on the carbonyl carbon reduces its positive charge and so reduces its attraction for a nucleophile. Formaldehyde has two H atoms and no alkyl group, so its carbon is the most positive.
2. Steric effect
Bulky groups crowd the carbonyl carbon and hinder the approaching nucleophile. In the product the carbon becomes sp$^3$ with four groups, which is more crowded when the groups are large. A ketone has two alkyl groups; an aldehyde has one; formaldehyde has none.
Resulting order
$$\mathrm{HCHO} > \mathrm{CH_3CHO} > \mathrm{CH_3CH_2CHO} > \mathrm{CH_3COCH_3}$$
| Compound | Groups on C=O | Reactivity to nucleophiles |
|---|---|---|
| Formaldehyde, HCHO | H, H | Highest |
| Acetaldehyde, $\mathrm{CH_3CHO}$ | $\mathrm{CH_3}$, H | High |
| Propionaldehyde, butyraldehyde | larger alkyl, H | Slightly lower |
| Acetone, $\mathrm{CH_3COCH_3}$ | $\mathrm{CH_3}$, $\mathrm{CH_3}$ | Lower |
| Carboxylic acids (HCOOH, $\mathrm{CH_3COOH}$) | OH donates by resonance | Low: no typical carbonyl reactions |
Acetone is less reactive than acetaldehyde because its two methyl groups both push electrons towards the carbonyl carbon and crowd it sterically. The methyl groups are electron-releasing, not electron-withdrawing.
Carboxylic acids are different
In HCOOH and $\mathrm{CH_3COOH}$ the lone pair of the $-\mathrm{OH}$ oxygen is delocalized into the C=O by resonance. This reduces the positive charge on the carbonyl carbon, so acids and their derivatives do not give the addition reactions (HCN, $\mathrm{NaHSO_3}$, 2,4-DNPH) typical of aldehydes and ketones. Among HCOOH, HCHO, acetone and acetic acid, HCHO has the most electrophilic carbonyl carbon.
Worked reasoning
Which reacts faster with HCN, butanal or butanone? Butanal: it has one alkyl group and one H on the carbonyl carbon, so less electron release and less crowding than butanone with two alkyl groups.
Common MDCAT traps
- Methyl groups are electron-donating; "electron-withdrawing methyl groups" is a distractor.
- The sp$^2$ hybridization and oxygen's electronegativity are the same in aldehydes and ketones, so they do not explain the difference in reactivity.
- The most reactive aldehyde is formaldehyde, not a larger one.
- The bond distorted towards oxygen is the $\pi$ bond, not a C–H or C–C $\sigma$ bond.
Quick revision
- Carbonyl carbon is $\delta+$ and electrophilic.
- Aldehydes are more reactive than ketones (electronic + steric reasons).
- HCHO is the most reactive carbonyl compound.
- Resonance lowers the electrophilicity of carboxylic acids.