Nucleophilic addition

Nucleophilic addition is the reaction of aldehydes and ketones in which a nucleophile attacks the slightly positive carbon of the C=O group. The double bond opens, the nucleophile joins on, and the oxygen picks up a hydrogen ion to become an OH group. At A-level the two examples are reduction with NaBH₄, where the nucleophile is a hydride ion (H⁻), and the reaction with KCN and dilute acid, which makes hydroxynitriles.

What is nucleophilic addition?

A nucleophile is an electron pair donor: a species with a lone pair that it can use to form a new bond to a positive or slightly positive atom. In an addition reaction, two molecules join to make one, and nothing is lost. So in nucleophilic addition, a nucleophile adds across a double bond.

AQA's specification puts it simply: aldehydes, ketones, carboxylic acids and their derivatives all contain the carbonyl group, which is attacked by nucleophiles. Oxygen is much more electronegative than carbon, so the C=O bond is polar, with the carbon δ+ and the oxygen δ−. The δ+ carbon attracts nucleophiles, and the second bond of the double bond (the π bond) can break, letting the oxygen take both of its electrons.

Nucleophilic addition vs electrophilic addition

Both are additions across a double bond, but the attacking species is opposite, because the double bonds are different.

Don't mix it up with nucleophilic substitution either. In substitution, the nucleophile replaces a leaving group such as a halogen. In addition, nothing leaves: the nucleophile and a hydrogen ion are simply added on.

The nucleophilic addition mechanism

Whatever the nucleophile, the mechanism has the same two steps. Call the nucleophile :Nu⁻.

1. Attack. Draw a curly arrow from the lone pair on the nucleophile to the δ+ carbon of the C=O. Draw a second curly arrow from the C=O double bond onto the oxygen. The π bond breaks and the oxygen now carries a negative charge.
2. Protonation. Draw a curly arrow from a lone pair on the O⁻ to an H⁺ ion (from water or added acid). The oxygen becomes an OH group.

The product has the nucleophile and an OH group on the carbon that was the carbonyl carbon, and that carbon now has four single bonds instead of a double bond.

Reduction of aldehydes and ketones with NaBH₄

Sodium tetrahydridoborate, NaBH₄, is a reducing agent. The BH₄⁻ ion is a source of hydride ions, H⁻, so in mechanisms it is shown simply as H⁻. AQA's specification states that aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH₄ in aqueous solution.

Equations with [H]

In organic equations the reducing agent is written as [H], meaning hydrogen from the reducing agent. Two [H] are added across the C=O:

• CH₃CH₂CH₂CHO + 2[H] → CH₃CH₂CH₂CH₂OH
• CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Check the equation balances: the product has exactly two more hydrogen atoms than the starting compound.

The mechanism with H⁻

1. A curly arrow goes from the lone pair on the hydride ion, H⁻, to the δ+ carbonyl carbon. A second arrow goes from the C=O bond to the oxygen, which becomes O⁻.
2. The O⁻ takes an H⁺ from water, forming the OH group of the alcohol.

Reaction with KCN and dilute acid

The other nucleophilic addition on the A-level course uses potassium cyanide followed by dilute acid. The nucleophile is the cyanide ion, CN⁻, which attacks through the lone pair on its carbon, so the product gains a carbon atom. The product is a hydroxynitrile, with an OH group and a C≡N group on the same carbon: ethanal gives 2-hydroxypropanenitrile, CH₃CH(OH)CN. Cyanide is highly toxic and acid releases poisonous HCN gas from it, so the reaction needs great care. The hydroxynitrile guide covers naming and conditions in detail.

Nucleophilic addition and optical isomers

The carbonyl group is planar, so the nucleophile is equally likely to attack from above or below. If the product has a chiral centre (four different groups on the old carbonyl carbon), the two directions of attack give the two enantiomers in equal amounts: a racemic mixture, which is optically inactive.

• Reducing butanone, CH₃COCH₂CH₃, gives butan-2-ol, CH₃CH(OH)CH₂CH₃, whose central carbon carries CH₃, C₂H₅, H and OH: four different groups, so a racemic mixture forms.
• Reducing propanone gives propan-2-ol, whose central carbon carries two CH₃ groups, so it isn't chiral.
• Reducing an aldehyde gives a primary alcohol with two hydrogens on that carbon, so it isn't chiral either.

With KCN, it's different: the CN group is a new, different group, so aldehydes (except methanal) and unsymmetrical ketones give racemic mixtures.

Summary of the reactions

Common mistakes

• Calling it electrophilic addition. Carbonyls are attacked by nucleophiles because their carbon is δ+.
• Drawing H⁺ as the attacking species with NaBH₄. The nucleophile is H⁻, a hydride ion with a lone pair.
• Starting the second arrow on the oxygen. It starts on the C=O double bond and ends on the oxygen.
• Forgetting the second step. The O⁻ intermediate must pick up H⁺ to give the OH group.
• Unbalanced [H] equations. Reducing one C=O always needs 2[H].

To practise carbonyl mechanisms alongside the rest of the second-year course, try today's A-level Chemistry: Year 2 short timed paper.

Read next

Chemistry · Organic chemistryNucleophilic substitution: mechanism, reactions, SN1 and SN2Nucleophilic substitution for A-level Chemistry: the mechanism with curly arrows, halogenoalkanes with OH⁻, CN⁻ and NH₃, reaction rates, and SN1 vs SN2.