Aldehydes & Ketones: Oxidation, Reduction & Nucleophilic Addition (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about the oxidation, reduction and nucleophilic addition in aldehydes and ketones.

Oxidation, reduction and the reaction with cyanide all take place at the same polarised carbon in the carbonyl group.

Aldehydes and ketones both contain the carbonyl group, which is a carbon-oxygen double bond, which is polar because the oxygen is more electronegative than the carbon. Oxidation converts an aldehyde's carbonyl into a carboxylic acid, while reduction and the reaction with cyanide are both nucleophilic addition reactions that add a nucleophile across that same carbon.

We'll start with aldehydes and ketones and how oxidation tells them apart, then look at how NaBH4 reduces a carbonyl to an alcohol, and finish with nucleophilic addition and why it can produce a racemic mixture.

Aldehydes and ketones both contain the carbonyl group, C double bond O. In an aldehyde, the carbonyl is always at the end of the carbon chain. In a ketone, it sits within the chain, with an alkyl group on both sides. Aldehydes are readily oxidised to carboxylic acids, using acidified potassium dichromate under reflux, but ketones resist oxidation because they have no hydrogen atom bonded to the carbonyl carbon. This difference is used to distinguish them: warming with Tollens' reagent gives a silver mirror with an aldehyde but no reaction with a ketone, and Fehling's solution turns from blue to a brick-red precipitate with an aldehyde but stays blue with a ketone.

Sodium tetrahydridoborate, NaBH4, reduces both aldehydes and ketones to alcohols. An aldehyde is reduced to a primary alcohol, and a ketone to a secondary alcohol. In aqueous solution, NaBH4 generates the hydride ion, H minus, which is the nucleophile that attacks the carbonyl carbon, so this is an example of nucleophilic addition. The hydride ion is not strong enough to reduce a carbon-carbon double bond, because it is repelled by the double bond's high electron density. It is also not strong enough to reduce a carboxylic acid.

The carbonyl carbon is delta positive, so it attracts nucleophiles such as the cyanide ion. Cyanide attacks the carbonyl carbon to form a negatively charged intermediate, which then reacts with a proton to give a hydroxynitrile. Because the carbonyl group is planar, cyanide can attack from either face with equal likelihood, producing a racemic mixture: this is how nucleophilic addition to aldehydes and unsymmetrical ketones can create enantiomers even though the starting material itself is not optically active.

You can simply say that ketones cannot be oxidised. Reduction of a carboxylic acid looks like a two-stage process through the aldehyde, but in reality it goes straight to the primary alcohol, so check the wording of the question.

The polarised carbonyl carbon is what oxidation, reduction and nucleophilic addition all react at. Aldehydes are oxidised to carboxylic acids, while ketones resist oxidation. NaBH4 reduces a carbonyl to an alcohol by nucleophilic addition of hydride. Cyanide's nucleophilic addition to a planar carbonyl can produce a racemic mixture.

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