Aldehydes and Ketones

What are Aldehydes, how can you test for them and how do they form?

Aldehydes are organic compounds with the functional group -CHO containing the C=O bond. The are produced from the initial oxidation of primary alcohols and oxidise further to form carboxylic acids, in the presence of potassium dichromate (Shown in Image).

Aldehydes can be tested using either Tollen’s reagent or Fehling’s solution as they produce a positive test for both.

What are Ketones, how can you test for them and how do they form?

Ketones are organic compounds with the functional group -C=O and contains a C=O bond with two R groups attached to the carbon instead of a hydrogen atom. These are produced from the oxidation of secondary alcohols with acidified potassium dichromate (Shown in Image), with no further oxidation.

Ketones show a negative test result for both Tollen’s reagent and Fehling’s solution.

How can Aldehydes and Ketones be Reduced with NaBH₄?

Oxidation used to produce Aldehydes and Ketones can be reversed with a reduction reaction, where a reducing agent of NaBH₄ is used in nucleophilic addition. The reducing agent provides the :H^- nucleophile, but a H⁺ ion is also required so the reaction takes places under aqueous conditions.

How can Aldehydes and Ketones be Reduced with KCN?

Oxidation used to produce Aldehydes and Ketones can be reversed with a reduction reaction, where a reducing agent of KCN is used in nucleophilic addition. The reducing agent provides the :CN^- nucleophile and is a form of synthesis and it extends the carbo chain by one carbon atom, producing a hydroxy nitrile.

This reaction produces a racemic mixture of two optical isomers when carried out with an aldehyde or an unsymmetrical ketone as the :CN^- ion may attack above or below the flat C=O group.