Carbonyl Group

Aldehydes and Ketones

Aldehydes and Ketones

Known collectively as carbonyls, aldehydes and ketones are represented by the general formula:


And have the functional group:

Aldehydes and Ketones

  • In aldehydes the carbon is attached to a hydrogen atom and the other either a hydrogen atom or an alkyl group.
  • In ketones the carbon is attached to two alkyl groups.

Redox reactions

As carbonyl redox reactions were already described in AS Chemistry, they are only summarised below.

Oxidation reactions

The oxidation of alcohols forms carbonyls.

  • If a primary alcohol is only partially oxidised then an aldehyde is formed.
  • If a secondary alcohol is oxidised a ketone is formed.

The oxidation of primary and secondary alcohols, and aldehydes is carried out with potassium dichromate (K2Cr2O7) in sulphuric acid (H2SO4). A gentle heat is required to get the reaction started.

To form an aldehyde care is needed to stop further oxidation of the carboxylic acid. To prevent this from occurring distillation apparatus can be used to distil off the aldehyde as it is produced.

There is a colour change from orange to green as Cr2O72- is reduced to Cr3+.

When writing out an oxidation reaction the oxidant can be represented simply as [O]. For example: R-CHO + [O] ? R-COOH

An important difference between aldehyde and ketone reactions is that:

  • aldehydes are be readily oxidised to carboxylic acids
  • ketones are not readily oxidised

This principle is used to distinguish them.

Tollen’s reagent or ammoniacal silver nitrate, [Ag(NH3)2]+, is a mild oxidising agent. Aldehydes can reduce it to create carboxylic acids:

R-CHO + 2[Ag(NH3)2]+ + H2O ? RCOOH + 2Ag + 4NH3 + 2H+

When boiled, aldehydes produce a grey precipitate known as the ‘silver mirror’.

As ketones cannot be oxidised using this method it is a standard test for an aldehyde.

Fehling’s solution, an alkaline solution which contains Cu2+ ions, is another mild oxidising agent. It can also be reduced by aldehydes to create carboxylic acids:

R-CHO + 4OH + 2Cu2+ ? R-COOH + Cu2O + 2H2O

When gently warmed, a brick-red precipitate of Cu2O forms.

As ketones cannot be oxidised using this method it is another standard test for an aldehyde.

Reduction reactions

The reducing agent sodium tetrahydroborate (III), NaBH4, can be used to reduce aldehydes and ketones into primary and secondary alcohols respectively. Using [H] to represent the reductant, the simplified equations of these reactions are as follows:

  • aldehyde ? primary alcohol: R-CHO + 2[H] ? R-CH2OH
  • ketone ? secondary alcohol: R1-CO-R2 + 2[H] ? R1-CH(OH)-R2

It is not possible to reduce carboxylic acids with NaBH4. However, using the reducing agent LiAlH4 it is possible to reduce them to primary alcohols.

Nucleophilic addition

Due to the fact that carbonyls are unsaturated they can undergo addition reactions. The positive centre of the polar C=O bond enables carbonyls to react with nucleophiles and, therefore, undergo nucleophilic addition.

Mechanism of nucleophilic addition

Example 1: addition of HCN to make hydroxynitriles

Hydrogen cyanide (HCN) is a nucleophile. It reacts with carbonyls as follows:

R1-CO-R2 + HCN ? R1-C(CN)(OH)-R2

HCN is a very toxic gas and is, therefore created in situ:

KCN(s) + HCl(aq) ? HCN(g) + KCl(aq)

There are three steps to the mechanism:

  • Step 1: the weak acid HCN dissociates:

? HCN ? H+ + CN

  • Step 2: the CNacts like a nucleophile and attacks the carbonyl.
  • Step 3: the H+ ion is collected up by the O atom.

Example 2: reduction by NaBH4 to make alcohols

NaBH4 consists of hydrogen in a negative oxidation state. It is, therefore, able to behave like a nucleophile.

There are two steps to the mechanism:

  • Step 1: the ?+ carbon atom is attacked by the Hion.
  • Step 2: a H atom is pulled of a water molecule by the O atom.