Carbonyl Compounds

 
 

Aldehydes and ketones

Aldehydes and ketones are carbonyl compounds — they contain the carbonyl (C=O) functional group.

  • Aldehydes have the carbonyl group at the end of the compound (-CHO) while ketones have the carbonyl group in the middle of the chain (R-C=O-R).

  • Aldehydes are named by adding the ending –al and ketones are named with the ending –one.

 
 

Reactions of aldehydes and ketones

Aldehydes are made by oxidising primary alcohols using acidified potassium dichromate. You prepare aldehydes using distillation, in which the oxidation product is immediately removed through evaporation and condensation. Repeated cycles of evaporation and condensation through reflux would further oxidise the aldehyde into a carboxylic acid.

 
 

Ketones are prepared by refluxing secondary alcohols with acidified potassium dichromate.

 
 

The reverse of this reaction can be carried out using a reducing agent such as sodium borohydride (NaBH4). Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols. This is a nucleophilic addition reaction with the hydride ion acting as a nucleophile. Nucleophiles are species with a lone pair of electrons that are attracted to positive charges – in this case it’s the slightly positive carbon on the carbonyl bond.

 
 

The reaction can be broken down into the following steps:

  1. The hydride nucleophile attacks the slightly positive carbon atom on the carbonyl functional group.

  2. The double bond of the C=O group breaks, with the bonding electrons being taken by oxygen. A negatively charged intermediate is formed.

  3. The lone pair of electrons on the oxygen attacks a water molecule, forming the alcohol and a hydroxide ion.

Hydrogen cyanide reacts with aldehydes and ketones in the same way, except the cyanide ion is the nucleophile. A hydroxynitrile is the product of this nucleophilic addition reaction.

 
 

Here’s a breakdown of the steps involved:

  1. The cyanide nucleophile attacks he slightly positive carbon atom on the C=O group.

  2. The double bond between carbon and oxygen breaks, with both electrons from the double bond transferred to oxygen. A negatively charged intermediate is formed.

  3. The lone pair of electrons on the oxygen are donated to a hydrogen ion (from HCN) to form a hydroxyl group. A hydroxynitrile is formed.

In the lab, sodium cyanide would be used instead of hydrogen cyanide as it is less toxic. These preparations should always be done in a fume cupboard.

Tests for carbonyl compounds

Test for the presence of the carbonyl functional group (C=O) using 2,4-dinitrophenylhydrazine, 2,4-DNP (Brady’s reagent.)

  • 2,4-DNP is dissolved in methanol and concentrated sulfuric acid then added to the unknown sample.

  • If an orange precipitate forms, the sample contains a ketone or an aldehyde.

You can then identify the compound — different aldehydes and ketones have established crystalline melting points.

  • Crystallise the compound and record its melting point.

  • Compare to published data book values to identify the carbonyl compound.

There’s another test that can be used to distinguish between an aldehyde and a ketone:

  • Add Tollens’ reagent to the unknown sample — it forms a silver mirror if an aldehyde is present. Nothing will be seen if the sample is a ketone.

  • Tollens’ reagent contains silver nitrate dissolved in ammonia solution — this oxidises aldehydes into carboxylic acids. Ketones cannot be further oxidised, which is why they don’t give a result in this test.