Chemistry, Third edition

336 18 · COMMON CLASSES OF ORGANIC COMPOUNDS

Acidified sodium dichromate is an example of an oxidizing agent which will bring

about the oxidation of primary alcohols through to carboxylic acids and secondary alcohols

to ketones. If an aldehyde is required as the product of oxidation of a primary alcohol, the

apparatus must be designed so that the aldehyde will be distilled off as it is formed.

Carbonyl compounds can be reduced back to alcohols by using the reducing

agents lithium aluminium hydride (LiAlH 4 ) or sodium borohydride (NaBH 4 ).

Tests to distinguish between aldehydes and ketones

The fact that aldehydes are easily oxidized to carboxylic acids, whereas ketones are

not, can be used to distinguish between them:

1.Fehling’s solution may be regarded as containing Cu2+ions in basic solution.

When this solution is boiled with an aldehyde, a brick-red precipitate of copper(I)

oxide is formed – the aldehyde is oxidized to a carboxylic acid by the copper(II)

ions. Ketones arenotoxidized by this reagent.

RCHO6H 2 O2Cu^2 RCOOH4H 3 O Cu 2 O

2.Tollens’ reagent contains Agions dissolved in aqueous ammonia. If the reagent

is warmed with an aldehyde, the silver ions are reduced to silver metal and a dis-

tinctive ‘silver mirror’ is deposited on the reaction container. Again, ketones do

not react with this reagent.

aqueous

ammonia

RCHO3H 2 O 2Ag——RCOOH2H 3 O2Ag

Identification of aldehydes and ketones

The carbonyl group in aldehydes and ketones will react with hydrazinederivatives.

Hydrazine is related to ammonia:

The products of the reaction of hydrazine with an aldehyde or ketone are known as a

hydrazones:

Because water is ‘eliminated’ in the reaction, the reaction is known as a condensation

reaction. A slight variation of this reaction can be used to identify aldehydes or

ketones using 2,4-dintrophenylhydrazine because the hydrazones formed are orange

solids that can be recrystallized easily.