6.1.2 (d,e) Characteristic tests for Carbonyl compounds

(d) use of 2,4-dinitrophenylhydrazine to:

(i) detect the presence of a Carbonyl group in an organic compound

(ii) identify a Carbonyl compound from the melting point of the derivative

{The equation for this reaction is not required. Structure of derivative not required.}

(e) use of Tollens’ Reagent (ammoniacal Silver Nitrate) to:

(i) detect the presence of an Aldehyde group

(ii) distinguish between Aldehydes and Ketones, explained in terms of the Oxidation of Aldehydes to Carboxylic acids with reduction of Silver ions to Silver.

{In equations involving Tollens’ reagent, [O] is acceptable.}

What does this mean?

The Tollens test distinguishes aldehydes from ketones by making use of the fact that aldehydes can be oxidised to carboxylic acids while ketones cannot.

Tollens reagent is sometimes referred to as Ammoniacal Silver Nitrate.

This is because it is prepared from Silver Nitrate.

The silver ions are precipitated as Silver Oxide with Sodium Hydroxide

This leaves the Nitrate ions in solution.

The Silver ions are then redissolved with ammonia solution to form the [H₃N-->Ag<--NH₃]⁺ ion, which also dissolves.

So, the solution now contains this ion and the Nitrate ion - Ammoniacal Silver Nitrate - as well as OH- ions.

The silver ions are then be reduced to silver metal when the aldehyde is oxidised to a carboxylic acid

- this coats the inside of the test tube with shiny silver (a silver mirror).

Or, it does in theory - it's often quite hit and miss and so is a better test in theory than practice.

This equation is not required knowledge.

This result is:

As far as A' level Chemistry is concerned, there are no ketones that produce a Silver Mirror (not entirely true).

At worst, an examiner can ask for the oxidation using [O] to represent the Tollens (or any other Oxidising agent)

So, you could equally distinguish between something you know is either an aldehyde or a ketone by adding acidified dichromate.