6.1.2 (d,e) Characteristic tests for Carbonyl compounds

Syllabus

(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?

2,4-DNP

You don't need to learn the structure of 2,4-dinitrophenylhydrazine.

Just as well.

You don't even need to learn its full name (though examiners use it).

You do need to understand its use.

It could be used to distinguish a carbonyl from other substances since it produces a yellow or orange ppt with all carbonyls but with nothing else.

So, the presence of a DNP derivative may be enough.

On the left we see a negative DNP test (no ppt).

Don't be fooled by the yellow solution - this happens to everything you add DNP to.

On the right, we see a yellow ppt in a yellow solution.

We know for sure that a Carbonyl is present.

But we don't know which one.


Fortunately, you no longer need to be able to draw the structure of the derivatives either.

The real advantage of DNP is that purified DNP derivatives have very different Melting Points.

So, these can be used to identify the exact carbonyl we started with.

Image result for 2,4-dinitrophenylhydrazine
Image result for dnp derivative melting point

You should know that to make full use of DNP you would have to recrystalise and filter your derivative to purify it, dry it, melt it and (everyone forgets this bit in exams) check the melting point against a database.

You absolutely don't need to learn any melting points.

But learn the sequence:

RECRYSTALISE, FILTER, DRY, MELT, CHECK DATABASE

Tollens Reagent (Silver Mirror Test)

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.

Image result for tollens test

This equation is not required knowledge.

This result is:

Image result for tollens test

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.

Image result for aldehyde oxidation [O]

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