Part 5. The chemistry of ALDEHYDES and KETONES

Selected condensation reactions

Part 5.6 Examples of the nucleophilic addition - elimination condensation reactions of aldehydes and ketones - including reaction product with 2,4-dinitrophenylhydrazine for identification

Sub-index for this page Part 6.6

5.6.1 A basic addition-elimination condensation reaction of aldehydes or ketones with compounds containing an amino/amine group

5.6.2 The reaction between 2,4-dintrophenylhydrazine with aldehydes or ketones

5.6.3 The preparation and use of the product from reacting aldehydes and ketones with 2,4-dinitrophenylhydrazine

5.6.1 A basic addition-elimination condensation reaction of aldehydes or ketones with compounds containing an amino group

In this case, molecules with an amino group (-NH₂), can undergo a condensation reaction with aldehydes and ketones.

R₂C=O  +  H₂N-X  ===>  R₂C=N-X  +  H₂O

R = H or alkyl for aliphatic aldehydes and ketones

This is also an example of a addition - elimination reaction, where two molecules are joined together by the elimination of a small molecule - in this case water.

e.g. aldehydes and ketones react with hydrazine (H₂N-NH₂), so X = -NH₂

R₂C=O  +  H₂N-NH₂  ===>  R₂C=N-NH₂  +  H₂O

(CH₃)₂C=O  +  H₂N-NH₂  ===>  (CH₃)₂C=N-NH₂  +  H₂O

The product here is called a hydrazone e.g. if R = CH₃, the product is called propanone hydrazone

In these reactions, the lone pair on the nitrogen of the -NH₂ group, enables it to act as a nucleophile that can attack the delta plus carbon atom of the carbonyl group, the polar bond (^δ+C=O^δ-) of the aldehydes and ketones functional group.

Unlike the addition of HCN, there is no addition of a proton (H⁺) to the oxygen of the carbonyl group.

Instead the initial product is a combination of the two molecules, before water is eliminated to give the final product, which has a C=N double bond in its molecular structure.

Note on E/Z isomerism - a type of stereoisomerism (old name geometric isomerism).

Around the carbon and nitrogen atoms of the C=N bond, there three groups of bonding and non-bonding electrons.

This gives a trigonal planar bond arrangement with bond angles of ~120^o.

Therefore you have a similar situation to that in alkenes (>C=C<) and the possibility of E/Z isomerism.

However, this is only possible if R and R' are different groups, so here the carbonyl compound must be an aldehyde (automatically unsymmetrical at the >C=O group) or an unsymmetrical ketone, where the two alkyl or aryl groups attached to the C=O are different e.g. all the '2-ones' like butanone or pentan-2-one.

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5.6.2 The reaction between 2,4-dintrophenylhydrazine with aldehydes or ketones

This is another example of an addition - elimination condensation reaction.

R₂C=O  +  H₂N-X  ===>  R₂C=N-X  +  H₂O

BUT, this time X is a bit more complicated - a lot bigger.

The general equation for the addition-elimination reaction between 2,4-dinitrophenylhydrazine and aldehyde or ketone - also an example of a condensation reaction. R, R' = H, alkyl or aryl.

The product is called a 2,4-dinitrophenylhydrazone or just 'dinitrophenylhydrazone' (one letter changed).

The equation for the addition-elimination reaction between 2,4-dinitrophenylhydrazine and ethanal.

The product is called ethanal 2,4-dintrophenylhydrazone.

Here there is the possibility E/Z isomers and it is theoretically the same for all aldehydes.

The equation for the addition-elimination reaction between 2,4-dinitrophenylhydrazine and propanone.

The product is called propanone 2,4-dintrophenylhydrazone.

Here there is no possibility of E/Z isomers,

and it's the same for other symmetrical ketones like pentan-3-one CH₃CH₂COCH₂CH₃

BUT, unsymmetrical ketones like butanone CH₃COCH₂CH₃ and pentan-2-one CH₃COCH₂CH₂CH₃, would theoretically form E/Z isomers.

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5.6.3 The preparation and use of the product from reacting aldehydes and ketones with 2,4-dinitrophenylhydrazine

Brady's reagent is an aqueous orange solution of 2,4-dinitrophenylhydrazine and is used as a test for aldehyde and ketone carbonyl compounds - BUT, it does NOT distinguish between them.

When a few drops of an aldehyde or ketone is shaken with a few cm³ of Brady's reagent a yellow-orange precipitate is formed if the compound is an aldehyde or ketone - a simple test.

The dinitrophenylhydrazone precipitate is insoluble in water.

However, you can filter of the precipitate and recrystallise with the minimum volume of hot ethanol.

Filter off again and dry on a filter paper or watch glass.

You can then determine the melting point of the dinitrophenylhydrazone.

Each dinitrophenylhydrazone has a unique melting point.

So you can match the melting point of the dinitrophenylhydrazone with a data table and identify the aldehyde or ketone.

More on the tests for aldehydes and ketones