Test for Carbonyl Compounds

In this video we want to discuss the distinguishing test for carbonyl compounds which include aldehydes and ketones.

To identify the presence of carbonyl compound we can add 2,4-DNPH at room temperature.

Positive test will be the observation of an orange precipitate.

test for carbonyl compounds test and observation

2,4-DNPH or Brady's Reagent

We need to know the structure of 2,4-DNPH for writing balanced equations, and it is easier to remember the full name and structure together.

The full name of 2,4-DNPH or Brady's Reagent is 2,4-dinitrophenylhydrazine.

To break down the functional groups:

nitro is -NO2
phenyl is benzene
hydrazine is NH2-NH2

So we can piece everything together to work out the structure of 2,4-dinitrophenylhydrazine.

test for carbonyl compounds 24DNPH structure

Type of Reaction - Condensation

The reaction between carbonyl compound and 2,4-DNPH is condensation, where both compounds combine and a small molecule H2O is eliminated.

To draw the product easily we can point the carbonyl functional group towards the hydrazine functional group, box up water, and join the 2 fragments via a C=N bond.

The product with the C=N bond is an imine functional group and this gives the distinct orange colour that indicates the presence of carbonyl functional group.

test for carbonyl compounds condensation reaction

For the detailed step-by-step discussion on how to draw 2,4-DNPH structure and work out the condensation reaction with carbonyl compound, check out this video!

Topic: Carbonyl Compounds, Organic Chemistry, A Level Chemistry, Singapore

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