What is 2,4-DNPH used to test for, and what is observed?

That a carbonyl group is present, in either an aldehyde or a ketone; crystals of yellow to orange precipitate appear.

Why does 2,4-DNPH not distinguish an aldehyde from a ketone?

Both contain the C=O group that the reagent reacts with, so both give the same coloured precipitate. A further test with Tollens' or Fehling's is needed to tell them apart.

How is 2,4-DNPH used to identify which carbonyl compound is present?

The derivative is filtered, purified by recrystallisation and its melting temperature measured. Comparing that value with tabulated melting temperatures of known derivatives identifies the original compound.

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Organic Chemistry II

Testing for a carbonyl group: 2,4-DNPH and the iodoform reaction

3 things to know
Papers 2 and 3

Overview

Recall questions on the 2,4-DNPH test and its orange precipitate, on identifying a carbonyl from its melting temperature, and on what a positive iodoform test proves.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why must the derivative be recrystallised before its melting temperature is taken?

Key term

What reagents are used in the iodoform reaction?

Key term

What is observed in a positive iodoform test?

Questions from this subtopic

Each one with the model answer underneath.

1
What is 2,4-DNPH used to test for, and what is observed?
Model answer
That a carbonyl group is present, in either an aldehyde or a ketone; crystals of yellow to orange precipitate appear.
2
Why does 2,4-DNPH not distinguish an aldehyde from a ketone?
Model answer
Both contain the C=O group that the reagent reacts with, so both give the same coloured precipitate. A further test with Tollens' or Fehling's is needed to tell them apart.
3
How is 2,4-DNPH used to identify which carbonyl compound is present?
Model answer
The derivative is filtered, purified by recrystallisation and its melting temperature measured. Comparing that value with tabulated melting temperatures of known derivatives identifies the original compound.
4
Why must the derivative be recrystallised before its melting temperature is taken?
Model answer
Impurities lower the melting temperature and widen the range over which the solid melts, so an impure sample would not match the tabulated value.
5
What reagents are used in the iodoform reaction?
Model answer
Iodine under alkaline conditions, or potassium iodide together with sodium chlorate(I).
6
What is observed in a positive iodoform test?
Model answer
A pale yellow precipitate of triiodomethane forms, which has an antiseptic smell.

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