Question

An organic compound gives orange precipitate with \[\text{2,4 -DNPH}\]. What is the functional group of the compound?

Answer 1

Hint:\[\text{2,4 -DNPH}\] is the abbreviated name for 2,4 – Dinitrophenyl Hydrazine and this chemical reagent is used to test the presence of an ketone and an aldehyde, basically a carbonyl group.

Complete answer:
Both aldehydes and ketones react with 2,4 – Dinitrophenyl Hydrazine to form yellow, orange, or reddish-orange precipitates, while any other functional group would not react with the reagent. An aqueous solution of 2,4 – Dinitrophenyl Hydrazine is also known as the “Brady’s Reagent”. To perform the DNP test, the compound to be tested is simply mixed with an acidic solution of Brady’s Reagent in methanol. If an orange or yellow colored precipitate is formed then it indicates the presence of the carbonyl group. The compounds that are formed from the reaction are “2,4 – DinitrophenylHydrazones” which have sharp melting points due to their crystalline nature from which the parent compound can be verified.
The colour of the precipitate also indicates the nature of the compound. If the precipitate is “yellow” in colour then it indicates that the parent carbonyl compound is “non-conjugated” while if the precipitate is “orange” in colour then the compound should be a conjugated compound, however there are certain exceptions to this as well.

Note:
Not only the colour of the precipitate tells about the nature of the compound taken but the time taken for the appearance of the precipitate is also indicative of whether the carbonyl compound taken is an aldehyde or a ketone. If the time required for the reaction is less and the precipitate appears quickly then it means that the group is “aldehyde” and if the precipitate requires time to appear, then the compound should be a “ketone”.