Question

When identifying an unknown compound in organic lab, will the tests for aldehyde and ketone work for anything with a carbonyl? For example, will the DNP test be negative if there are no carbonyls present at all or will it be positive if there is a carbonyl even included esters or amides? I️ need a suggestion for a test to narrow down if my unknown has a carbonyl at all. The unknown will be an amide, an ester, aldehyde, ketone, or alcohol. I️ think I️ have an alcohol, but I️ am nervous my tests would have also worked for an aldehyde or ester. Thanks.

Answer 1

Carbonyl group contains carbon-oxygen double bond (>C=O)

In case of a carbonyl group oxygen is more electronegative than carbon, thus carbon will act as a electrophile.

Hence the reaction will follow the following steps in a aldehyde and ketone.

The NH2 group in the 2,4-dinitrophenylhydrazineb will react with electrophilic carbon atom and get precipitated.

Comparing with the aldehyde and ketone the carbonyl carbon in ester, amide or carboxylic acid are less electrophile or no electrophile in nature (In aldehydes and ketones, carbonyl carbons are adjacent to either alkyl groups or hydrogen atoms Alkyl groups may donate some electrons density to carbonyl carbon, but it is still electrophilic enough to react with -NH2 group and additionally there are one -OR group in ester adjacent to carbonyl carbons)

So 2,4-dinitrophenylhydrazine test will be positive only with aldehyde and Ketone.