Aldehydes and ketones
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I'd like to know why is 2,4-dinitrophenylhydrazine the most useful reagent for identification of early members of aldehydes and ketones?
250509
Corrie writes ....
2,4-dinitrophenylhydrazine (used as Brady's reagent) is useful because it forms brightly coloured (yellow - orange - red) insoluble products with many aldehydes and ketones, often just on mixing. The formation of a coloured precipitate is used as a test to distinguish molecules containing a >C=O group, aldehydes and ketones giving a coloured precipitate, whereas a negative test is given by carboxylic acids, esters etc.
Small aldehyde and ketones have low boiling points and would be difficult to identify by boiling point alone, especially as some have very similar boiling points. The precipitates from a Brady's reagent test (called 2,4-dinitrophenylhydrazones) can be filtered off, purified by recrystallisation, and their (much higher) melting points determined accurately. In most cases these melting points can then be used to identify the original aldehyde or ketone - older books on organic analysis often contain tables listing M.Pts. of the 2,4-dinitrophenylhydrazones of many aldehydes and ketones.
Risk assessment
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updated: 26 May 2009
