Aldehydes and ketones
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2,4-Dinitrophenylhydrazine reacts with the carbonyl group found on aldehydes and ketones to form a red precipitate. Why is it that it does not react with the carbonyl group of carboxylic acids or esters to form a precipitate?
Carboxylic acids and their derivatives can be represented as R-COX where R is a hydrocarbon group and X is -OH in an acid, -OR' in an ester, -Cl in an acid chloride and so on.
In every case X includes an electronegative atom (O, N, Cl) with a lone pair of electrons. This has a marked effect on the chemistry of the C=O group for two main reasons:
(1) Interaction between the p (pi) electrons in the C=O double bond and the lone pair on X give rise to a degree of delocalisation such that it is often better to regard the -COOH, -COCl or -COR' group as a whole as the functional group. This effect is most marked when a carboxylic acid ionises so that X = O-.
(2) In these compounds X is a possible leaving group with the polar C-X bond being much easier to break than the non-polar C-C or C-H bond in an aldehyde or ketone. As a result the characteristic reactions take the form:
R-COX + Nu- -> R-CO-Nu + X-
where Nu- is an attacking nucleophile.
So far as 2,4-DNP is concerned we think that the first argument is the key to this situation when it comes to explainingthat no reaction with acids or esters. This must be a matter of interference between the p (pi )electrons and the lone pairs.
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updated: 21 August 2003
