Question 3

[anon3543]

Hi, I completely understand the explanation, but I also added the fact that it is soluble in water, so I thought it would be an amide, which is must more likely to be soluble in water than just a ketone.... Does this not make sense because an amide doesn't count as ketone/aldehyde positive test from 2,4 DNP? Thanks!


3. An unknown water soluble compound is insoluble in 5% NaHCO3, gives a positive 2,4-DNP test and a negative Fehling's test. Which of the functional groups are present?

1. Carboxylic acid [x]
2. Amide [x]
3. Aldehyde [x]
4. Ketone [x]

INCORRECT
Your Answer: B
Correct Answer: D
Explanation
Insolubility in NaHCO3 eliminates carboxylic acid as a potential candidate. A positive 2,4-DNP test and a negative Fehling's test must necessarily indicate that a ketone is present according to Table 1.

[admin]

You hit the nail on the head in terms of the problem with an amide as the solution. The 2,4 DNP being only positive for aldehydes and ketones would not be expected to react with an amide. Furthermore, water solubility, as you know, is a relative term (after all, not all amides are more soluble than all ketones/aldehydes; the substituents, length/size of the alkyl chains make a difference). And the Table is telling us, that according to their data, that the ketones and aldehydes which are soluble flow in one direction, and if they were not soluble then they would follow the right side of the table.

[admin]

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