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Since the Brady test will be positive for both aldehydes and ketones, we need a way to distinguish between the two. The Tollen's test is only positive for aldehydes (and $\alpha$-hydroxyketones). I was taught in class that there's a nitroprusside test for ketones, but I'm not sure why it's specifically said to be "for ketones", and not aldehydes.

The mechanism I've been taught is this:

$$\ce{CH3COCH3 + OH- -> CH3COCH2- + H2O}$$ (the hydroxide ion is present because we add NaOH to the mixture) $$\ce{CH3COCH2- + [Fe(CN)5NO]^2- -> [Fe(CN)5NO.CH3COCH2]^3-}$$

where the final complex is red (at least for the case of acetone shown above).

This abstraction of the acidic hydrogen using base doesn't seem unique to ketones. Does this test work for aldehydes too? If it doesn't, why not?

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  • $\begingroup$ en.wikipedia.org/wiki/Urine_test_strip#Ketone_test $\endgroup$
    – Mithoron
    Feb 6 at 16:11
  • $\begingroup$ You know all these tests are lame and pretty much irrelevant as far as actual chemical practice goes? $\endgroup$
    – Mithoron
    Feb 6 at 16:15
  • $\begingroup$ Thanks for the link, but I'm not sure that it answers my question. As for the relevancy of this test to actual practice, that's a separate issue. $\endgroup$
    – AVS
    Feb 8 at 14:15

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