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I have read that Tollens' test is given by aldehydes. The aldehyde is oxidised to a carboxylate ion and silver is deposited.

My teacher had given me notes which pointed that salicylaldehyde does not give this test. However, I cannot find any explanation for the same. I have tried to search online and through books but I did not get anything.

I think it could be related to resonance from the −OH group as something similar is mentioned in my notes, but it is not elaborated. Can anyone please explain this?

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Of course, the answer is resonance. As described by Benet et al., a basic medium (pH>10) favours the Tollen's reaction. On the other hand, if we want to dissolve phenol in water, we will need OH- to generate phenolate. It means the hydroxyl group of salicylaldehyde is sensitive to the high pH, and this hinders attacking the other hydroxides to the carbonyl group by further stabilizing it.

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  • $\begingroup$ Okay, so what I understood from your answer is that because of resonance from the hydroxyl group the carbonyl is further stabilized which prevents other reactions. This would also mean that if there were other groups like -NR2 that can stabilize the carbonyl then also the test would not be obtained. Is that correct ? $\endgroup$
    – Aurav S Tomar
    Commented Jul 18, 2020 at 4:45
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    $\begingroup$ I don't think -NR2 or -OR (R is not H) has the same effect, but a negative response, too. I couldn't find a reference to prove this but in my opinion, the stability of the final structure has a significant influence on blocking the carbonyl group. $\endgroup$
    – Reihani
    Commented Jul 18, 2020 at 6:03

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