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You have a bottle of salicylaldehyde. And you want to make salicylic acid from salicylaldehyde. You can use all reagents and catalyst to make salicylic acid.

I think salicylic acid can be synthesized from salicylaldehyde since salicylaldehyde has a structure similar to salicylic acid. I'm thinking that it might be possible to use $\ce{KMnO4}$, but I'm afraid the hydroxyl group attached to the benzene would be oxidized too.

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  • $\begingroup$ You can try using Tollens reagent. Not sure. $\endgroup$ – user24149 Dec 27 '15 at 18:13
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There are a large number of reagents out there that will transform aldehydes into the corresponding acids (benzaldehydes and benzoic acids in this case) and $\ce{KMnO4}$ is indeed one of them.

Oxidation of the phenolic hydroxide is only possible if at the same time the aromatic system is destroyed. However, it is rather hard to come up with a good product of this oxidation since a simple quinone isn’t possible. It therefore takes much harsher conditions than the oxidation of an aldehyde.

Things would change if a second hydroxy group were attached to the same aromatic system either ortho or para to the first. Then the oxidation to the corresponding quinone is much easier.

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Salicylic acid can be synthesized from salicylaldehyde using $\ce {KMnO4}$ without oxidising the hydroxy group ($\ce {-OH}$).

The hydroxy group that attached to the benzene isn't oxidized, because the C atom that bonded with the hydroxy has already use 2 electrons for sigma bonding and 1 electron for pi bonding. The electron for pi bonding is used for resonance on benzene cycle, so it can't be used for bond with O atom to make oxo group ($\ce {=O}$).

If the electron of pi bonding is used for bonding with O atom to make oxo group, the benzene will change to a cyclohexadiene.

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    $\begingroup$ You can oxidize phenols to quinones, but the conditions for that are more forcing than oxidation of aldehydes to carboxylic acids. $\endgroup$ – Abel Friedman Oct 8 '14 at 18:24
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I did the réaction for my project but it didn't work. You can't oxidize your salycaldehyde just by using $\ce{KMnO4}$ (its maybe due to the $\ce{OH}$). I think there is a protocol by using an $\ce{Fe[CN}$] complex for the ciclique molecule.

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