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In the following mechanism, nitric acid acts as an oxidising agent to remove 2 H atoms from the intermediate (in the blue square) mechanism My first instinct if given only the reagents would be to protonate something, such as the amine group for example, rather than removing hydrogens.

So my question is how can I identify if $\ce{HNO3}$ will act as an acid or an oxidising agent, given only the reagents? Do I just have to try out both and see which one leads to a reaction? Or is there a property of nitric acid that I'm missing?

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  • $\begingroup$ If compound can be easily oxidised and you add strong oxidating agent... $\endgroup$ – Mithoron May 28 '16 at 23:26
  • $\begingroup$ This is kind of a special case, the dihydropyridine is extremely easily oxidised because the pyridine product is aromatic, nitric acid usually behaves as an acid but it also has oxidising properties so as @Mith said... $\endgroup$ – orthocresol May 28 '16 at 23:53
  • $\begingroup$ @orthocresol so there's no real way of telling, unless I know the full reaction? (eg. knowing that this reaction is thermodynamically favourable due to aromaticity) $\endgroup$ – name.disp May 29 '16 at 0:12
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The reactions of nitric acid could be due to 1) its acid nature, 2) its oxidizing properties, or both.

Orthocresol was correct that the substrate could predetermine which course was more likely: acid-susceptible substances (like alkalies, or alkaline materials) would respond to the acid nature; oxidizable substances would respond to the oxidizing power (like organic alcohols and sugars).

An interesting case is pure iron: in dilute nitric acid (less than 10%), it dissolves. In concentrated nitric acid, it is oxidized (passivated) and does not dissolve.

Increased temperature would also increase the likelihood of an oxidation reaction compared to an acidic reaction, because the activation energies for oxidations are usually higher.

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