A basic trend for acid: if the conjugate base is more stable, then the acid has a lower $\ce{pKa}$. Therefore, it would quite intuitive to think that resonance in the conjugate base will make the acid more acidic.

e.g. Cyclohexanol $\mathrm p K_\mathrm a$ -16 ; phenol $\mathrm p K_\mathrm a$ -10

But, there are exceptions. For $\ce{H3PO4, H3PO3, H2PO2}$ the effect is reversed. There is a more spacious resonance in $\ce{H3PO4}$ than in $\ce{H3PO3}$. But, still $\mathrm p K_\mathrm a$ of the former acid is greater than the latter.

Edit: Mithoron commented that my claim about P acids isn't true. Though I don't what claim he is referring, I think even his linked question supports my claim of the order of $\mathrm p K_\mathrm a$ as $\ce{H3PO4>H3PO3>H3PO2}$

So, my question is : can the effect of resonance on acidity be generalised?


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.