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I have to compare the acidic strength of these compounds:

A: 2‐hydroxybenzoic acid; B: 2,6‐dihydroxybenzoic acid; C: 2‐methylbenzoic acid

Now, in A and B hydrogen bonding will be present which will help stabilize the negative charge of the conjugate base. However, the hydroxyl group is not bulky and won't cause steric inhibition of resonance, hence the +M effect of phenyl will still be present.

In C, methyl will cause steric inhibition of resonance and hence stop/reduce the +M effect of phenyl which will stabilize the conjugate base.

How do I compare the relative stabilization offered by these two effects (H-bonding and SIR)?

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I think the main effect in play here is straight electron withdrawing potential of the oxygens. In my opinion this is a classical BSc question that tries to avoid the grey zones of pka eye balling.

I would invite you to review the concept of steric inhibition of resonance since I'm not entirely sure I understand your argument there.

Here's a good starting point if you do not have a text book covering the subject https://www.quora.com/What-is-steric-inhibition-of-resonance

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