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I have to compare the acidity of the above two carboxylic acids. I wish to know which one has a stronger -I effect, the benzylic group or allylic group?

  • $\begingroup$ For future visitors, note that the difference in acidity of these two compounds is very small. The pKa of phenylacetic acid is 4.31, while that of 3-butenoic acid (page 3) is 4.34. One could thus argue that phenyl has a stronger -I effect than a simple alkene, which is expected as it has 6 sp2 carbons vs a single sp2 carbon in alkene. $\endgroup$ – Gaurang Tandon Feb 24 '18 at 10:23

Phenyl has a tendency to withdraw electrons.
see the link for the reasoning: Inductive effect of phenyl ring Also the allylic group would be donating via inductive effect, thus making the carboxylate ion unstable( after acid donates H+ ions) But an sp2 carbon is more electronegative than an sp3 carbon, hence it will donate less.(more electronegative element tries to retain electrons) So 1 has an electron withdrawing group,2 has an electron donating group which is not as good at donating as 3.This gives us the option A

An important point to note here is that electron withdrawing groups increase H+ concentration. Think why?

Edit: Now that the Question has been edited, an edit for the solution is required.

Benzylic group has -I inductive effect as in the link. For the second compound in the question, with the allylic group, since less number of sp2 carbon are present(2 instead of 6). So we can infer that -I effect will be less. Infact it should show +I effect, especially since there is an sp2 Oxygen attached 2 carbons away, which will pull electrons as it is highly electronegative.

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