Arrange the following in increasing order of their acidic strength.
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Electron-withdrawing groups increase the acidic strength of carboxylic acids by stabilizing the carboxylate ion:
\[
-NO_2 \gt -Br \gt -PhCH_2 \gt -CH_3
\]
Greater \(-I\) effect means stronger acid.
All four compounds are acetic acid with one hydrogen on the alpha carbon replaced, so the acidity differences come entirely from how strongly that substituent pulls electron density away through the inductive effect, the better it stabilises the negative charge left behind after losing a proton, the stronger the acid.
A plain methyl group actually pushes electron density in, working against stabilising the carboxylate, so plain acetic acid is the weakest acid of the set.
A benzyl group is only mildly electron withdrawing compared to methyl, giving a small boost in acidity.
Bromine is a genuinely electronegative atom sitting right on the alpha carbon, pulling electron density away strongly and stabilising the conjugate base noticeably more.
The nitro group is the strongest electron withdrawer of the four, thanks to both its inductive pull and its ability to spread the negative charge further through resonance assisted stabilization, making the nitro substituted acid the strongest.
Ranking weakest to strongest, that is acetic acid, then the benzyl acid, then the bromo acid, then the nitro acid, that is a less than b less than c less than d.
