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With respect to the compounds I–V, choose the correct statement(s).

List of compounds

  1. Compound II becomes more acidic when it has a $\ce{-NO2}$ substituent.
  2. The acidity of compounds follows the order I > IV > V > II > III.
  3. The conjugate base of compound IV is aromatic.
  4. The acidity of compound I is due to delocalization in the conjugate base.

I am not able to decide about the first statement properly. I compared the acidity by considering the conjugate base, which is the same (benzyne) even if $\ce{NO2}$ group is added.

However, the solution says that it will increase acidity because of being an electron withdrawing group.

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    $\begingroup$ Do you really think conjugate base of II is benzyne? $\endgroup$
    – Aniruddha Deb
    Sep 29, 2020 at 16:48

1 Answer 1

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Both the predictions are good. The $\ce{NO2}$ will actually be an effective electron withdrawing group, and the lost of that $\ce{H}$ will stabilize the compound (if there is and $\ce{NO2}$ on it). It is also important to rescue, that both effects will be stronger just on para or ortho position. You can make the different structures to check it.

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    $\begingroup$ It is talking about the acidity of the compound, so ortho para don't matter here, the acidity of the compound in general matters. $\endgroup$
    – P K
    Sep 29, 2020 at 16:20
  • $\begingroup$ @PK Objection. The relative position of the nitro group to the deprotonated carbon influences the acidity of the compound. Draw the deprotonated form and count the mesomeric formulae possible for each of them and you find o and p differ from the m-pattern. It isn't limited to benzoic acids, although often the ortho effect is taught on them as example. $\endgroup$
    – Buttonwood
    Sep 29, 2020 at 17:13
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    $\begingroup$ @Buttonwood The deprotonated form holds the extra electron (i.e. the negative charge) in an $\ce{sp^3}$ orbital which is parallel to the plane, whereas the pi system is perpendicular to the plane. So they can't interact with each other and conjugation is not a factor here. Simple inductive effect would predict ortho > meta >para in order of high to low stabilization of the anion. $\endgroup$
    – S R Maiti
    Mar 28, 2021 at 20:20
  • $\begingroup$ Indeed... the answer is wrong (-1 from me) and @SRMaiti is absolutely spot on. $\endgroup$
    – orthocresol
    Mar 24, 2022 at 0:34

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