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Questions
Give reasons for the following:
o-nitrophenol is more acidic than o-methoxyphenol.
Explain why is ortho nitrophenol more acidic than ortho methoxyphenol?
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Solution 1
Due to the strong –R and –I effect of the NO2 group, the electron density on the O–H bond decreases; hence, the proton is released easily.
After losing the proton, the remaining o-nitrophenoxide ion becomes stable by resonance.
Ortho-nitrophenoxide ion resonance is stable; hence, o-nitrophenol is a strong acid. On the other hand, due to the +R effect of the OCH3 group, the electron density on the O–H bond increases; hence, the removal of the proton becomes difficult.
Now, the o-methoxyphenoxide ion, which remains after the loss of the proton, gets destabilised due to resonance.
The two negative charges repel each other and destabilise the o-methoxyphenoxide ion, so o-nitrophenol is more acidic than o-methoxyphenol.
Solution 2
The acidity of phenol is due to the dissociation of a hydrogen ion (H+) from the hydroxyl (O–H) group. This loss increases if the O–H bond polarity intensifies and H carries a significant δ+ charge.
The presence of the strongly electron-withdrawing NO2 group at the ortho position in (I) increases the polarity of the O–H bond.
This helps in the release of H+ and increases acidity.
The reaction (II) is less acidic because –OCH3 is an electron-releasing group and increases the electron density on O of OH, and H+ release is suppressed. This can be understood from the resonance structures of II, which are:
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\[\begin{array}{cc}
\phantom{...................................}\ce{Br}\\
\phantom{..................................}|\\
\ce{CH3 - CH - CH - CH3 ->[HBr] CH3 - C - CH2 - CH3}\\
|\phantom{.........}|\phantom{...................................}|\phantom{...........}\\
\ce{CH3}\phantom{...}\ce{OH}\phantom{...............................}\ce{CH3}\phantom{.......}\\
\end{array}\]
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| (a) |  |
| (b) |  |
| (c) |  |
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