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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
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 destabilized due to resonance.
The two negative charges repel each other and destabilize the o-methoxyphenoxide ion, so o-nitrophenol is more acidic than o-methoxyphenol.
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\[\begin{array}{cc}
\phantom{.......................}\ce{Br}\\
\phantom{......................}|\\
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\end{array}\]
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