Question

The correct order of decreasing acidity of nitrophenols will be:
a. m-nitrophenol p-nitrophenol o-nitrophenol
b. o-nitrophenol m-nitrophenol p-nitrophenol
c. p-nitrophenol m-nitrophenol o-nitrophenol
d. p-nitrophenol o-nitrophenol m-nitrophenol

Answer 1

Hint: The general formula of nitrophenols is: ${ HOC }_{ 6 }{ H }_{ 5-x }{ (NO }_{ 2 }{ ) }_{ x }$
The structure of nitrophenol is shown below:

Here the acidity depends on the presence of electron-withdrawing or electron releasing groups and their respective positions.

Complete step-by-step answer:
An inductive effect is an electronic effect due to the polarisation of σ bonds within a molecule or ion. This is typically due to an electronegativity difference between the atoms at either end of the bond.
${ -NO }_{ 2 }$ is an electron-withdrawing group.
${ -NO }_{ 2 }$ group at ortho and para position pulls back electrons of the OH bond towards itself by the stronger -R effect while the ${ -NO }_{ 2 }$ group at m-position pulls back electrons of the OH bond by the weaker effect.
Thus, ortho and para-nitrophenols are more acidic than m-nitrophenol is a little less acidic than p-nitrophenol because of intramolecular hydrogen bonding which makes the loss of a proton difficult to remove.

Figure: Showing intramolecular hydrogen bonding in m-nitrophenol.

Figure: Showing intermolecular hydrogen bonding in p-nitrophenol

Hence, The correct order of decreasing acidity of nitrophenols will be:
p-nitrophenol o-nitrophenol m-nitrophenol
The correct option is D.

Additional Information:
Resonance effect: It can be defined as the delocalization of π electrons due to the presence of electron releasing or electron-withdrawing substituents, which results in canonical forms with different electron distribution.
It is designated as the R or M effect.

Application: It gives extra stability to the compound by delocalization of charges. Some time it activates or deactivates the benzene ring towards electrophilic substitution.

Note: The possibility to make a mistake is that you may choose option C. But p-nitrophenol shows both -R and -I effects, so it will be more acidic than m-nitrophenol as it does not show any such effect.