Question

The correct order of acidity of the following elements is $\left(\mathrm{i}\right)$Phenol $\left(\mathrm{ii}\right)$Methyl phenol $\left(\mathrm{iii}\right)$Meta-nitro phenol $\left(\mathrm{iv}\right)$ Para-nitro phenol

• A. Phenol$>$Meta-nitrophenol$>$ Methyl phenol$>$Para nitrophenol
• B. Para-nitrophenol $>$Meta-nitrophenol $>$Phenol$>$Methyl phenol
• C. Methyl phenol$>$Phenol$>$Meta-nitrophenol$>$Para-nitrophenol
• D. Phenol$>$Para-nitrophenol$>$Meta-nitrophenol$>$Methyl phenol

Answer 1

The correct option is B

Para-nitrophenol $>$Meta-nitrophenol $>$Phenol$>$Methyl phenol

The explanation for the correct option:

Option (B): Para-nitrophenol $>$Meta-nitrophenol$>$ Phenol$>$Methyl phenol

1. The kind of substituents present in the benzene nucleus determines the acidic strength of phenols.
2. Electron withdrawing group like ${\mathrm{NO}}_2,\mathrm{CN},\mathrm{X},\mathrm{CHO}$ when present to phenol increases the acidic strength
3. Electron donating group like $\mathrm{R},\mathrm{OR},{\mathrm{NH}}_2$ when attached to phenol decreases the acidic strength.
4. The base removes the proton from the phenol $\mathrm{OH}$ group, making para-nitro phenol the most acidic. As a result, the nitro group, which is present at the para position through resonance action, stabilizes the phenoxide ion that is created and increases its acidity.
5. When compared to para-nitrophenol, meta-nitrophenol is less acidic because the base abstracts the proton to create the phenoxide ion, which is stabilized by the inductive effect of the electron-withdrawing group, which is less effective than the resonance
6. Due to the absence of an electron-withdrawing group and the absence of an inductive or resonance action, phenol is less acidic than para nitrophenol and meta nitrophenol.
7. Methyl phenol has one electron-donating methyl group which causes the $+\mathrm{I}$ effect and hyperconjugation, increasing the nucleophilicity of negatively charged oxygen, allowing it to take the proton fast. As a result, it was unable to release the proton as easily as phenol.
8. Hence, it is the correct option

The explanation for the incorrect options

Option (A): Phenol$>$ Meta-nitrophenol$>$ Methyl phenol$>$ Para-nitrophenol

1. The base removes the proton from the phenol $\mathrm{OH}$ group, making para-nitro phenol the most acidic. As a result, the nitro group, which is present at the para position through resonance action, stabilizes the phenoxide ion that is created and increases its acidity.
2. Hence, it is the incorrect option.

Option (C): Methyl phenol$>$ Phenol$>$ Meta-nitrophenol$>$Para-nitrophenol

1. Methyl phenol has one electron-donating methyl group which causes the $+\mathrm{I}$ effect and hyperconjugation, increasing the nucleophilicity of negatively charged oxygen, allowing it to take the proton fast. As a result, it was unable to release the proton as easily as phenol and is least acidic.
2. Hence, it is the incorrect option.

Option (D): Phenol $>$Para-nitrophenol$>$ Meta-nitrophenol$>$ Methyl phenol

1. Phenol does not contain any electron-withdrawing group that's why it is less acidic whereas Meta-nitro phenol and Para nitro phenol contain an electrons withdrawing group which increases their acidity.
2. Hence, it is the incorrect option.

Therefore, The correct order of acidity is Para-nitrophenol$>$Meta-nitrophenol $>$Phenol$>$ Methyl phenol