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Question
Phenol is less acidic than ______________.
Phenol is less acidic than ______________.
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Solution
The correct option is B o−nitrophenol
Analysing the factors affecting the acidic strengths of given compounds
- Electron withdrawing group increases the acidic strength and electron donating group decreases the acidic strength.
- Acidic strength ∝ stability of conjugate base
Analysing the options
Option (A):
Conjugate base of ethanol is ethoxide ion which is not stabilized by resonance:
Phenol is more acidic as compared to ethanol because conjugate base of phenol is stabilized by resonance:
Option (B):
Phenol will be less acidic as compared to o−nitrophenol due to the presence of the electron withdrawing group (−NO2) in o−nitrophenol. −NO2 group decreases electron density on oxygen atom of −OH and makes release of H+ easier.
Option (C):
o−methylphenol will be a little less acidic as compared to phenol because of the presence of electron donating group (−CH3) at ortho position. −CH3 group increases electron density on oxygen atom of −OH and makes release of H+ difficult.
Option (D):
o−methoxyphenol will be less acidic as compared to phenol because of the +R effect of the methoxy group present at ortho position which makes the release of H+ difficult.
So, the correct answer is option (B).
Analysing the factors affecting the acidic strengths of given compounds
- Electron withdrawing group increases the acidic strength and electron donating group decreases the acidic strength.
- Acidic strength ∝ stability of conjugate base
Analysing the options
Option (A):
Conjugate base of ethanol is ethoxide ion which is not stabilized by resonance:
Phenol is more acidic as compared to ethanol because conjugate base of phenol is stabilized by resonance:
Option (B):
Phenol will be less acidic as compared to o−nitrophenol due to the presence of the electron withdrawing group (−NO2) in o−nitrophenol. −NO2 group decreases electron density on oxygen atom of −OH and makes release of H+ easier.
Option (C):
o−methylphenol will be a little less acidic as compared to phenol because of the presence of electron donating group (−CH3) at ortho position. −CH3 group increases electron density on oxygen atom of −OH and makes release of H+ difficult.
Option (D):
o−methoxyphenol will be less acidic as compared to phenol because of the +R effect of the methoxy group present at ortho position which makes the release of H+ difficult.
So, the correct answer is option (B).
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