Question

Arrange the following compounds in increasing order of acidity and give suitable explanation:-
Phenol, Ortho nitrophenol and para niteophenol

Answer 1

Dear student

Acidity of substituted phenols depends upon the type of group substituted. A group showing -I or -R effect increases the acidity of phenol as these groups withdraws electrons from ring which weakens the O-H bond and thus removal of H⁺ is easier. On the other hand, groups showing +I and+R effect decreases the acidity as they give electrons to rhe ring and so strengthens the O-H bond.
NO₂ shows -I as well as -R effect, and therefore will increase the acidity of phenol .
So, phenol < o-nitrophenol
Now, in ortho nitrophenol, intramolecular hydrogen bonding between the adjacent hydroxyl and nitro groups significantly affects release of the proton. So, it is a weaker acid compared to para nitrophenol.
So, the increasing order is
phenol < o-nitrophenol< p-nitrophenol



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