Relative acidic strength of Carboxylic acids and their derivatives
Carboxylic acids are weak acids. Mineral acids like HCl or HNO3 are very strong acids and non-edible, but many carboxylic acids are edible acids. Carboxylic acids are more acidic than phenol. The conjugate base of carboxylic acid is stabilized by two equivalent resonating structures in which the negative charge is at the more electronegative oxygen atom. Electron withdrawing group (EWG) increases the acidity of carboxylic acids while electron donating group (EDG) decreases the acidic strength of carboxylic acids. The acidic nature of carboxylic acid is also affected by direct attachment of groups such as phenyl or vinyl to the carboxylic acid. They increase the acidity of corresponding carboxylic acids.
It is due to the greater electronegativity of sp2 hybridized carbon to which carboxylic carbon is attached. This acidic character of carboxylic acid is based on its pKa values. The smaller the pka values, the stronger the acid. The effect of the following groups in increasing acidic order is
Ph < I < Br < Cl < F < CN < NO2 < CF3
Which of the following conditions will not result in esterification ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Reaction of phenol with alcohol in the presence of mineral acid.
Esterification forms an ester, R–COO–R′: an alcohol or phenol supplies the –OR′ part, and a carboxylic acid or one of its derivatives supplies the acyl group (R–CO–). A phenol and an alcohol have no acyl group between them, so no ester can form.
Option (1) Reaction of alcohol with acid in the presence of mineral acid. — Gives an ester. This is the usual acid-catalysed esterification; it is reversible, so water is removed as it forms.
\( \displaystyle \ce{CH3COOH + C2H5OH} \xrightarrow{\text{conc. }\ce{H2SO4}} \ce{CH3COOC2H5 + H2O} \)
Option (2) Reaction of alcohol with acid anhydride in the presence of mineral acid. — Gives an ester.
\( \displaystyle \ce{(CH3CO)2O + C2H5OH} \xrightarrow{\ce{H+}} \ce{CH3COOC2H5 + CH3COOH} \)
Option (3) Reaction of phenol with acid chloride in the presence of pyridine . — Gives an ester. Pyridine removes the HCl and drives the reaction forward.
\( \displaystyle \ce{C6H5OH + CH3COCl} \xrightarrow{\text{pyridine}} \ce{CH3COOC6H5 + HCl} \)
Option (4) Reaction of phenol with alcohol in the presence of mineral acid. — Does not give an ester. Neither phenol nor an alcohol has an acyl group, so no ester can form.
NCERT (2026-27 edition):
Esterification – “Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. … The reaction with carboxylic acid and acid anhydride is carried out in the presence of a small amount of concentrated sulphuric acid. The reaction is reversible, and therefore, water is removed as soon as it is formed. The reaction with acid chloride is carried out in the presence of a base (pyridine) so as to neutralise HCl which is formed during the reaction.”