Question

Carboxylic acids are more acidic than phenol and alcohols because of:
(A) Intermolecular hydrogen bonding
(B) Formation of dimers
(C) Highly acidic hydrogens
(D) Resonance stabilization of their conjugate base

Answer 1

Hint: To answer this question, you must recall the factors on which the acidity of a compound depends. The acidic strength of an organic compound depends upon two major factors, namely the ease of loss of a proton and the stability of the conjugate base so formed.

Complete step by step solution
The strength of an acid varies as follows. The presence of an electron withdrawing group increases the acidic strength as it decreases the electron density on the atom bonded to the hydrogen atom which causes the loss of proton. Similarly the presence of an electron donating group decreases the tendency to lose a proton.
Also the stability of the conjugate base formed increases the acidic strength. The stability and thus the acidity increases by resonance or due to the presence of an electronic withdrawing effect.
Carboxylic acids are the most stable of all organic compounds. When a carboxylic acid loses a proton molecule, a carboxylate ion is formed. The conjugate base carboxylate ion is extremely stable due to the formation of two equivalent resonating structures. The negative charge is delocalized effectively on both the electronegative oxygen atoms thus providing stability. However, no such resonance stabilization of the conjugate base is present in alcohols.
The correct answer is D.

Note:
Phenols however are strongly acidic but not as strong as carboxylic acids. The phenoxide ion formed after the loss of a proton is stabilized by resonance effect of the benzene ring. However, the exceptional stability of equivalent resonance structures lead to carboxylic acids being stronger acids.