Read the passage carefully and answer the Questions.
Carboxylic acids are the earliest organic compounds isolated from nature. These are also known as organic acids but are weaker acids as compared to mineral acids. However, they are more acidic than alcohols and phenols because of greater resonance stabilization of the parent acid and its conjugate base. The acidic strength is determined by the Ka, which is called the acidity constant. The greater the value of Ka, the greater the tendency of the acid to ionize and hence the stronger the acid. The acidic strength of the saturated aliphatic carboxylic acid depends upon the inductive effect and its position with respect to - COOH groups. Electron-donating substituents tend to decrease whereas electron -withdrawing substituents tend to increase the acidic strength. The acidic strength of the aromatic carboxylic acid, on the other hand depends upon both the inductive effect and the resonance effect of the substituents.
Arrange the following acids in decreasing order of acidity
(A) \( CH_3CH_2OH \)
(B) \( CH_3COOH \)
(C) \( BrCH_2COOH \)
(D) \( C_6H_5CH_2COOH \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C), (D), (B), (A)
Two factors decide this order.
Carboxylic acid or alcohol. The carboxylate ion is stabilised by two equivalent resonance structures with the negative charge on oxygen, but the ethoxide ion has no resonance at all. So every carboxylic acid is far more acidic than ethanol (pKa about 16).
The substituent on the acid. An electron-withdrawing group stabilises the carboxylate ion and strengthens the acid; an electron-donating group weakens it.
Applying the second factor to the three acids:
(C) \( \ce{BrCH2COOH} \): bromine withdraws electrons strongly (−I): pKa about 2.9.
(D) \( \ce{C6H5CH2COOH} \): the phenyl group (sp2 carbon) withdraws electrons only weakly, and through a \( \ce{CH2} \) group: pKa 4.31.
(B) \( \ce{CH3COOH} \): the methyl group donates electrons (+I): pKa 4.76.
(A) \( \ce{CH3CH2OH} \): an alcohol, the weakest acid of the four.
Shortcut: The bromo acid is the strongest and ethanol the weakest, so the order starts with (C) and ends with (A): Options (2) and (4). Phenylacetic acid is stronger than acetic acid, so (D) comes before (B), which leaves Option (2).
NCERT (2026-27 edition):
Reactions Involving Cleavage of O–H Bond – “… Carboxylic acids are weaker than mineral acids, but they are stronger acids than alcohols and many simple phenols (pKa is ~16 for ethanol and 10 for phenol). In fact, carboxylic acids are amongst the most acidic organic compounds you have studied so far.”
Effect of Substituents on the Acidity of Carboxylic Acids – “… The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3”