Question

Arrange the following labelled hydrogens in decreasing order of acidity :

• A. b > c > d > a
• B. b > a > c > d
• C. c > b > d > a
• D. c > b > a > d

Hint:

The carboxylic acid H (b) is the most acidic, then the two phenolic OH hydrogens (the one para to NO₂ is stronger), and the terminal alkyne H (a) is the least acidic (pKa about 25).

Answer 1

The Correct Option is A

Labelled hydrogens: a = terminal alkyne C-H, b = COOH, c = phenolic OH (para to NO₂), d = phenolic OH (meta to NO₂).
pKa order: carboxylic acid (about 4-5) < phenol (about 7-10) < terminal alkyne (about 25). So b is the most acidic and a the least.
Between the phenols c and d: NO₂ is para to OH(c), so the phenoxide formed on losing H_c is stabilised by both -M and -I effects of NO₂. For OH(d) the NO₂ is meta, so only the -I effect operates. Hence c is more acidic than d.
Order: b > c > d > a.
Answer: (1) b > c > d > a