Question

How many of the following compounds have less $p{K}_a$ than benzoic acid:
(i)HCOOH (ii)$C{H}_{3}COOH$
(iii)
(iv)
(v)(vi)(vii)
(viii)

• A. 4
• B. 4.0
• C. 4.00

Answer 1

Answer: (A), (B), (C)

The $p{K}_a$ is the $pH$ value at which a chemical species will accept or donate a proton. The lower the pK_a, the stronger the acid and greater the ability to donate a proton in aqueous solution.
An electron withdrawing group will generally lower the $p{K}_a$ of the molecule (making it more acidic). However electron donating group decreases acidity.
(1) After removal of proton, species resonance stabilize so more acidic than benzoic acid.

(2)$C{H}_{3}COOH$ is less acidic than benzoic acid due to +I effect of $-C{H}_3$ group.

(3)Less acidic then benzoic acid due to +I effect of -OH group.

(4)Due to strong intermolecular H-bonding it will become strong acid than benzoic acid.

(5)Due to ortho effect of methyl group (electron donating group).
Note: Ortho-effect: ortho substituted benzoic acid is more acidic than other benzoic acid.

(6)$-Cl$ group is electron withdrawing group which decreases the electron density on ring and makes it more acidic than benzoic acid .

(7, 8)Due to strong +R effect, all amino acids are weaker acids than benzoic acid.
Since $-N{H}_2$ group cannot exert its +R effect from m-position, but can exert -I weak effect.

Therefore only (i, iv, v, vi) are strong acids and have less $p{K}_a$ values than benzoic acid.