Question

Select the acid with the highest $K_a$ (i.e., lowest $p{K}_a$).

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

More the -I effect more is the acidic strength.
Strong acid has lesser $p{K}_a$ value.

Here all the compounds are carboxylic acid which has equivalent resonance structures.
More the acid strength , easier is the $H^{+}$ release from that acid.
Halogens are highly electron withdrawing group whereas methyl and cyclo alkane groups are electron donating groups.
In compound (a), methyl group show +I effect so ease of $H^{+}$ release is poor.
In compound (c) and (d), the halogens are located too far from the carboxylic acid group so its electron withdrawing power is little so these are less acidic.
In compound (b), two chlorine atom attached to adjacent carbon of carboxylic acid. These withdraw the electron density toward itself and $H^{+}$ is released easily. Thus it has the most acid nature of the given compounds. Thus, it has the lowest $p{K}_a$ value.