Question

Arrange the following in order of ${\mathrm{K}}_{\mathrm{a}}$ value

$$\begin{gathered} \left(\mathrm{P}\right){\mathrm{FCH}}_2\mathrm{COOH} \\ \left(\mathrm{Q}\right){\mathrm{ClCH}}_2\mathrm{COOH} \\ \left(\mathrm{R}\right){\mathrm{O}}_2{\mathrm{NCH}}_2\mathrm{COOH} \\ \left(\mathrm{S}\right){\mathrm{NCCH}}_2\mathrm{COOH} \end{gathered}$$

• A. P > Q > R > S
• B. R > S > P > Q
• C. R > P > S > Q
• D. R > S > Q > P

Answer 1

The correct option is B

R > S > P > Q

Explanation for correct option:

We know,

Acidity $\mathrm{\alpha }$ ${\mathrm{K}}_{\mathrm{a}}$ (where ${\mathrm{K}}_{\mathrm{a}}$ is ionization constant of acid)

So it is clear with increasing acidity, ${\mathrm{K}}_{\mathrm{a}}$ value increases.

Now we know,

Acidity also increases with increases inductive effect ($-\mathrm{I}$)

so, the order of $-\mathrm{I}$ effect:

$-{\mathrm{NO}}_2>\mathrm{CN}>\mathrm{F}>\mathrm{Cl}$

Hence, the correct order is R > S > P > Q.

Therefore, option (A) is the correct.