Higher the value of K_a or lower the value pK_a, stronger will be the acid.


Presence of electron donating groups destabilises the phonoxide ion, thus, reduces acidic nature of phenol. Presence of electron withdrawing group stabilises the phenoxide ion thus increase the acidic strength of phenol.

Thus, order of acidic strength is:

$$\therefore$$ Order of $$\mathrm{pK_a}$$ will be = b > d > a > c

Note :

Acid Strength : -

Acids : Acids are those molecules which donates H⁺ ion in aqueous solution (water) and increases concentration of H⁺ ion in water.

$$H - A(aq)$$ $$\rightleftharpoons$$ $${H^ + }(aq) + {A^ - }(aq)$$

Dissociation constant of this reaction is K_a. It is called ionisation constant.

$$\to$$ So we can say if a acid form more H⁺ ion after adding it in H₂O then that acid is called strong acid. If acid produce less H⁺ ion in H₂O then that acid is weak acid.

$$\to$$ Acid Strength $$\propto$$ Stability of conjugate base.

$$\to$$ Acid Strength $$\propto$$ K_a

$$\to$$ Acid Strength $$ \propto {1 \over {p{K_a}}}$$

Remember :

$$p{K_a} = - \log {K_a}$$

$$ = \log {({K_a})^{ - 1}} = \log {1 \over {{K_a}}}$$