1 Three phenol derivatives and their pkas are shown below Ex

1. Three phenol derivatives and their pkas are shown below. Explain why the meta isomer has a lower pka and the para isomer a higher pka as compared to the ortho isomer. 9.45 9.2 10.9 OH OH OH OH OH OH

Solution

Pka value of substituted phenol depends upon two factors

1) Inductive effect

2) resonance

Inductive effect can be observed in all the three position that is ortho,para and meta but being nearest ortho position has strong inductive effect.that is -I effect.

Now let us consider resonance,it is more important factorthan inductive effect to determine pka .

At ortho and para position there is equal + R effect and there is no effect of resonance at meta position.

Since ortho and para has same +R effect but ortho has more -I effect, therefore ortho substituted phenol is more acidic than para substituted phenol . hence,ortho substituted phenol has

lowerpka value than para substituted phenol.

In case of meta substituted phenol ,there is only -I effect which could stabilise conjugate phenoxide ion make it more acidic than ortho substituted phenol and hence it has lower pkabvalup than ortho substituted phenol.

So,the order of pka value is

Meta substituted phenol<ortho substituted phenol<para substituted phenol