Question

Arrange the following in decreasing order of their acidic strength and give reason for your answer.

$$\mathrm{CH}{\phantom{A}}_3\mathrm{CH}{\phantom{A}}_2\mathrm{OH},\mathrm{CH}{\phantom{A}}_3\mathrm{COOH},\mathrm{ClCH}{\phantom{A}}_2\mathrm{COOH},\mathrm{FCH}{\phantom{A}}_2\mathrm{COOH},\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{CH}{\phantom{A}}_2\mathrm{COOH}$$

Answer 1

The order is 

$$\mathrm{ClCH}{\phantom{A}}_2\mathrm{COOH}>\mathrm{FCH}{\phantom{A}}_2\mathrm{COOH}>\mathrm{C}{\phantom{A}}_6\mathrm{H}{\phantom{A}}_5\mathrm{CH}{\phantom{A}}_2\mathrm{COOH}>\mathrm{CH}{\phantom{A}}_3\mathrm{COOH}>\mathrm{CH}{\phantom{A}}_3\mathrm{CH}{\phantom{A}}_2\mathrm{OH}$$

When compared to other alcohols $$\mathrm{CH}{\phantom{A}}_3\mathrm{CH}{\phantom{A}}_2\mathrm{OH}$$ is the least acidic. Because of the halogen, $$\mathrm{FCH}{\phantom{A}}_2\mathrm{COOH}$$ is quite acidic. It is either an electron-donating group or an electron receiving group in acidic strength. Acidic strength increases as the number of electron withdrawing groups increases. The acidic strength of electron-donating groups diminishes.