Which one is the correct order of acidity? [2017]

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Solution

Q.
Which one is the correct order of acidity [2017]

1 $CH \equiv CH > {CH}_{3} - C \equiv CH > {CH}_{2} = {CH}_{2} > {CH}_{3} - {CH}_{3}$

2 $CH \equiv CH > {CH}_{2} = {CH}_{2} > {CH}_{3} - C \equiv CH > {CH}_{3} - {CH}_{3}$

3 ${CH}_{3} - {CH}_{3} > {CH}_{2} = {CH}_{2} > {CH}_{3} - C \equiv CH > CH \equiv CH$

4 ${CH}_{2} = {CH}_{2} > {CH}_{3} - CH = {CH}_{2} > {CH}_{3} - C \equiv CH > CH \equiv CH$

Ans.
(1)

Alkanes, alkenes and alkynes follow the following trend in their acidic behaviour:

$\overset{sp}{HC} \equiv \overset{sp}{CH} > \overset{{sp}^{2}}{H_{2} C} = \overset{{sp}^{2}}{{CH}_{2}} > \overset{{sp}^{3}}{{CH}_{3}} - \overset{{sp}^{3}}{{CH}_{3}}$

This is because sp-hybridised carbon is more electronegative than $s p^{2}$ -hybridised carbon, which is further more electronegative than $s p^{3}$ -hybridised carbon. Hence, in ethyne, proton can be released more easily than in ethene and ethane.

Among alkynes, the order of acidity is:

$HC \equiv CH > {CH}_{3} - C \equiv CH > {CH}_{3} - C \equiv C - {CH}_{3}$

This is due to the $+ I$ effect of the $- {CH}_{3}$ group.

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