Arrange the following in decreasing order of their acidic strength and give reason for your answer. $C H_{3} C H_{2} O H, C H_{3} C O O H, C l C H_{2} C O O H, F C H_{2} C O O H, C_{6} H_{5} C H_{2} C O O H .$

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Solution

Conjugate bases of the given acids

Acidic strength depends on the stability of the conjugate base after the release of $H^{+}$ ion.

The given compounds are $C H_{3} C H_{2} O H, C H_{3} C O O H, C l C H_{2} C O O H, F C H_{2} C O O H a n d C_{6} H_{5} C H_{2} C O O H$

Their respective conjugate bases are $C H_{3} C H_{2} O^{-}, C H_{3} C O O^{-}, C l C H_{2} C O O^{-}, F C H_{2} C O O^{-} a n d C_{6} H_{5} C H_{2} C O O^{-}$

Comparing the stabilities of the conjugate bases

1. Among the conjugate bases, $C H_{3} C O O^{-}$ is more stable than $C H_{3} C H_{2} O^{-}$ because the negative charge developed in $C H_{3} C O O^{-}$ is stabilised by two equivalent resonating structures.

2. For $X C H_{2} C O O^{-} [X = - C l, - F, - C_{6} H_{5}],$ the $- I$ (inductive) effect of X group will stabilise the negative charge developed in the following order: $F C H_{2} C O O^{-} > C l C H_{2} C O O^{-} > C 6 H 5 C H_{2} C O O^{-}$

Determining the acidic strength of the given acids

The order of stability of the conjugate bases of the given acids is: $F C H_{2} C O O^{-} > C l C H_{2} C O O^{-} > C_{6} H_{5} C H_{2} C O O^{-} > C H_{3} C O O^{-} > C H_{3} C H_{2} O^{-}$

Since acidic strength depends on the stability of the conjugate base, the decreasing order of acidic strengths will be: $F C H_{2} C O O H > C l C H_{2} C O O H > C_{6} H_{5} C H_{2} C O O H > C H_{3} C O O H > C H_{3} C H_{2} O H$

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