Question

Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids?
$C{H_3}C{H_2}COOH$ > ${(C{H_3})_2}CHCOOH$ > ${(C{H_3})_3}CCOOH$

Answer 1

Hint: The Inductive effect is the sigma electron displacement effect while the electromeric effect involves the pi electrons. Both the effects can be classified into electron releasing and electron-withdrawing groups.

Complete step by step answer:
First, let us understand what are Inductive and Electromeric effects.
Inductive Effect :
It may be defined as the displacement of sigma electrons toward the more electronegative atom in the bond. This results in the formation of one end as positively charged and the other as negatively charged.
It is a permanent effect. It is represented by an arrow on the bond. The arrow points toward the direction in which the electrons are displaced.
It is a distance dependent effect and dies after a short-range.
The Inductive effect can be classified into two types:-
+ I effect:- It involves all the electron releasing or electron-donating groups. The alkyl groups show +I effect.
-I effect:- It involves the electron-withdrawing groups. The cyano group, nitro, amino and all halogens show -I effect.
Now let us see the Electromeric effect.
This effect is temporary. It involves the transfer of $\Pi $ electrons on demand of attacking reagents towards more electronegative atoms. It dominates to Inductive effect.
The Electromeric effect can be classified into two types:-
+E effect:- In this, the $\Pi $ electrons are transferred toward the attacking reagent.
-E effect:- In this, the $\Pi $ electrons are transferred away from the attacking reagent.
The Electromeric effect is shown by compounds having double bonds . Such as $C = C$, $C = N$ and $N = N$ etc.

As we are done with the Inductive and Electromeric effects.
Now, let us see the electron displacement effect that explains the following decreasing order of acidity of the carboxylic acids.
$C{H_3}C{H_2}COOH$ > ${(C{H_3})_2}CHCOOH$ > ${(C{H_3})_3}CCOOH$
From the above points, we can say that here inductive effect will take place and not electromeric because in this case, we have sigma electron movement.
So, an inductive effect will occur. Further, we observe that the COOH group is attached to alkyl groups and the alkyl groups are electron releasing in nature. So, they will decrease the acidity of COOH. The alkyl group will donate electrons to COOH and with the increase in the number of alkyl groups, the negative charge will intensify on the Carboxylic group. Thus, stability and acidic strength will decrease. So, the order can be explained based on +I effect.


Note: It must be noted that after donation of a proton from carboxylic acid, there is a negative charge on the oxygen atom which is stabilised by resonance. The presence of +I effect due to the alkyl group increases the charge thereby decreasing its stability. This leads to an even decrease of acidity of the carboxylic acids.