Question 17

Explain the terms Inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids?

(a) Cl3CCOOH > Cl2CHCOOH > ClCH2COOH

(b) CH3CH2COOH > (CH3)2CHCOOH > (CH3)3C.COOH

Answer

Inductive effect

The permanent displacement of sigma electrons along a saturated chain, whenever an electron withdrawing or electron donating group is present, is called inductive effect.

Inductive effect could be + I effect or - I effect. When an atom or group attracts electrons towards itself more strongly than hydrogen, it is said to possess - I effect. For example,

F —← CH2 —← CH2 —← CH2 —←CH3

When an atom or group of atoms attracts electrons towards itself less strongly than hydrogen, it is said to possess + I effect. For example,



CH3  —→  CH2 →→ -Cl

Electrometric effect

It involves the complete transfer of the shared pair of  π-electrons to either of the two atoms linked by multiple bonds in the presence of an attacking agent. For example,

Electrometric effect could be + E effect or - E effect.

+ E effect: When the electrons are transferred towards the attacking reagent

- E effect: When the electrons are transferred away from the attacking reagent

 

(a) Cl3CCOOH > Cl2CHCOOH > ClCH2COOH

The order of acidity can be explained on the basis of Inductive effect (- I effect). As the number of chlorine atoms increases, the - I effect increases. With the increase in - I effect, the acid strength also increases accordingly.

 

(b) CH3CH2COOH > (CH3)2 CHCOOH > (CH3)3 C.COOH

The order of acidity can be explained on the basis of inductive effect (+ I effect). As the number of alkyl groups increases, the + I effect also increases. With the increase in + I effect, the acid strength also increases accordingly.

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