Question 8

Which acid of each pair shown here would you expect to be stronger?

(i) CH3CO2H or CH2FCO2H

(ii)CH2FCO2H or CH2ClCO2H

(iii) CH2FCH2CH2CO2H or CH3CHFCH2CO2H

(iv)

Answer

The +I effect of -CH3group increases the electron density on the O-H bond. Therefore, release of proton becomes difficult. On the other hand, the -I effect of F decreases the electron density on the O-H bond. Therefore, proton can be released easily. Hence, CH2FCO2H is a stronger acid than CH3CO2H.

(ii)

F has stronger -I effect than Cl. Therefore, CH2FCO2H can release proton more easily than CH2ClCO2H. Hence, CH2FCO2H is stronger acid than CH2ClCO2H.



(iii)

Inductive effect decreases with increase in distance. Hence, the +I effect of F in CH3CHFCH2CO2H is more than it is in CH2FCH2CH2CO2H. Hence, CH3CHFCH2CO2H is stronger acid than CH2FCH2CH2CO2H.

(iv)

Due to the -I effect of F, it is easier to release proton inthe case of compound (A). However, in the case of compound (B), release of proton is difficult due to the +I effect of -CH3group. Hence, (A) is a stronger acid than (B).

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