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Grade: 12
        My question is that carboxyl group(-COOH) is an electron withdrawing group right???


And dioic acids have higher acidity than monoacids by the -I effect then how is it possible that the pKa value of Benzoic acid is 4.19 and the pKa value of Benzene 1,4-dicarboxylic acid(Terephthalic acid) is 4.46 ???
This is given so in the class 12 ncert book in the chapter no.14.

Help me hurry....Aieee approaching plz fellas !!!
Ans will be approved
8 years ago

Answers : (2)

vikas askiitian expert
509 Points
							

dear geoffery , u are right dicarboxylic acids are more acidic than mono carboxylic acid due to inductive effect ..

benzene 1,4 dicarboxylic acid cannot be less acidic than benzoic acid ...

The name they themselves have written is ( below the image) 4-Methoxy benzoic acid which has an Electron

releasing group) and hence it makes sense for it to be less acidic than benzoic acid , but they have given structure 

of benzene 1,4 di carboxylic acid instead of 4 metoxy benzoic acid ....

To doubly confirm that you may check the value of pKa for 4-Methoxy benzoic acid and I am sure it should be more than 4.19 and should be close to or equal to 4.46.

Sometimes, even standard texts have errors and can create confusion.

8 years ago
Geoffrey Richards
34 Points
							

Yeah Vikas u r right 4-methoxy bezoic acid has a pKa value of 4.47 dats close to 4.46 !!!!

Thanks alot man !!!

8 years ago
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