Question

I need help with this question please. we preformed EFFECT OF SUBSTITUENTS ON pKa OF CARBOXYLIC ACIDS experiment

Explain the ranking of your substituted benzoic acids in the table you constructed from data obtained in this experiment and explain any differences between the effect of ortho-, meta-, and para-substitution.

    Compound                            MM                  MP       Solubility

    Benzoic Acid                          122.12              122      0.322

    2-methylbenzoic Acid            136.15              104      0.283

    4-methylbenzoic Acid            136.15              181      0.105

    4-hydroxybenzoic Acid           138.12              215      0.274

    2-nitrobenzoic Acid                167.12              147      0.280

    4-nitrobenzoic Acid                167.12              239      0.023

    4-chlorobenzoic Acid             156.57              241      0.027

Answer 1

If am electron withdrawing group is attached to the benzene ring of the carboxylic acid; the acid strength increases because the electron withdrawing ability of the group stabilises the conjugate base of the corresponding acid.

On the other hand; if electron donating group is attached to the benzene ring of the carboxylic acid; then acidity decreases and pKa increases.

For the reason; nitro benzoic acids and chloro benzoic acids are stronger acid than methyl benzoic acids and hydroxy benzoic acids. Stronger acid has lower pKa.

In between nitro and chloro group; nitro is stronger electron withdrawing group than chkoro and hence nitro benzoic acid is stronger acid than chloro benzoic acid.

Now the position of the group in the benzene ring also affects the acidity.electron withdrawing group at ortho position with respect to the carboxylic acid group stabilises the conjugate base more than at the para position.

Hence 2- nitro benzoic acid is stronger acid than 4- nitro benzoic acid.