Which is more acidic, 3 methyl benzoic acid or 2 methyl benzoic acid?

Increasing Acidic Order:

4–Methylbenzoic acid has a pKa of 4.362

3–Methylbenzoic acid has a pKa of 4.269

Benzoic acid (pKa=4.19).

2–Methylbenzoic acid has a pKa of 3.90

The methyl group at ortho & para-position(4-position) destabilises Negative charge of corresponding Carboxylate ion both +I (Inductive ERG)) as well as by +H (hyperconjugation).

The +H effect by methyl at meta position does not affect the carboxylate.(From here only +I effect operates)

Hence m-isomer is more acidic than para-isomer.

2-Methyl Benzoic acid (or o-toluic acid) is stronger than either because steric hindrance results in the carboxyl group being out of the plane of the aromatic ring. [Ortho effect- o-toluic acid is even stronger than benzoic acid.] The carboxy group being out of the plane in o-toluic acid eliminates the factor of +H & increase acidic character by also inhibiting +R effect of benzene (which somewhat destabilises negative charge of carboxylate ion by cross-conjugation).

Advertisements

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s