The pKa values are 4.21 (benzoic acid) and 4.38 (p-methylbenzoic acid), in other words, the former is slightly more acidic than the latter. Why is that?
Benzoic acid, unlike p-methylbenzoic acid, has no methyl substituents on the benzene ring so its aromatic system is electron-poorer. Methyl groups, indeed, are known as electron-releasing groups that increase the electron density of the atom groups they are next to (the aromatic ring, in this case).
This causes the negative charge on the carboxylate groups to be less “spreadable” on the aromatic ring of p-methylbenzoic acid which, in turn, means more basic behaviour (or less acidic, put it differently).
As benzoic acid does not have such an atom group that increases its electron density, the negative charge of its -COO⁻ group can be delocalised more efficiently on the benzene ring nearby and this, therefore, enhances its willingness to release the proton.
The pKa values are 4.21 (benzoic acid) and 4.38 (p-methylbenzoic acid), in other words, the former is slightly more acidic than the latter. Why is that?
Benzoic acid, unlike p-methylbenzoic acid, has no methyl substituents on the benzene ring so its aromatic system is electron-poorer. Methyl groups, indeed, are known as electron-releasing groups that increase the electron density of the atom groups they are next to (the aromatic ring, in this case).
This causes the negative charge on the carboxylate groups to be less “spreadable” on the aromatic ring of p-methylbenzoic acid which, in turn, means more basic behaviour (or less acidic, put it differently).
As benzoic acid does not have such an atom group that increases its electron density, the negative charge of its -COO⁻ group can be delocalised more efficiently on the benzene ring nearby and this, therefore, enhances its willingness to release the proton.
This is because the acid group interacts with the ring electrons. This enables the benzoate ion formation to have a lower activation energy. Ie- the breaking of the O-H bond.
The methyl group in the para or 4th position in the ring is electron donating; so this increases the electron density in the acid group increasing the activation energy of the breaking of the O-H bond making para-methyl benzoic acid less acidic.
This is because the acid group interacts with the ring electrons. This enables the benzoate ion formation to have a lower activation energy. Ie- the breaking of the O-H bond.
The methyl group in the para or 4th position in the ring is electron donating; so this increases the electron density in the acid group increasing the activation energy of the breaking of the O-H bond making para-methyl benzoic acid less acidic.
You are comparing two different acid-base equilibria:
C₆H₅-COOH ⇄ C₆H₅-COO⁻ + H⁺ (benzoic acid)
CH₃-C₆H₄-COOH ⇄ CH₃-C₆H₄-COO⁻ + H⁺ (p-methylbenzoic acid)
The pKa values are 4.21 (benzoic acid) and 4.38 (p-methylbenzoic acid), in other words, the former is slightly more acidic than the latter. Why is that?
Benzoic acid, unlike p-methylbenzoic acid, has no methyl substituents on the benzene ring so its aromatic system is electron-poorer. Methyl groups, indeed, are known as electron-releasing groups that increase the electron density of the atom groups they are next to (the aromatic ring, in this case).
This causes the negative charge on the carboxylate groups to be less “spreadable” on the aromatic ring of p-methylbenzoic acid which, in turn, means more basic behaviour (or less acidic, put it differently).
As benzoic acid does not have such an atom group that increases its electron density, the negative charge of its -COO⁻ group can be delocalised more efficiently on the benzene ring nearby and this, therefore, enhances its willingness to release the proton.
You are comparing two different acid-base equilibria:
C₆H₅-COOH ⇄ C₆H₅-COO⁻ + H⁺ (benzoic acid)
CH₃-C₆H₄-COOH ⇄ CH₃-C₆H₄-COO⁻ + H⁺ (p-methylbenzoic acid)
The pKa values are 4.21 (benzoic acid) and 4.38 (p-methylbenzoic acid), in other words, the former is slightly more acidic than the latter. Why is that?
Benzoic acid, unlike p-methylbenzoic acid, has no methyl substituents on the benzene ring so its aromatic system is electron-poorer. Methyl groups, indeed, are known as electron-releasing groups that increase the electron density of the atom groups they are next to (the aromatic ring, in this case).
This causes the negative charge on the carboxylate groups to be less “spreadable” on the aromatic ring of p-methylbenzoic acid which, in turn, means more basic behaviour (or less acidic, put it differently).
As benzoic acid does not have such an atom group that increases its electron density, the negative charge of its -COO⁻ group can be delocalised more efficiently on the benzene ring nearby and this, therefore, enhances its willingness to release the proton.
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This is because the acid group interacts with the ring electrons. This enables the benzoate ion formation to have a lower activation energy. Ie- the breaking of the O-H bond.
The methyl group in the para or 4th position in the ring is electron donating; so this increases the electron density in the acid group increasing the activation energy of the breaking of the O-H bond making para-methyl benzoic acid less acidic.
This is because the acid group interacts with the ring electrons. This enables the benzoate ion formation to have a lower activation energy. Ie- the breaking of the O-H bond.
The methyl group in the para or 4th position in the ring is electron donating; so this increases the electron density in the acid group increasing the activation energy of the breaking of the O-H bond making para-methyl benzoic acid less acidic.
More
VOTE