Chemistry Reference
In-Depth Information
actuality, the pK
a
of p-methylbenzoic acid is 4.36, thus supporting the conclusion of
this problem.
i. m-CHO
The structure of m-carboxybenzaldehyde is shown below with partial charges assigned
to the ring system. Because the electron-withdrawing aldehyde group is located meta
to the carboxylic acid, electron density is increased adjacent to the acid functionality,
effectively rendering the aromatic ring electron donating meta to the carboxy
(aldehyde) group. An electron-donating group attached to a carboxylic acid destabilizes
the anion resulting from deprotonation, thus decreasing its acidity and raising its pK
a
.
In actuality, the pK
a
of m-formylbenzoic acid is 3.85, reflecting the electron-withdrawing
nature of the carboxyphenyl group. In fact, formyl groups (aldehydes) are so electron-
withdrawing that they render the phenyl ring electron-withdrawing in its entirety.
j. p-OCH
3
The structure of p-methoxybenzoic acid is shown below with partial charges
assigned to the ring system. Because the electron-donating methoxy group is
located para to the carboxylic acid, electron density is increased adjacent to the
acid functionality, effectively rendering the aromatic ring electron donating para
to the methoxy group. An electron-donating group attached to a carboxylic acid
destabilizes the anion resulting from deprotonation, thus decreasing its acidity and
raising its pK
a
. In actuality, the pK
a
of p-methoxybenzoic acid is 4.47, thus support-
ing the conclusion of this problem.
