why is 4-methoxybenzoic acid less acidic than 4-fluorobenzoic acid?

Answer 1

Okay, first picture a benzene ring - six carbons, flat topology thanks to its aromaticity. By definition, the benzene carbon with the carboxylic acid group attached is carbon #1. So, you have two cases here, both with a ring substituent "para" (or directly opposite) the carboxylic acid. Everybody on the same page?

Now, acidity is defined, in simple terms, as the willingness of a molecule to give up a proton, aka an H+ ion. The more easily it will part with that H+, the more acidic a compound is. So, the question, really, is what makes a proton stay stuck to an acid?

A carboxylic acid anion (COO-) has a large electron density delocalized between its two oxygen atoms. A hydrogen ion will tend to stay stuck to this negatively charged group to balance out it's negative charge. In layman's terms, a carboxylic acid is more comfortable with a positively charged bit hanging around to make all those nonbonding electrons on the oxygens less unwieldy.

With your two compounds, the difference between them is the substituent in the 4 position. Remember that an aromatic system like this has a flat electron cloud above and below the ring, and this electron cloud will interact directly with any ring substituents.

The methoxy group is an electron donor. It is not, overall, very electronegative, so it will tend to "push" its electron density into the benzene ring, and that will tend to prevent the carboxylate group from sharing its negative charge density with the aromatic system. With the negative charge localized to the carboxylate, it will be difficult for the hydrogen ion to dissociate, and 4-methoxybenzoic acid will be very weak.

Fluorine, on the other hand, is an excellent electron sink; it is one of the most electronegative substituents you can put on a benzene ring. It will tend to draw electron density towards itself, allowing the carboxylate to push some of its negative charge into the aromatic ring. The negative charge, then, will be more delocalized, making it easier for the acidic proton to dissociate from the carboxylate group. 4-fluorobenzoic acid will, therefore, be a stronger acid than 4-methoxybenzoic acid.

Does that make sense? I hope this helps.

Answer 2

4 Methoxybenzoic Acid

Answer 3

ok, really pH is on a scale of one million to fourteen. A pH of one million is extremely acidic, at the same time as a pH of 14 is extremely uncomplicated. A pH of seven is impartial (neither acidic or uncomplicated). The decrease the pH the added uncomplicated. So a pH of 8 is way less acidic than a pH of four. there is your answer :)

Answer 4

The methoxy is electron donating into the ring
The fluorine is electron withdrawing so it better stablizes the negative charge of the deprotonated acid