Comparing acidity among 3 compounds - pic included

[pizzadude]

Homework Statement

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My book says that if I attach an electron-donating group to an acid, it would decrease the acidity of that acid because the electrons would cause the already negative ion to be more unstable.

Well, -OH is an electron-donating group. I don't understand why when I attach an -OH group to acetic acid to make it into 2-hydroxyacetic acid, its acidity actually increases. It is suppose to decrease, according to the book. Does anybody know how to resolve this conflict?

Also, if I compare phosphoric acid with carboxylic acid, the phosphoric acid is a lot stronger than carboxylic acid. I don't understand why. If I deprotonate 1 acid, the negative charge would form resonance with one other oxygen on both acids (phosphoric acid and carboxylic acid). I know phosphoric acid has 4 oxygens attached, but two of them would probably not be involved in resonance stabilization because they are too busy bonding to Phosphorus and Hydrogen. So, if both phosphoric acid and carboxylic acid can only form 2 resonance structures when 1 H is deprotonated, then why is phosphoric acid so much stronger than carboxylic acid?

Thanks in advance.

 

[Kushal]

i would use the inductive effect to explain the acidity of acetic acid and 2 hydroxy acetic acid. the oxygen on the hydroxyl group of 2 hydroxy acetic acid is of course electronegative. it tends to pull electrons towards itself. the carbon of -COOH will have a lesser electron density, and the O-H bond of the -COOH is weakened that is the hydrogen is held less strongly and is easily removed.

i think it is like when you attach a chlorine atom instead of the -OH.

 

[Blueshift5]

I would first like to commend you for actively seeking to resolve conflicts and understand the concepts in your textbook. It shows a strong curiosity and dedication to learning.

Now, to address your concerns about the effects of attaching an -OH group to acetic acid, it is important to consider the overall electronic structure of the molecule. While -OH is indeed an electron-donating group, in the case of 2-hydroxyacetic acid, the presence of the -OH group increases the overall electron density in the molecule, making it more stable and thus more acidic. This is because the -OH group can participate in resonance with the carbonyl group, stabilizing the negative charge on the oxygen atom.

Regarding the comparison between phosphoric acid and carboxylic acid, it is true that both molecules can form two resonance structures when one H is deprotonated. However, the strength of an acid also depends on the stability of the resulting conjugate base. In the case of phosphoric acid, the negative charge can be stabilized by resonance with all three oxygen atoms, making the conjugate base more stable and therefore the acid stronger.

I hope this helps to resolve your conflicts and further your understanding of acidity and electron-donating groups. Remember, in science, it is always important to question and seek clarification when things seem contradictory. Keep up the good work!