Amino Acid Being Amphoteric

[Silverfalcon]

So I read that the amino group is protonated in acidic solution and
deprotonated in basic solution while the carboxylic acid group of the amino acid loses a proton in basic solution. And, at some intermediate pH, the amino group is protonated while the carboxylic acid is deprotonated.

What i don't get is why. Can anyone explain this? Thank you.

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[loveoforganic]

carboxylic acids are acidic. amines are basic. Amine + carboxylic = reaction. Doesn't matter they're on the same molecule.

 

[Silverfalcon]

carboxylic acids are acidic. amines are basic. Amine + carboxylic = reaction. Doesn't matter they're on the same molecule.

Oh okay.

This sounds dumb, but somewhat confused. How does pH affect whether amino/carboxyl group is deprotonated or protonated? Like:

So I read that the amino group is protonated in acidic solution and deprotonated in basic solution

Amino group = basic = protonation. But, it gets deprotonated in basic. It seems obvious, but I can't explain to myself why.

 

[venacontracta]

Oh okay.

This sounds dumb, but somewhat confused. How does pH affect whether amino/carboxyl group is deprotonated or protonated? Like:



Amino group = basic = protonation. But, it gets deprotonated in basic. It seems obvious, but I can't explain to myself why.

All amino acids have two sites that can either have a proton or not have a proton (amino and carboxylic acid groups).

Start with the amino acid in a very low pH environment. The amino acid will be protonated at the amino site and protonated at the carboxylic acid site. This makes sense - low pH means there's a large proton concentration in the solutions. If you have an abundance of protons floating around, both sites that could possibly have an extra proton will grab onto one. The shear quantity of protons that are floating around makes this favorable, even for groups that don't really want a proton that badly.

Now, start to gradually raise the pH of the environment. The concentration of protons in solutions goes down... in other words, protons are gradually becoming more scarce. In order for a site to hold onto its proton when protons are scarce, that site has to really want a proton badly. How do you quantify how badly a site wants to hang onto its proton? You use acidity. The more acidic a proton is, the more readily it will be lost when pH starts to rise. The carboxylic acid proton is more acidic than the amino group proton -- in other words, the amino group hangs onto its proton more stubbornly than the carboxylic acid group. So, when the pH gets high enough, the carboxylic acid proton will be the first that is lost from our amino acid.

Keep increasing the pH. Protons become more and more scarce in solution (if you haven't picked up on it yet, you can apply concepts of equilibrium and Le Chatlier's principle when thinking this through). Eventually, the pH becomes high enough that even the amino group proton, which is not that acidic, can't keep hold of its proton anymore. So the amino group is then deprotonated.

 

[Chemdude]

Do you know what pKa is?

pKa=-log[Ka], where Ka is the acid dissociation constant.

pKa tells you about the level of dissociation. The larger the pKa value the less likely an acid is to dissociate(more basic). The smaller the pKa value the more likely an acid is to dissociate (more acidic).

If my memory serves me right, the pKa of the carboxylic acid is ~5. The pKa of the amino group is ~10. So the carboxylic acid is definitely more acidic than the amino group. This means that it will be easier for a carboxylic acid to lose a proton than the amino group. Thus, it will be deprotonated at lower pH levels than the amino group.

When do the carboxylic acid/amino group get deprotonated?

When their pKa<pH of the system. If the pH was 6, the carboxylic acid would be deprotanted while the amino group will still have its proton. If the pH was 11, both the amino group and carboxylic acid will be deprotonated.

Therefore, at very low pH's(acidic solution), the carboxylic acid will be protonated. At pH levels greater than the pKa of the carboxylic acid, the carboxylic acid will be deprotonated. The amino acid is only deprotonated when the pH is greater than its pKa.

Hope that helps!