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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Nandosch3m on July 19, 2019, 08:12:32 AM

Title: Why does the OH break first (pH buffering of 2-ammino acids)
Post by: Nandosch3m on July 19, 2019, 08:12:32 AM

So here's what I think I know:
2-amino acids have a carboxyl group and an amine group.

These are a weak acid and a weak base respectively.

In low pH (high [H⁺]) the carboxyl group has it's hydrogen attached and the amine has accepted a proton.

When weak bases accept protons, they become strong acids.


Thus in low pH the amino acid has a weak acid (COOH) and a strong acid (NH₃⁺).

So my question:
Why, in a moderate pH environment, is the carboxyl group the first to give off it's proton for pH buffering? Since the NH₃⁺ is the stronger acid, shouldn't it ionise preferably?

Cheers,

Title: Re: Why does the OH break first (pH buffering of 2-ammino acids)
Post by: Babcock_Hall on July 19, 2019, 10:51:59 AM

"When weak bases accept protons, they become strong acids."  This statement is not correct.

The best way to think about this question is first to remember the relationship between acid strength and the value of the pK_a.  Second, one has to find the pK_a values of the carboxylic acid and the ammonium group, respectively.  There are tables in most biochemistry textbooks.