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Author Topic: pKa and determining basicity  (Read 3081 times)

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bonkers

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pKa and determining basicity
« on: October 30, 2010, 11:43:01 AM »

Hi,

I had this question when I had to solve a problem where one point was which is more basic carboxylate anion or amino group. The pKa-s are 36 for amino and 3.7 for HCOOH, but one of the value is for NH3 and other for HCOOH, how to i get for HCOO-
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Schrödinger

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Re: pKa and determining basicity
« Reply #1 on: October 31, 2010, 06:49:09 AM »

Well, pKa's will determine which one of them is the stronger acid... Now, what do you know about conjugate acid-base pair??
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bonkers

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Re: pKa and determining basicity
« Reply #2 on: October 31, 2010, 09:07:01 PM »

Well, pKa's will determine which one of them is the stronger acid... Now, what do you know about conjugate acid-base pair??
The stronger the acid the weaker it´s conjugated base. But i want to know how to compare if the conjugated base is stronger base than the other acid which is actually a base.
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rabolisk

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Re: pKa and determining basicity
« Reply #3 on: November 01, 2010, 09:40:12 AM »

You're trying to compare whether HCOO- or NH3 is a stronger base. Of the data you gave us, only one is helpful, and the other is useless. You need something else.
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bonkers

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Re: pKa and determining basicity
« Reply #4 on: November 01, 2010, 11:34:15 AM »

The question arose from the following exercise:
Proteins are built up from units called amino acids. The simplest amino acid is glycine, the structure of which can be written in two ways: One has NH3+ and COO- in the molecule and other NH2 and COOH.
a) consult the table of pKa values and decide which is the better representation of the structure of the compound. (Hint: Which is more basic, an amino group or carboxylate anion?)
The exercise is from Seyhan Ege´s Organic chemistry. Sadly i don´t have the study guide and can´t find it anywhere.
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rabolisk

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Re: pKa and determining basicity
« Reply #5 on: November 01, 2010, 05:38:05 PM »

To anyone who has studied amino acids in organic chemistry or biochemistry, the answer is simple. Regardless, look for the relevant pKa values in the table, and you should be able to solve the problem. Remember that pKa of an acid and pKb of its conjugate base add up to 14.
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