Question 42

[matthew]

I thought glycine was around neutral and alcohols were weak acid.

I saw the COOH and NH2 groups but said that it might average 2.3 and 9.2ish to around 7. Can you please explain this one.

[admin]

Let's put glycine aside for a moment in order to underline a frequently tested MCAT point:

Carboxylic acids are 10^10 times more acidic than the relevant alcohol. Yes, that is 10 to the power of 10! Why? Because of the gained resonance energy: when the proton is booted out, the carboxylate anion has two equivalent resonance forms meaning increased stability because of electron delocalisation.

Given the perspective of the enormity of the difference in acidity, adding an amino group would not be sufficient to decrease the acidity to below that of an alcohol.

To be more specific, alcohols are hardly more acidic than water. The concentration of hydrogen ions in pure ethanol is about 1.58 ? 10^-10 M, so ethanol is neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water!

And glycine, taking 2.3 and 9.5, averages to a pI of less than 6.

[yikang_xu4881]

I had a hard time understanding this question too.

But now I see what you mean.

[admin]

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