acid and base strength of alcohols

[bobo]

In Solomons Organic Chemistry (6th edition ad I think 7th too) I encountered a statement that water would be a weaker acid than methanol and stronger than ethanol. There is a table in this book which lists PK values: methanol at 15.5, water at 15.74 and ethanol at 15.9. The value for water is a bit strange, this looks as if they took the log of the equilibrium constant not corrected for water concentration as I am used to ([H2O]=55M, log(55)=1.74). This is OK as long as they do it for each compound in the table. In the book they use this PKa for water all the time.

Which is really the stronger acid? I had the impression that methanol was in fact many times less deprotonated than water in a basic solution of both.

Also related to this topic, would a methylester be susceptible to nucleophilic attack by water in slightly basic environment?

[megalomania]

Eh, try again... its the negative log of the acid dissociation constant, the Ka, which for water is 1.8 x 10-16. That comes out to a pKa of 15.7. The 1.8 figure is actually derived by dividing waters true Ka of 1 x 10-14 by 55.5. This should be assumed for any pKa chart to normalize the concentrations with respect to the other substances.

Ethanol and methanol both actually have a pKa of 16, so they are less acidic than water.

I don't think your methylester will budge in any environment expect extremly basic conditions, if indeed you are referring to acid/base reactivity ala saponification.

[bobo]

Originally posted by megalomania
I don't think your methylester will budge in any environment expect extremly basic conditions, if indeed you are referring to acid/base reactivity ala saponification. [/B]

The nucleophile would be OH- and the leaving group CH3O-. The reaction I want is with a primary amine. The ester I try to displace is a n-hydroxy succinimidyl group. This is a bulky, good leaving group. Hydrolysis with -OH takes place readily.

Amines react readily with certain esters, as they are good nucleophiles, but not so well with acids. But, since amines are also basic, they cause the water and alcohols to deprotonate. I think I can live with deprotonated methanol if it will compete with the amine and then get kicked out by the amine in the end. Whether or not this happens, I do not know. If all esters are about as reactive to amines, this will happen but I do not know.

I am sure though, that the acid (the product of reaction with OH-) is not reactive to amines in my environment.

My environment is methanol so there will be CH3O- ions floating around who must compete with the amines for reaction. But I noticed traces of water in the methanol bottle, so if water is more likely to deprotonate than methanol, considerable hydrolysis might occur. This is why I ask this question.

[John Ashcroft]

MeOH is just barely more acidic than water, while EtOH's disassociation constant is a tad smaller than water.

You're not going to have anywhere near significant quantities of MeO- ions in the solution you describe.