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Acid strength of Alcohols
#1 Posted : Friday, July 03, 2020 2:24:15 AM
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Hi, for page 172 of CC, it ranks the acidity of the alcohols: methyl > water > 1o > 2o > 3o. So a primary alcohol is a stronger acid than a tertiary alcohol.

However, doesn't a more stable conj base form a stronger acid? so if the alcohol loses the proton, wouldn't the tertiary deprotonated alcohol form a more stable conj base than a primary alcohol and therefore, form a stronger acid?

#2 Posted : Saturday, July 04, 2020 1:41:57 AM
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Good question!

Yes, a stronger acid has a more stable conjugate base.

Recall that when deprotonating an alcohol, you form a negatively charged oxgyen species known as an alkoxide.

A tertiary alkoxide is less stable than a primary alkoxide because it has more electron donating C groups attached, donating a bit more electon density in and destabilizing the negative charge relative to the primary alkoxide.

Recall that oxygen is electronegative and does like negative charge, but not enough to want to be fully formal negative - the electron donating nature of C exarerbates this, destabilizing the conjugate base and rendering the tertiary alcohol to be a weaker acid.

Stability can be tricky - what stabilizes an electron deficient speccies will in fact destabilize an electron rich species. That's why this trend is opposite to the postivie carbocation - here you are forming a negative oxygen anion.

I hope this helps!

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