What is acidity and basicity

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Alcohols: acidity and basicity

Similar to water, alcohols are both weak acids and weak bases. They can both take up and release protons. The self-dissociation is weaker than that of water.

Accordingly, protonated alcohols (oxonium ions) are very strong acids and deprotonated alcohols (alcoholates) are very strong bases. The acidity of alcohols is defined like that of inorganic acids:

Compared to the typical inorganic acids such as HCl or H2SO4 or carboxylic acids such as acetic acid are alcohols very weak acids:

Tab. 1
Acid strength of alcohols
acid
H2O14,0
CH3OH15,5
CH3CH2OH15,9
(CH3)3COH18,0
CF3CH2O12,4
C.6H5OH10,0
CH3COOH4,8
HCl-2,2

Methanol and ethanol are just as strongly acidic as water. tert-Butanol is an even less strong acid, i.e. the conjugate bases methanolate and ethanolate are similarly strong bases as OH-, while tert-Butanolate (CH3)3CO- is about 2 orders of magnitude more basic.

In the laboratory, these bases are usually produced in a reaction immediately before they are used by dissolving metallic sodium in an excess of the corresponding alcohol.

tert-Butanolate is also commercially available in its pure form (white powder). Not only is it more basic than methanolate (MeO-) and ethanolate (EtO-), but because of the bulky tert-Butyl group is also less nucleophilic. It is mainly used when nucleophilic substitution is to be suppressed as a disruptive side reaction compared to deprotonation.

Phenols are stronger acids than methanol and ethanol. The negative charge in the anion can be delocalized over the benzene ring, thereby stabilizing the phenolate ion: