How do we prove that alcohols are weaker acids than water?

Question

Luna Chen · Accepted Answer

My response will be somewhat lengthy because I want to give a thorough explanation of why this occurs.

Taking into account the fact that an alkyl group is actually an EDG, or electron donating group (sometimes referred to as an ERG, or electron releasing group), is crucial to demonstrating why alcohols are weaker acids than water.

I'll explain why this matters from two perspectives - the bond between oxygen and hydrogen in the #"-OH"# group and the stability of the conjugate base.

When comparing the acid strength of alcohols and water, you must look at how "eager" the acidic proton is to jump off. An alcohol's acidity comes from the important difference in electronegativity between the oxygen and the hydrogen in the attached #"-OH"# group.

Since oxygen is more electronegative than the other atom in the functional group, it will draw the bond electrons to itself, which will facilitate the proton's removal.

In the case of an alcohol, on the other hand, the alkyl group will "push" electrons towards the oxygen; this will cause the oxygen atom's electron density to increase and its pull on the two bonding electrons it shares with the acidic proton to decrease.

Because of the strengthened bond between hydrogen and oxygen, the hydrogen will be more difficult to remove as a direct result.

So, the acidic proton is less "eager" to jump off #-># weaker acid.

Regarding the conjugate bases, the variation in stability can be attributed to the identical alkyl group's positive inductive effect.

Conjugate base stability is directly related to acid strength; the more stable a conjugate base, the less reactive it is; the less reactive it is, the weaker it will be, which implies that the acid will be stronger. In other words, stronger acids have weaker conjugate bases and vice versa.

The conjugate base of a generic alcohol is called an alkoxide ion, or #"R-O"^(-)#. Compare this with water's conjugate base, the hydroxide ion #"HO"^(-)#.

The oxygen can accommodate the negative charge in both scenarios; however, because of the alkyl group's previously mentioned positive inductive effect, oxygen in the case of an alkoxide ion has to accommodate an additional negative charge.

This will reduce its stability when compared with the hydroxide ion. A less stable conjugate base is a more reactive conjugate base, and thus a stronger base #-># the acid is weaker.

Avery Adams · Answer

undefined Alcohols are weaker acids than water because their conjugate bases are more stable due to the electron-donating alkyl groups attached to the oxygen atom, which stabilize the negative charge. This can be shown through experiments comparing the acidity of alcohols and water using pH measurements or acid-base titrations.