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#1 2008-10-25 03:55:39


Why do tertiary alcohols react faster with HCl than sec or primary?

Yahoo Answers Chemistry

Why would tertiary alcohols react much faster with concentrated HCl than do secondary or primary alcohols?

I expect there are two factors in play. One is because carbon is a better electron donor than hydrogen, the oxygen of a tertiary alcohol would be more basic than a secondary or primary alcohol. This effect is similar to the pKa of amines with tertiary amines being the most basic and ammonia the least. (Their pKa data belies this principle. If anyone wishes an explanation, they should contact me.)

The mechanism of reaction of a tertiary alcohol is an SN1 reaction (see The kinetics of an SN1 reaction are the rate is proportional to the concentration of the oxonium ion concentration.

Rate = k * [ROH2(+)]

Since the concentration of ROH2(+) is higher for a tertiary alcohol than the primary or secondary (due to the greater basicity of a tertiary alcohol), its reaction rate will also be higher.

Secondly, since carbons are electron donors, they donate electrons to the tertiary carbon. That will weaken the C-O bond making it more easily cleaved. This effect will be greatest with a tertiary alcohol as it contains three electron donating groups. Since this is the rate determining effect, tertiary alcohols should react the fastest to form the carbocation.

The formation of the chloride is a fast reaction, but it is not part of the rate equation.

The basicity effect of a teriary alcohol is also seen in the acidity of methanol, ethanol to t-butanol. t-Butanol is the least acidic (most basic alkoxide) of the alcohols. Again, this is consistent with the methyl groups donating electrons to the tert-carbon and inductively increasing the electron density on the oxygen decreasing its acidity.

Learn reaction mechanisms with "A Guide to Organic Chemistry Mechanisms" available at, Barnes and Noble or . While this answer is an application of atomic principles, "A Guide to Organic Chemistry Mechanisms" lays out these atomic principles from which you can deduce the answer to this problem.


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