Base induced E2 eliminations:

Base induced E2 eliminations of alcohols may be achieved by their sulfonate ester derivatives. This has the advantage of avoiding strong acids, which may cause molecular rearrangement and or double bond migration in some cases.

*Since 3º-sulfonate derivatives are sometimes unstable, this procedure is best used with 1º and 2º-mesylates or tosylates.

Application of this reaction sequence is shown here for 2-butanol.

The Zaitsev Rule favors formation of 2-butene (cis + trans) over 1-butene.

*Eliminations often favor the more stable trans-product over the cis-product (stereoselectivity).

*The E2 elimination of 3º-alcohols under relatively non-acidic conditions may be accomplished by treatment with phosphorous oxychloride (POCl₃) in pyridine.

*This procedure is also effective with hindered 2º-alcohols, but for unhindered and 1º-alcohols an S_N2 chloride ion substitution of the chlorophosphate intermediate competes with elimination.

*Some examples of these and related reactions are:

*The first equation shows the dehydration of a 3º-alcohol. The predominance of the non-Zaitsev product (less substituted double bond) is presumed due to steric hindrance of the methylene group hydrogens, which interferes with the approach of base at that site.

*The second example shows two elimination procedures applied to the same 2º-alcohol.

The first uses the single step POCl₃ method, which works well in this case because SN² substitution is retarded by steric hindrance.

*The second method is another example in which an intermediate sulfonate ester confers halogen-like reactivity on an alcohol. In every case the anionic leaving group is the conjugate base of a strong acid.