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Unformatted text preview: + (Eq. 3) heat
major minor There is, of course, another type of elimination reaction, known as an E2 elimination, which
occurs under basic conditions. However, alcohols will not undergo E2 elimination because the
hydroxide ion (-OH) is not a good leaving group. The rules for leaving groups in an E2 reaction
are the same as for an SN2 reaction. The leaving group must be stable and be a weak base. While
hydroxide ion is reasonably stable, it is a strong base, and is therefore a poor leaving group.
Alcohols can be converted into good leaving groups, however, in the same manner as described
for the SN2 reaction. Often the simplest way is to convert the alcohol to the corresponding alkyl
bromide via treatment with PBr3. However, conversion to the tosylate or mesylate and use of
these in the E2 reaction is also useful. Since the E2 reaction proceeds via a transition state
involving both the alkene precursor and the base, there is no rearrangement. The major product
is the product resulting from direct Zaitsev elimination from the starting material. A typical
sequence is illustrated below for the conversion of 3-methyl-2-butanol to 2-bromo-3methylbutane which is then subjected to treatment with potassium t-butoxide to give 2-methyl-2butene as the major product (Eq. 4). K +t-BuO- PBr 3
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