EXPT 9 Dehydration

3 ho conc h3po 4 eq 3 heat major minor there

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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 OH (Eq....
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This note was uploaded on 12/15/2012 for the course CHE 301 taught by Professor Lucy during the Spring '12 term at VCU.

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