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Unformatted text preview: 4) Br To illustrate the use of alcohols in elimination reactions, you will carry out the formation of
cyclohexene from cyclohexanol. This particular reaction was chosen because there is no possibility of rearrangement of the intermediate carbocation, thus allowing only one product to
form. This avoids the necessity of a complex separation scheme to purify the product.
You will also carry out a qualitative analysis test for double bonds to confirm that you have
formed cyclohexene. The first test is the reaction of the alkene with bromine (Br2) in methylene
chloride. Alkenes react readily with bromine to add one bromine atom to each carbon of the
double bond. The reaction involves the pi-bond of the alkene acting as a nucleophile to attack a
slightly polarized bromine molecule at the more positive end. This leads to the formation of a
bromonium ion intermediate. This bromonium ion, which is a three-member ring, is attacked
from the least hindered side of one of the carbons to give trans- (or anti-) addition of bromine
across the double bond, as indicated for cyclohexene (Eq. 5). Positive results of the test are easy
to determine. The bromine solution is a deep orange/red. As the reagents are mixed, the pr...
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