Alkenes react with hydrogen bromide (HBr)

15 Pages·2010·1.89 MB·English

by  Craig Mossman

Alkenes react with hydrogen bromide (HBr) to yield alkyl bromides
and with hydrogen chloride (HCl) to yield alkyl chlorides in what are
called hydrohalogenation reactions:
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2-Methylpropene could add HBr to give 1-bromo-2-
methylpropane, but it does not; it gives only 2-
bromo-2-methylpropane. Only one of the two
possible addition products is obtained.
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Markovnikov’s rule: In the addition of HX to an alkene, the H
attaches to the carbon that already has the most H’s, and the
X attaches to the carbon that has fewer H’s.
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Hydration: The addition of water, in the presence of a strong
acid catalyst, to a multiple bond to give an alcohol product.
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How Alkene Addition Reactions Occur
• Reaction mechanism: A description of the individual steps by
which old bonds are broken and new bonds are formed in a
reaction.
• Detailed studies show that alkene addition reactions take
place in two distinct steps and involve a carbocation
intermediate.
• The addition of HBr to ethylene is an example.
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In the first step, two electrons move from the double
bond to form a C-H bond. In the second step, Br- uses
two electrons to form a bond to the carbocation.
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Alkene Polymers
A polymer is a large molecule formed by the repetitive
bonding together of many smaller molecules called
.
monomers
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Aromatic Compounds and the Structure of Benzene
• Aromatic: The class of compounds containing benzene-like
rings.
• Benzene and other aromatic compounds are much less
.
reactive than alkenes
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Naming Aromatic Compounds
• Substituted benzenes are named using -benzene as the
parent.
•
• No number is needed for monosubstituted benzenes because
all the ring positions are identical.
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