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-2methylpropane, but it does not; it gives only 2bromo-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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• Disubstituted aromatic compounds are named using
one of the prefixes ortho-, meta-, or para-.
• An ortho- or o-disubstituted benzene has its two
substituents in a 1,2 relationship on the ring.
• A meta- or m-disubstituted benzene has its two
substituents in a 1,3 relationship on the ring..
• A para- or p-disubstituted benzene has its
substituents in a 1,4 relationship on the ring..
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Occasionally, the benzene ring itself may be considered a
substituent group attached to another parent compound.
When this happens, the name phenyl is used for the unit:
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Reactions of Aromatic Compounds
Unlike alkenes, which undergo addition reactions, aromatic
compounds usually undergo substitution reactions.
That is, a group Y substitutes for one hydrogen atom on the aromatic
ring without changing the ring itself.
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• Halogenation is the substitution of a halogen
atom, usually bromine or chlorine, for one of
the ring hydrogens.
• The reaction occurs when benzene reacts with
Br2 or Cl2 in the presence of iron as catalyst:
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• Sulfonation is the substitution of a sulfonic
acid group for one of the ring hydrogens.
• The reaction occurs when benzene reacts with
concentrated sulfuric acid and SO3 .
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