© Prentice Hall 2001Chapter 31 Addition of Hydrogen Halides to Alkenes What about the following reaction? Which sp 2 carbon gets the hydrogen and which.
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© Prentice Hall 2001 Chapter 3 1
Addition of Hydrogen Halides to Alkenes
What about the following reaction?
Which sp2 carbon gets the hydrogen and which gets the chlorine?
CH3 C
CH3
CH2HCl CH3 C
CH3
Cl
CH3 CH3 CH
CH3
CH2Cl+ or
© Prentice Hall 2001 Chapter 3 2
Addition of Hydrogen Halides to Alkenes The more substituted carbocation is
preferred
© Prentice Hall 2001 Chapter 3 3
Stability of Carbocations Alkyl groups (“R”s) tend to stabilize the
positive charge on the sp2 carbon of a carbocation
© Prentice Hall 2001 Chapter 3 4
Stability of Carbocations Alkyl groups are more polarizable than
hydrogen (i.e. they tend to release electrons more easily than does hydrogen)
Also, alkyl groups can release electrons via hyperconjugation
© Prentice Hall 2001 Chapter 3 5
Stability of Carbocations Alkyl groups bonded to the sp2 carbon of
a carbocation tend to spread out the positive charge, thereby stabilizing the carbocation
© Prentice Hall 2001 Chapter 3 6
The Hammond Postulate
Carbocation formation is endergonic - hence transition state is closest to the carbocation
© Prentice Hall 2001 Chapter 3 7
The Hammond Postulate Formation of a tertiary carbocation should
be faster than formation of a primary carbocation
© Prentice Hall 2001 Chapter 3 8
Regioselectivity - Markovnikov’s Rule
“When a hydrogen halide adds to an asymmetrical alkene, the addition occurs such that the halogen attaches itself to the carbon atom of the alkene bearing the least number of hydrogen atoms”
© Prentice Hall 2001 Chapter 3 9
Markovnikov’s Rule Modern equivalent statement: The electrophile adds to the sp2 carbon
that is bonded to the greater number of hydrogens
© Prentice Hall 2001 Chapter 3 10
Addition of Water
© Prentice Hall 2001 Chapter 3 11
Rearrangements of Carbocations
1,2-Hydride Shift
© Prentice Hall 2001 Chapter 3 12
Rearrangements of Carbocations
1,2-Methyl Shift
© Prentice Hall 2001 Chapter 3 13
Addition of Halogens
The atoms that make up the Br2 or Cl2 molecules are very electronegative
The halogen-halogen bond is weak
© Prentice Hall 2001 Chapter 3 14
Addition of Halogens
When electrons approach a halogen molecule, one end of the halogen molecule accepts them releasing the shared bonding pair to the other halogen atom to form a halonium ion and a halide ion
© Prentice Hall 2001 Chapter 3 15
Addition of Halogens
The remaining halide ion is a good nucleophile which attacks the positively charged halonium ion
© Prentice Hall 2001 Chapter 3 16
Addition of Halogens
Chlorine and bromine are the only useful halogen used for this reaction
Fluorine reacts explosively, and iodine doesn’t react
Addition reactions typically are carried out in CCl4
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