6.4 electrophilic addition of hydrogen halides to alkenes · electrophilic addition of hydrogen...
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6.46.4Electrophilic Addition of Electrophilic Addition of
Hydrogen Halides to Hydrogen Halides to AlkenesAlkenes
++ EE——YYδ+δ+ δδ––
CC CC EE YYCC CC
General equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic addition
++ HH——XXδ+δ+ δδ––
CC CC HH XXCC CC
When EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halide
CHCH33CHCH22 CHCH22CHCH33
HH HH
CHCH33CHCH22CHCH22CHCHCHCH22CHCH33
BrBr
(76%)(76%)
CHClCHCl33, , --30°C30°CCC CC
ExampleExampleExample
HBrHBr
Electrophilic addition of hydrogen halides Electrophilic addition of hydrogen halides to alkenes proceeds by rateto alkenes proceeds by rate--determining determining formation of a carbocation intermediate.formation of a carbocation intermediate.
MechanismMechanismMechanism
Electrons flow fromElectrons flow fromthe the ππ system of thesystem of thealkene (electron rich) alkene (electron rich) toward the positivelytoward the positivelypolarized proton of polarized proton of the hydrogen halide.the hydrogen halide.
MechanismMechanismMechanism
XXHH
HH
CC CC
MechanismMechanismMechanism
....
.... ::
CC CC++
....
.... ::XX::––
XXHH
HH
CC CC
MechanismMechanismMechanism
....
.... ::
CC CC++
....
.... ::XX::––
HHCC CC........XX::
6.56.5Regioselectivity ofRegioselectivity of
Hydrogen Halide Addition:Hydrogen Halide Addition:Markovnikov's RuleMarkovnikov's Rule
When an unsymmetrically substituted When an unsymmetrically substituted alkene reacts with a hydrogen halide, alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that the hydrogen adds to the carbon that has the greater number of hydrogen has the greater number of hydrogen substituents, and the halogen adds to substituents, and the halogen adds to the carbon that has the fewer hydrogen the carbon that has the fewer hydrogen substituents.substituents.
Markovnikov's RuleMarkovnikov's Rule
acetic acidacetic acidBrBr
CHCH33CHCH22CHCHCHCH33
Example 1Example 1Example 1
Markovnikov's RuleMarkovnikov's Rule
CHCH22CHCH33CHCH22CHCHHBrHBr
(80%)(80%)
CHCH33
CHCH33
CHCH33 CC
BrBr
(90%)(90%)
CC CC
Markovnikov's RuleMarkovnikov's Rule
Example 2Example 2Example 2
acetic acidacetic acid
HBrHBrCHCH33
CHCH33
HH
HH
0°C0°CCHCH33 ClCl
CHCH33
(100%)(100%)
Markovnikov's RuleMarkovnikov's Rule
Example 3Example 3Example 3
HClHCl
6.66.6Mechanistic BasisMechanistic Basis
forforMarkovnikov's RuleMarkovnikov's Rule
Protonation of double bond occurs in Protonation of double bond occurs in direction that gives more stable of two direction that gives more stable of two
possible carbocations.possible carbocations.
BrBr
CHCH33CHCH22CHCHCHCH33
Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1
CHCH22CHCH33CHCH22CHCHacetic acidacetic acid
HBrHBr
BrBr
CHCH33CHCH22CHCHCHCH33
Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1
CHCH22CHCH33CHCH22CHCH
HBrHBr
CHCH33CHCH22CHCH——CHCH33 + Br + Br ––++
BrBr
CHCH33CHCH22CHCHCHCH33
Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1
CHCH22CHCH33CHCH22CHCH
HBrHBr
CHCH33CHCH22CHCH——CHCH33 + Br + Br ––++
CHCH33CHCH22CHCH22——CHCH22
++
primary carbocation is less stable: not formedprimary carbocation is less stable: not formed
ClCl
CHCH33
CHCH33
HH
Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3
0°C0°C
HClHCl
ClCl
CHCH33
CHCH33
HH
CHCH33
HH HH
++ Cl Cl ––
Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3
HClHCl
ClCl
CHCH33
CHCH33
HH
CHCH33
HH HH
++
++
HH
CHCH33
HH
Cl Cl ––
Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3
HClHCl
secondary secondary carbocation is carbocation is less stable: less stable: not formednot formed
6.76.7Carbocation Rearrangements in Carbocation Rearrangements in
Hydrogen Halide Addition Hydrogen Halide Addition to Alkenesto Alkenes
HCl, 0°CHCl, 0°C
CHCH33CHCCHCHH(CH(CH33))22
++
CHCH33CHCCHCHH(CH(CH33))22
ClCl (40%)(40%)
CHCH33CHC(CHCHC(CH33))22
++HH
CHCH33CHCH22C(CHC(CH33))22
ClCl(60%)(60%)
Rearrangements sometimes occurRearrangements sometimes occurRearrangements sometimes occur
HH22CC CHCCHCHH(CH(CH33))22