6.4 electrophilic addition of hydrogen halides to alkenes

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6.4 6.4 Electrophilic Addition Electrophilic Addition of of Hydrogen Halides to Hydrogen Halides to Alkenes Alkenes

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Page 1: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

6.46.4Electrophilic Addition of Electrophilic Addition of

Hydrogen Halides to Hydrogen Halides to AlkenesAlkenes

Page 2: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

++ EE——YY ––

CC CC EE YYCC CC

General equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic addition

Page 3: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

++ HH——XX ––

CC CC HH XXCC CC

When EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halide

Page 4: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

CHCH33CHCH22 CHCH22CHCH33

HH HH

CHCH33CHCH22CHCH22CHCHCHCH22CHCH33

BrBr

(76%)(76%)

CHClCHCl33, -30°C, -30°CCC CC

ExampleExampleExampleExample

HBrHBr

Page 5: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

Electrophilic addition of hydrogen halides Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining to alkenes proceeds by rate-determining formation of a carbocation intermediate.formation of a carbocation intermediate.

MechanismMechanismMechanismMechanism

Page 6: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

Electrons flow fromElectrons flow fromthe the system of the system of thealkene (electron rich) alkene (electron rich) toward the positivelytoward the positivelypolarized proton of polarized proton of the hydrogen halide.the hydrogen halide.

MechanismMechanismMechanismMechanism

Page 7: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

XXHH

HH

CC CC

MechanismMechanismMechanismMechanism

....

.... ::

CC CC++

....

.... ::XX::––

Page 8: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

XXHH

HH

CC CC

MechanismMechanismMechanismMechanism

....

.... ::

CC CC++

....

.... ::XX::––

HHCC CC........XX::

Page 9: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

6.56.5Regioselectivity ofRegioselectivity of

Hydrogen Halide Addition:Hydrogen Halide Addition:Markovnikov's RuleMarkovnikov's Rule

Page 10: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

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 RuleMarkovnikov's RuleMarkovnikov's Rule

Page 11: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

acetic acidacetic acidBrBr

CHCH33CHCH22CHCHCHCH33

Example 1Example 1Example 1Example 1

Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule

CHCH22CHCH33CHCH22CHCHHBrHBr

(80%)(80%)

Page 12: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

CHCH33

CHCH33

CHCH33 CC

BrBr

(90%)(90%)

CC CC

Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule

Example 2Example 2Example 2Example 2

acetic acidacetic acid

HBrHBrCHCH33

CHCH33

HH

HH

Page 13: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

0°C0°CCHCH33

ClCl

CHCH33

(100%)(100%)

Markovnikov's RuleMarkovnikov's RuleMarkovnikov's RuleMarkovnikov's Rule

Example 3Example 3Example 3Example 3

HClHCl

Page 14: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

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.

Page 15: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

BrBr

CHCH33CHCH22CHCHCHCH33

Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1

Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1

CHCH22CHCH33CHCH22CHCHacetic acidacetic acid

HBrHBr

Page 16: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

BrBr

CHCH33CHCH22CHCHCHCH33

Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1

Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1

CHCH22CHCH33CHCH22CHCH

HBrHBr

CHCH33CHCH22CH—CHCH—CH33 + Br + Br – – ++

Page 17: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

BrBr

CHCH33CHCH22CHCHCHCH33

Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1

Mechanistic Basis for Markovnikov's Rule:Mechanistic Basis for Markovnikov's Rule:Example 1Example 1

CHCH22CHCH33CHCH22CHCH

HBrHBr

CHCH33CHCH22CH—CHCH—CH33 + Br + Br – – ++

CHCH33CHCH22CHCH22—CH—CH22

++

primary carbocation is less stable: not formedprimary carbocation is less stable: not formed

Page 18: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

ClCl

CHCH33

CHCH33

HH

Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3

Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3

0°C0°C

HClHCl

Page 19: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

ClCl

CHCH33

CHCH33

HH

CHCH33

HH HH

++ Cl Cl ––

Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3

Mechanistic Basis for Mechanistic Basis for Markovnikov's Rule:Markovnikov's Rule:Example 3Example 3

HClHCl

Page 20: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

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

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

Page 21: 6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

6.76.7Carbocation Rearrangements in Carbocation Rearrangements in

Hydrogen Halide Addition Hydrogen Halide Addition to Alkenesto Alkenes

Page 22: 6.4 Electrophilic Addition of Hydrogen Halides to 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 occurRearrangements sometimes occur

HH22CC CHCCHCHH(CH(CH33))22