6.46.4Electrophilic Addition of Electrophilic Addition of

Hydrogen Halides to Hydrogen Halides to AlkenesAlkenes

++ EE——YY ––

CC CC EE YYCC CC

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

++ 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

CHCH33CHCH22 CHCH22CHCH33

HH HH

CHCH33CHCH22CHCH22CHCHCHCH22CHCH33

BrBr

(76%)(76%)

CHClCHCl33, -30°C, -30°CCC CC

ExampleExampleExampleExample

HBrHBr

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

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

XXHH

HH

CC CC

MechanismMechanismMechanismMechanism

....

.... ::

CC CC++

....

.... ::XX::––

XXHH

HH

CC CC

MechanismMechanismMechanismMechanism

....

.... ::

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 RuleMarkovnikov's RuleMarkovnikov's Rule

acetic acidacetic acidBrBr

CHCH33CHCH22CHCHCHCH33

Example 1Example 1Example 1Example 1

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

CHCH22CHCH33CHCH22CHCHHBrHBr

(80%)(80%)

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

0°C0°CCHCH33

ClCl

CHCH33

(100%)(100%)

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

Example 3Example 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

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

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

CHCH22CHCH33CHCH22CHCH

HBrHBr

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

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

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

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

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

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 occurRearrangements sometimes occur

HH22CC CHCCHCHH(CH(CH33))22