The "peroxide effect"The "peroxide effect"

6.86.8Free-radical Addition of HBr to Free-radical Addition of HBr to

AlkenesAlkenes

acetic acidacetic acidBrBr

CHCH33CHCH22CHCHCHCH33CHCH22CHCH33CHCH22CHCHHBrHBr

(80%)(80%)

Markovnikov's RuleMarkovnikov's Rule

only product inonly product in

absence of peroxidesabsence of peroxides

CHCH33CHCH22CHCH22CHCH22BrBr

only product when only product when

peroxides added to peroxides added to

reaction mixturereaction mixture

BrBr

CHCH33CHCH22CHCHCHCH33

CHCH22CHCH33CHCH22CHCH

HBrHBr

Addition of HBr to 1-ButeneAddition of HBr to 1-Butene

CHCH33CHCH22CHCH22CHCH22BrBr

only product when only product when

peroxides added to peroxides added to

reaction mixturereaction mixture

CHCH22CHCH33CHCH22CHCH

HBrHBr

addition opposite to addition opposite to Markovnikov's ruleMarkovnikov's rule

occurs with HBr (not occurs with HBr (not HCl or HI)HCl or HI)

Addition of HBr to 1-ButeneAddition of HBr to 1-Butene

Addition of HBr with a regiochemistry oppositeAddition of HBr with a regiochemistry oppositeto Markovnikov's rule can also occur whento Markovnikov's rule can also occur wheninitiated with light with or without added peroxides.initiated with light with or without added peroxides.

+ HBr+ HBrhh

(60%)(60%)

CHCH22

CHCH22BrBr

HH

Photochemical Addition of HBrPhotochemical Addition of HBr

Addition of HBr opposite to Markovnikov's rule Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism.proceeds by a free-radical chain mechanism.

Initiation steps:Initiation steps:

....

....OO RRRR OO........

OO ..RR ........

....

....OO RR..++

MechanismMechanism

Addition of HBr opposite to Markovnikov's rule Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism.proceeds by a free-radical chain mechanism.

Initiation steps:Initiation steps:

....

....OO RRRR OO........

++ BrBr

OO ..RR ........

....

....OO RR..++

OO ..RR ........

HH ........:: ++OORR ....

....HH .. BrBr....

....::

MechanismMechanism

Propagation steps:Propagation steps:

++CHCH33CHCH22CHCH CHCH22.. BrBr........::

..CHCH33CHCH22CHCH CHCH22 BrBr::

....

....

MechanismMechanism

Propagation steps:Propagation steps:

++

++

CHCH33CHCH22CHCH CHCH22.. BrBr........::

..CHCH33CHCH22CHCH CHCH22 BrBr::

....

....

..CHCH33CHCH22CHCH CHCH22 BrBr::

....

....

HH BrBr........::

....CHCH33CHCH22CHCH22CHCH22 BrBr::....

.. BrBr........::

MechanismMechanism

6.96.9Addition of Sulfuric Acid to AlkenesAddition of Sulfuric Acid to Alkenes

follows Markovnikov's rule:follows Markovnikov's rule:yields an alkyl hydrogen sulfateyields an alkyl hydrogen sulfate

CHCH33CHCHCHCH33

OSOOSO22OHOH

CHCH33CHCH CHCH22

HOSOHOSO22OHOH

IsopropylIsopropylhydrogen sulfatehydrogen sulfate

Addition of HAddition of H22SOSO44

++

slowslow

++

fastfast

CHCH33CHCH CHCH22 HH....

SOSO22OHOH....

++CHCH33CHCH CHCH33

....SOSO22OHOH

....::

––

OO

OO

CHCH33CHCHCHCH33

OSOOSO22OHOH:: ....

MechanismMechanism

H—OHH—OH

HO—HO—SOSO22OHOH++

CHCH33CHCHCHCH33

OO SOSO22OHOH

heatheat

CHCH33CHCHCHCH33

OO HH

++

Alkyl hydrogen sulfates undergo Alkyl hydrogen sulfates undergo hydrolysis in hot waterhydrolysis in hot water

1. H1. H22SOSO44

2. H2. H22O, heatO, heat

(75%)(75%)

Application: Conversion of alkenes to alcoholsApplication: Conversion of alkenes to alcohols

OHOH

not all alkenes yield alkyl hydrogen sulfatesnot all alkenes yield alkyl hydrogen sulfateson reaction with sulfuric acidon reaction with sulfuric acid

these do: these do:

HH22C=CHC=CH22, RCH=CH, RCH=CH22, and RCH=CHR', and RCH=CHR'

these don't:these don't:

RR22C=CHC=CH22, R, R22C=CHC=CH22, and R, and R22C=CRC=CR22

But...But...