8-1Chpt.8 Alkyl Halides & Radical Rx’s

StructureNomenclaturePhysical PropertiesHalogenation of AlkanesMechanism of HalogenationAllylic Halogenation

Note the Chapter Summary and Key Rx’s

ClCl2

8-28.1 Structure of ‘R-X’

Haloalkene

sp2

(vinyl halide)

C

CH X

H H

Haloalkane

sp3

(alkyl halide)

R-XH3C-Cl

Haloarene

sp2

(aryl halide)

X

8-3

IUPAC - halides (X) are substituentsSubstituent names: halo

fluoro, chloro, bromo, iodo

8.2 Nomenclature

#-haloalkane

#-halocycloalkane

(R)-4-bromo-1-chloro-4-fluoro-1-cyclopentenestructure ?

Br

F

Cl

8-4Common Names: (alkyl halide)

or (special names)

(isopropyl bromide) vs 2-bromopropane

(chloroform) vs trichloromethane

H-CCl3

H3C C

Br

CH3

H

8-58.1 Structure of ‘R-X’

Haloalkane (alkyl halide) sp3

R-X

H3C-Clmethyl chloride

R C

H

X

H1o halide

R C

H

X

R'2o halide

X

R C

R"

R'3o halide

8-68.3 Physical Properties

“polar covalent bond” - dipole- mismatch of electronegativity -size

H3C

C BrH3C

H

8-7

8.4 Halogenation of Alkanes

X2 = Cl2, Br2 seldom F2 (too reactive - exothermic)or I2 (endothermic, unreactive)

hv = ultraviolet light, = heat

Substitution of X for H

H3C

CH3C

H3CH

+X X

orhv

H3C

CH3C

H3CX

H X+

8-8

Substitution, products and by-products+ other

R-X’s

H

CHH

H

+ Cl Clorhv

HC

HH Cl

H X+

Cl

C

ClH

Cl

Cl

CHH Cl

ClC

ClCl

Cl

hvCl2

hvCl2

hv Cl2

8-9Generally halogenation not useful - mixtures (separate)

A few rx’s are useful, e.g.:

+ Br2

Br + HBr + other Br's

Others - allylic & benzylic

8-10

Substitution, products and by-products

H3C CH3 + Br2hv

H3C CH2Br + HBr + diBr + etc

monobromination

bromination favors 3o > 2o > 1o

+ HBr + diBr +...

(92) (8)

CH3

CH2 + Br2

CH3

CH3

C

CH3

Br

CH3

CCH2Br

+

H H H

CH3C

BrCH3

CH3C

HH

+ HBr + diBr +...

CH3

C

HCH3

CH3C

HH

+ Br2

8-11Br+Br Br Brinitiation

H3CC

H3C HBr Br+

H3CC

H3C H

Br+ Br

H3CC

H3CH

HBr+

H3CC

H3C HH Br+propagation

terminationsH3C

CH3C H

Br H3CC

H3C H

Br

Br + BrBrBr

H3C

CH3C

H CH3

C CH3

H H3C

CH3C

H

CH3

C CH3H

and/or

and/or

8-12

CH3

CH2 + Br2

CH3

CH3

CH

CH3

Br

CH3

CH2

CH2Br

+ + HBr + diBrs +

(92) (8)

Regioselective for 3o > 2o > 1o C-H

(57) (43)

CH3

CH2 + Cl2CH3

CH3

CH

CH3

Cl

CH3

CH2

CH2Cl

+ + HCl + diCls +

>

>

8-13radical stability like carbocation - electron deficient

R CR

RR C

H

R>

R. is electron deficient (not charged)

> etc.

R CR

RR C

H

R> > etc.

8-14Order of stability of R(+)

R CR

R

R CH

R

R CH

H

H CH

H

>

>

>

E

R CR

R

R CH

R

R CH

H

H CH

H

>

>

>

E

‘Same’ for R.

write condensed

8-15

EE >R3C

+ R2CH+ RCH2

+ H3C+

>>

Order of stability of R(+)

E

>R3C

. R2CH. RCH2

. H3C.

>>

‘Same’ for R.

8-16

Selectivity 3o > 2o > 1o, but Cl and Br are different

Br2 1600 80 1Cl2 5 4 1

3o 2o 1o

CH3

CH2 + X2

CH

H3C CH3

CH3

CH2

CH3C CH3

X

+ HX + other RX

major mono-X product

8-17

Hammond’s Postulate

8-18

Hammond’s Postulate Cl vs Br

• Hammond’s Postulate: the structure of the transition state:

– for an exothermic reaction looks more like the reactants of that step

- for an endothermic reaction looks more like the products of that step

8-19

Hammond’s Postulate• In halogenation of an alkane, the rate-

limiting step is hydrogen abstraction

– this step is endothermic for bromination

H°(kcal/mol)

-88 +10.0+98++CH3CH3 •Br CH3CH2• HBr

-5.0-103+98+ +CH3CH3 •Cl CH3CH2• HCl

and exothermic for chlorination.

8-20

Hammond’s Postulate

• transition state resembles the alkane and chlorine atom

• little radical character on carbon in t.s.

• regioselectivity only slightly influenced by radical stability

For chlorination (hydrogen abstraction is exothermic):

8-21Hammond

R

C H

HR

Br

late t.s.

PE

SM prog rx Prod

stability of R. important SM prog rx Prod

PE

early t.s. - like SMR

C H

HR

Cl

site of collision important

8-22

Halogenation (free radical substitution)

H H

+ Br2

(heat)

H Br

+ H-Br

cool

H H

Br

Br

8-23Allyl Radical - resonance

CC

C

H

H

H H

H CC

C

H

H

HH

H

CC

C

H

H

HH

H

8-24NBS

Br

NBS for Br2

N

O

O

N

O

O

Br X+

hv orROOR HBr

N-Br

O

O

N-H

O

O

HBr+ Br2

X+XXX R. + HX

8-25NBS for Br2

CH

H

+ HBrBr

CH3Br2

CH

H

Br

Br Br Br Br+

NBS

+ H-BrN

O

O

H+

N

O

O

Br + N

O

O

CH

H

H

8-26Free Radical Stability

C

H

H

C

H

HC

H

HC

H

HC

H

H

8-27

CC

C

H

H

HH

H

allylic

Free Radical Stability

C

H

H

benzylic

>>

>

R CR

R

R CH

R

R CH

H

H CH

H

8-28

H3C

H3C

H H

H

HH H

Br2

H3C

H3C

Br HH

HH H

H3C

H3C

H

H

BrH H

H

+H-Br

H3C

H3C

H HH

HH Br

H3C

H3C

H H

Br

H

H

H

+ + +

Reaction

8-29

H3C

H3C

H H

H

HH H

Br2

Br-Br

H3C

H3C

H HH

HH H

Br

H3C

H3C

Br HH

HH H

Br

H3C

H3C

H

H

BrH H

H

Br

+H-Br

H3C

H3C

H HH

HH Br

H3C

H3C

H H

Br

H

H

H

+ + +

+

Reaction

initiation

propagation

step 1rate determining

8-30

H HH

HH H

H3C

H3C

H3C

H3C

H HH

HH H

Br

BrH-Br

H-Br

H

H

HH H

H3C

H3C

H3C

H3C

H

H

HH H

H

H

HH H

H3CH3C

H3C

H3C

H

H

HH H

Br Br

Br Br

Br Br

Br Br

H3C

H

H

BrH H

H

H3C

H3C

Br HH

HH H

H3C

H3C

H3C

H

H

BrH H

H

and

H3C

H3C

Br HH

HH Hand

Br

Br

+

+

+

+

propagation

step 1rate determining

step 1rate determining

step 2

step 2

two different allylic sites

8-31

H

H

HH H

Br BrH3C

H

H

BrH H

H

H3C

Br HH

HH H

and

H3CH3C

H3C

H3C

Br+

Br

H

H

HH H

H3CH3C

H

H

HH H

H3CH3C Br

step 2

termination - one of many

8-32Radical Addition of HBr to Alkenes• Addition of HBr to alkenes gives either

Markovnikov addition or non-Markovnikov addition depending on reaction conditions– Markovnikov addition occurs when radicals are absent– non-Markovnikov addition occurs when peroxides or

other sources of radicals are present

+ HBr

no peroxides

Br

2-Methyl-propene

2-Bromo-2-methylpropane

Markovnikovaddition

HBrBrperoxides

+

2-Methyl-propene

1-Bromo-2-methylpropane

Non-Markovnikovaddition

8-33Radical Addition of HBr to Alkenes– addition of HCl and HI gives only

Markovnikov products– to account for the the non-Markovnikov

addition of HBr in the presence of peroxides, chemists proposed a radical chain mechanism

• Chain initiation

R O

R-O O-R

H Br

R O RO

HR O Br

Two alkoxy radicalsA dialkylperoxide

Bromineradical

Step 1: +

+Step 2: +

8-34

Radical Addition of HBr to Alkenes

• Chain propagation

Br

BrBr H

Br

BrH Br+

Step 3:

Step 4:

+

+

A 3° radical

1-Bromo-2-methylpropane

8-35Radical Addition of HBr to Alkenes

• Chain termination

• This pair of addition reactions illustrates how the products of a reaction can often be changed by a change in experimental conditions– polar addition of HBr is regioselective, with Br adding to the more

substituted carbon– radical addition of HBr is also regioselective, with Br adding to the

less substituted carbon

Br BrBr Br

BrBr Br

BrStep 6:

Step 5: +

+