Al Kanes Radicals

Suggested Problems:Suggested Problems:

10.1-10.18, 10.21, 10.24, 10.2810.1-10.18, 10.21, 10.24, 10.28(in text)(in text)

H O

H

H O H+H2O

Bond Dissociation Energies.Bond Dissociation Energies.

KKaa = 10 = 10--

1616

H O

H

H O H+ ΔH° = 498 kJ/molΔH° = 498 kJ/mol

heterolytic bond breaking is not bond heterolytic bond breaking is not bond dissociationdissociation

homolytic bond breaking is bond dissociationhomolytic bond breaking is bond dissociation


H O

H


+C H H

H

H

H C

H

H

H ΔH° = 435 ΔH° = 435 kJ/molkJ/mol

ΔH° = 410 ΔH° = 410 kJ/molkJ/mol

+C C H

H

H

C C

H

H

H

H

H

H

C

H

H

H

H

H C

H

H


+C H H

H

H

H C

H

H

H ΔH° = 435 ΔH° = 435 kJ/molkJ/mol


+C C H

H

H

C C

H

H

H

H

H

H

C

H

H

H

H

H C

H

H


+C H H

H

C

CH

HH

HH

H

C

H

C

CH

HH

HH

H

Ener

gy (H

)

Reaction

C H

H

C

CH

HH

HH

H

+ HC

H

C

CH

HH

HH

H

+ HC

H

C

CH

HH

HH

H

410 kJ/mol410 kJ/mol


1515

Ener

gy (H

)

Reaction

C H

C

C

CH

HH

HH

H

H HH

+ HC

C

C

CH

HH

HH

H HH

H

410 kJ/mol410 kJ/mol + HC

C

C

CH

HH

HH

H

H HH


2929

A 2° radical is more stable than a 1° radical but A 2° radical is more stable than a 1° radical but less stable than a 3° radical.less stable than a 3° radical.radical stability:radical stability:

CHH

HCRH

HCRH

RCRR

R> > >

Groups that delocalize electrons are important in Groups that delocalize electrons are important in the stabilization of double bonds, cations and the stabilization of double bonds, cations and radical. These groups include alkyl and radical. These groups include alkyl and conjugated substituents.conjugated substituents.

CHH

HCRH

HCRH

RCRR

R> > >

Alkyl group stabilization of radicals.Alkyl group stabilization of radicals.

C CHH

H

HH

C CHH

HH

H

electron delocalization electron delocalization resonanceresonance

Alkyl group stabilization of radicals.Alkyl group stabilization of radicals.

C HHH

••

C CHH

H

HH

••

electron delocalization electron delocalization orbitalsorbitals

Give an explanation for the difference in the Give an explanation for the difference in the following bond dissociation energies.following bond dissociation energies.

CH2 CH CH2 CH2 CH CH2 H+H

CH2CHCH2



CC

CHH

HH

H

CH3 CH2 CH3 CH3 CH2 CH2 H+

C H bonds are reactive towards free C H bonds are reactive towards free radicals.radicals.

C HHH

HCl C ClH

H

HH Cl+ +

or hCl ΔH° = -102 kJ/molΔH° = -102 kJ/mol

ΔG° = ΔH° - TΔS°ΔG° = ΔH° - TΔS°

435435 243243 349349 431431 = bond = bond dissociation dissociation energies in kJ/molenergies in kJ/mol

when T = 298 °K ΔG° = -103 when T = 298 °K ΔG° = -103 kJ/molkJ/mol

In most organic reactions the TΔS° term is small.In most organic reactions the TΔS° term is small.

bonds broken (678)bonds broken (678) bonds made (780)bonds made (780) = -102 = -102 kJ/molkJ/mol

--

Table of Bond Dissociation Energies page 434

ΔG° ~ ΔH°ΔG° ~ ΔH°

The reaction mechanism for the chlorination of The reaction mechanism for the chlorination of methane. How are the bonds made and broken?methane. How are the bonds made and broken?

H CH

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +

435435 431431

ΔH° = +4 kJ/molΔH° = +4 kJ/mol

ΔH° = -106 kJ/molΔH° = -106 kJ/mol

243243 349349

C HHH

HCl C ClH

H

HH Cl+ +

or hCl ΔH° = -102 kJ/molΔH° = -102 kJ/mol

Ener

gy (H

Ener

gy (H

))

-102-102

+4+4

-106-106

The reaction mechanism for the chlorination of The reaction mechanism for the chlorination of methane.methane.

ReactionReaction

H CH

H+Cl Cl ClH C

H

HCl +

H CH

HH + Cl H C

H

H+ ClH

+16+16

activation energyactivation energyΔHΔH‡ ‡ (E(Eaa))

An alternative mechanism can be postulated for An alternative mechanism can be postulated for the chlorination of methane. the chlorination of methane.

Cl + C HHH

HC ClHH

H+ H

H + Cl Cl ClH Cl+435435 349349


H CH

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +

431431435435


Ener

gy (H

)

H CH

HH + Cl H C

H

H+ ClH

+4+4

ReactionReaction

Cl + C HHH

HC ClHH

H+ H

86


H CH

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +

Cl Cl Cl2 or h

H CH

H+ HC

H

HH C

H

HCH

HH

initiationinitiation

Only one Cl• is Only one Cl• is required to produce required to produce many CHmany CH33Cl Cl molecules. This is a molecules. This is a radical chain radical chain mechanism.mechanism.

Cl


H CH

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +

Cl Cl Cl2 or h

H CH

H+ H C

H

HClCl

propagationpropagation


terminationtermination

Only one Cl• is Only one Cl• is required to produce required to produce many CHmany CH33Cl Cl molecules. This is a molecules. This is a radical chain radical chain mechanism.mechanism.

Cl Cl

H CH

HHH C

H

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +


H CH

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +

Cl Cl Cl2 or h

H CH

H+ H C

H

HClCl

Cl Cl

H CH

HClH C

H

HH

ClH

++ ++


Stereochemistry of chlorination.Stereochemistry of chlorination.

+H Cl

ClCl

Cl Cl

+Cl

+H Cl

Cl H


.

Cl

Cl Cl

+Cl

H

Cl

H

+


.Cl

H H

H Cl

Cl HCH3

H

Cl

Cl Cl

H

H Cl

+

Cl

Cl H

H

The reaction of the other halogens with alkanes.The reaction of the other halogens with alkanes.

H CH

HH + Cl H C

H

H+ ClH

H CH

H+Cl Cl ClH C

H

HCl +

435435 431431



243243 349349

H CH

H+ F F FH C

H

HF +

H CH

HH + F H C

H

H+ FH

435435 569569

ΔH° = - 134 kJ/molΔH° = - 134 kJ/mol

ΔH° = - 293 kJ/molΔH° = - 293 kJ/mol

159159 452452

Ener

gy (H

)

ReactionReaction

H CH

HH + Cl H C

H

H+ ClH+4+4

-134-134

H CH

HH + F H C

H

H+ FH

H CH

HH + Br H C

H

H+ BrH

+69+69

H CH

HH + I H C

H

H+ IH

-138-138

transition statetransition state

ΔH ΔH ‡‡

Not all C-H bonds have the same reactivity. Not all C-H bonds have the same reactivity. Secondary C-H bonds are more reactive Secondary C-H bonds are more reactive than primary C-H bonds.than primary C-H bonds.

actual yield: actual yield: 71%71%

29%29%

Cl2

Cl

Cl

h+

H H

H H

H

expected yield based on available hydrogens: 40% 60%


Cl2

Cl

Cl

h+

71%71% 29%29%

H

H

Cl ClCl

Cl

ClCl

1°1°

2°2°


Conclusion: 2° hydrogens are more Conclusion: 2° hydrogens are more reactive than 1°hydrogens.reactive than 1°hydrogens.


29%29%

Cl2

Cl

Cl

h+

H H

H H

H




29%29%

Cl2

Cl

Cl

h+

H H

H H

H


71=29

2°1°

60x40

=3.7 2°1°

reactivityratio ?=

Relative reactivities of C-H bonds in Relative reactivities of C-H bonds in chlorination.chlorination.

3°3° 2°2° 1°1°ClCl22 5.05.0 3.7 3.7 11

Bromination is more selective than Bromination is more selective than chlorination.chlorination.

Cl2

Cl

Cl

h+

71%71% 29%29%

Br2

Br

Br

h+

98%98% 2%2%

3°3° 2°2° 1°1°BrBr22 1600 1600 8282 11

Relative reactivities of C-H bonds in Relative reactivities of C-H bonds in Bromination.Bromination.

Relative reactivities of C-H bonds in Relative reactivities of C-H bonds in chlorination.chlorination.

3°3° 2°2° 1°1°ClCl22 5.05.0 3.8 3.8 11

Radical addition of H-Br to alkenes.Radical addition of H-Br to alkenes.

RO

RO OR

H Br

RO

ROH

OR

Br

+

+ +

Br + C C C C

Br

C C

Br

+ H Br C C

Br

H

Br + Br Br Br




Radical addition of H-Br to alkenes. Radical addition of H-Br to alkenes. Regiochemistry.Regiochemistry.

Br

Br

Br

Br

H

HH Br

H Br

majormajorproductproduct

RO OR

H Br

Br

RO

Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.

R

OO

RR

O

O

+ R



R

OO

RR

O

O

+ R



R

OO

RR

O

O

+

R

R

ethyleneethylene

polyethylenepolyethylene




R

OO

RR

O

O

+ RR

H


+R

OO

RR

O

O

R

H

R


+R

OO

RR

O

O

R


R

OO

RR

O

O

+n

ethyleneethylene polyethylenepolyethylene


R

OO

RR

O

O

+n

polystyrenpolystyrenee

styrenestyrene


R

OO

RR

O

O

+

F

F

F

F nF F

F F

teflonteflontetrafluoroethylenetetrafluoroethylene polytetrafluoroethylenpolytetrafluoroethylen

ee

DNADNA

Free radicals can damage cells.Free radicals can damage cells.

DNA DNA damagdamagee

Free radicalsFree radicals

DNA and RNA are DNA and RNA are polyesters. polyesters.

H OH

H CH3

H CH2OH

H° = 435

H° = 498

H° = 387

H

H

H OH

CH3

CH2OH+

+

+

Free radicals can damage cells.Free radicals can damage cells.

N

NN

N

NH2

O

HO

HHH

O

O

HO

HH

H

O

N

N

NH2

OPO

O H

H

R RH

N

NN

N

NH2

O

HO

HH

O

O

HO

HH

H

O

N

N

NH2

OPO

O H

H singlesingleDNADNA

strandstrandscissionscission

DNADNA

OH

OH

O

OH

+ H ΔΔH° = 336kJ/mol H° = 336kJ/mol

H O

H


Free Radical TrapsFree Radical Traps

Many chemicals can destroy free Many chemicals can destroy free radicals.radicals.

OOH

vitamin E.vitamin E.

BHTBHTOH

Vitamin CVitamin COH

OH

OH

OO

HO

OO

H

O2

O

O O

HOO

OO

H

There are necessary biological processes that There are necessary biological processes that proceed by free radical reaction mechaniamsproceed by free radical reaction mechaniams

Arachadonic Arachadonic acidacid

Prostaglandin Prostaglandin HH22

fattyfattyacidsacids

bloodbloodclottingclotting

inflammationinflammation

painpain


OO

H

O

H

H

O O

OO

H

OO

H

H

O O


O O

OO

H

O O

OO

H

O

O

O O

OO

H

O

OO

H

O O

HOO

OO

H


OO

H

O2

O

O O

HOO

OO

H


Arachadonic Arachadonic acidacid


.

Summary:Summary:Free radicals are among the few species that will react with C-H bonds.

Free radical reactions usually occur by a chain reaction mechanism.

C H + X X C X + XH

C H + X X C+H

C + X X C X + X

Free radical stability can be estimated from bond energies.

H O

H

H O H+

ΔH° = 498 kJ/mol

+C H H

H

H

H C

H

H

H

ΔH° = 435 kJ/mol

The reaction of chlorine with butane gives The reaction of chlorine with butane gives two monochlorinated derivatives.two monochlorinated derivatives.

Cl2two monochloro derivatives

(a) Give the structures of these two products.(a) Give the structures of these two products.(b) Postulate reaction mechanisms for their (b) Postulate reaction mechanisms for their formation.formation.(c) Predict the major product of the reaction.(c) Predict the major product of the reaction.



Cl2

Cl

+

Cl



H

Cl

+ H Cl

Cl Cl

Cl2

ClCl+



Cl Cl

Cl2

ClCl+

+ H Cl

+

Cl

Cl Cl

Cl

+ H Cl

H

The chlorination of (The chlorination of (RR)-2-butane gives a mixture )-2-butane gives a mixture of isomers. of isomers.

Cl HCl2

a mixture of chlorinated compounds

(a) Draw all of the isomers with the formula (a) Draw all of the isomers with the formula CC44HH88ClCl22..

(b) Circle those that are chiral.(b) Circle those that are chiral.

The chlorination of (The chlorination of (RR)-2-butane gives a mixture )-2-butane gives a mixture of isomers. of isomers.

(a) Draw all of the isomers with the formula (a) Draw all of the isomers with the formula CC44HH88ClCl22..

(b) Circle those that are chiral.(b) Circle those that are chiral.

Cl HCl2

Cl H

Cl +Cl Cl

+Cl H

Cl H

Cl H

H Cl

Cl H+

Cl

Al Kanes Radicals

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