Post on 10-Apr-2016
description
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
Bond Dissociation Energies.Bond Dissociation Energies.
H O
H
H O H+ ΔH° = 498 kJ/molΔH° = 498 kJ/mol
+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
Bond Dissociation Energies.Bond Dissociation Energies.
+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
ΔH° = 395 ΔH° = 395 kJ/molkJ/mol
+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
395 kJ/mol395 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
381 kJ/mol381 kJ/mol
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
ΔH° = 356 ΔH° = 356 kJ/molkJ/mol
ΔH° = 395 ΔH° = 395 kJ/molkJ/mol
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
ΔH° = +86 kJ/molΔH° = +86 kJ/mol
H CH
HH + Cl H C
H
H+ ClH
H CH
H+Cl Cl ClH C
H
HCl +
431431435435
ΔH° = +4 kJ/molΔH° = +4 kJ/mol
Ener
gy (H
)
H CH
HH + Cl H C
H
H+ ClH
+4+4
ReactionReaction
Cl + C HHH
HC ClHH
H+ H
86
The reaction mechanism for the chlorination of The reaction mechanism for the chlorination of methane.methane.
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
The reaction mechanism for the chlorination of The reaction mechanism for the chlorination of methane.methane.
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
initiationinitiation
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 +
The reaction mechanism for the chlorination of The reaction mechanism for the chlorination of methane.methane.
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
++ ++
ΔH° = -102 kJ/molΔH° = -102 kJ/mol
Stereochemistry of chlorination.Stereochemistry of chlorination.
+H Cl
ClCl
Cl Cl
+Cl
+H Cl
Cl H
Stereochemistry of chlorination.Stereochemistry of chlorination.
.
Cl
Cl Cl
+Cl
H
Cl
H
+
Stereochemistry of chlorination.Stereochemistry of chlorination.
.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
ΔH° = +4 kJ/molΔH° = +4 kJ/mol
ΔH° = -106 kJ/molΔH° = -106 kJ/mol
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%
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.
Cl2
Cl
Cl
h+
71%71% 29%29%
H
H
Cl ClCl
Cl
ClCl
1°1°
2°2°
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.
Conclusion: 2° hydrogens are more Conclusion: 2° hydrogens are more reactive than 1°hydrogens.reactive than 1°hydrogens.
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%
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%
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
initiationinitiation
propagationpropagation
terminationtermination
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
initiationinitiation
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+ R
propagationpropagation
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+ R
propagationpropagation
initiationinitiation
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+ R
propagationpropagation
initiationinitiation
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+
R
R
ethyleneethylene
polyethylenepolyethylene
terminationtermination
initiationinitiation
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+ RR
H
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
+R
OO
RR
O
O
R
H
R
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
+R
OO
RR
O
O
R
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+n
ethyleneethylene polyethylenepolyethylene
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
R
OO
RR
O
O
+n
polystyrenpolystyrenee
styrenestyrene
Free Radical Polymerization of Alkenes.Free Radical Polymerization of Alkenes.
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
H O H+ ΔH° = 498 kJ/molΔH° = 498 kJ/mol
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
There are necessary biological processes that There are necessary biological processes that proceed by free radical reaction mechaniamsproceed by free radical reaction mechaniams
OO
H
O
H
H
O O
OO
H
OO
H
H
O O
There are necessary biological processes that There are necessary biological processes that proceed by free radical reaction mechaniamsproceed by free radical reaction mechaniams
O O
OO
H
O O
OO
H
O
O
O O
OO
H
O
OO
H
O O
HOO
OO
H
Prostaglandin Prostaglandin HH22
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
.
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.
The reaction of chlorine with butane gives The reaction of chlorine with butane gives two monochlorinated derivatives.two monochlorinated 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
The reaction of chlorine with butane gives The reaction of chlorine with butane gives two monochlorinated derivatives.two monochlorinated 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.
H
Cl
+ H Cl
Cl Cl
Cl2
ClCl+
The reaction of chlorine with butane gives The reaction of chlorine with butane gives two monochlorinated derivatives.two monochlorinated 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.
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