Chemistry 125: Lecture 40 January 15, 2010 Predicting Rate Constants, and Reactivity - Selectivity...
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Transcript of Chemistry 125: Lecture 40 January 15, 2010 Predicting Rate Constants, and Reactivity - Selectivity...
Chemistry 125: Lecture 40January 15, 2010
Predicting Rate Constants, and Reactivity - Selectivity Relation.
Rates of Chain Reactions. This
For copyright notice see final page of this file
-10
-8
-6
-4
-2
0
2
Ph Me Et iPr tBu Allyl
H
Me
Et
iPr
tBu
R-X BDE : Alkyl Variation in Detail
X
R
Me-R
Et-R
i Pr-R
t Bu-R
H-R
If this trend is due to radical stabilization by substitution, other X-R bond strengths should show the same trend.
BD
E r
elat
ive
to C
H3X
(kc
al/m
ole)
12.2
8.3
5.0
2.3
t Bu-R
MolecularMechanics
Strain Energies inStarting Material
t-Bu t-Bu
van der Waals Energy 26.9 kcal/mole5.2
“Idealized”Bond
Lengthsand
Angles
“Relaxed”Structure
Crunch!“steric hindrance”
van der Waals Energy drop by 16.8 to 5.2 kcal/mole
comes at the expense of bond stretching and bending.
van der Waals Energy drop by 16.8 to 5.2 kcal/mole
1.52Å
1.57Å
109.5°
112.3°
comes at the expense of bond stretching and bending.(which total 4.8 kcal/mole)
van der Waals Energy drop: 16.8 to 5.2 kcal/mole
ResultantTotal Strain (incl. 2.2 torsion) 12.2 kcal/mole
-10
-8
-6
-4
-2
0
2
Ph Me Et iPr tBu Allyl
H
Me
Et
iPr
tBu
R-X BDE : Alkyl Variation in Detail
X
R
For X = alkylalmost all of the Me to t-Bu change is
due to strain energy inthe starting material.
Me-R
Et-R
i Pr-R
t Bu-R
H-R
If this trend is due to radical stabilization by substitution, other X-R bond strengths should show the same trend.
BD
E r
elat
ive
to C
H3X
(kc
al/m
ole)
12.2
8.3
5.0
2.3
t Bu-R
0.8
1.51.9
2.3Me-R
molecularmechanics
strain energies
1.9
1.5
0.8
0
9.9
kcal
/mol
e di
ff. i
n in
itia
l Str
ain
w
hole
8.9
kca
l/m
ole
diff
. in
BD
E
Dit
to1.
5
2.6
But not for H-R
-10
-8
-6
-4
-2
0
2
Ph Me Et iPr tBu Allyl
Me
Et
iPr
tBu
R-X BDE : Corrected for R-X Strain
X
R
Me-REt-Ri Pr-Rt Bu-R
BD
Eco
rr r
elat
ive
to C
H3X
(kc
al/m
ole)
Alternative to hypothesis of radical stabilization by substitution
Intrinsic C-C bond strength (corrected for strain) is practically insensitive to substitution.
-10
-8
-6
-4
-2
0
2
Ph Me Et iPr tBu Allyl
H
Me
Et
iPr
tBu
R-X BDE : Corrected for R-X Strain
X
R
Me-REt-Ri Pr-Rt Bu-R
H-R
Alternative to hypothesis of radical stabilization by substitution
But C-H bonds are weakened by alkylation of the carbon.
Intrinsic C-C bond strength (corrected for strain) is practically insensitive to substitution.
BD
Eco
rr r
elat
ive
to C
H3X
(kc
al/m
ole)
-7
-5
-3
-1
1
3
5
Ph Me Et iPr tBu Allyl
H
Me
Et
iPr
tBu
Cl
Br
I
R-X BDE : Corrected for R-X Strain
X
R
Intrinsic C-C bond strength (corrected for strain) is practically insensitive to substitution.
Me-REt-Ri Pr-Rt Bu-R
H-RBut C-H bonds are weakened by alkylation of the carbon.
Alternative to hypothesis of radical stabilization by substitution
While C-Cl and C-Br are strengthened by alkylation of the carbon.
Cl-RBr-R
I-R
BD
Eco
rr r
elat
ive
to C
H3X
(kc
al/m
ole)
No one I know of understands this, but the textbooks are clearly wrong.
Can we use energies of stable structures that we “understand” to
infer the energies of other structures (e.g. transition states),so as to predict reactivity?
How can we predict activation energy?
Might exothermic reactions be faster than endothermic ones?
How can we predict activation energy?
This is no easy task a priori, especially when interaction with
solvent is important.But often one can say something
sensible about relative values of Ea (or G‡).
Compared to What?
The Hammond Postulate (1955)
George S. Hammond (1921-2005)
“If two states, as for example, a transition state and an unstable interme-diate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the molecular structures.”
This stimulated organic chemists to think about transition states and try to generalize plausibly
about reaction coordinates.
by p
erm
issi
on, E
. Men
ger
The more exothermic a reaction - the more similar the transition state to starting material
(in both energy and structure)
StartingMaterial
Product
endoproduct
At least among one-step reactions
that are closely analogous, such as
X• + H-R.
.X-H + •R…..
The more exothermic a reaction - the more similar the transition state to starting material
(in both energy and structure)
There is “likely” a continuum
between starting material and product with respect to the factors that
influence stability.
endoproduct
An effect mostly influencing the
energy of the product of an endothermic
reaction should have a similar (slightly
smaller) influence on its (late) transition
state.
Rates of slower reactions should be more sensitive to overall G!
An effect mostly influencing the
energy of the product of an exothermic
reaction should have a small influence on its (early) transition
state.
e.g. resonance stabilization
HPhCH2
PhCH2
H
PhCH2
Reactivity/Selectivity “Principle”
More Reactive
Less Reactive
More Selective
Less Selective
‡
G 1
G Reactivity‡
k e -G /RT 10
13 - 3/4 G /sec‡ ‡
G Relative Reactivity‡
k1 e -G /RT‡
1
k2 e -G /RT‡
2
k1/k2 = e -G /RT 10
-3/4 G‡ ‡
‡
G 2
‡
Gor G -4/3 (log(k) -13)
‡
or G -4/3 log(k1/k2)‡
Rates converge with increasing T Increase discrimination by lowering T !
Reactivity & Selectivity
+ X•H3C-CH2-CH3
+ HXH3C-CH2CH2
•
+ HX
•H3C-CH-CH3
43% X = Cl• 57%
8% X = Br• 92%
‡G 4/3 log(4) = 0.8 kcal/mole
‡G 9/3 log(35) = 4.5 kcal/mole@ 330°C( N.B. : Br• shows greater selectivity despite increased T )
k1° 6 k2° 2Note correction for
number of primary & secondary H atoms.
k2°/k1° = 4
k2°/k1° = 35(3 14)
n-Pr-X i-Pr-X
136.3103.287.571.3
1°
35.22.1
13.629.8
FClBrI
101.1”””
Reactivity & Selectivity
2°
37.74.6
11.127.3
98.6”””
136.3103.287.571.3
+ X•
+ HX + HXH2C-CH2CH3
•
H3C-CH2-CH3
•H3C-CH-CH3
Reactivity/Selectivity “Principle”
More Reactive
Less Reactive
More Selective
Less Selective
22.14.6 Cl
713.611.1Br
1° abstraction
2° abstraction
11.1
13.6
-4.6
-2.1
k2/k1 ~ 4 k2/k1 ~ 35
G‡ 4.5G‡ 0.8
Reactivity/Selectivity “Principle”
Less Reactive
More Selective
713.611.1Br
1°
2°
11.1
13.6
k2/k1 ~ 35
G‡ 4.5
How can transition states be more different than
products are?
Br H •R
Br• H R
Br– H• R+
Maybe polar character helps transition states.
(2° cations are much more stable than 1°)
G 2.5!
Sometimes factors involved in stabilizing Transition States can
be different from those involved in stabilizing either starting materials or products.
Chain H-X Addition to Alkene
C=C
H-X
•X •C-CX
C-CX H
cyclicmachinery
X• + C=C(+ X-H)
X-C-C-H+ X•
0
10
-10
-20
-30
-40
-50 RX HX116 13581 10368 8752 71
FClBrI
X-C-C•(+ X-H)
83146 99
Chain H-X Addition to Alkene
83
Only HBr works fast enough in
both steps.
Average Bond Energies
146135
8399
116
281 298 = 17
But only HBr works.Why?
End of Lecture 40Jan. 15, 2010
Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
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