ChE 551 Lecture 28
Solvents As Catalysts
1
Solvents As Catalysts
Literature does not usually consider
solvents to be catalysts but I think of
them as catalysts.


Solvents can stabilize intermediates
Solvents can raise rates by 1040
2
Solvents as Catalysts


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Solvents can initiate reactions
Solvents stabilize intermediates
Solvents stabilize transition states and thereby
modify the intrinsic barriers to reactions
Solvents act as efficient means for energy
transfer
Solvents can donate or accept electrons
Mass transfer limitations are more important
when solvents are present.
3
Effect of Solvents


Large for ionic reactions
Small for radical reactions
4
Examples of the Role of Solvents
Table 13.1 The rate of the Deils Alder reaction (2 cyclopentadiene  cyclopentadiene dimer) at
300 K. Data of A. Wasserman, Montash Chem, 83 (1952) 543.
Solvent
gas phase
Rate
constant,
lit/molesec
6.9  10-7
ethanol
19  10-7
nitrobenze 13  10-7
ne
paraffin oil 9.8  10-7
Solvent
Rate
constant,
lit/molesec
9.3  10-7
carbon
disulfide
tetrachloro 7.9  10-7
methane
benzene 6.6  10-7
"neat
liquid"
5.2  10-7
Effect small for non-ionic reactions.
5
Example of the Role of Solvents
Table 13.2 The rate of the SN2 reaction NaCl + CH3I  CH3Cl + NaI at 350 K. Data from A. J.
Parker, Chem Rev 69 (1969) 1. The gas phase rate in the table is estimated from the abinitio
calculations of Glukhovsten et al
Solvent
Rate
Solvent
constant,
lit/molesec
gas phase about 10-45
Water
3.5  10-6 methylcya
nide
Methanol 3.1  10-6 dimethylfo
rmamide
Rate
constant,
lit/molesec
0.13
2.5
Big effect for ionic reactions
6
More Examples
Solvent dielectric
constant
hexane
benzene
chlorobe
nzene
methoxy
benzene
acetone
nitroben
zene
1.89
2.28
5.62
(C2H5)3N + CH3COOCOC
H3 +
CH3I
[(C2H5)3NCH 2 C2H5OH
+
2
3] +[I]
CH3COOC2H5
0.0119
0.5  10-5
0.0053
39  10-5
0.0046
160  10-5
9
400  10-5
0.0029
20.7
35
265  10-5
1383  10-5
0.0024
Intermediate effect when Ions form during reaction.
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Ionic Strength Also
Affects Rate
0.6
A
A.
B.
C.
D.
E.
ln(rate of reaction)
0.4
B
0.2
C
0
Co (NH3)5 Br2+ + Hg2+ 
S2O82- + I- 
[(Cr(urea)6]3+ + H2O 
Co(NH3)5 Br2+ + OH-  products
Fe2+ + Co (C2 O4)3-  products
-0.2
D
-0.4
E
0
0.05
0.1
0.15
0.2
Sqrt(ionic strength, moles/lit)
8
Why Do Solvents Change Rates?
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Solvents stabilize intermediates
Solvents can initiate reactions
Solvents stabilize transition states and thereby
modify the intrinsic barriers to reactions
Solvents act as efficient means for energy
transfer
Solvents can donate or accept electrons
Mass transfer limitations are more important
when solvents are present.
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Example: Cl+CH3BrClCH3+BrTransition
state
8 0
Enthalpy, Kcal/mole
complex
complex
Gas Phase
6 0
88 kcal/mole
4 0
Transition
state
2 0
24.6 kcal/mole
0
Aqueous Solution
Figure 13.3 A comparison of the free energy changes
during the reaction Cl- + CH3Br  ClCH3 + Br-. Data
from p131 in Reichardt[1988]
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SN2 Reactions Prefer Polar Aprotic
Solvents
Table 13.2 The rate of the SN2 reaction NaCl + CH3I  CH3Cl + NaI at 350 K. Data
from A. J. Parker, Chem Rev 69 (1969) 1. The gas phase rate in the table is estimated
from the abinitio calculations of Glukhovsten et al
Solvent
Rate
constant,
lit/molesec
gas
phase
water
about
10-45
methano
l
Solvent
Rate
consta
nt,
lit/mol
e-sec
3.5  10-6
methylcya
nide
0.13
3.1  10-6
dimethylfo
rmamide
2.5
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SN1 Reactions prefer Protic Solvents
Table 13.5 The rate constant a prototypical SN1 reaction: the solvolysis of p-methoxyneophyltolunesulfonate in a number of solvents at 75 C. Data of Smith, Fainberg and Winstein JACS 83
618 (1961)
Solvent
formic
acid
water
acetic acid
k, min-1
7.1
methanol
ethanol
C7H15COO
H
DMSO
nitrometha
ne
0.1
3.810-2
4.410-3
4.0
0.19
1.110-2
7.210-3
Solvent
methylcya
nide
DMF
acetic
anhydride
pyridine
acetone
ethylacetat
e
dioxane
diethylethe
r
k, min-1
3.610-3
3.010-3
2.010-3
1.310-3
5.110-4
6.810-5
5.110-5
~310-6
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Basis For Which Solvent Is Best
Want the solvent to
 Stabilize intermediates
 Lower the energy of the transition state
relative to the reactants
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How Do Solvents Affect The Stability Of
Transition States?


Reactants and transition state can have
different solvation energy.
If solvation energy of transition state is
greater than the solvation energy of the
reactants, the transition state will be
stabilized by the solvent

Leads to a higher rate
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Solvation Energy


Energy to take a molecule out of a pure
phase and put the molecule into a
solution
Three step process

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Remove molecule from pure solute phase
Create a hole in solvent to hold solute
Transfer the solute into the hole
 Gain energy due to solute/solvent
attractions
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Solvation Energy
Solvation energy= (Energy of the solutesolvent bonds formed) – (Energy of the
solvent-solvent bonds broken) – (Energy
of solute-solute bonds broken)
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Eaxmple: Solubility Of Salt In Water
Table . Solubility of some alkyl
halides in water
salt
solubility, moles/liter
Lattice energy (Hsolute),
kcal/mole
LiF
0.10
247
NaCl 6.11
188
KBr
4.49
167
CsI
1.69
144
LiI
12.32 174
CsF
24.16 177
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Which Solvent to Use?
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Protic solvents: good for SN1 reactions of
anions water, ethanol, methanol, acetic acid,
formic acid, ammonia, ethane thiol
Polar Aprotic solvents: good for SN2 reactions
of anions acetone, dimethyl sulfoxide (DMSO)
[(CH3)2S=O], dichloromethane, ethers,
Dimethylformamide (DMF) [(CH3)2NCHO],
cyclohexanone, acetaldehyde
Non-polar Aprotic solvents: good for radical
reactions ethylene, benzene
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Hydrophobic Effects:
SN2 - TST larger than reactants (want
aprotic solvent)
SN1 -TST smaller than reactants (want
protic solvent)
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Hughes-Ingold Rules
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If the transition state for a reaction has a larger
charge than the reactants, then the rate of reaction
will increase as the polarity of the solvent increases.
If the transition state for a reaction has a smaller
charge than the reactants, then the rate of reaction
will decrease as the polarity of the solvent increases.
If the net charge remains the same, but the charge is
dispersed, then there will be a small decrease in rate
as the polarity of the solvent increases.
If the net charge remains the same, but the charge is
localized, then there will be a small increase in rate as
the polarity of the solvent increases.
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Equations For Solvent Effect

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Single sphere model Assume solvent-solute
Double sphere model interactions control rate
Regular solution theory
Assume
hydrophobic
interactions control
rate
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Double Sphere Model
YYR
rAB
‡
rAB
R3C+
R3C+
Reactants
2
 kS 
Z
Z
e
BT ln   G‡  A ‡B
rAB
 k 
Transition
State
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Single Sphere Model
2
2
2

 kS 




B 
   1   A 
‡ 
‡
‡


BTln
 GS  Gg  



3
3
3 
 kg 
 r 
2


1






r
r
B
‡
 
 A

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Fit to Data Mixed
(CH 3CH 2 ) 3 N + CH 3CH 2 I  [(CH 3CH 2 ) 3 NI]- +[CH 3CH 2 ]+ Kirkwood constant
Kirkwood constant
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.1
0.2
0.3
0.4
0.5
0.6
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22.5
22.4
26
22.3
25
 G‡s, kcal/mole
 G‡s, kcal/mole
22.2
22.1
22
21.9
21.8
21.7
24
23
22
21
21.6
20
21.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
(dielectric constant)-1
Figure 13.6 A plot of G‡s for reaction 13.36
in a series of dioxane acetone mixtures.
() data plotted against 1 
() data plotted against (1) (21)
0.0
(dielectric constant)-1
Figure 13.7 A plot of G‡s for reaction 13.36
in a series of different solvents.
() data plotted versus 1 
() data plotted versus (1) (21)
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Regular Solution Theory

Assume hydrophobic interactions
dominate
E cavity  VA  (CEDs)
After pages of algebra
G  G
  V      V    

‡
S
‡
ideal
2
‡
‡
S
A
A
2
S
 VB  B   S  
2
A  (CEDA )1/ 2
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Predictions
1E+1
rate constant
1E+0
1E-1
1E-2
1E-3
1E-4
1E-5
1E-6
5
10
15
20
25
Solubility parameter
SN1 reaction: the solvolysis of p-methoxyneophyl-toluene-sulfonate
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Summary
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Solvents are catalysts
Solvents enhance initiation reactions, stabilize
intermediates, transition states
Key energy: solvation energy


includes solvent-solvent, solvent-solute, solute-solute
interactions
Leads to large variations in rate for ionic
reactions

Much smaller effects for radical reactions
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Query

What did you learn new in this lecture?
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