CHAPTER 2 SOLVENT EFFECTS ON ORGANIC
RATES AND EQUILIBRIA
References
1.
Carey and Sundberg, 5th ed section 3.8
2.
Carroll, p. 329
3.
L & R pp. 177-189; 335-340
4.
Isaacs Chapter 5
5.
E. Buncel and H. Wilson, J. Chem. Ed., 57, 629(1980).
Importance
a)
Changes in solvent can produce changes in reaction rate of up to 1010 or more.
Some understanding of solvent effects is therefore useful in minimizing reaction
times, or maximizing yields where two or more products are formed by competing
pathways
b)
Solvent effects on reaction rate can yield information about the reaction
mechanism (specifically, information about the TS structure).
I SOLVENTS AND SOLVATION
Classification of solvents:
a)
b)
polar
dielectric constant (ε) greater than about 15
nonpolar
ε < about 15
protic
contain a proton attached to N or O that is usually rapidlyexchangeable in D2O and that can form a hydrogen bond with an
electron pair donor.
aprotic
no rapidly-exchangeable proton
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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Some common solvents are classified in this way in the Table.
Classification of Solvents
Dipolar Protic
ε
Nonpolar Protic
ε
H2O
78.5
CH3CO2H
6.2
HCO2H
58.5
(CH3)3COH
12.5
CH3OH
32.7
cyclohexanol
15.0
CH3CH2OH
24.6
phenol
9.8
HC(O)NH2
111.0
Dipolar Aprotic
Nonpolar Aprotic
acetone
20.7
CCl4
2.2
HC(O)N(CH3)2 (DMF)
36.7
pyridine
2.4
CH3S(O)CH3
46.7
n-hexane
1.9
CH3CN
37.5
cyclohexane
2.0
CH3NO2
35.9
ethyl ether
4.3
[(CH3)2N]3PO (HMPA)
30
dioxane
2.2
CH3OCH2CH2OCH3 (DME)
7.2
benzene
2.3
THF
7.6
Solvation involves the same forces as are involved in all intermolecular interactions,
namely dispersion (London), dipole-dipole, and ion-dipole forces. In general, solutes
and solvents in which the intermolecular forces are of the same kind and similar in
magnitude are mutually-miscible (i.e., "like dissolves like").
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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Two generalizations:
a)
Uncharged solutes are usually more soluble in organic solvents than in water,
while ionic solutes are usually more soluble in water.
b)
Salts (at least at lower concentrations) tend to make ionic solutes more soluble
and uncharged solutes less soluble.
Ions are solvated by ion-dipole interactions. Consider, for example, a solvent such as
methanol or water:
+
+
-
-
+
+
-
- M
-
-
+
-
+
X
R
+
O
-
H
+
-
+
+
-
Solvation of ions is stronger the more concentrated the charge. Ionic or dipolar solutes
are stabilized by ion-dipole or dipole-dipole attractions in dipolar solvents. With anions,
particularly small anions, hydrogen bonding is particularly important.
The hydrogen
bond is a dipole-dipole attraction but is stronger than other dipole-dipole attractions
because the positive end of the bond dipole (the proton) is very exposed and permits
very close approach.
Dipolar aprotic solvents such as dimethyl sulfoxide (DMSO)
are not nearly as effective at solvating anions because the positive end of the dipole is
less exposed.
-
CH3
Oδ
+
Sδ
CH3
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II QUALITATIVE ELECTROSTATIC MODEL OF SOLVENT EFFECTS ON RATES
(Hughes & Ingold)
The largest solvent effects are seen where reactant and transition state differ greatly in
polarity, i.e., when charges are developed or neutralized in going from reactants to the
transition state. Hughes and Ingold reasoned that reactions leading to increased charge
separation in the transition state should be increased in rate by increased "solvent
polarity" (increased ion-solvating ability) while those leading to decreased charge
separation should respond in the opposite way.
These ideas were developed for
nucleophilic substitution reactions and are summarized in the Table.
Solvent Effects in Nucleophilic Substitutions (Hughes & Ingold)
Reactants
T.S.
+
δ
R
R X
X
+
R X
δ
R
+
R X
δ
Y + R X
δ
-
Y +
-
Y + R X+
-
Y + C O
X
change in charge
distribution
effect of increased “solvent
polarity” (ion solvating
ability)
-
increased charge
separation
large increase
+
charge dispersal
small decrease
−
charge dispersal
small decrease
−
increased charge
separation
large increase
+
decreased charge
separation
large decrease
charge dispersal
small decrease
δ
δ
−
Y
R
X
+
δ
Y
R
X
−
Y
δ
δ
R
X
C
δ
O
−
δ
Y
δ
−
These rules very often lead to a correct qualitative prediction of a change in solvent
polarity on the rate. The reason they work is that they usually correctly predict the
solvent effect on the difference G‡ - G reactants (ΔG‡) even though both may increase
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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or decrease.
For example, in the solvolysis of benzyl chloride in methanol-water
mixtures, the rate increases by a factor of 200 in going from 100% methanol to 100%
water.
Increasing the % H2O actually increases both γR-Cl and γT.S., but γT.S. is
increased less than γR-Cl.
More examples of kinetic solvent effects follow.
Me3S+ + OH- → MeOH + Me2S
(Et)3N + EtI → (Et)4N+ + I-
% EtOH (v/v) in
H2O/EtOH system
rel. rate (100 °C)
solvent
ε
rel. rate (100 °C)
0
1.00
hexane
1.9
1
60
4.01 X 10
benzene
2.3
8.1 X 10
80
4.82 X 102
acetone
20.5
8.4 X 102
100
1.96 x 10 4
nitrobenzene
34.6
2.8 X 103
Acetyl Peroxide Decomposition
nBu-I + *I-→ nBu-I* + I-
medium
rel. rate (85 °C)
solvent
ε
rel. rate (25 °C)
gas
1.0
methanol
32.7
0.20
cyclohexane
0.57
ethanol
24.2
1.00
isooctane
0.67
n-butanol
17.3
5.1
benzene
0.72
n-hexanol
12.8
5.7
acetic acid
0.58
n-dodecanol
6.15
6.8
propionic acid
0.74
carbon tetrachloride
0.54
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tBuCl → tBuOS + HCl
MeI + Cl- → MeCl + I-
(solvolysis in SOH)
SOH
rel. rate
(25 °C)
solvent
rel. rate (25
°C)
EtOH
1
methanol
1.0
MeOH
9.3
formamide
1.6 X 10
EtOH/H2O = 80/20 (v/v)
1.08 x 102
N-methyl
formamide
4.5 X 10
EtOH/H2O = 60/40 (v/v)
1.43 x 103
N,N-dimethyl
formamide
7.9 X 105
EtOH/H2O = 40/60 (v/v)
2.02 x 104
N,N-dimethyl
acetamide
2.5 X 106
H2O
5.5 x 105
N-methylpyrrolidine
7.9 X 106
acetonitrile
4.0 x 104
acetone
1.6 X 106
nitromethane
1.6 X 104
The qualitative rules do not correctly predict the large differences between aprotic and
protic solvents in reaction involving anions (see CH3I + Cl- example). The term "solvent
polarity" is not precisely defined. Several solvent properties, particularly the ability of
the solvent to solvate anions and cations, must be considered.
The origin of the
dramatic increase in reactivity of anions in changing from a protic to an aprotic solvent is
the poor solvation of anions in aprotic solvents.
The qualitative rules can also lead to an incorrect prediction for reactions of ionic
nucleophiles Y- unless the reactions compared actually involve free Y-. In very poorly
ion-solvating media the nucleophile may be present entirely as ion pairs, M+Y-, or higher
aggregates.
In such situations reaction of Y- with a substrate RX can be strongly
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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increased in rate by an increase in solvent ion-solvating ability by increasing the
concentration of much more reactive Y- ions.
III EMPIRICAL SCALES OF ION-SOLVATING ABILITY
The solvent dielectric constant is in general a very poor indicator of ion-solvating ability
unless the solvents are very similar in structure.
H Br
H OS
SOH
+ HBr
-
Br
H
δ
+
via this T.S.:
δ
SOH = H2O, EtOH, CH3CO2H, etc.
Nitrobenzene (ε = 35) added to ethanol (ε = 25) decreases the rate. Acetic acid (ε =
6.1) solvent reacts faster than ethanol (ε = 25).
The reason for these seemingly
anomalous observations is that solvation of the anion and cation are both important.
Ethanol (protic solvent) solvates the incipient Cl- better than nitrobenzene, while acetic
acid is a better anion-solvating solvent than ethanol.
There are several quantitative scales of ion-solvating ability based on model reactions
or physical processes (analogous to σ values for substituent effects).
These ion-
solvating power or "polarity" scales can be used to develop linear free energy
relationships for solvent effects.
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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A GRUNWALD-WINSTEIN Y VALUES
model reaction:
H3C
H3C C Cl
H3C
H3C
+
δC
SOH
25 °C
Cl
δ
H3C
H3C C OS
H3C
+ HCl
-
H3C CH3
T.S.
Y = log (k/ko)t-BuCl, 25°
where k = rate constant in the solvent whose Y value is to be measured.
ko = rate constant in the standard solvent, 80% aqueous ethanol, i.e., 80/20 (v/v)
ethanol/water.
Y = 0 (by definition) for 80% ethanol-water, positive for solvents of greater ionizing
power, and negative for solvents of lower ionizing power.
The Y scale is limited to solvolyzing (i.e., protic) solvents. It therefore cannot be used to
correlate or predict rate constants in aprotic solvents. Y values for some common
solvents are listed in the table on p. 2-11.
B KOSOWOR Z VALUES
O
C
OMe
O
+
-
I
+
N
CH2CH3
C
OMe
-
I
+
N
CH2CH3
charge-transfer complex
N-ethyl-4-carbomethoxypyridinium iodide
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The spectrum of N-ethyl-4-carbomethoxypyridinium iodide (and also some other
quaternary pyridinium and related halides) shows a strong maximum in the UV-visible
which is very sensitive to solvent (examples: λmax = 342 nm in methanol, 450 nm in
CHCl3). The new maximum is absent in the chloride or bromide and the absorbance
does not follow Beer's law. The absorption corresponds to a charge-transfer transition
corresponding approximately to
-
Excited State
Py.I.
hν
Py+I
Py+I
.
minor
Py.I.
major
hν (charge transfer
Ground State
Py+I
.
major
Py.I.
minor
Since the ground state is stabilized by solvation of ions, the energy of the transition
increases with increasing ion-solvating ability.
Z = ΔE
= Nhν =
Nhc
λ
in kcal/mol
hνct
hνct
CHCl3
MeOH
Some values are listed in the Table on p. 2-11.
(reference: E.M. Kosowor, J. Am.
Chem. Soc. 80, 3253(1958).)
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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C DIMROTH ET VALUES
reference: C. Reichardt, Angew. Chem. Intl. Ed. Engl., 4, 29(1965)
(a review of solvent "polarity" scales)
Ph
-
O
N+
Ph
Again, the long wavelength maximum is very
Ph
sensitive to solvent because there is less
Ph
Ph
charge separation in the excited state than in
the ground state. ET is the transition energy in
kcal mol-1 (as with Z values).
For reactions of unknown mechanism a plot of log k vs Y, Z, or ET gives some idea of
the sensitivity of reaction rate to ion-solvating ability and therefore some indication of
the difference in charge separation between the reactant(s) and transition state.
Interpretation of the slopes of such plots requires a comparison with slopes for reactions
of "established" mechanisms or well-characterized transition states. The slopes are
analogous to ρ values, which measure sensitivity to substituent effects. Again, we are
comparing the reaction to some model reaction or model process. If the correlation is
good, the model is a good one.
The original Dimroth solvent parameters are called ET(30) values in more recent
papers. Several new betaine dyes having different substituents on the pyridinium and
phenoxide rings (compared to the structure noted above) have been synthesized and
the new compounds have permitted considerable extension of the original scale.
Reichardt & Harbusch-Görnet Liebigs (Ann., 721 (1983)) have defined a scale called
ETN values based on all of these compounds. In the ETN scale, tetramethylsilane is
assigned a value of 0.00 and water a value of 1.00, so that all other values fall between
0 and 1.0. ETN values for 243 solvents are listed.
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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Selected Solvent Polarity Parameters
ET
Z
Y
63.1
94.6
3.49
formic acid
-
-
2.05
trifluoroacetic acid
-
-
1.84
trifluoroethanol
-
-
1.06
80/20 (v/v) EtOH/H2O
53.6
84.8
0.00
methanol
55.5
83.6
-1.09
ethanol
51.9
79.6
-2.03
acetic acid
51.2
79.2
-1.64
1-propanol
50.7
78.3
-2.73
nitromethane
46.3
71.3
-
acetonitrile
46.0
71.2
-
DMSO
45.0
71.1
-
DMF
43.8
68.4
-3.5
acetone
42.2
65.5
-
chloroform
39.1
63.2
-
ethyl acetate
38.1
59.4
-
THF
37.4
58.8
-
ethyl ether
34.6
-
-
benzene
34.5
54
-
carbon tetrachloride
32.5
-
-
n-hexane
30.9
-
-
Solvent
water
Y values: Fainberg & Winstein, JACS 78, 2770(1956)
ET values: Reichardt Angew. Chem. Int. Ed. Engl. 4, 29(1965)
Z values: Kosower JACS 78, 3253(1956)
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IV PHASE TRANSFER CATALYSIS
references: W.P. Weber and G.W. Gokel, Phase Transfer Catalysis in Organic
Synthesis, Springer-Verlag, Berlin, 1977.
C.M. Starks and C. Liotta, Phase Transfer Catalysis, Principles and
Techniques, Academic Press, N.Y. 1978.
Phase transfer catalysis is a way of carrying out reactions between an organic
compound, soluble in an organic solvent, and an ionic compound not appreciably
soluble in the organic solvent. The reaction is carried out in a two-phase
organic/aqueous system using a large quaternary ammonium or related salt (phase
transfer catalyst) which carries the ionic reactant into the organic phase as an ion pair.
Using this method it is possible to carry out reactions ordinarily requiring a scrupulously
anhydrous medium.
example:
+
-
Na OH in H2O
aqueous phase
organic phase
PhCH2CN + CH3I in CH2Cl2
No reaction occurs until PhCH2N+Et3 Cl- is added. After addition of the phase transfer
catalyst the reaction is over in a few minutes.
Procedure:
product: Ph CH(CH3)CN
Add aqueous Na+OH- + R4N+OH- to a solution of PhCH2CN + CH3I in
CH2Cl2 and stir.
General Mechanism:
+
aqueous phase
organic phase
-
-
Q Cl + OH
+
Q PhCHCN + H2O
+
Q PhCHCN + CH3I
+
-
+
-
-
[Q OH ] + Cl
[Q OH ] + PhCH2CN
+-
PhCHCN + [Q I ]
CH3
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The method depends on the fact that R4N+OH- is much more soluble in organic
solvents than Na+OH-.
another example:
aqueous phase
+
-
Cl Cl
+
Na OH , PhCH2N Et3, in H2O
organic phase
C C
C C
in CHCl3
Crown ethers and cryptands can also be used a phase transfer catalysts:
+
-
Na OH
+ crown
[crown.Na ] OH
+
-
(soluble in organic phase
V SOLVENT EFFECTS ON PRODUCT COMPOSITION
The solvent can have a large effect on the product composition in reactions where two
or more products are produced by competing pathways that respond differently to
changes in solvent. One familiar example is competition between concerted nucleophilic
substitution (SN2) and nucleophilic substitution by a stepwise ionization (SN1)
mechanism. Depending on the substrate and nucleophile, the solvent can effect the %
rearrangement or % racemization, both of which require a carbocation intermediate.
Two other examples are shown:
(i)
Concerted vs Ionic Cycloaddition
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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CN
+
Cl
Cl
C
C
Cl
+
+
CN
CN
NC
Cl
2
1
in: benzene
acetonitrile
3
50%
9%
38%
26%
2%
64%
Adduct 1 is produced by a thermally-allowed π2s +
π2s + π2s cycloaddition, while 2 and 3 are
presumed
to
arise
from
the
+
accompanying
C
zwitterionic intermediate.
-
C CN
Cl
(ii) Reactions of Ambident Anions
Ambident anions have two or more nucleophilic sites and can react with alkyl halides,
acyl halides, etc. at two or more positions.
examples:
-
-
O
O
N
CN
-
O
O
R
-
O
-
O
-
C
H
O
R
R
etc.
O
C
H
R
etc.
enolate anions
General rule:
the more free the anion the greater the tendency to react at the
most electronegative atom
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For enolate anions, protic solvents strongly solvate the oxygen site favouring Calkylation. Polar aprotic solvents, by leaving the anion relatively unsolvated, favour Oalkylation.
Examples:
O
Ph
in
C
O
Ph
+ CH3I
C
- Ph
tBuOH
Ph
C
OCH3
Ph
C
Ph
C Ph + Ph
C
CH3
Ph
96%
DME/DMSO
4%
50%
50%
CH2Ph
-
O
in:
OCH2Ph
PhCH2Br
OH
+
DMF
97%
0%
methanol
57%
24%
CF3CH2OH
7%
85%
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PRACTICE PROBLEMS 3
1. Predict the effect on the rate accompanying a change to a more polar solvent. Is
the rate change considered to be large or small?
a)
Et2S + EtBr
Et3S+Brb)
-
OH + Et4N+
H2O + CH2 CH2 + Et3N
c)
CH2 CH2 + Br2
BrCH2 CH2Br
via rate determining bromonium ion formation
2. Account for the trends in Keq for the following equilibrium.
O
O
Keq
O
OH
solvent
Keq
% enol
n-hexane
19
95
1,4-dioxane
4.6
82
methanol
2.8
74
3. Consider the accompanying substitution reaction, which follows the additionelimination mechanism introduced in CHEM*3750.
Br
Br N3
+
NO2
Na+N3-
N3
+
-
NO2
Na+Br-
NO2
When the reaction is done in DMF, the rate of disappearance of the starting material is
rapid. If increasing incremental amounts of water are added to the mixture, the reaction
slows down. Please explain.
4. Rank the following compounds for their propensity to solvolyze under the conditions
indicated (fastest to slowest). Give reasons for your rankings. The temperature is the
same in each instance.
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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Situation A
Situation B
Situation C
Situation D
CH3O
CH3
Cl
CH3
CH3O
80% EtOH/20% H2O
CH3
Cl
CH3
80% EtOH/20% H2O
CH3O
CH3O
CH3
Cl
CH3
CH3
Cl
CH3
50% EtOH/50% H2O
Situation E
CH3
Cl
CH3
80% EtOH/20% H2O 90% iPrOH/10% H2O
5. For the following reaction that proceeds by way of a dipolar (or zwitterionic)
intermediate,
a) Match the relative rate with the appropriate solvent.
ClSO2
ClSO2
CH3
N C O
C CH2
+
CH3 C CH2
CH3
CH3
measured relative rates
1,
31,
5000
N C O
reaction solvents
diethyl ether, n-hexane, nitrobenzene
a) Draw the structure of the probable intermediate.
6. You have been introduced to the Hughes-Ingold model for predicting solvent
polarity effects on reaction rates.
Consider the following reaction:
Et3N + Et-I
Et4N+ + I-
which is for the 2nd table of page 2-5 of the course notes. Draw a potential energy
level diagram (as on course notes p. 3-10 at the top) demonstrating the reaction path
when this reaction proceeds in benzene vs. nitrobenzene. Place both curves on one
diagram and be sure to label each curve.
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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7. Although it was only briefly mentioned in class, the solvent susceptibility
parameter m can assume a negative value, as shown in the two similar reactions
below.
Me3S
+
Et3S
+
+
-
OH
MeOH + Me2S
m = -0.78
+
-
OH
EtOH + Et2S
m = -0.84
a) Briefly explain, in general, what kinds of chemical reactions give a positive m value
and hence the kinds of chemical reactions that give provide a negative m value.
b) Briefly justify the negative m value in the particular instance of the reactions
above.
8.
As indicated by the symbolic drawing below, benzyltrimethyl ammonium chloride is
complexed by a crown ether (18-crown-6) and the equilibrium constant (K) for the
complexation is solvent dependent (298 K). The interaction of the ether oxygens with
the ammonium ion is through hydrogen bonding. The table showing the data also
shows the entropy change for the reaction, ΔSr, which is presented as TΔS; that is,
the temperature component has been incorporated.
O
O
O
O
O
K
+
PhCH2NH3
O
+
O
+
H3N
O
O
O
CH2 Ph
O
O
solvent
log K
TΔS (kJ/mol)
H2O
1.44
--
iPrOH
4.14
-26.4
DMSO
1.34
-23.7
You are required to:
a) Explain why the entropy for the complexation equilibrium is negative.
b) Explain why the log K for iPrOH is much larger than that for water.
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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Explain why water and DMSO have comparable equilibrium values despite being
solvents of such different structure.
SOLUTIONS TO PRACTICE PROBLEMS 3
1. a)
Et2S + EtBr
Et3S+Br-
-an example of decreased charge separation
-large decrease in rate
b)
-
OH + Et4N+
H2O + CH2 CH2 + Et3N
-another example of decreased charge separation
-large decrease in rate
c)
CH2 CH2 + Br2
BrCH2 CH2Br
via rate determining bromonium ion formation
-an example of increased charge separation
-large increase in rate
2.
solvent
Keq
% enol
n-hexane
19
95
1,4-dioxane
4.6
82
methanol
2.8
74
O
O
O
Keq
OH
H
O
O
Much like one of the labs in CHEM*3750, we see a dependence
of internal H-bonding on solvent. With the least polar solvent,
there in no opportunity for solvent participation through H- H-bonding arrangemen
bonding or dipole alignment, so the molecule satisfies itself somewhat by tautomerizing
to the enol and doing the internal H-bonding. With 1,4-dioxane, increased stabilization of
ORGANIC REACTIVITY - CHEM*4720 - COURSE NOTES W2010
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the polar carbonyl form is possible through interaction with the solvent, (but not Hbonding). With MeOH there is good chance to H-bond to the carbonyl and to solvate it
well through polar interaction. Hence self-stabilization by enol formation is reduced.
3. When sodium (or K+) azide is added to DMF, the sodium is solvated due to the
direction of the dipole moment of the solvent. The azide is poorly solvated and readily
does chemistry; hence the efficient addition elimination reaction. The addition of water
creates a new solvent system that can assist in the stabilization of the positive sodium
ion, but will probably have a greater effect on the azide which is poorly solvated in
DMF alone. Water can perform H-bonding to the azide, which stabilizes it and reduces
its reactivity.
4.
Situation A
Situation B
Situation C
Situation D
CH3O
CH3
Cl
CH3
CH3O
CH3
Cl
CH3
CH3O
CH3O
CH3
Cl
CH3
CH3
Cl
CH3
80% EtOH/20% H2O
80% EtOH/20% H2O
50% EtOH/50% H2O
ranks third,
boring
substituent and
common solvent
system
2nd fastest case, is
in common solvent
system and bears
+R group for
through resonance
stabilization in the
TS for Cl loss
fastest case, is in
most ionizing
solvent and bears
+R group for
through resonance
stabilization in the
TS for Cl loss
Situation E
CH3
Cl
CH3
80% EtOH/20% H2O 90% iPrOH/10% H2O
2nd slowest
case, bears -I
group and is in
common
solvent system
slowest case,
bears -I group and
is in least ionizing
solvent
5.
a) A dipolar intermediate would suggest that more polar solvents would accelerate
the reaction.
The accompanying molecule is the proposed
intermediate. Note that the tertiary carbon bears the
cation while the collection of electronegative atoms
bears the anion.
ClSO2
-N C O
CH2
+
CH3 C
CH3
Measured relative rates reaction solvents
1 n-hexane,
31 diethyl ether,
5000 nitrobenzene
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6. Here is my thought. The important issues are that the starting materials should be
close, the overall ΔG‡ should be less for nitrobenzene and the ionic products in
nitrobenzene should be of lower energy that in benzene. I also believe the energy
difference between the products in the different solvents should be greater than the
energy difference between the two transitions states in the different solvents
7.
Me3S
+
Et3S
+
+
-
OH
MeOH + Me2S
m = -0.78
+
-
OH
EtOH + Et2S
m = -0.84
a) The concept of m arises from solvolysis reactions. One can take the rate of
solvolysis of a standard reaction, determine its’ rate in a number of solvents and
then establish a solvent parameter, m. Since in a solvolysis, there is creation of
charge, so more polar solvents create a large m and anything that creates charge
will have a positive m based on how m was established in the first place.
So when m is measured for a reaction that destroys charge, then m comes out to be
negative.
b) In the case of the reactions at hand, the negative m value is applicable since charged
ion are reacting to create neutral compounds.
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8.
a) Two reasons: Two molecules come together to form one.
The flexibility of the ring is lost when it closes to complex with the
ammonium ion.
b) iPrOH is not as good of an H-bonding solvent as water because of steric reasons,
polarity and the number OH bonds per molecule. Note that for the ammonium ion to
bind with the crown ether, it must give up its interaction with the solvent. In water
more so than iPrOH, that interaction is through H-bonding and hence it is more
difficult to leave that H-bonding interaction behind in favour of the crown ether. The
weaker interaction of the ion with iPrOH therefore will more readily concede the
ammonium to the crown ether.
Both water and DMSO have oxygens capable of offering electron density towards
cations. When that cation is something like an ammonium ion, then both can participate
in H-bonding. Despite their structural differences, both can H-bond very well.
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