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Chapter 8: Nucleophilic Substitution |
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Solvent Effects
In general terms, the choice of solvent can have a significant effect
on the performance of a reaction.
Factors when chosing a solvent are:
-
solubility : need to get reagents in the same phase, the molecules need
to collide to react !
-
usually, the solvent needs to unreactive towards the reagents
(except in reactions where the solvent is the Nu : "solvolysis")
-
how will the solvent affect the rate of reaction ?
For an SN1 reaction, the polarity and ability of the solvent to stabilize
the intermediate carbocation is of paramount importance, as shown by the
relative rate data for the solvolysis of tBuCl.
|
Solvent
|
Dipole moment, m
|
Dielectric constant, e
|
Relative Rate
|
|
CH3CO2H
|
1.68
|
6
|
1
|
|
CH3OH
|
2.87
|
33
|
4
|
|
H2O
|
1.84
|
78
|
150,000
|
| Dipole moment, m
in debye |
For an SN2 reaction, the effect of solvent polarity is usually much
less, but the ability (or really lack there of) of the solvent to solvate
the nucleophile is the important criteria, as shown by the relative rate
data for the SN2 reaction of nBuBr with N3-.
|
Solvent
|
Dipole moment, m
|
Dielectric constant, e
|
Relative Rate
|
Type
|
|
CH3OH
|
2.87
|
33
|
1
|
protic
|
|
H2O
|
1.84
|
78
|
7
|
protic
|
|
DMSO
|
3.96
|
49
|
1,300
|
aprotic
|
|
DMF
|
3.82
|
37
|
2,800
|
aprotic
|
|
CH3CN
|
3.92
|
38
|
5,000
|
aprotic
|
|
|
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POLAR PROTIC SOLVENTS (polar and ability to be H-bond donor)
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have dipoles due to polar bonds
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can H atoms that can be donated into a H-bond
-
examples are the more common solvents like H2O and ROH
-
remember basicity is also usually measured in water
-
anions will be solvated due to H-bonding, inhibiting their ability to function
as Nu
POLAR APROTIC SOLVENTS (polar but no ability
to be H-bond donor)
-
have dipoles due to polar bonds
-
don't have H atoms that can be donated into a H-bond
-
examples are acetone, acetonitrile, DMSO, DMF
-
anions are not solvated and are "naked" and
reaction is not inhibited
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Overall
-
All nucleophiles will be more reactive in aprotic than protic
solvents
-
Those species that were most strongly solvated in polar protic solvents
will "gain" the most reactivity in polar aprotic (e.g.
F-).
-
Polar aprotic solvents are typically only used when a polar protic
solvent gives poor results due to having a weak Nu, (esp. F-,
-CN,
RCO2-)
