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23
Optimizing Flash Separations
Vary Solvent Selectivity to Maintain
R
f
Values Between 0.15 and 0.40
Frequently, the R
f
of one component can be changed relative
to that of another by varying the “type” of solvent used at a
constant solvent strength. The approximate solvent strength
of a mixture can be derived by taking the weighted average
of the individual solvent strengths. See Table 6 for a listing of
commonly used solvents and their strengths.
Solvent Strength Parameters
Methanol
0.95
Ethanol
0.88
2-Propanol
0.82
Acetonitrile
0.65
Ethyl acetate
0.58
Tetrahydrofuran
0.57
Acetone
0.56
Dichloromethane
0.42
Chloroform
0.40
Diethyl ether
0.38
Toluene
0.29
Hexane
0.01
Heptane
0.01
Iso-Octane
0.01
Example:
1:1 Hexane:Ethyl acetate = (0.5 x 0.01) + (0.5 x 0.58) = 0.295
1:2 Hexane:Dichloromethane = (0.33 x 0.01) + (0.67 x 0.42) = 0.285
Table 6. Solvents commonly used for flash chromatography and their
relative strengths.
Solvent Selectivity Groups
Methanol
II
Ethanol
II
2-Propanol
II
Acetonitrile
VIb
Ethyl acetate
VIa
Tetrahydrofuran
III
Acetone
VIa
Dichloromethane
V
Chloroform
VIII
Diethyl ether
I
Toluene
VII
Hexane
--
Heptane
--
Iso-Octane
--
Table 7. Solvents commonly used for flash chromatography and their
relative selectivity group.
In Figure 29, the R
f
of the more polar component was changed
by replacing ethyl acetate with dichloromethane and resulted in
the upper (i.e. less polar) spot being essentially unchanged.
In some instances, varying the polar solvent component can
have a much more dramatic effect on the separation, including
reversing the elution order of the sample components as shown
in Figure 30. Table 7 lists commonly used solvent and their
selectivity groups.
For a more detailed discussion of solvent selectivity groups,
see
Introduction to Modern Liquid Chromatography by
L. R. Snyder and J. J. Kirkland.
Figure 29
Origin
Solvent Front
Origin
Solvent Front
Hexane:EtOAc
1:1
Hexane:CH
2
Cl
2
1:2
. Different solvent mixtures with similar strengths (0.28 and 0.29)
will frequently provide different selectivities.
Figure 30
Origin
Solvent Front
Origin
Solvent Front
?
A
B
C
?
C
B
?
?
A
Hexane:EtOAc
2:1
100% CH
2
Cl
2
. By changing the solvent type (i.e. Group VIa versus Group V),
the resolution between B, the compound of interest, and the impurities,
changed dramatically. Note that while the R
f
values of B are similar on the
two plates (0.65 and 0.53) in the example above, the elution order of A
and C were reversed.
Summary of Contents for Flash 150
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