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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:

For an SN1 reaction, the polarity and ability of the solvent to stabilise the intermediate carbocation is of paramount importance, as shown by the relative rate data for the solvolysis of tBuCl.
 
Solvent Dipole moment, μ Dielectric constant, ε Relative Rate
CH3CO2H 1.68 6 1
CH3OH 2.87 33 4
H2O 1.84 78 150,000
Dipole moment, 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, μ Dielectric constant, ε 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





POLAR PROTIC SOLVENTS (polar and ability to be H-bond donor)

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
polar aprotic solvents

Overall


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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary