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Solvent Effects

Article ID: 145
Last updated: 11 May, 2012
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In general terms, the choice of solvent can have a significant effect on the performance of a reaction.

Factors to considering when choosing a solvent for a specific reaction 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 reagent
    (except in reactions where the solvent is the Nu :  "solvolysis")

  • how will the solvent affect the rate of reaction, specific to that reaction

    For an SN1 reaction, for instance, 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, μ Dielectric constant, ε Relative Rate
CH3CO2H 1.68 6 1
CH3OH 2.87 33 4
H2O 1.84 78 150,000
Dipole moment, µ 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 (which would minimize the nucleophiles ability to donate its electrons) 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)

  • have dipoles due to polar bonds
  • have H atoms that can be donated into a H-bond
  • examples are 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 include acetone, acetonitrile, DMSO, DMF
  • anions are not solvated and are "naked" and electron donation is not inhibited

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Department of Chemistry & Chemical Biology