Solvation

Interaction of a solvent has profound influences that are often overlooked.  For example, the simple act of dissolving a molecule may provide the energetic change needed for SN1 ionization to occur.  Also, changing solvents from polar to nonpolar (or vice versa) can change the rate constant by many orders of magnitude.

Look at an example of a carbocation surrounded by acetone solvent molecules:

solvcom.pdb
Solvation shell around the 1-phenylethyl cation (acetone is the solvent in this example).solvcom.gif (2678 bytes)

In this case, the dipole of the acetone molecules tends to orient itself to stabilize the positive charge and disperse it.

 

Another impact of solvation is that the solvent shell has to reorganize itself as ionization occurs.  The solvent molecule on the side of the leaving group has to "get out of the way" as the leaving group departs.  This leads to the opportunity for solvent (or other external nucleophiles) to react at any of several distinct stages:

No ionization Contact or tight ion pair Solvent-separated ion pair Free ions
Sn2.pdb TIP.pdb SSIP.pdb freeions.pdb
Consequences:
  • Second order kinetics (substrate, nucleophile)
  • Clean inversion for solvolysis
  • Clean inversion for an external nucleophile
 Consequences:
  • First-order behavior (breaking C-X is RDS)
  • Inversion for both solvolysis and attack by external nucleophiles (leaving group blocks front side)
 Consequences:
  • First-order behavior
  • Solvolysis leads to both stereoisomers (approach from both sides)
  • Other external nucleophiles give inversion (LG still blocks front side)
 Consequences:
  • First order
  • Racemization regardless of nucleophile
Example: Goering, H. L; Towns, D. L.; Dittmar, B. J. Org. Chem., 1962, 27, 736-739. Example: Diaz, A. F.; Lazdins, I;, Winstein, S. J. Am. Chem. Soc., 1968, 90, 1904-5. Example:Richard, J. P.; Amyes, T. L; Vontor, T. J. Am. Chem. Soc., 1991, 113, 5871-3 (electron-withdrawing substituents). Example: Richard, J. P.; Amyes, T. L; Vontor, T. J. Am. Chem. Soc., 1991, 113, 5871-3 (electron-donating substituents).

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Last updated:  09/21/2000