Authored By: Abby L. Parrill and Jacquelyn Gervay
Funded by NSF, The University of Arizona, and The Department of Chemistry

SN2 Reaction Mechanism

SN2 stands for Substitution Nucleophilic Bimolecular. This reaction results in the net replacement of one atom or group for another. The reaction requires a nucleophile to collide with an electropositive atom with an attached leaving group. The collision must occur so that the nucleophile approaches the electropositive carbon from the side opposite the leaving group. The reaction mechanism can be visually examined as a Quicktime Movie. In this movie, the bromine atom is red and the chlorine atom is green. Watch specifically for the following key features:

• Transition State: The transition state of the SN2 reaction is a trigonal bipyramid with partial bonds from the carbon atom to both the leaving group and the nucleophile.
• Concerted Mechanism: The mechanism of the SN2 reaction involves a single step.
• Bimolecular: The slow step (and the only step!) of the SN2 reaction involves a collision between two different molecules.
• Inversion of Configuration: The carbon is tetrahedral at the beginning and end of the reaction, but "umbrellas" in the middle so that the three hydrogen atoms which were pointing to the nucleophile end up pointing to the leaving group.

SN2 Reaction Energetics

The SN2 reaction has a very simple reaction energy diagram because the reaction is concerted. The reaction energy diagram is shown below. The left side of the reaction energy diagram, where the energy is the lowest, represents chloride ion and methylbromide prior to the reaction beginning. The right side of the reaction energy diagram, where the energy is almost as low, represents bromide ion and methylchloride after the end of the reaction. The center of the diagram, where the energy is the highest, represents the transition state. It is important to note, however, that this highest energy is actually the highest point on the lowest energy path from starting material to product. Think of it as being the top of a pass from one side of a mountain range to another. It is the highest point a traveler will cover, but a very low point compared to the mountain peaks. Click on the datapoints in the figure to view molecular coordinates for the structures along the reaction coordinate. Note: If you are using Rasmol to view these coordinates, not all partial bonds will be shown. Use space filling models or ball and stick models for best impact.

Computational Details

This reaction pathway was calculated using Gaussian 94, Revision C.3, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995.
The calculation was performed at the 6-31G level on the CRAY-YMP supercomputer at The North Carolina Supercomputer Center.