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SN2 Reaction

The first reaction encountered by a student in an organic chemistry course is invariably SN2, and what is stressed is the inversion at carbon.

This can be shown "on a blackboard", but a computer animation perhaps leaves a more lasting impression.


Open "Sn2 cyanide+methyl iodide" (Open from the File menu or click on ). One frame of an animation will appear on screen together with an energy profile. To animate, click on at the bottom left of the screen. Alternatively, step through the frames by clicking on and .
The computer animation . . . but not the blackboard . . . can also show the "flow" of negative charge throughout the course of the reaction.
Select Surfaces from the Display menu, and double click on the line "density potential... " in the dialog.
An electrostatic potential map is displayed. This gives the energy of a positive charge located at different locations on a surface of electron density. "Red" areas demark regions where the surface is most negatively charged, while "blue" areas demark regions where the surface is least negatively charged. Colors in between . . . orange, yellow, green . . . depict intermediate charge. Note that the negative charge "flows smoothly" from cyanide to iodide during the reaction. Note also, that cyanide (as the reactant) is "more red" than iodide (as the product). Iodide is more able to carry the negative charge, meaning that it is the better leaving group.

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