CH610B Sophomore Organic II

Dr. Brian Pagenkopf

The Making of a Good Leaving Group: Charge Delocalization

When a leaving group departs from an intermediate, it usually carries with it a negative charge. Putting a charge on a molecule violates a golden rule of chemistry: molecules hate charges. However, better negative charge delocalization makes a more stable the anion. The more stable the anion, the better the leaving group (it costs less energy to leave). Thus, ¯:CCl₃, with the negative charge distributed partially onto the electronegative chlorine atoms is a really good leaving group compared to ¯:CH₃, where no charge delocalization occurs.

The haloform reaction is the first case where we’ve seen a carbon leaving group in collapse of the tetrahedral intermediate.

In the electrostatic potential surfaces plotted above, note how the negative charge (red color) is localized on the carbon atom of ¯:CH₃ (this is bad and makes ¯:CH₃ a very bad leaving group), while there is much less localized negative charge (the color is more yellow than red) on the carbon atom of ¯:CCl₃ (this makes ¯:CCl₃ a very good leaving group).

Sources include http://www.cm.utexas.edu/academic/courses/Spring2001/CH610B/Iverson/index.html