Leaving Groups

[OneDay]

What makes a good leaving group in orgo reactions? Is it the presence of lone pairs or electronegativity? I keep running into opposite explanations, so I'm confused!

[Avinash]

Good leaving groups are weak bases, or in other words, the conjugate bases of strong acids. It's best if you just memorize the table of good leaving groups.

[ballofnerves]

What makes a good leaving group in orgo reactions? Is it the presence of lone pairs or electronegativity? I keep running into opposite explanations, so I'm confused!

 

A good leaving group is one that is stable once it leaves with an extra pair of electrons. So, in general, a good leaving group is one that can stabilize negative charge well.

 

For example, if you have an alcohol R-C-OH, by protonating it with acid, you get R-C-OH2+. At this point, the -OH2+ group is a very good leaving group because if it took the pair of electrons from the C-O bond and left, you would obtain H2O.

 

If you did not protonate the alcohol, you would still have R-C-OH. If the -OH group were to get up and leave with the electrons from the -OH bond, you would have hydroxide ions OH- in solution. Hydroxide ions are strong bases because they cannot stabilize that additional negative charge very well. OH2+ is a much better leaving group because it is electron deficient (as is evident by its positive charge).

 

One more point: I- is a better leaving group than Br-, which is better than Cl-, which is better than F-. This is because iodine atoms have a large atomic radius, so they can stabilize the negative charge over a large area. F- is much smaller, so the additional negative charge is very concentrated and not spread out. This concentrated negative charge is unstable, so F- is a poor leaving group, but I- is very good.

 

I hope this makes sense. If you need clarification, please let me know