Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: rednukleus on July 02, 2005, 09:24:22 AM
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I know that when determining whether a reaction will go through substitution or elimination, we decide if the leaving group is good or bad. We also decide if we have a nucleophile, or a base. I'm not sure how to determine either. Here's what I think:
Leaving group - if the difference in electronegativity btwn the leaving group and the adjacent atom is larger, the leaving group is better. I do know for sure that it has something to do with the ability of the group to bear the burden of a negative charge.
Nucleophile/Base - I don't know the difference. molecules with O- anions on them are bases. That's all I know.
Please explain the criteria in full for deciding these two things. Thank you!
James Hansen
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Weaker the base, better the leaving group.
Nucleophiles attack electrophilic carbon atoms of the molecules.
Bases abstract protons.
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Your electronegativity analogy is flawed since things with very high electronegativity (e.g. F) are very poor leaving groups. As you alluded to, you really have to think about the ability to bear a negative charge in solution. Something like F- just has too much negative charge in a small surface area. Something like I-, however, has the same negative charge dispersed over a much larger area, so it doesn't take as much energy to cram all of that charge in. As Winga mentioned, leaving groups that are conjugate bases of strong acids are generally very good leaving groups. That's probably the simplest way to assign them as such.
The nucleophile vs. base thing is a lot more complicated though. Some good bases are not good nucleophiles (and vice versa). The key is that nucleophilicity is a kinetic measurement, while basicity is a thermodynamic measurement. To restate: nucleophilicity is based on how rapidly the nucleophile will attack an electrophile, basicity is a measure of how stable the molecule is when it is protonated.
Think of something like triethylamine. The lone pair on nitrogen is a good base and can readily "mop up" protons (pKa of TEA is ~11). However, since there are three ethyl groups surrounding the nitrogen, TEA is also a relatively sterically hindered molecule and therefor is kinetically slow when attacking nucleophiles.
At the opposite end of the spectrum is something like iodide. Iodide is a poor base (pKa of HI is ~-10) but a good nucleophile since it has a relatively concentrated charge.
I hope this is helpful, this is a very difficult topic!