wannabadr10

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Oct 21, 2008
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I never understood the difference between the two. I've looked up the definitions for both and it seems that a strong base would be a strong nucleophile and a strong nucleophile would always be a strong base. My prep book states that Sn2 rxns are favored by strong nucleophiles and E2 rxns are favored by strong bases. What is the difference and why?

Thanks!
 

futuredoctor10

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A strong nucleophiles can better attack electrophiles.
A strong bases can better donate electrons.

There are some strong nucleophiles which are also strong bases.
 

Yope

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Jun 2, 2006
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Like you said, a strong base is often a strong nucleophile, which complicates matters. However, determining SN2 vs. E2 requires considering more factors. Let's assume that the nucleophile is a strong base.

The substrate, for instance, plays a big role. Bulky substrates that hinder access of the nucleophile/base to the carbon will favor elimination, since the nucleophile/base can more easily get to the hydrogens than the carbon. Furthermore, for an E2 reaction to happen, a hydrogen has to be antiperiplanar to the one the nucleophile/base pulls off. If that antiperiplanar hydrogen is not there, an SN2 reaction will dominate.
 
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