Chem3x11 Lecture 5
Under construction Sat May 5.
This lecture is about Neighbouring Group Participation (sometimes also called Anchimeric Assistance).
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- Groups/atoms with available electrons positioned near the carbon involved in a substitution reaction can influence the reaction
Simple Examples of NGP
A sulfur atom in the 2-position of an alkyl halide greatly increases the reactivity of that alkyl halide.
This is not terribly surprising when you realise that S is a good nucleophile. Though we give these kinds of influences the fancy name of neighbouring group participation (NGP) or anchimeric assistance, keep in mind it's just intramolecular nucleophilic attack. The reaction goes through a high energy intermmediate (a sulfonium ion), and you might want to pause and think what that will mean in terms of its rate behaviour and whether this is an SN1 or SN2 reaction.
These kinds of structures (2-chlorothioethers) are sometimes called mustard gases, and were used with terrible effect as chemical weapons in World War I. The sulfonium ion can react with many biomolecules, yet if it reacts with water it produces HCl which can burn and blister the skin. This is an unusual example - chemistry is typically a Force for Good.
Besides effects on rates of reactions, NGP has very considerable influence on reaction stereochemistry, which we'll now look at.
Stereochemical Effect of NGP
If we imagine a molecule that could display NGP, but for some reason did not, then a regular SN2 nucleophilic displacement would go with inversion of configuration. Obviously the diastereomer of the starting material behaves the same way.
In fact what happens is this:
Whatever you do, don't memorize this. We need to look at the mechanism, when everything will become clear and obvious.
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