ORGO: What does it mean when a molecule is 'quenched?'

[alphaj]

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Just a question. I keep seeing that in SN2 reactions, sometimes a molecule is quenched by H2O or H3O. I've seen it turn an O- into an OH and also a heteroatom into an H. What exactly is going on here?

 

[jedishrfu]

I found this on Wikipedia:

https://en.wikipedia.org/wiki/Quenching_(fluorescence)

Normally when I hear quench, I think of a samurai sword heated until it glows the color of the sun and is then thrust into to water to quench it ie cool it to room temperature. This process fixates the molecular structure of the metal. The faster-cooled metal is more flexible than the slower-cooled metal. Prior to heating, clay is applied to the sharp edge of the blade so that during quenching the sharp edge will cool more slowly and thus retain its sharpness and hardness.

http://www.pbs.org/wgbh/nova/samurai/swor-nf.html

 

[Borek]

Normally when I hear quench

Verb[edit]
quench
(...)
(transitive, chemistry) To terminate or greatly diminish (a chemical reaction) by destroying or deforming the remaining reagents.

(not that I understand what the OP asked about, @alphaj, can you give an example and explain precisely what is the question?)

 

[TeethWhitener]

In this case, quenching means that you've essentially stopped the reaction. For example, say you're doing a Grignard reaction. The product that you get from addition of an organomagnesium halide to a carbonyl is an alkoxide. When you quench the reaction with water, the water will destroy all the Grignard reagents, halting the reaction, and the excess water will protonate the extremely basic alkoxide, giving a hydroxyl group.

 

[Ygggdrasil]

The term quench has many different meanings in chemistry from fluorescence quenching to reacting away excess reactant to terminate the chemical reaction. In the context of organic chemistry, the latter meaning is more common. It sounds like you are discussing the typical workup performed after a chemical reaction has completed:
https://en.wikipedia.org/wiki/Work-up_(chemistry)

See also this useful page discussing the practical aspects of setting up a reaction in organic chemistry, including the quenching and workup steps:
http://chem.chem.rochester.edu/~nvd/pages/reaction.php?page=roadmap