what is a quenching agent?

a quenching agent is a compound that is added at the end of the reaction when u want the reaction to stop. in organic chemistry it depends on what reaction u are doing precisely as there are many different types of quenching. u can end the reaction with hydrolysis under cold/hot temperature and acidic/basic environment. there are so many more and again it depends what type of reaction are u particularly doing.

in the synthesis of polystyrene, u really need to understand the mechanism of how it is synthesized before going any further. manufacturers use a thing call radical to start the reaction. like (CH3)3C-O-OH, note that it is not a C=O-OH (not a carboxylic acid) which means the 3rd carbon is only singlely bonded to the oxygen, now that molecule will break down into (CH3)3C-O‧ and ‧OH which both oxygen only contain ONE electron from the breakage of bond!! now u must be familiar that hydroxide ion contain a negative charge but remember this is a radical agent and not OH-. now what happens is that the (CH3)3C-O‧takes one electron from the double bond of CH2=CH, and one electron is left at the CH:

(CH3)3C-O‧ CH2=CH-Ph = (CH3)3C-O-CH2-CHPh‧<----note that that single electron is no the C, not on the Ph group, so esseintially, from the double bond breakage, one electron goes to the carbon of CH2 and one electron goes to the carbon that has the benzene ring

now u can see how another radical is essentiall created which it can do the same step as previously done. and the chain goes on and on and on. NOW TO STOP the reation, there are 3 ways, 2 ways naturally and 1 way purposely.

the long carbon chain can either go through *chain combination* which two radical long chains react with each other and ends the reaction:

2 (CH3)3C-O-CH2-CHPh‧ = (CH3)3C-O-CH2-CHPh-CHPh-CH2-O-C(CH3)3

the reaction can also end with *disproportionation*, that is two radical long chains go over redox reaction:

2 (CH3)3C-O-CH2-CHPh‧ = (CH3)3C-O-CH2=CHPh + (CH3)3C-O-CH2-CH2Ph

2008-04-28 07:03:01 補充：
so one chain is oxidized into alkene and one chain is reduced into alkane. those two are the natural ways. the last way is to add some impurities:

(CH3)3C-O-CH2-CHPh‧ + impurity = (CH3)3C-O-CH2-CHPh-impurity

Quenching agent is an agent that can stop the reaction proceed.

To start with, in the view of kinetic factors, the rate of reaction is related to several factors:Temperature,Pressure,Concentration of the reactants, Contact surface area of reactant particles and Prescence of catalyst.

So in the quenching process, maybe the chemicals are decreased by concentration or the surrounding temperature is decreased .And the rate of reaction is affected to a very low way and the reaction seems to be stopped.