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Say for example you are performing a halogenation reaction with dichloromethane (DCM) as the solvent:

ethylene + chlorine $\stackrel{DCM}\longrightarrow$ 1,2-dichloroethane

I have 2 questions:

  1. Is the DCM acting as a catalyst to the reaction, or just a solvent? And if just a solvent, why do we have to use an organic solvent, why not water?
  2. Are solvents like DCM necessary if the reactants are liquids already, or only for gaseous or solid reactions?

Thank you

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    $\begingroup$ As for not using water, try dissolving some non-polar organics in water, then you'll see why. Also, water+chlorine would spark a whole bunch of side reactions we don't want to have. $\endgroup$ – Ivan Neretin Oct 13 '15 at 7:58
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In the example you give, the dichloromethane is acting as a solvent. Solvents are usually chosen to be inert under the reaction conditions so that they do not react and produce unwanted products.

There are examples of reactions in which no solvent is used, or in which the solvent is a reagent. An example could be forming an ester from a carboxylic acid, using an excess of the alcohol as both solvent and reagent, in the presence of an acid catalyst. That said, solvents are usually used as they severely effect the outcome of a reaction. Polar solvents (THF, diethyl ether) promote very different reactions to non polar ones (hexane) due to the stabilisation they provide for intermediates during the reaction, dilution can also play a role, deciding whether inter or intra-molecular reaction occurs.

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  • $\begingroup$ Thank you very much, I was wondering though, are solvents like DCM necessary if the reactants are liquids already, or only for gaseous or solid reactions? $\endgroup$ – Jason Chami Oct 13 '15 at 7:35
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To answer the second part of your question, solvents are also used as heat sinks. If no solvent is used, the heat given off from a particular reaction can lead to it being uncontrollable, potentially runaway. If engineering controls are used, no solvent is fine (usually in an industrial setting). However, in a laboratory setting, the heat evolution could cause flashing off (instant vaporisation) , in this instance of DCE, causing a large expansion of material changing from liquid to gas. DCM in itself may not be a great heat sink, but if the reaction is very dilute, the specific heat capacity is supplemented by the volume/mass present.

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