Simple and Multiple Chlorinations

A chemical transformation that takes place via exactly one transition state is called an elementary reaction. This holds regardless of whether it leads to a short-lived intermediate or to a product that can be isolated. According to the definition, an n-step reaction consists of a sequence of n elementary reactions. It takes place via n transition states and (n 1) intermediates. 

If only methyl chloride is needed, it can be produced essentially free of multiple chlorination products only if a large excess of methane is reacted with chlorine. In this case, there is always more unreacted methane available for more monochlorination to occur than there is methyl chloride available for a second chlorination. 

Another preparatively valuable multiple chlorination is the photochemical perchlorination of methyl chloroformate, which leads to diphosgene  

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