Video: EC17-18-S1-Q20A

Write the chemical equation for the Friedel-Crafts methylation of benzene.

02:37

Video Transcript

Write the chemical equation for the Friedel-Crafts methylation of benzene.

The Friedel-Crafts methylation technique is a way of adding a methyl group to an aromatic compound. The general term for this type of reaction is a Friedel-Crafts alkylation. So we know, in this chemical reaction, we are going to be adding a methyl group to benzene. This is the structure of benzene. We know that, by the end of this question, we should’ve added a methyl group to benzene. Another name for methyl benzene is toluene. So what do we need to react benzene with in order to methylate it?

You should be able to recall that the basic principle of a Friedel-Crafts alkylation is the addition of an alkyl halide to an aromatic. One such example is chloromethane. Using chloromethane ensures that you’ll be doing a methylation. If you want to do an ethylation, you would use ethyl chloride. So a suitable reagent for the methylation of benzene is chloromethane. It would be possible to use other halomethanes. But chloride is the most commonly used.

The next thing you should know about Friedel-Crafts alkylation is that a catalyst is involved. The most common catalysts for Friedel-Crafts alkylations are aluminum chloride and iron three chloride. Here, I’m going to use aluminum chloride. Now, we have one last question to answer. What happens to the hydrogen that is substituted with a methyl group? Ultimately, this hydrogen pairs up with the halide on the alkyl halide and in this case forms HCL. So the full word equation for the Friedel-Crafts alkylation is that an aromatic plus an alkyl halide forms an alkylated aromatic plus a hydrogen halide. One important feature of a Friedel-Crafts alkylation is that the reaction has to be performed under anhydrous conditions. Otherwise, the catalyst is destroyed.

So the full answer is benzene plus chloromethane react well, catalyzed by aluminum chloride under anhydrous conditions, forming toluene plus hydrogen chloride. This is the Friedel-Crafts methylation of benzene.

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