Video Transcript
Consider the following reaction. What product is formed from it?
Let’s begin by identifying the reactants of the chemical reaction. The first reactant is a monosubstituted arene. It has a single substituent directly bonded to an aromatic ring. The IUPAC name of this compound is chloromethyl benzene. The chloromethyl part of the name represents the substituent that is bonded to the benzene ring. Because of the presence of this substituent, this molecule is classified as an alkyl chloride. The second reactant is benzene.
Over the reaction arrow, we see the chemical formula of the catalyst aluminum chloride. This catalyst is used for a variety of substitution reactions with benzene. However, when the reactants are an alkyl chloride and benzene, the substitution reaction that will take place is an alkylation reaction, also called Friedel–Crafts alkylation. The alkylation of benzene results in two products: an alkylbenzene and hydrogen chloride. We can see that the chemical formula of hydrogen chloride is already provided in the reaction. So our job is to determine the identity of the alkylbenzene that forms.
We mentioned that an alkylation reaction is a type of substitution reaction. When benzene undergoes a substitution reaction, one or more hydrogen atoms are removed and replaced with other groups. We can also see that the hydrogen chloride molecule contains a chlorine atom. The aluminum chloride catalyst helps to weaken the carbon-to-chlorine single bond in the alkyl chloride. In a series of steps during the reaction, the hydrogen atom from benzene and the chlorine atom from the alkyl chloride will join to form a hydrogen chloride molecule. But what group takes the place of the hydrogen atom in benzene?
Well, it’s actually the very large alkyl group that was left over from when the first reactant lost a chlorine atom. Let’s go ahead and draw the chemical structure of the alkylbenzene product. Here, we can see the alkyl group, which is outlined, has chemically bonded to benzene. The name of this alkylbenzene is benzylbenzene. Chemists often simplify the structural formulas of complex aromatic molecules like benzylbenzene. One way that they do this is by using lines and angles only to represent the carbon atoms and the bonds between them. In conclusion, the product formed from the alkylation reaction provided is the structure we’ve drawn in blue here.
