Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.
Reacting with Benzene
Benzene is extremely stable, and it does not want to break the aromaticity of the ring. So how does a compound such as benzene ever react? There are many different substituted benzene rings, how did these rings form? The mechanism by which a new R group is added to an aromatic ring was discovered by Charles Friedel and James Crafts, and named after them. These Friedel-Crafts reactions use an alkyl halide or another carbocation to add the R group onto the benzene ring.
There are two types of Friedel-Crafts reactions, alkylation and acylation. Alkylation reaction add a simple carbon chain to the benzene ring. Acylation adds an acyl group, creating a ketone or aldehyde.
The Friedel-Crafts reaction starts with a benzene and an alkyl chloride. The alkyl chloride reacts with AlCl3 (aluminum chloride) or FeCl3 (Ferric chloride), which removes the chorine from the alkyl chloride, making a positive charge on the carbon. This positive charge now needs electrons. So even though the benzene doesn't like to break the aromaticity the positive charge on the carbon allows it to break the aromaticity in the benzene ring.
The aromaticity isn't broken for long, because the aluminum chloride (or ferric chloride) can take the hydrogen off the benzene ring making it aromatic again.
When using Ferric chloride, the reaction will often keep adding more R groups to the benzene ring until the entire ring is filled up with R groups, so you need to be careful using Ferric chloride as the catalyst.
In general, the alkylation reactions occur by first making a carbocation. We can either use aluminum chloride or ferric chloride to remove the chlorine from the alkyl chloride. Now the R group has a positive charge on it. The electron rich benzene can give some electrons to the electron deficient R group, forming a new bond. But this then creates a positive charge on the benzene ring. The ferric chloride (or aluminum chloride) has an extra chlorine attached, so this chlorine can take the hydrogen, forming hydrochloric acid. Left behind we have the benzene ring with a new R group attached and the catalyst has been re-formed.
This is the way the reaction occurs when the alkyl chloride is either a secondary or a tertiary, because the carbon can hold a positive charge.
When the alkyl chloride is a primary this primary carbon is not able to hold the positive charge, even for a little while. So, a complex forms where the R group is still attached to the chlorine while also attached to the ferric chloride. Although the carbon doesn't have a positive charge in the instance the reaction can still occur. The chlorine doesn't like having a positive charge, so it is drawing a lot of electrons away from the carbon, making it electron deficient. The reaction then occurs normally.
The general reaction for acylation is just like alkylation, a positive charge is formed on the carbon, the electrons from the benzene form a new bond, and the hydrogen is removed.
This reaction occurs in the same way that secondary and tertiary alkylation reaction occurs. This is because the carbon can never be a primary carbon because it is double bonded to an oxygen.
Benzene is extremely stable, as such it can be difficult to react it with anything. The Friedel-Crafts reactions explains how to create a new carbon-carbon bond on benzene. It occurs by creating a carbocation which needs more electrons even more than benzene needs to keep those electrons. It can occur as an alkylation where a new carbon chain is added or as an acylation where a new ketone is formed.
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