Friedel-Crafts Reaction

Figure 60.

This is reversible so that, by heating benzenesulfonic acid, the sulfuric component can be drawn off the molecule in the presence of dilute sulfuric acid, and benzene is produced once again.

FRIEDEL-CRAFTS REACTION

This is a type of alkylation reaction that involves the replacement of a hydrogen ion with an alkyl group. There are four limitations you need to know about with regard to this type of reaction:

• It cannot be done with vinyl and aryl halides

• There cannot be a strong deactivating group on the benzene ring (such as NH2,

NH with a side chain, or Nitrogen plus two side chains) because they deactivate the catalyst necessary for the reaction to take place.

• More than one alkylation can take place with this type of reaction.

• Carbo-cation rearrangements can occur in any reaction that involves a carbocation. This leads to isomers of the alkyl side chain attaching to the benzene molecule.

In this reaction, benzene is mixed with an alkyl chloride plus Aluminum (III) chloride (AlCl3) to make benzene plus an alkyl group attached to it. The catalyst aluminum chloride acts very similar to the halogenation process. The aluminum chloride makes a carbo-cation, which is an intermediary step allowing for the carbon atom on the alkyl side chain to be electrophilic. It looks like this as seen in figure 61:

Figure 61.

Alkylation ability increases as one goes up the halogen group on the periodic table. In the Friedel-Crafts alkylation process, catalysts like BF3, SbCl5, AlCl3, and AlBr3 are commonly used as these are Lewis acids that make the alkyl group more electrophilic.

As you can see, there can be rearrangements of the alkyl group when adding a carbon chain greater than two carbons in length. This can lead to the alkyl group being an isomer of unbranched alkyl chain. In addition, a deactivating nitric oxide group or other nitrogen group cannot be also attached to the benzene ring. This is because the lone pair of electrons on the nitrogen will react with the catalyst (such as aluminum chloride), forming a molecule that will not allow the alkylation reaction to occur.

Finally, when one alkyl side chain is attached to the benzene ring, this activates the benzene ring, making it increasingly likely to add still more alkyl groups (called polyalkylation). This doesn’t happen in a Friedel-Crafts reaction involving an acyl group (RCO side chain). The addition of the acyl side chain deactivates the ring, preventing polyacylation. Figure 62 shows the Friedel-Crafts reaction as it applies to acylation.

Figure 62.

So far, we have talked about the activation and deactivation of the benzene ring. Things that activate the ring make it more likely to add more side chains, while the deactivation of the ring makes a ring that is less likely to add more side chains. One can determine whether something is activating or deactivating by looking at the dipole moment created between the benzene ring and the side chain.

Electron-donating side chains have a dipole moment that points toward the benzene ring. These will activate the ring because it will make it more susceptible to electrophilic attack. Activating side chains include CH3, OCH3, OH, and NH2—all of which have a dipole moment that makes the benzene ring more electrically negative.

Electron-receiving side chains have a dipole moment that points toward the side chain, making the benzene ring less likely to be susceptible to electrophilic attack. These include NO2, CN, CO2CH3, and Cl. These will effectively “deactivate” the substituted benzene ring.