The Friedel-Crafts alkylation reaction is used to add alkyl groups such as methyl (-CH3) and ethyl groups (-C2H5) on to aromatic ring. During a Friedel-Crafts reaction the alkyl group substitutes for one of the hydrogen atoms on the aromatic ring. The type of mechanism as you might expect from an aromatic ring is electrophilic substitution. The alkylation reaction can be easily thought of as occurring in three separate steps:
As in any electrophilic substitution reaction the delocalised pi(π) electrons in the aromatic ring act as a nucleophile and attack a carbocation. The positively charged carbocation (the electrophile) in these reactions is generated by the reaction of an alkyl chloride with a Lewis acid catalyst such as aluminium chloride (AlCl3) or iron (III) chloride (FeCl3) at around 800C. The mechanism of this reaction was covered in the page on halogenation of benzene rings, but essentially the Lewis acid (electron pair acceptor) will help to further polarise the halogenalkane and depending on the nature of the alkyl group (-R) the Lewis acid will aid in the formation of a carbocation as shown in the equation below.
Once the Lewis acid helps form the carbocation (R+) then the pi(π) electrons from the aromatic ring will attack the electrophile, this is a typical electrophilic substitution reaction, the mechanism is shown below:
Polystyrene or poly(phenylethene) is a widely used polymer or plastic. It is used to make a wide variety of everyday items such as plastic cutlery, car parts, kids toys as well as thousands of other items. Polystyrene is made by the addition polymerisation of the monomer styrene (phenylethene).
We could imagine that styrene (phenylethene) could simply be made by the addition of chloroethene to benzene via a Friedel-Crafts alkylation reaction as shown below, however Friedel-Crafts alkylation reactions involving the addition of vinylic group (contains a C=C ) fail, they simply don't work, it is also worth mentioning perhaps that Friedel-Crafts reactions also fail with aryl halides, that is halides joined to an aromatic ring, this is outlined below:
This is a little disappointing since it would seem to be the most obvious route for the synthesis of styrene. However recall from the electrophilic substitution mechanism that one of the key steps is the attack of the pi(π) delocalised electrons in the aromatic ring on a carbocation, so all that is needed to manufacture styrene (phenylethene) is an alternative route to get an ethyl group (-C2H5) onto a benzene ring. The attached ethyl group (-C2H5) could then be dehydrogenated to form the desired phenylethene or styrene.
One of the first mechanisms you probably learned in organic chemistry was electrophilic addition of hydrogen bromide and hydrogen chloride to alkenes. Here the polar hydrogen halide adds across the carbon carbon double bond (C=C) to form an alkyl carbocation, this is shown in the diagram below. In electrophilic addition reactions the intermediate carbocation reacts with a chloride or bromide ion to give the final product. However we require the intermediate alkyl carbocation to add to an aromatic ring instead of reacting with a chloride or bromide ion. To achieve this a Lewis acid is added. The Lewis acid will intercept any chloride or bromide ions present, leaving the alkyl carbocation free to react with the aromatic ring. This is shown below:
Cumene (2-phenylpropane or 1-methylethylbenzene) is an intermediate in the preparation of phenol. Phenol is an important chemical which has many uses including in the manufacture of:
Cumene being an alkyl substituted aromatic compound can be prepared by a Friedel-Crafts alkylation reaction. The reaction will be similar to that for preparation of ethylbenzene except that we can use propene or 2-chloropropane to produce cumene as shown below:
Friedel-crafts alkylation reactions are not particularly useful as a general rule simply because of the limitations of this type of reaction. The main problems with Friedel-Crafts alkylation reactions are:
Polyalkylation: The product of this reaction, alkyl substituted aromatic rings are more susceptible to electrophilic attack than the starting material. This is simply because the alkyl group attached to the aromatic ring is an activating group, this makes the aromatic ring much more willing to undergo electrophilic substitution and leads to polyalkylation products as shown below:
It is possible to try and reduce the possibility of polyalkylation by using a large excess of the starting reactant. We have seen that any substituent that can feed electron density into an aromatic ring results in a mixture of polyalkylated products being produced. However there are many substituents that withdraw electron density from aromatic rings. These substituents will make the aromatic ring less able to attack an electrophile, that is the aromatic ring will be deactivated. The most common deactivating groups that you are likely to meet include:
