Friedel-Crafts alkylation

Electrophilic substitution-alkylation of benzene rings

The Friedel-Crafts alkylation reaction is used to add alkyl groups such as methyl (-CH3) and ethyl groups (-C2H5) on to an aromatic ring. During a Friedel-Crafts reaction the alkyl group replaces or 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 Friedel-Crafts alkylation reaction is just another example of an electrophilic substitution reaction and can be thought of as occurring in three separate steps:

Friedel-Crafts alkylation reactions

The equation below outlines an equation that shows a Friedel-Craft reaction using benzene as an example, here the benzene ring reacts with an electrophile (R+) which simply substitutes or replaces one of the hydrogen atoms present on the benzene ring. The electrophile in a Friedel-Crafts reaction is produced by the reaction of a halogenalkane (an alkyl halide) with a Lewis acid catalyst. Aluminium chloride (AlCl3) is a common Lewis acid catalyst used in these reactions.

Equation to show a typical Friedel-Crafts reaction, the reaction of benzne with a haloalkane to form an alkyl substituted aromatic ring, an arene

Or if you prefer to use the circle notation to show the benzene ring rather than the Kekulé structure then we have:

Equation to show a typical Friedel-Crafts reaction, the reaction of benzne with a haloalkane to form an alkyl substituted aromatic ring, an arene

Making the electrophile

Defintion of Lewis acid and Lewis base

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 a halogenalkane (alkyl halide) with a Lewis acid catalyst such as aluminium chloride (AlCl3) or iron (III) chloride (FeCl3) at around 800C.


Activating the electrophile

In a Friedel-Crafts alkylation reaction the electrophile is a carbocation which is often given the symbol (R+); it is formed by the reaction of a halogenalkane molecule with a Lewis acid (an electron pair acceptor) such as aluminium chloride (AlCl3) or iron (III) bromide (FeBr3), both of these molecules are electron deficient and have empty orbitals that are capable of accepting a lone pair of electrons.

The halogenalkane molecule (R-X) will supply the carbocation (R+) that will add to the aromatic ring. X is typically a halogen atom such as Cl, Br or I. The steps which take place in forming the carbocation electrophile can be summarised as:

Mechanism for the reaction of a Lewis acid with a halogenalkane molecule.

Self-check

Match up the terms with their correct definitions. Simply click the term and then its correct definition, correct responses will turn green.

Terms

Definitions

The synthesis of cumene (isopropylbenzene)

Image explains the fact that primary carbocations are unstable while secondary and tertiary carbocations are much more stable.

The example below shows how the industrially important chemical cumene can be synthesised in a Friedel-Crafts reaction between the halogenalkane or alkyl halide 2-chloropropane and benzene. Cumene (isopropylbenzene) is an important intermediate in the industrial production of many important chemicals including phenol and acetone (propanone). The main points to consider in this reaction; which is shown in the image below are:

Image to show the reaction of an alkyl halide with a benzene ring to produce an alkyl substituted aromatic ring- Friedel-Crafts alkylation reaction.

An alternative to Friedel-Crafts alkylation reactions

Unfortunately Friedel-Crafts alkylation reactions have a number of limitations and do not always produce the product you hoped for, some of these limitations are shown below. However there are a number of alternative routes/reactions that offer better routes to produce arenes.

One of the first mechanisms you probably learned in organic chemistry was the electrophilic addition of hydrogen bromide and in particular the addition of hydrogen chloride to an unsaturated alkene such as ethene; as outlined below:

Mechanism for the electrophilic addition of HCl to ethene.

In the above mechanism the addition of the chloride ion (Cl-) to the carbocation is a relatively slow step since the chloride ion is a poor nucleophile. If a Lewis acid was added to this reaction then it could be used to "intercept the chloride ion"; this would leave the alkyl carbocation to react with something else- for example an aromatic ring! This is outlined below:

Use of a Lewis acid to prevent attack by chlorid eion on a carbocation

The ethyl cation which is produced in the above reaction could as mentioned be attacked by the delocalised pi(π) electrons in an aromatic ring such as benzene to form ethylbenzene, this is outlined below using both the Kekulé representation of benzene and the circle notation:

Image show the mechanism for the addition of an ethyl cation to benzene to form ethylbenzene

The synthesis of ethylbenzene is a particularly useful reaction since ethylbenzene can be dehydrogenated using steam and an iron(III) oxide catalyst to form phenylethene or as it is more commonly called styrene. Styrene is very useful since it is the monomer used to make the polymer polystyrene.

Image shows the dehydrogenation of ethylbenzene to form styrene or phenylethene

Problems or limitations of Friedel-Crafts reactions

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:

Making phenylethene or styrene

The reaction above gave one possible way in which to make ethylbenzene which was then dehydrogenated to form styrene or phenylethene. We could however 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 a vinylic group (that is a molecule which contains a C=C ) fail, they simply don't work.

limitations of the Friedel-Crafts reactions- reactions involving vinylic and aryl halides both fail and will not undergo Friedel crafts reactions.

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 mainly due to the fact that the C-X bond in aryl halides is much stronger than the corresponding C-X bond in a halogenalkanes or alkyl halides and also the Lewis acid catalyst, such as AlCl3, is not strong enough to effectively pull off a halogen atom from an aryl halide since this would disrupt the delocalised pi(π) electrons in the aromatic ring and result in the formation of an unstable aryl carbocation, this is a pity since this would be a very useful reaction to be able to carry out in practice since it would provide a way to build large molecules containing many aromatic rings linked together, this is outlined below:

Image shows that aryl halides do not under Friedel-Crafts reactions.

Polyalkylation

The product of a Friedel-Crafts reaction; an alkyl substituted aromatic ring or an arene is 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. The alkyl group will push electrons into the aromatic ring, this makes the alkyl substituted aromatic ring much more willing to undergo further electrophilic substitution reactions which leads to polyalkylation products as shown below:

polyalkylation of aromatic rings is a problem with Friedel-Crafts alkylation reactions.

It is possible to try and reduce the possibility of polyalkylation by using a large excess of the starting reactant. However it is still likely that a mixture of polyalkylated products will be produced.

Deactivated aromatic rings

While an alkyl substituent will activate an aromatic ring 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: Aromatic rings containing these deactivating groups are not able to attack electrophiles; that is carbocations. This means that aromatic rings containing these deactivating groups are not able to undergo Friedel-Crafts substitution reactions. Image shows aromatic mlecules which have decativating gripus attcahed and so will not undergo Friedel-Crafts reactions.

Aromatic rings containing basic groups

Another issue comes into play with aromatic rings that contain basic groups such as the amino group (-NH2); as shown below. Aromatic rings which contain a basic group will react with the Lewis acid catalyst needed for a Friedel-Crafts reaction and so these molecules will not undergo a Friedel-Crafts reaction.

addition of a Lewis acid to aromatic rings 
containing a basic amino group will result in the formation of a complex ion and so Friedel Craft reactions reactions will fail.

Self-check summary

Answer the two questions below to review your understanding of a few of the main points on Friedel-Crafts alkylation reactions covered above, click the blue boxes to reveal the answers to the questions.


1. What is the electrophile in a Friedel-Crafts alkylation reaction and how is it formed?
+
Image part of the mechanism for the generation of the electrophile in a Friedel-Crafts alkylation reaction.

In a Friedel-Crafts alkylation reaction the electrophile is a carbocation or a carbocation like species formed by the reaction of a halogenalkane (which is also called an alkyl halide) with a Lewis acid such as aluminium chloride (AlCl3), ferric bromide (FeBr3) or born trifluoride (BF3). Recall that a Lewis acid is an electron deficient molecule that is capable of accepting a lone pair of electron from another species (a Lewis base).

Step by step explanation of how the electrophile forms:


2. What are the major drawbacks or limitations of the Friedel-Crafts alkylation reaction?
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Image shows an experiemnt not going to plan.

The major disadvantages of Friedel-Crafts alkylation reactions are:



Key Points


Practice questions

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