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:

• Step 1-The generation of the electrophile that will add to the aromatic ring.


• Step 2- The attack on the electrophile by the delocalised pi(π) electrons in the aromatic ring. This results in the temporary loss of the delocalisation stability associated with the delocalisation of electrons in aromatic rings.


• Step 3- The final step is the loss of a hydrogen ion (H⁺) from the aromatic ring to regenerate the delocalised pi(π) electron system and to leave the final product, an alkylated aromatic ring.

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.

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

Making the electrophile

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:

• The C-X bond present in the halogenalkane molecule is already a polar one but it can be further polarised by reaction with a Lewis acid. The halogen atom present in the halogenalkane molecule will use one of its lone pairs of electrons to attack the electron deficient Lewis acid. The halogen atom will form a dative covalent bond with the Lewis acid molecule to form a stable complex; as shown in the image below. The formation of this complex will weaken the carbon-halogen bond in the halogenalkane molecule (C-X) and further polarise the C-X bond and make the halogenalkane (alkyl halide) a much better electrophile.


• If the alkyl halide is a secondary or a tertiary one then it is likely to ionise further to form a carbocation (R⁺) and an ion such as the tetrachloroaluminate ion (AlCl₄^-) if the Lewis acid used is AlCl₃ and the halogenalkane is a chloride. However if the alkyl halide is a primary one then it will not ionise since primary carbocations are unstable and any reaction that forms them will have a high activation energy, instead the complex formed is often described as a "carbocation like species"; where the C-X bond is highly polarised and stretched making the carbon atom present in the halogenalkane molecule strongly electrophilic, this is outlined below using a halogenalkane (alkyl chloride) and the Lewis acid aluminium chloride (AlCl₃) in the image below:

• The carbocation (R⁺ ) is a highly reactive species with an empty p-orbital making it a strong electrophile (electron loving species) which will be immediately attacked by the delocalised pi(π) electrons in the aromatic ring. In essence we can say that the Lewis acid acts as a "halogen carrier" pulling electrons away from the halogenalkane molecule to further polarise it and generate either a reactive complex or a carbocation that are both able to react with molecules containing aromatic rings.

Self-check

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

The synthesis of cumene (isopropylbenzene)

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:

• Step 1- The halogenalkane or alkyl halide 2-chloropropane; which is commonly called isopropyl chloride; reacts with the Lewis acid aluminium chloride (AlCl₃). Since the alkyl halide is a secondary one it will ionise to form an isopropyl carbocation, which is a secondary carbocation that will act as the electrophile in step 2 in the image below.


• Step 2- The delocalised electrons in the benzene ring attack the isopropyl carbocation and form a dative covalent bond with it. The delocalisation pi(π) electron system in benzene will be temporarily disrupted as a result of this and the benzene molecule will end up with positive charge. Despite the aromaticity of the benzene ring being disrupted by the addition of the electrophile the carbocation which forms can be stabilised by undergoing resonance (this is covered in more detail on the page detailing electrophilic substitutions reactions.)


• Step 3- The AlCl₄^- ion now removes a hydrogen ion (H⁺) from the aromatic ring to restore the delocalised pi(π) electron system in the benzene ring and regenerate the aluminium chloride catalyst. The final product, cumene or isopropyl benzene is formed.

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:

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:

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:

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.

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.

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:

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:

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

• -NO₂
• -CHO
• -CN

Aromatic rings containing basic groups

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?

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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:

• The halogen atom in the halogenalkane will attack the Lewis acid and donate a lone pair of electrons; this will result in the formation of a complex where the C-X has been weakened.


• Depending on the nature of the halogenalkane molecule, that is whether it is a primary, secondary or tertiary haloalkane then it may well result in the halogen leaving as a halide ion (X^-); taking it bonding electrons with it, this will leave behind a positively charged carbocation (R⁺) which will act as the electrophile in the Friedel-Crafts alkylation reaction.


• However if the halogenalkane molecule is a primary halogenalkane such as chloromethane (CH₃Cl) then it will not fully ionise since the formation of primary carbocations is generally an unfavourable reaction with a high activation energy, instead a complex forms where the C-X bond is stretched and further polarised which results in the carbon atom present in the halogenalkane molecule being a strong enough electrophile to be attacked by the delocalised electrons in an aromatic ring.

2. What are the major drawbacks or limitations of the Friedel-Crafts alkylation reaction?

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The major disadvantages of Friedel-Crafts alkylation reactions are:

Carbocation Rearrangement
• The alkyl carbocation intermediate can rearrange to form a more stable carbocation (e.g. primary → secondary or tertiary), leading to unexpected products.

This makes product prediction and control difficult.

Polyalkylation
• The initial alkylation makes the benzene ring more reactive (activates it), increasing the chance of further substitution.
  This leads to multiple alkyl groups being added unless the reaction is carefully controlled.


Limited to Alkyl Halides
• Only certain alkyl halides (mainly those that can form stable carbocations) are suitable.
  Aryl halides and vinyl halides do not react due to the instability or lack of carbocation formation.


Inactivation by Strong Electron-Withdrawing Groups
• Benzene rings with strong electron-withdrawing groups (e.g. the nitro group (-NO₂) are too deactivated to undergo alkylation.


Poor Yield with Deactivated Arenes
• If the benzene ring is deactivated, the yield can be very low or the reaction may not proceed at all.

Key Points

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