electrophillic aromatic substitution

The reactions of aromatic compounds are closely related to the stabilization effects given from the conjugated pi system. 


The non-equivalence of the carbon atoms in the benzene ring leads to the unique properties and different substitution effects. 

Each of these groups represent a slightly different chemical environment.

Substitution to an aromatic benzene ring


Benzene almost always reacts via a substitution mechanism. This is due to the energy favourability in maintaining the aromatic ring after the reaction. This means that there is usually a loss of a hydrogen atoms from the ring re-forming the aromatic ring.

             Electrophilic substitution

Electrophilic substitution is one of the most common ways that benzene undergoes aromatic substitution. This is when a positively charged particle is reacted with the electron dense benzene causing a positively charged benzene to form. These reactions then involve the loss of the hydrogen on the benzene to maintain aromaticity.

This means that a positive charge is introduced to the benzene enviornement which then undergoes a reaction. The position that this electrophile attacks is based on where the position of any substituents are in the reaction. The chemical nature of these substituents effect the electron density on the carbon positions of the benzene making some more electron rich than others. Electrophiles will react where there can be best stabalisation of the charge produced.

The diagram below doesn't need to be memorised although having a good idea of the boundaries is very useful.

A better understanding of why each of these substituents have the following properties by looking at the different resonance states for each of the substituents.

The stability of each can be shown after electrophilic substitution by the following diagrams:

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Reactions of benzene and its derivatives

There are a number of reaction of benzene and its derivatives that need to be taken into account. The final two mechanisms show how to achieve a reasonable level of selectivity through the reactions.

Friedel-Crafts addition

Friedel-Crafts addition is an effective way to add alkyl chains onto a benzene group. There is the problem of carbocation rearrangement meaning the addition of secondary or tertiary alkyl groups becomes impossible. 

Friedel-Crafts acylation

This reaction has the advantage of not allowing carbocation rearrangement to occur. This means that the use of this reaction is usually favoured.

These are two very effective methods although past simple alkyl groups they do not allow the addition of anything more complex. This is seen very strongly in the attempted production of the following compound. This is because the acyl chloride derivative of formaldehyde does not exist.  

This means a different reaction mechanism needs to be used. A good example of this being the use of the Vilsmever – Haack reaction. This is where an amide group can be reacted with a halogenated phosphoxide group. This forms a strong electrophile which can then react with the benzene ring. This forms an imine which can then be hydrolysed forming an aldehyde group.

The Mannick reaction also utilises this imine intermediate. 

This shows how there is great number of uses for a nitrogen ion. These have the possibility to react easily as either an electrophile or a nucleophile. This places great importance on the formation of the electrophilic amine group.

The Sand Meyer reactions are also important this involves the addition of a nitrogen molecule onto a benzene ring and then the addition of a number of different reactants to form a number of products. The cation for this reaction can be produced by the following method.

These diazonium ions can be easily removed. This produces the reactions known as the SandMeyer reactions.

Protecting groups

It is important in a lot of these reactions that the electrophile attacks the correct location on the benzene ring. Ortho protection can be carried out by reacting an acyl chloride with the amine group. This stops ortho reactions by kinetic effects.

This shows how a protecting group can be used to allow the formation of a para positioned electrophile on the benzene ring.

It is also worth remembering that for meta substitution there needs to be a powerful electron withdrawing group. This means that the initial nitro substituted benzene can be present for the nitro addition and then the addition and then reduction forming the amine.

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Heteroaromatic reactions

Pyridine

Pyridine is very similar to benzene and is likewise an aromatic compound. The nitrogen lone pair is not involved in the aromatic π system. 

So this leaves the need to understand whether pryridine acts as an ortho/para director or a meta director. This is found by looking at the resonance structures and remembering that the nitrogen is unstable when positively charged.  

There are other heteroaromatic systems such as pyrrole, furan and thiophene. These tend to react at the alpha carbon.

It is important to note that pyrrole in an acidic mixture reacts to form a polymeric tar.