Electrophilic substitution

3d model showing the delocalised electrons in a benzene ring

Aromatic compounds all contain aromatic rings. Aromatic compounds are susceptible to attack by electrophiles and undergo electrophilic substitution reactions. Recall that electrophiles are electron deficient species. Now recall that aromatic rings contain clouds or molecular orbitals containing delocalised electrons above and below the plane of the carbon atoms; as shown in the image opposite. These clouds of electron density are prone to attack by electrophiles. The clouds of delocalised electrons are sterically unhindered and act as a large target for any electrophile.

The benzene or aromatic ring acts as a source of electron, that is it is a nucleophile. In an electrophilic substitution reaction:

One or more of the hydrogen atoms in the aromatic ring are replaced or substituted for another group.
An intermediate positively charged carbocation is produced. This cation will quickly lose a hydrogen ion (H⁺) and then regenerate the delocalised electron system within the aromatic molecule. Recall that this delocalisation of electrons is associated with extra stability within aromatic molecules.

Electrophilic substitution reactions

The delocalised electrons in an aromatic molecule will attack an electrophile. The electrophile will then replace or substitute for one of the hydrogen atoms in the benzene ring to generate a positively charged intermediate carbocation (an ion with a positive charge). This intermediate cation is resonance stabilised.
The final part of the reaction mechanism involves the loss of a hydrogen ion (H⁺) which will result in the formation of the delocalised pi(π) electrons system in the aromatic ring. An outline of a typical electrophilic substitution reaction is shown below:

general mechanism of electrophilic subsitution reaction- Friedel Crafts

Electrophilic substitution of benzene rings

Electrophilic substitution is a very versatile reaction and it is possible to add a wide variety of substituents onto a benzene ring using this reaction, for example:

Halogenation: addition of F, Cl, Br or I onto a benzene ring.
Nitration: addition of the nitro group (NO₂) to form nitrobenzene.
Hydroxylate: addition of a hydroxyl group (OH) to form phenol.
Friedel-Crafts alkylation: addition of an alkyl group onto a benzene ring.
Friedel-Crafts acylation: addition of an acyl group (RCO) onto a benzene ring to form ketones.

We will cover each of the reactions above in this website but all these reactions proceed via the same mechanism, that is electrophilic substitution and the final result of this is simply that a hydrogen atom on the benzene ring is replaced by an electrophile, as outlined in the diagram above:

The mechanism of electrophilic substitution

The mechanism for a typical electrophilic substitution reaction is outlined in the steps below:

Step 1 addition of the electrophile to the aromatic ring. This results in the formation of a positively charged cation (which is resonance stabilised). It should be noted that addition of the electrophile destroys the stability associated with the delocalisation of the pi (π) electrons, so this process will be endothermic.

Step 2: Loss or elimination of a hydrogen ion (H⁺) to regenerate the delocalised pi(π) electron system in the aromatic ring.

The diagram below shows the mechanism for an electrophilic substitution reaction using both the Kekulé representation and the circle representation for the benzene ring. It is up to you as to which one you prefer to use, personally I prefer the Kekulé representation simply because I feel that it enables you keep track of how the electrons are moving throughout a mechanism, but it's up to you as to which representation you prefer to use!

Electrophilic substitution mechanism

Diagrams to show the mechanism of electrophilic substitution reactions using both the Kekule and circle notation for the aromatic ring.

Resonance hybrid structures

The intermediate cation formed in step 2 is as we have mentioned is resonance stabilised. Addition of the electrophile to the benzene ring is likely to be the slow step in the above reaction since it will remove or destroy the aromatic stabilisation that results from the delocalisation of the pi(π) electrons in the ring. Resonance is simply where the nuclei of the atoms stay in the same place and while the electrons move. In the diagram below it is possible to draw 3 resonance hybrid structures for the intermediate cation. Recall that resonance helps to stabilise the ion and generally the more resonance structures you can draw the more stable the ion is likely to be.

It is important to mention that the double head arrow used to indicate resonance does NOT imply that all the structures actually exist, rather it is suggesting that the structure of the intermediate ion is a combination of all the resonance structures.

resonance stabilised structures
for electrophilic substitution

Key Points

Aromatic compounds undergo electrophilic substitution reactions.
Aromatic compounds are resonance stabilised molecules due to the delocalisation of the pi(π) electrons in the aromatic ring. To remove this delocalisation of the pi(π) electrons will require an input of energy.
Electrophilic substitution can be thought of as occurring in two steps:

addition of the electrophile which results in the formation of a cation and the loss of the delocalisation of the pi electrons.
loss or elimination of a hydrogen ion (H⁺) or proton to regenerate the delocalisation stability within the aromatic molecule