ELECTROPHILIC REACTIONS

In this blog, we will be considering reactions in which the electrophilic reagents become bonded to substrates that are electron rich, especially those that contain multiple bonds such as alkenes, alkynes and aromatic compounds. The π electrons in these systems provide regions of high electron density, and electrophilic reactions feature as the principle reactivity in these classes of compounds. These reactions are known as electrophilic reactions, rather than nucleophilic reactions (due to the electron richness in these multiple bonds), is because the electrophile are the ones that provide the reactive series.

There are two kinds of electrophilic reactions, namely;

1. Electrophilic Addition (occurs in alkenes, alkynes)
2. Electrophilic Substitution (occurs in aromatic compounds)

So what is an electrophile?

An electrophile is a reagent which is attracted to electrons. Since it is attracted to a negative region, an electrophile tend to carry either a full positive charge, or a partial positive charge. It participates in a chemical reaction by accepting an electron pair in order to bond to an electron rich region, also known as the nucleophile.

Electrophilic addition

The key to understanding electrophilic addition reactions lie in the geometry of an alkene group. Each of the C atoms in the alkene group is sp² hybridised, meaning that each C atom has 3 sp²  hybrid orbitals extending out in the same plane, with a single unhybridised p orbital perpendicular to the plane. The unhybridised p orbitals will overlap sideways to form a π bond.

The electrons in the π bond are held further away from the nuclei of the atoms than that of a σ bond. This means that less energy is required to pull the π electrons out of their orbitals. In other words, the π bond in the C=C alkene group is very reactive.

Now imagine that an electrophile, being electron deficient, will be able to pull out the electron from the π bond as it approaches the C=C bond in the alkene. As a result, a new σ bond is formed between one of the C atom and the electrophile (at the same time the C atom becomes sp³ hybridised now). The other C atom in the alkene is still sp²  hybridised, but now bears a positive charge due to its empty unhybridised p orbital. This is also known as a carbocation. The carbocation is very reactive and a nearby nucleophile will quickly donate an electron pair to the carbocation, thus forming a new σ bond as well and the C atom becomes sp³ hybridised too.

As you may have noticed, an unsaturated organic compound can be made a saturated compound via the electrophilic addition reaction. And as the name suggest, it involves the addition of atoms onto the unsaturated compound.

Electrophilic substitution

As mentioned above, electrophilic substitution occurs in aromatic compounds such as benzene. Aromatic compounds do not undergo electrophilic addition reactions. This is explained below.

The electrophilic attack on an aromatic ring is less favourable than attack on an alkene. This is because the initial addition reaction leading to the formation of a carbocation formation uses up one of the p orbitals that normally contributes to the π electron system, and thereby creates an sp³ hybridised centre. This means that the π electron delocalisation characteristic of an aromatic system is destroyed. Hence, in order to maintain the aromatic system, aromatic compounds such as benzene, will lose one of its proton during an electrophilic attack.

Generally, a stronger electrophile is required for an electrophilic substitution reaction with the aromatic compound as the delocalised π electron system in the aromatic ring is much less reactive than that of an isolated alkene.