Electrophilic Substitution Reaction

The process in which an electrophile (an electron pair acceptor) replaces the functional group attached to a compound is called an electrophilic substitution reaction. Examples of electrophilic species are the hydronium ion (\[H_{3}O^{+}\]), the hydrogen halides (HCl, HBr, HI), the nitronium ion (\[NO{_{2}}^{+}\]), and sulfur trioxide (\[SO_{3}\]).

In an electrophilic substitution, the displaced functional group is usually a hydrogen atom. Electrophilic substitution occurs in many reactions of arenes (compounds containing benzene rings), and it is called electrophilic aromatic substitution reactions. The electrophilic aliphatic substitution reaction is another primary type of electrophilic substitution. The three steps involved in the electrophilic substitution reaction are the generation of an electrophile, then the formation of carbocation that acts as an intermediate, and the removal of a proton from the medium.

Basic examples of electrophilic substitution reactions of benzene are alkylation, acylation, halogenations, nitration, sulphonation, etc.

Types of Electrophilic Substitution Reactions

There are two types of electrophilic substitution reactions undergone by organic compounds. They are:

1. Electrophilic Aromatic Substitution Reactions

2. Electrophilic Aliphatic Substitution Reactions

Electrophilic Aromatic Substitution Reactions

Electrophilic aromatic substitution reactions are the organic reactions in which an electrophile replaces an atom connected to an aromatic ring. The replacement of a hydrogen atom from a benzene ring with an electrophile is involved in these reactions. Examples include aromatic nitrations, aromatic sulphonation, and Friedel-Crafts reactions.

In an electrophilic aromatic substitution reaction, the aromaticity of the aromatic system is preserved. The stability of the aromatic ring is not lost when bromobenzene is produced from the reaction of benzene and bromine, for example.

Electrophilic aromatic halogenation reactions of aromatic rings with iodine, chlorine, or bromine can produce aryl halides or haloarenes.

Electrophilic Aliphatic Substitution Reactions

In electrophilic aliphatic substitution reactions, an electrophile replaces the functional group (generally hydrogen) in an aliphatic compound. These reactions are classified into five types.

• Halogenation of ketones

• Nitrosation

• Keto-Enol tautomerism

• Insertion of a carbene into a carbon-hydrogen bond

• Diazonium coupling (aliphatic)

If the electrophilic attack happens at an angle of 180 degrees to the leaving group, this electrophilic substitution reaction may result in a configuration inversion. 

Reaction of Amines

Firstly to answer what are amines- Amines are the organic derivatives of ammonia (NH3), in which the hydrogen is replaced by the alkyl, cycloalkyl, or aromatic groups to bond with the Nitrogen atom. The simplest case of aromatic amines is aniline, where amine-type nitrogen is bound to an aromatic ring.

Some examples of the reaction of amines are,

• Amine reacts as a base with acid to form a salt.

• Amine reacts as a nucleophile with alkyl halide through the substitution reaction of SN2 is called alkylation.

• Primary aliphatic amines, when oxidized by KMNO\[_{4}\], produce ethanol.

Types of Electrophilic Substitution Reactions of Anilines

Aniline is an organic compound that consists of a phenyl group attached to an amino group, and it has the molecular formula of \[C_{6}H_{5}NH_{2}\]. In aniline, the functional group (NH\[_{2}\]) is very active towards electrophilic substitution reactions because it is an electron-donating type group. And Aniline having several resonating structures also has an excess of negative charge or electron in the ortho and para positions of a benzene ring than that of the meta position; hence the o and p positions are directed towards the electrophilic substitution reaction of an aniline. The types of electrophilic substitution reactions that aniline can perform are,  

Nitration of Aniline   

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In the nitration of aniline reaction, meta isomer is also observed along with para because the aniline molecule gets protonated in an acidic medium to become an Anilion ion. Though the NH\[_{2}\] group is an o and p indicator, when the reaction takes place with acid, it produces meta-Nitroaniline.

Sulphonation of Aniline

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Aniline reacts vigorously with Sulphuric Acid to form anilinium hydrogen sulfate, which on heating gives 4-amino benzene sulphonic acid, which also has a resonating structure with zwitterion. This reaction is called the sulphonation of aniline.

Halogenation of Aniline

Aniline, when reacted with bromine water, then at room temperature, produces a white-coloured precipitate named the 2, 4, 6–tribromoaniline. This happens because polarity is developed within the bromine molecule, and bromine acts as an electrophile for having a slightly positive charge. It attacks the electron-rich ortho and para position of the aniline.

Solved Examples

In the Chlorination and Bromination of Aromatic Rings, what catalysts are used?

Ans. Lewis acid catalysts like AlCl\[_{3}\] or FeCl\[_{3}\] are used for the chlorination of an aromatic ring. The catalysts used for the Bromination of Aromatic Rings are AlBr\[_{3}\] or FeBr\[_{3}\].  

Fun Fact

One of the only electrophiles that will only accept electrons is H+ because it has no electrons.  

Water molecule is both an electrophile and a nucleophile for the oxygen molecule in water is more electronegative (because the oxygen molecule has two lone pairs and a d- charge, which makes it nucleophilic) and each of the hydrogen molecules behaves as an electrophile for it bears a d+ charge.