Electrophilic Substitution Reaction Mechanism

When an electrophile (an electron pair acceptor) replaces the functional group connected to a molecule, an electrophilic substitution reaction occurs. In an electrophilic substitution, the displaced functional group is typically a hydrogen atom. Many arenes (benzene-ring compounds) undergo electrophilic substitution, also known as electrophilic aromatic substitution processes.

The electrophilic aliphatic substitution reaction is another type of electrophilic substitution reaction. The electrophilic substitution reaction consists of three steps: the formation of an electrophile, the formation of a carbocation that acts as an intermediate, and the removal of a proton from the medium.

What is Electrophilic substitution at unsaturated carbon centres?

Electrophilic substitution is a significant reaction due to its wide applicability, particularly in aromatic systems. It is possible for one of several mechanisms to react. One of the more common is depicted here; these reactions involve the replacement of a group designated Y (often a hydrogen atom) in an aromatic molecule by an electrophilic agent designated E. Both substituents can belong to any of several groups (e.g., hydrogen atoms or nitro, bromo, or tert-alkyl groups).

In this case, Y represents a ring substituent, and the arrow from the ring centre indicates coordination.

The reaction starts with the formation of a pi complex, in which the electrons associated with the aromatic ring or other unsaturated centres (pi electrons) coordinate weakly with the electrophile. This complex forms quickly in an equilibrium preceding the rate-determining step, which leads to a carbonium ion intermediate and then to the product via a second pi complex. There are examples where the proton removal from the carbonium ion intermediate (to form the second pi complex) becomes rate-determining.

A Mechanism for Electrophilic Substitution Reactions

For these electrophilic substitution reactions, a two-step mechanism has been proposed. The electrophile forms a sigma-bond to the benzene ring in the first, slow or rate-determining step, resulting in a positively charged benzenonium intermediate. A proton is removed from this intermediate in the second, quick step, yielding a substituted benzene ring.

This mechanism for electrophilic aromatic substitution should be considered in conjunction with other carbocation intermediate-based mechanisms. These include alkyl halide SN1 and E1 reactions, as well as alkene Bronsted acid addition reactions.

To summarise, when carbocation intermediates form, they can be expected to react in one or more of the following ways:

• The cation may form a bond with a nucleophile, resulting in a substitution or addition product.

• The cation may transfer a proton to a base, resulting in the formation of a double bond product.

• The cation may rearrange to a more stable carbocation, at which point it will react in mode #1 or #2.

• The first two modes of reaction are represented by the SN1 and E1 reactions, respectively. The second step of the alkene addition reactions is carried out.

• Characteristics of Specific Substitution Reactions

The electrophilic reactivity of these various reagents differs. We discover, for example, that nitrobenzene nitration occurs smoothly at 950 C, yielding meta-dinitrobenzene, whereas nitrobenzene bromination (ferric catalyst) requires a temperature of 1400 C. Also, as previously stated, toluene nitrates about 25 times faster than benzene, but chlorination of toluene is over 500 times faster than benzene. We can conclude from this that the nitration reagent is more reactive and less selective than the halogenation reagents.

Water is produced as a byproduct of both sulfonation and nitration.

This has no effect on the nitration reaction (note the presence of sulfuric acid as a dehydrating agent), but sulfonation is reversible and is accelerated by the addition of sulphur trioxide, which converts the water to sulfuric acid. The reversibility of the sulfonation reaction can be used to remove this functional group on occasion.

Electrophilic Aromatic Substitution Reactions

Although aromatic compounds have multiple-

Despite having multiple double bonds, aromatic compounds do not undergo addition reactions. Their lack of reactivity to addition reactions is due to the high stability of the ring systems produced by complete electron delocalization (resonance). Aromatic compounds react via electrophilic aromatic substitution reactions, which preserve the aromaticity of the ring system. Benzene, for example, reacts with bromine to form bromobenzene.

Electrophilic aromatic substitution reactions can add a variety of functional groups to aromatic compounds. A functional group is a substituent that causes chemical reactions that the aromatic compound does not exhibit.

Conclusion 

When an electrophile replaces the functional group connected to a molecule, an electrophilic substitution reaction occurs. In an electrophilic substitution, the displaced functional group is typically a hydrogen atom. The electrophilic aliphatic substitution reaction is another type of electrophilic substitution reaction. The electrophilic substitution reaction consists of three steps: the formation of an electrophile, the formation of a carbocation that acts as an intermediate, and the removal of a proton from the medium. Aromatic compounds react via electrophilic aromatic substitution reactions, which preserve the aromaticity of the ring system.