Electrophilic Substitution Reaction

An electrophilic substitution reaction occurs when a compound’s useful group is replaced with an electrophile. In other words, an electrophile replaces the ‘functional group’ of the molecule. The electrophile could be any pair of electrons accepting a group. However, the functioning group is usually a hydrogen atom.

Some electrophiles are not electrophilic enough to react alone, or the reaction is delayed. With the help of Lewis acids, we may additionally speed up the electrophilic substitution reaction. For example, in the presence of AlCl3 and FeCl3, the electrophilic substitution process of benzene with Cl2 can proceed at a substantially higher pace.

This article will cover the discussion on the electrophilic substitution reaction and its importance, types, and mechanism to make the concept understandable. 

Electrophilic substitution reaction meaning

Electrophilic substitution reactions occur when an electrophile removes a functional group in a molecule, typically but not always a hydrogen atom. Electrophilic aromatic substitution processes are frequently adding functional groups to benzene rings and are characteristic of aromatic molecules. Some aliphatic molecules can also be electrophilically substituted. 

In ‘aromatic compounds’, an electrophile replaces an atom attached towards the aromatic ring, which is commonly hydrogen. The most important are aromatic nitration, aromatic acylation, aromatic sulfonation, aromatic halogenation, and alkylating Friedel-Crafts reactions. It also includes acylation and alkylation.

An electrophile replaces a functional group in an ‘electrophilic substitution’ in aliphatic molecules. This reaction is analogous to nucleophilic aliphatic substitution, except that the donor is a nucleophile instead of an electrophile. ‘SE1, SE2 (front), SE2 (back), and SEi is, known as Substitution Electrophilic’ are the four potential electrophilic aliphatic substitution reaction’ mechanisms, which are similarly comparable to the nucleophile counterparts SN1 and SN2. The substrate ionises first in the SE1 course of action, forming a carbanion and a positive charge organic residue. After that, the carbanion immediately reassembles with the electrophile. The ”SE2 reaction mechanism’ includes a unique transition state that contains both the old and newly created bonds.

The following are examples of electrophilic aliphatic substitution reactions:

• Nitrosation
• Aliphatic diazonium coupling
• Keto-enol tautomerism
• Ketone halogenation
• Carbonyl alpha-substitution reactions
• Carbene insertion into C-H bonds

Electrophilic substitution reaction importance

One of the most important reactions in synthetic organic chemistry is the ‘electrophilic aromatic substitution’. These reactions produce a significant intermediate, which can be used as antecedents in manufacturing medicinal, agrochemical, and industrial goods. However, many commercial procedures for producing such materials continue to rely on science that was created many years ago. Such methods frequently result in mixes of regioisomers, and numerous new ways have been developed in recent years to obtain influence over the radiochemistry of the processes.

Zeolites can operate as heterogeneous catalysts, assist reagents, entrain by-products, improve product para-selectivity by shape-selectivity, and minimise watery work-ups. Under mild circumstances, zeolites can benefit from halogenation, alkylation, para-regioselective nitration, acylation, and methanesulfonyl reaction processes. Furthermore, they can usually be removed easily from the reaction medium by simple filtration and rejuvenated by heating, allowing them to be reused numerous times with nearly the same productivity and selectivity as samples.

Types of electrophilic substitution reactions

Electrophilic substitution reaction on aromatic compounds

An electrophilic substitution reaction involving aromatic compounds occurs when the aromatic ring itself is substituted or displaced by an electrophile. The aromaticity of molecules is preserved in such reactions. Chemical reactions including sulfonation, Friedel-Crafts reactions, and nitration in aromatic compounds are examples of electrophilic aromatic substitution reactions.

Electrophilic substitution reaction in aliphatic compounds

 ‘Electrophilic aliphatic substitution reactions’ are chemical reactions in which an electrophile displaces a functional group in an aliphatic molecule. The electrophile assaults the aliphatic molecule in these reactions, resulting in a version, like 180° inversion.

Electrophilic substitution reaction mechanism

• Generation of electrophile: Anhydrous aluminium chloride is a precious Lewis acid for producing electrophiles from an aromatic ring’s alkylation, chlorination, and acylation. The resultant electrophiles from the reaction of dehydrated aluminium chloride and the opposing reagent are Cl+, R+, and RC+O.

• Formation of intermediate carbocation: The electrophile assaults the aromatic ring, resulting in the formation of an arenium ion or sigma complex. Another of the carbons in this arenium ion has undergone sp3 hybridisation. In a resonance structure, this arenium ion achieves stability. Because electron delocalisation terminates at the ‘sp3 hybridised carbon’, the ‘sigma complex or arenium ion’ loses its aromatic property.

• Removal of proton from the intermediate carbocation: When attacked by AlCl4, the sigma complex emits a proton from the sp3 hybridised carbon to regain the aromatic property. As a result, the electrophile takes the place of the hydrogen ion inside the benzene ring. Because the notion of electrophilic substitution has been used in most organic name reactions, it is an exceptionally essential reaction in organic chemistry.

Conclusion

The shifted functional group in an ‘electrophilic substitution’ is commonly a hydrogen atom. Many abstract compounds containing benzene rings undergo electrophilic substitution, which is referred to as electrophilic aromatic substitution processes. Another form of electrophilic substitution reaction is the ‘electrophilic aliphatic substitution reaction. The electrophilic substitution process involves three steps: producing an electrophile, synthesising a covalent bond that acts as an intermediary, and eliminating a proton from the medium.