Electrophilic Aliphatic Substitution Reactions

There are two types of electrophilic substitution reactions- Aliphatic and Aromatic.

In an aliphatic electrophilic substitution reaction, an electron-rich molecule, the electrophile, reacts with an electron-deficient molecule, the nucleophile. The electrophile attacks the most electron-deficient atom in the nucleophile, and the reaction results in the formation of a new carbon-carbon bond. 

Electrophilic aromatic substitution reactions require the use of a chemical mechanism. The main elements of this reaction are the electrophile, aromatic compound, and substitution. The complexity of the aromatic compounds is taken into consideration when analysing this reaction. This article explores the mechanism and the basic components of electrophilic aromatic substitution, and it concludes with a comparison of this reaction to nucleophilic aromatic substitution.

Analysis of sigma-complex and pi-complex in electrophilic aromatic substitution

The sigma-complex exhibits a unique relationship between acidity and electrophilicity, but further disintegration depends on participants’ reaction conditions and types. The presence of nucleophilic species with potential basic character is a clue to the further disintegration of the complex. This process may also be controlled by controlling the electrophilicity of the non-aromatic benzene nucleus.

While analysing the s-complex and p-completion in electrophilic aromatic substitution reactions, Prof. Olah and his colleagues based their observations on arene stability and high positional selectivity in SEAr reactions. These observations suggest that the s-complex may be the rate-determining complex in the electrophilic aromatic substitution reaction.

The stability of s and p-complexes during the electrophilic aromatic substitution reaction depends on the electron-donor power of substituents and the strength of coordination bonds. The more coordinated the compound, the more stable it is. However, some e4 and e5 complexes are unstable and may show structural deformation during construction. This is because of the possibility of deformation of shape and homogeneity during the process.

A more detailed study of the pi-complex in electrophilic benzene and sigma-complex in benzene is necessary to understand the mechanism of SEAr. SEAr reactions require higher temperatures than benzene reactions. Further, mercuric sulphate and HgSO₄ reactions require harsher conditions.

Comparison with nucleophilic aromatic substitution

What is an electrophilic aliphatic substitution reaction? The answer is a reaction in which a nucleophile is introduced into the process. The nucleophile reacts with the aromatic ring, forming two products, one of which is an aliphatic alcohol. In the case of nucleophilic substitution, substituted hydrogen is left unsubstituted. The nucleophile is introduced by introducing an electrophilic group into the aromatic ring.

The rate of nucleophilic substitution depends on the concentration of the nucleophiles, leaving capacities, and the reaction mechanism. This reaction has two stages. The first step is characterised by forming an intermediate – the carbocation. The second step involves the formation of a lone pair. The nucleophile has two types of lone pairs, one on the carbon atom and one on the oxygen atom.

The remaining two steps are similar in structure and rate, but the former has two steps. The first step is the attack of the nucleophile on the leaving group, resulting in an anionic intermediate, the Meisenheimer complex, that is resonance stabilised. The second step, the formation of the substituted molecule, is characterised by the departure of the leaving group. In both reactions, the nucleophile shares an electron with the atom whose leaving group is attached.

Mechanism of reaction

The electrophilic aliphatic substitution reaction mechanism is not entirely clear, but it consists of two basic steps. The first step is the pi-bond electron attack, in which a proton abstracts from a carbon atom and makes a sigma-bond with the electrophile. After this, the two react to restore the aromatic p-system. The second step is the displacement of the proton from benzene to form an allylic carbocation.

The electrophilic aliphatic substitution reaction proceeds by the initial addition of an electrophilic compound to the aromatic ring. The electrophilic compound forms cationic sH adducts, and the de-aromatization step is related to this. As the proton departs, the aromaticity regains, and the substitution products form. Sometimes, the electrophilic substitution process proceeds in the same position that the substituent occupies and is known as the ipso substitution.

The underlying physicochemical parameters determine the overall mechanism of the reaction. The simplest type of substitution is electrophilic, where an atom is replaced with an electron-donating atom. The second type is nucleophilic substitution, which involves attacking a positively charged atom. Regardless of the electrophilic aliphatic substitution reaction, the basic steps are the same.

Conclusion

An electrophilic aliphatic substitution reaction is an organic reaction in which an electron-rich aliphatic molecule, or substrate, reacts with an electron-poor electrophile to form a new molecule. This article has looked at electrophilic aliphatic substitution reactions, and we have looked at the mechanisms involved and the different types of reactive centres that can be involved. To learn more about this topic, do visit our website.