Electrophillic Substitution Reactions

Electrophilic substitution reactions are the chemical reactions in which an electrophile replaces a functional group in a compound, which is generally, but not always, a hydrogen atom. Electrophilic aromatic substitution reactions are representative of aromatic compounds and are common ways of presenting functional groups into benzene rings. Some aliphatic compounds can experience electrophilic substitution as well.

Substitution reaction is the class of chemical reactions in which an atom, ion, or group of atoms or ions in a molecule is substituted by another atom, ion, or group of atoms or ions. An example of substitution reaction is a reaction in which the chlorine atom in the chloromethane molecule is replaced by the hydroxide ion ultimately forming methanol. 

TYPES OF ELECTROPHILIC SUBSTITUTION REACTION

The types of electrophilic substitution reactions undergone by organic compounds are the electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions.

Electrophilic Aromatic Substitution Reaction

Electrophilic aromatic substitution is a naturally occurring reaction in which an atom that is involved in an aromatic reaction generally hydrogen is replaced by an electrophile. Some of the most significant electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and alkylation and acylation Friedel-Crafts reaction.

Electrophilic Aliphatic Substitution Reaction

Electrophilic aliphatic substitution is the chemical reaction where the electrophile replaces the functional group in aliphatic compounds called electrophilic aliphatic substitution reactions. In these reactions, the electrophile outbreaks the aliphatic compound, which results in a 180⁰ inversion.

ELECTROPHILIC SUBSTITUTION REACTION OF BENZENE

These electrophilic substitution reactions are proposed by a two-step mechanism. In the first step, which is a slow or rate-determining step, a positively charged benzenonium intermediate is generated as the electrophile forms a sigma-bond to form the benzene ring. In the second step, which is the last step, a proton is excluded from this intermediate, resulting in a substituted benzene ring.

The relative reactivity of the compound is the first factor where the compound is compared with benzene itself. Experiments have shown that substituents on a benzene ring can greatly affect reactivity. For example, a hydroxy or methoxy substituent enhances the rate of electrophilic substitution by about ten thousand-fold, as in the case of anisole. 

In opposition, a nitrogen substituent reduces the reactivity of the ring by roughly a million quantities. The activation or deactivation of the benzene ring in the direction of electrophilic substitution may be interrelated with the donating or withdrawing of electrons which influence the substituents, as measured by molecular dipole moments.

The second factor that becomes essential in reactions of substituted benzenes alarms the place at which electrophilic substitution happens. As a mono-substituted benzene ring has two equal ortho-sites, two equal meta-sites, and an exclusive para-site, three possible lawful isomers may be formed in such a substitution. If a reaction arises similarly well at all available places, the expected numerical mixture of isomeric products would be 40% ortho, 40% meta, and 20% para. 

Again, we find that the nature of the substituent affects the product ratio in a unique pattern. Bromination of methoxybenzene or simply anisole that is very rapid and gives mainly the para-Bromo isomer, attended by 10% of the ortho-isomer and only a minority of the meta-isomer. Bromination of nitrobenzene needs a high amount of heating and produces the meta-Bromo isomer as the major product.

STEPS INVOLVED IN ELECTROPHILIC SUBSTITUTION REACTION

There are three steps involved in the electrophilic substitution reaction mechanism.

Step 1: Electrophile Generation

In the production of electrophiles from the chlorination, alkylation, and acylation of an aromatic ring, anhydrous aluminum chloride is a very useful Lewis acid. Electrophile production occurs due to the existence of Lewis acid. The electron pair from the reagent that attacks are accepted by the Lewis acid. The resulting electrophiles are chlorine cation Cl+, positive radical R+, and RC+O respectively from the combination of anhydrous aluminum chloride and the reagent that are attacking.

Step 2: Formation of carbocation

The electrophile, which forms a sigma complex or an arenium ion, outbreaks the aromatic ring. One of the hybridized carbons in this ion of uranium is sp3. This arenium ion finds stability in a resonance structure. The sigma complex or the arenium ion misplaces its aromatic character since the delocalization of electrons stops at the sp3 hybridized carbon.

Step 3: Deprotonation

The third step of electrophilic substitution is deprotonation. Deprotonation is the driving force of the reaction, making it possible to move energetically. The activation energy in this step is much lower, and the reaction happens very fast.

EXAMPLES OF ELECTROPHILIC SUBSTITUTION REACTION

Some examples of electrophilic substitution reactions are stated below.

Benzene Sulfonation

Benzene sulfonation is the process of fuming sulphuric acid (H2SO4 + SO3) to warm benzene at high temperatures to produce benzene-sulfonic acid. Sulfonation is an alterable reaction that creates benzene-sulfonic acid by adding sulfur trioxide and fuming sulfuric acid. The reaction is altered by adding hot liquid acid to benzene-sulfonic acid for the production of benzene.

Benzene Nitration

Through the protonation of nitric acid by sulfuric acid, the foundation of the nitronium ion encourages the loss of a water molecule and the formation of a nitronium ion. Naturally, this reaction is reversible. Benzene at nearly 323-333k reacts with concentrated nitric acid to form nitrobenzene.

Benzene Halogenation

Halogenation is an illustration of electrophilic aromatic substitution. In electrophilic aromatic substitutions, benzene is attacked by an electrophile which outcomes in the substitution of hydrogens. However, halogens are not electrophilic sufficient to break down the aromaticity of benzenes, which require a catalyst, say FeCl3 to activate. In the presence of FeCl3 or FeBr3, Benzene counters with halogens for the formation of aryl halides. This reaction is known as benzene halogenation.

Sulfuric Acid Activation of Nitric Acid

To activate HNO3 with sulfuric acid is the first step in benzene nitration that is done for the creation of a nitronium ion which is a stronger electrophile. As the nitronium ion is a strong electrophile, it is attacked by benzene to produce nitrobenzene.

CONCLUSION

In this article, we have discussed electrophilic substitution reactions are the chemical reactions in which an electrophile replaces a functional group in a compound, which is generally a hydrogen atom. We have discussed the two types of substitution reactions namely the electrophilic aromatic substitution reactions and electrophilic aliphatic substitution reactions. Electrophilic aromatic substitution is a naturally occurring reaction in which an atom that is involved in an aromatic reaction generally hydrogen is replaced by an electrophile. Electrophilic aliphatic substitution is the chemical reaction where the electrophile replaces the functional group in aliphatic compounds called electrophilic aliphatic substitution reactions.