An aromatic compound like benzene gives rise to unique stability because of its aromaticity. This stability can be attributed to the aromaticity of the compound, electrons being spread out in the ring system hence making the ring quite stable. But even then, aromatic compounds do not escape several chemical reactions, the principal of which is the electrophilic substitution reactions.
This blog will highlight the general steps and reactions that define electrophilic aromatic substitution and halogenation.
Understanding Aromaticity Reaction Mechanisms
Aromaity is a concept that results from the fact that cyclic compounds with electron-loose bonds/ rings are more stable than other compounds/ complexes.
The reaction mechanisms concerning aromatic compounds especially with the electrophilic aromatic substitution (EAS) demonstrate how aromatic rings behave with electrophiles.
In EAS, an aromatic ring carelessly attacks an electrophile to form a new bond in which a hydrogen atom involved in the aromatic sextet is replaced.
The mechanism follows a three-step process: The generation of the electrophile, the attack of this electrophile on the aromatic compound the formation of a non-aromatic sigma complex, and the rearomatization of the compound.
Electrophilic Aromatic Substitution Reactions
Electrophilic aromatic substitution reactions are considered to be the primary mechanism of reactivity of aromatic compounds.
Some of the frequently used are nitration, sulfonation, and alkylation processes. The general mechanism commences with the electrophilic attack on the ring through the pi electrons with the formation of what is called the arenium ion which is a carbocation.
This intermediate interferes with the aromatic system with this intermediate momentarily disrupting the system.
Last, the proton is eliminated from the carbocation intermediate being back to an aromatic state of the compound.
Benzene Loses Aromaticity in Its Reaction
Among the simplest aromatic compounds is benzene which undergoes reaction with strong electrophile agents such as halogens and acids.
In these reactions, the aromaticity of benzene is not permanently lost during the formation of the intermediate complex but regains it again at the end of the reaction.
For instance, in Friedel-Crafts alkylation, benzene interacts with an alkyl halide, and a catalyst causes the replacement of a hydrogen atom without the loss of an aromatic structure for a long time.
Aromatic Halogenation Reaction
Aromatic halogenation is one of the most numerous types of reactions of electrophilic substitution.
In this process, an aromatic compound brings it to a reaction with halogen (chlorine or bromine) in the presence of a Lewis acid catalyst such as FeCl3 or ACl3.
The electrophilic halogen species goes on to attack the fiduciary aromatic ring that contains pi electrons forms a sigma complex and loses its aromaticity temporarily.
As soon as the reaction is complete, stability is regained, and the product is an aryl halide.
Electrophilic Aromatic Substitution Mechanism
The mechanism of electrophilic aromatic substitution involves a few key steps:
Generation of the Electrophile:
The first one is the formation of a highly reactive electrophile which may be in the form of Cl+, or NO2+.
Formation of the Sigma Complex:
The electrophile reacts with the pi-electrons of the aromatic ring and produces a positively charged sigma complex. Frequently at this stage, the aromaticity of the ring is temporarily lost.
Restoration of Aromaticity:
One proton is released from the sigma complex; the aromaticity is returned, and the final substituted aromatic compound is produced.
Types of Electrophilic Substitution Reactions
There are several forms of electrophilic substitution reactions that aromatic compounds experience, including:
Nitration: The arcuate segment’s first appearance of a nitro group (NO 2).
Sulfonation: The existence of a benzyl-4-sulphonyl group (-C6H4-SO3H) is known as sulfonation.
Alkylation (Friedel-Crafts Reaction): The process of attaching an alkyl group to a molecule’s aromatic ring.
Friedel-Crafts Reaction: The addition of an acyl group to an aromatic molecule causes oxidation, or the Friedel-Crafts Reaction.
Although different electrophiles are employed in each reaction to justify a different reaction name, all of these reactions include electrophilic aromatic substitution.
Conclusion:
The reactions of aromatic compounds such as electrophilic aromatic substitution and aromatic halogenation, the complementarity of stability with reactivity in a chemical reaction emerges to be an interesting area of study. Nevertheless, benzene or other aromatic hydrocarbons, although usually very stable, are capable of participating in several reactions without the resultant product losing its aromatic nature. The knowledge of these reactions can be useful in explaining the behavior of aromatics systems in organic chemistry.
FAQS
Q1. What is the purpose of electrophilic aromatic substitution?
The intention is to build a substituent into an aromatic ring by displacing the hydrogen of the ring with an electrophile without causing the ring to lose its aromaticity.
Q2. Why do aromatic compounds undergo an electrophilic substitution reaction?
Aromatics undergo electrophilic substitution because the pi electron cloud of the ring attracts the electrophilic reagents yet the aromaticity of the ring is not jeopardized.
Q3. How to determine the reactivity of electrophilic aromatic substitution?
As with the effect of substituents on nucleophilic aromatic substitution, reactivity is increased with electron-donating groups and decreased with electron-withdrawing groups.
Q4. What is the role of catalyst in electrophilic aromatic substitution?
Another reason is that the substitution reaction is facilitated with the help of a catalyst, often a Lewis acid, which in turn strengthens the electrophile.
Q5. What is the general mechanism for electrophilic aromatic substitution?
The mechanism involves three steps: activation of the electrophile, formation of the sigma complex, and regeneration of aromaticity through the process of Deprotonation.
