Question

Nitration and chlorination of benzene are (a) nucleophilic and electrophilic substitution respectively (b) electrophilic and nucleophilic substitution respectively (c) electrophilic substitution in both the reactions (d) nucleophilic substitution in both the reactions.

Short answer

Both nitration and chlorination of benzene are electrophilic substitution reactions, making option (c) the correct answer.

Step-by-step solution

Understand the Nature of Nitration and Chlorination

Nitration of benzene involves the substitution of a hydrogen atom on the benzene ring with a nitro group (-NO2), typically using a mixture of nitric and sulfuric acids. Chlorination of benzene involves the substitution of a hydrogen atom on the benzene ring with a chlorine atom, typically using chlorine gas or iron(III) chloride as a catalyst. Both reactions involve the benzene ring acting as a nucleophile and the incoming group (nitro for nitration, chlorine for chlorination) behaving as an electrophile.

Examine the Mechanisms of Nitration and Chlorination

The mechanism for both nitration and chlorination involves the formation of an electrophilic species that attacks the π-electron-rich benzene ring to form a sigma complex. The loss of a hydrogen cation (proton) from this complex restores the aromaticity, resulting in the electrophilic substitution product.

Identify the Type of Substitution

Given that both reactions involve the attack of an electrophile on a rich electron system (the benzene ring), and the initial step is the addition of the electrophilic species to the aromatic system, they are classified as electrophilic substitution reactions.

Key concepts

Nitration of Benzene

Understanding the nitration of benzene is crucial for chemistry students as it represents a fundamental type of reaction involving aromatic hydrocarbons. Nitration is a process in which a nitro group (o{NO2}) is introduced into the benzene ring. This is typically carried out using a nitrating mixture, which is a combination of concentrated nitric acid (o{HNO3}) and sulfuric acid (o{H2SO4}).

The reaction proceeds through the generation of the nitronium ion (o{NO2+}), which is a potent electrophile created by the protonation of nitric acid by sulfuric acid. The electrophilic nitronium ion then attacks the electron-rich aromatic ring, forming a sigma complex — an intermediate where the ring temporarily loses its aromaticity. Lastly, the sigma complex releases a proton to regenerate the aromatic character of benzene, yielding nitrobenzene as the product. This electrophilic aromatic substitution mechanism is a hallmark of reactions involving the benzene ring.

Chlorination of Benzene

Chlorination is another typical reaction of benzene that exhibits the electrophilic substitution reaction mechanism. During chlorination, a chlorine atom replaces one of the hydrogen atoms on the benzene ring. The reaction is usually catalyzed by the presence of aluminum chloride (o{AlCl3}) or iron(III) chloride (o{FeCl3}), which help in the formation of the chloronium ion (o{Cl+}), the electrophilic species in this reaction.

Similar to nitration, the chloronium ion attacks the π-electron-rich benzene, forming a sigma complex. The complex then undergoes deprotonation, restoring aromaticity and producing chlorobenzene. The use of catalysts is crucial in this process, as they enhance the electrophilicity of the chlorine molecule, enabling it to undergo the substitution reaction with the stable benzene ring.

Aromatic Substitution Mechanisms

Aromatic substitution mechanisms detail how certain reactions occur within the context of an aromatic compound like benzene. The most common type of these reactions is electrophilic aromatic substitution (EAS), where an electrophile replaces one of the hydrogen atoms on the aromatic ring. EAS involves several key stages:

• Aromaticity drives the stability of benzene and makes it less reactive, necessitating the use of a strong electrophile for reaction.
• The formation of a sigma complex is an essential intermediate step where the electrophile is added to the ring, temporarily disrupting the consistent o{p}-electron overlap.
• The loss of a proton restores the compound's aromaticity, which is a low-energy, stable configuration.

Understanding the mechanism of EAS is vital as it serves as the basis for a wide variety of important organic reactions. These include not just nitration and chlorination, but also sulfonation, alkylation, and acylation, to name a few.

Benzene Ring Reactions

Benzene ring reactions are a fascinating topic in organic chemistry due to the unique stability and reactivity of the aromatic ring. The benzene ring's stability arises from the delocalization of six o{p}-electrons above and below the plane of six carbon atoms, forming what is known as aromaticity. This stability is central to the concept of EAS, wherein the benzene ring can undergo substitution reactions without losing its aromatic character.

Electrophilic substitution is the primary type of reaction that the benzene ring undergoes. This includes not only the nitration and chlorination reactions but also reactions such as Friedel-Crafts alkylation and acylation. Each of these reactions helps in introducing different functional groups onto the benzene ring, thereby altering its chemical properties and leading to derivatives that have significant applications in various industries, including pharmaceuticals, dyes, and polymers.