SYNTHESIS AND REACTIONS OF BENZENE: MECHANISMS.

ABSTRACT:

In this article, we will discuss about the synthesis and reactions of benzene. Benzene are derivatives of aromatic hydrocarbons due to presence of large amount of carbon and hydrogen. These are also unsaturated hydrocarbons. These reactions are very crucial and give the several pharmaceutical, industrial and other organic products. We will also discuss the mechanisms of these reactions. We will provide references to understand our concept deeply.

INTRODUCTION OF SYNTHESIS AND REACTIONS OF BENZENE:

Benzene, a six-membered aromatic hydrocarbon, is one of the fundamental compounds in organic chemistry. Its unique structure and reactivity have made it a subject of extensive research and application in various industries. In this article, we will discuss the preparations and reactions of benzene, along with the underlying mechanisms involved.

SYNTHESIS AND REACTIONS OF BENZENE:

1. SYNTHESIS OF BENZENE:

a) FROM PETROLEUM FRACTION:

Benzene commonly obtained from the distillation of petroleum fractions. The initial fractionation produces gasoline, followed by the extraction of benzene through various purification processes such as fractional distillation, extraction, or catalytic reforming. This process involves the dehydrogenation of cyclohexane, a cyclic hydrocarbon, under high temperatures and pressures using a catalyst such as platinum or palladium.

b) FROM CYCLOHEXANE:

Cyclohexane, a cycloalkane, can dehydrogenated to produce benzene. This process involves the use of catalysts, such as platinum or aluminum oxide, at elevated temperatures.

c) FROM PHENOL:

Phenol, an aromatic compound, can also use as a precursor to benzene. It can undergo decarboxylation, wherein the carboxyl group removed, resulting in the formation of benzene.

2. REACTIONS OF BENZENE:

a) ELECTROPHILIC SUBSTITUTION REACTIONS:

Benzene shows exceptional stability due to its aromatic nature. Consequently, it exhibits electrophilic substitution reactions, where a substituent replaces a hydrogen atom on the benzene ring. Some noteworthy reactions include:

1. NITRATION REACTION:

Benzene reacts with a mixture of concentrated nitric acid and sulfuric acid, resulting in the introduction of a nitro group (-NO2) on the benzene ring. The reaction is facilitated by a Lewis acid catalyst, typically concentrated sulfuric acid. The crucial step involves the interaction of the electrophile with the π-electron system of benzene. This interaction results in the formation of a sigma complex, followed by the removal of a proton to regenerate the aromaticity of the ring. Various mechanisms, such as the Wheland intermediate mechanism and the sigma complex mechanism, have been proposed to explain these reactions.

2. HALOGENATION REACTION:

Benzene readily undergoes halogenation in the presence of a halogen (e.g. chlorine or bromine) and a Lewis acid catalyst, such as iron(III) chloride. This results in the substitution of a hydrogen atom by a halogen atom.

b) ADDITION REACTIONS:

Benzene can also undergo addition reactions under specific conditions, leading to the formation of cyclic compounds. Notable examples are:

1. FRIEDEL-CRAFTS ALKYLATION:

Benzene reacts with an alkyl halide, in the presence of a Lewis acid catalyst (e.g., aluminum chloride), to form an alkylbenzene. Friedel-Crafts reactions involve the formation of a carbocation intermediate, which then reacts with the benzene ring to yield the desired product. The Lewis acid catalyst facilitates the formation of the electrophile and stabilizes the carbocation intermediate.

2. FRIEDEL-CRAFTS ACYLATION:

Benzene reacts with an acyl chloride, using a Lewis acid catalyst, to produce an aromatic ketone.

CONCLUSION:

Benzene, with its distinct aromatic structure, exhibits a wide range of preparations and reactions. Electrophilic substitution reactions and addition reactions are the key pathways for the transformation of benzene. Understanding the mechanisms behind these reactions is crucial for enhancing our knowledge and application of this vital compound in various fields of chemistry.

REFERENCES:

Clayden, J., Greeves, N., Warren, S. Organic Chemistry (2nd ed.). Oxford University Press, 2012. https://iroch.ir/wp-content/uploads/2020/12/Clayden-Organic-Chemistry-2nd-edition-c2012-txtbk.pdf

Solomons, T. W. G., Fryhle, C. B. Organic Chemistry (12th ed.). Wiley, 2016. https://iroch.ir/wp-content/uploads/2020/12/Solomons-12e-Organic-Chemistry-Wiley-2016.pdf

March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.). Wiley, 2007.