Question

Which of the following is the end product of the treatment of benzene with an excess of \(\mathrm{Cl}_{2}\) in the presence of \(\mathrm{I}_{2}\) ? (1) Monochlorobenzenc (2) Dichlorobenzme (3) Trichlorobenzene (4) IIexachlorocyclohexane

Short answer

Option (4) Hexachlorocyclohexane

Step-by-step solution

Understand the Reaction Conditions

When benzene is treated with \( \mathrm{Cl}_{2} \ \) in the presence of \( \mathrm{I}_{2} \), it promotes a substitution reaction where chlorine atoms replace hydrogen atoms on the benzene ring.

Identify Result of Multiple Chlorination

An excess of \( \mathrm{Cl}_{2} \ \) in the reaction mixture suggests that the chlorine atoms will fully saturate the benzene ring. This implies that each hydrogen on the benzene ring could be replaced by a chlorine atom.

Determine the Final Product

With all six hydrogen atoms being replaced by chlorine atoms, the end product will be fully chlorinated, resulting in \( \mathrm{C}_{6}\mathrm{Cl}_{6} \). This compound is called hexachlorocyclohexane.

Select the Correct Option

From the given options, the compound that matches \( \mathrm{C}_{6}\mathrm{Cl}_{6} \ \) hexachlorocyclohexane is option (4).

Key concepts

Substitution Reaction

In a substitution reaction, one atom or group of atoms in a molecule is replaced by another atom or group of atoms. This type of reaction is common in organic chemistry, particularly in aromatic compounds like benzene. Benzene, a hydrocarbon with the formula \( \text\bf\text\bfC_6H_6 \), undergoes substitution reactions when treated with different reagents. For instance, when benzene is exposed to chlorine, the chlorine atoms can replace hydrogen atoms on the benzene ring. This process is influenced by certain conditions—such as the presence of a catalyst like iodine (I\text\bf2\text\bf) – which help in promoting the substitution reaction.

During the chlorination process, each chlorine molecule (Cl\_\text\bf2\text\bf) can substitute a single hydrogen atom within the benzene ring. Hence, for each Cl\_\text\bf2\text\bf molecule introduced, you get one hydrogen atom replaced by a chlorine atom. This continues until all hydrogen atoms in the benzene molecule are replaced, depending on the amount of chlorine available.

Excess Chlorine

The term 'excess chlorine' refers to a situation where chlorine is available in a greater amount than is needed for a complete reaction. Introducing excess chlorine in a benzene chlorination reaction ensures that all available hydrogen atoms are replaced by chlorine atoms. This is crucial for achieving a fully chlorinated product.

In our specific reaction, when we use benzene (C_6H_6) and expose it to excess Cl_2 in the presence of I_2, we ensure that every hydrogen atom on the benzene ring is substituted by chlorine. The initial step sees one chlorine atom replacing one hydrogen atom, but because we have excess chlorine, the process continues until all six hydrogen atoms are replaced. Hence, using excess chlorine is key to fully saturating the molecule and reaching our final product, which is a completely substituted hexachlorocyclohexane.

Hexachlorocyclohexane

Hexachlorocyclohexane (HCH), also known as benzene hexachloride or by its chemical formula C_6Cl_6, is the final product when benzene undergoes complete chlorination with excess chlorine. It contains six chlorine atoms substituted in place of all six hydrogen atoms originally present in benzene.

This compound can exist in several isomeric forms, depending on the spatial arrangement of the chlorine atoms around the cyclohexane ring. The most notable isomer is the gamma-isomer, known as Lindane, which has been used historically as an insecticide. Hexachlorocyclohexane is known for its high chlorine content, which makes it a significant molecule in industrial chemical production.

Understanding how to synthesize such fully substituted compounds is crucial in organic chemistry, particularly for developing various applications in the pharmaceutical and agricultural industries. In the context of the given exercise, recognizing hexachlorocyclohexane as the end product of benzene chlorination in the presence of excess chlorine is essential.