Question

In the reaction \(\mathrm{HC} \equiv \mathrm{CH} \stackrel{\mathrm{x}}{\longrightarrow} \mathrm{Cl}_{2} \mathrm{CH}-\mathrm{CHCl}_{2}, \mathrm{X}\) is (1) \(\mathrm{NaOCl}\) (2) \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\) (3) chlorine water (4) \(\mathrm{NaCl}\)

Short answer

The correct reagent is \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\) (Option 2).

Step-by-step solution

- Understand the Reaction

Examine the given chemical reaction: \[ \mathrm{HC} \equiv \mathrm{CH} \stackrel{\mathrm{x}}{\longrightarrow} \mathrm{Cl}_{2} \mathrm{CH}-\mathrm{CHCl}_{2} \]Here, a molecule of acetylene (ethyne) is converted into 1,1,2-trichloroethane. The task is to identify the reagent 'X' that facilitates this transformation.

- Identify Applicable Reagents

List the given reagent options: 1) \(\mathrm{NaOCl}\), 2) \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\), 3) chlorine water, 4) \(\mathrm{NaCl}\). Determine which could add chlorine atoms to hydrogen and carbon atoms.

- Evaluate Each Reagent

Analyze each potential reagent:1) \(\mathrm{NaOCl}\) - Typically used for oxidizing reactions, not chlorination.2) \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\) - Known for adding chlorine atoms across a triple bond.3) Chlorine water - May add chlorine but does not specifically give a clean trichloroethane product.4) \(\mathrm{NaCl}\) - Common salt, no reaction for chlorination.

- Select the Correct Option

Among the provided options, \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\) is the reagent that facilitates the addition of chlorine to both carbon atoms and converts acetylene into 1,1,2-trichloroethane.

Key concepts

Chlorination Reaction

In organic chemistry, a chlorination reaction involves adding chlorine (Cl) atoms to an organic molecule. This process often changes the properties of the molecule significantly. When studying chlorination, consider:

• The type of bond where chlorine is added (single, double, or triple bond)
• The reaction conditions and reagents used
• The specific chlorination product formed

In our exercise, chlorination transforms acetylene (ethyne) into 1,1,2-trichloroethane. Chlorine atoms are added to the carbon atoms, saturating a triple bond. This reaction is essential in converting less reactive hydrocarbons into more chemically active chlorinated products.

Reagent Selection

Choosing the right reagent is crucial for a successful chemical reaction. Here, the right reagent converts acetylene into 1,1,2-trichloroethane. Our reagent options are:

• aOCl: Used mainly for oxidation, it wouldn't chlorinate acetylene.
• Cl2 / CCl4: Known for chlorinating triple bonds, making it a prime candidate.
• Chlorine water: Adds chlorine in a less controlled manner, leading to different products.
• NaCl: Common table salt, inert in this context.

Cl2 / CCl4 correctly adds chlorine across the triple bond of acetylene. Thus, selecting this reagent ensures efficient, controlled chlorination and yields the desired 1,1,2-trichloroethane.

Ethyne Reaction

Ethyne, also called acetylene, is a simple hydrocarbon with a carbon-carbon triple bond (eq C-HC). This highly reactive molecule undergoes different transformations depending on the reagents used. When subjected to the chlorination reaction with Cl2 / CCl4, the triple bond (qm) converts to single bonds, forming 1,1,2-trichloroethane:

• The triple bond breaks, allowing chlorine to add across the carbons.
• Chlorine atoms saturate the bond, forming three new C-Cl bonds.
• The final product is a stable, chlorinated alkane.

This reaction exemplifies how the reactivity of ethyne can be harnessed for significant molecular transformations, expanding the scope of organic synthesis.