Problem 100 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following reactions: \(\mathrm{X}+\mathrm{HCl} \frac{\text { Anhyd } \mathrm{AlCl}_{3}}{\text { (addition) }} \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) \(\frac{\text { Anhyd. } \mathrm{ZnCl}_{2} / \mathrm{HCl}_{3}}{\text { (substitution) }}\) \(\mathrm{Y}\) can be converted to \(\mathrm{X}\) on heating with \(\ldots \ldots\) at..........temperature. (a) \(\mathrm{Cu}, 300^{\circ} \mathrm{C}\) (b) \(\mathrm{Al}_{2} \mathrm{O}_{3}, 350^{\circ} \mathrm{C}\) (c) \(\mathrm{NaOH} / \mathrm{I}_{2}, 60^{\circ} \mathrm{C}\) (d) \(\mathrm{Ca}(\mathrm{OH})_{2}^{2}+\mathrm{CaOCl}_{2}, 60^{\circ} \mathrm{C}\)

Step-by-Step Solution

Answer

Y can be converted to X using \( \text{Ca(OH)}_2 + \text{CaOCl}_2 \) at \( 60^{\circ}C \). The answer is (d).

Step 1: Analyze the Given Reactions

The problem provides two reactions involving an unknown compound X and Y. The first reaction indicates the formation of \( \text{C}_2\text{H}_5\text{Cl} \) (ethyl chloride) through a process with HCl and anhydrous AlCl3, suggesting Friedel-Crafts alkylation where an alkane is involved. The second part describes that Y can be converted to X through a process involving substitution, using anhydrous ZnCl2 and HCl.

Step 2: Identify Compound X

From the first reaction, the use of HCl and anhydrous AlCl3 indicates that X is likely to be an unsaturated hydrocarbon (like an alkene or alkyne) that undergoes an addition reaction to form ethyl chloride, which suggests that X might be ethylene \( \text{C}_2\text{H}_4 \).

Step 3: Identify the Process and Temperature for Conversion

The problem states that Y can be converted to X. Knowing that X is \( \text{C}_2\text{H}_4 \), we need a mechanism to convert an alkane to an alkene. Given the options, \( \text{(d) Ca(OH)}_2 + \text{CaOCl}_2, 60^{\circ} \text{C} \) is a process used in the chemical industry to produce acetylene, which suggests that Y could be an acetylide, which when decomposed produces ethylene.

Step 4: Determine the Correct Answer Choice

Since option (d) involves producing \( \text{C}_2\text{H}_4 \) (ethylene), a known intermediate for the formation of ethyl chloride in option (d) using appropriate catalysts and conditions, this aligns with the information from the previous steps.

Key Concepts

Friedel-Crafts alkylation

Friedel-Crafts alkylation is a crucial reaction in organic chemistry, often used to attach an alkyl group to an aromatic ring. This reaction involves the use of a Lewis acid catalyst, such as aluminum chloride (AlCl₃), to facilitate the transfer of an alkyl group to the aromatic substrate. In our original exercise, the reaction utilizes anhydrous AlCl₃ along with HCl to form ethyl chloride, suggesting the presence of an unsaturated hydrocarbon which undergoes this process.

• The reaction can produce either an alkane or aromatic compound, although the mechanism differs slightly for each.
• In aromatic systems, the Friedel-Crafts alkylation involves a substitution process where the alkyl group replaces a hydrogen atom on the aromatic ring.
• In a non-aromatic substrate like an unsaturated hydrocarbon, the process transitions into an addition reaction, where the alkyl group attaches across the unsaturated bond, converting, for example, an alkene into an alkane.

Whether it involves substitution or addition, the presence of a strong catalyst like AlCl₃ is critical as it forms a complex with the alkyl halide, making it more electrophilic and ready to react.

Unsaturated hydrocarbons

Unsaturated hydrocarbons are organic compounds containing double or triple bonds between carbon atoms. These hydrocarbons include alkenes, alkynes, and aromatic hydrocarbons.

• Alkenes have one or more carbon-carbon double bonds, represented as C=C.
• Alkynes contain carbon-carbon triple bonds, represented as C≡C.
• Aromatic hydrocarbons represent a special class with alternating double bonds that create a delocalized \( \pi \) electron system.

In the given exercise, the compound X is likely an unsaturated hydrocarbon, most probably ethylene (C₂H₄), a simple alkene. This assumption is based on the process described: the addition of HCl in the presence of anhydrous AlCl₃ aligns with the behavior of alkenes reacting to form alkyl halides.
Unsaturated hydrocarbons participate in a variety of reactions, typically characterized by the addition across double or triple bonds. Such reactions convert the structure into a more saturated form, as seen when alkenes react with halogens.

Reaction mechanisms

Understanding reaction mechanisms is critical in organic chemistry. A reaction mechanism describes the step-by-step process by which reactants transform into products. This includes details about bonds breaking and forming, rearrangements, and the role of catalysts.

• Reaction mechanisms allow chemists to predict products of reactions.
• They provide insights into optimizing reaction conditions, including temperature, catalyst choice, and reactant concentrations.

In the original problem, the mechanism involves two key concepts—the transformation of Y to X, and X into ethyl chloride. The conversion of Y to X likely involves dehydrohalogenation, potentially using the combination of Ca(OH)₂ and CaOCl₂. This process is followed by Friedel-Crafts alkylation or addition reaction to form the final product.
The mechanism highlights the importance of selecting appropriate conditions and catalysts to direct the reaction pathway towards the desired outcome. Proper temperatures and catalyst selections enable process efficiency and product yield, crucial in both academic and industrial chemical syntheses.