Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

Propene when heated with \(\mathrm{Cl}_{2}\) at \(500^{\circ} \mathrm{C}\) forms (a) \(\mathrm{CH}_{3}-\mathrm{CHCl}-\mathrm{CH}_{2} \mathrm{Cl}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}-\mathrm{CH}=\mathrm{CH}_{2}\) (c) \(\mathrm{CH}_{2} \mathrm{Cl}-\mathrm{CHCl}-\mathrm{CH}_{2} \mathrm{Cl}\) (d) all of these

Step-by-Step Solution

Answer

All three products (a), (b), and (c) can form; thus, the answer is (d) all of these.

Step 1: Analyze the Reaction Conditions

In this step, we consider the reaction conditions: propene is being heated with chlorine at a high temperature of 500°C. Such conditions are typical for free radical halogenation reactions.

Step 2: Identify Possible Free Radical Substitution Sites

Propene (\( \text{CH}_3- ext{CH}= ext{CH}_2 \) ) contains both a double bond and allylic hydrogens. Under these conditions, chlorine radicals can replace allylic hydrogens.

Step 3: Determine Major Product

The high temperature suggests free radical halogenation will prefer allylic sites. Thus, the major product is likely allylic chlorination, such as \( \text{CH}_2 \text{Cl}- ext{CH}= ext{CH}_2 \) .

Step 4: Evaluate Other Products

All products mentioned could form in different proportions. At such high temperatures, even di-substitution like \( \text{CH}_2 \text{Cl}- ext{CHCl}- ext{CH}_2 \text{Cl} \) could occur alongside the major product and mono-substitution products.

Step 5: Conclusion on Reaction Outcome

Given the high temperature and free radical environment, all three products listed in options (a), (b), and (c) could indeed form through allylic and subsequent possible chlorination pathways.

Key Concepts

Propene Chlorination

When propene undergoes chlorination, it typically involves the addition or substitution of chlorine atoms on the propene molecule. This process mainly involves free radical halogenation. Propene (\(\text{CH}_3-\text{CH}=\text{CH}_2\)) has a double bond, which makes it a site of interest for chemical reactions. However, when heating with \(\mathrm{Cl}_2\) at high temperatures, it is the allylic hydrogen atoms that are more reactive and become the primary site for substitution.

In free radical halogenation, chlorine radicals are generated that seek out the weak allylic C-H bonds of propene rather than the stronger C-H bonds or C=C bonds. The reaction is initiated by the homolytic cleavage of \(\mathrm{Cl}_2\), forming chlorine radicals. These highly reactive radicals abstract hydrogen atoms from allylic positions, forming a more stable allylic radical, which then captures another chlorine atom to form chlorinated products. This is due to the formation of resonance-stabilized allylic radicals, which are more favorable in this high-energy environment.

Allylic Substitution

Allylic substitution is a significant reaction mechanism in organic chemistry. It specifically refers to the process where a hydrogen atom at the allylic position is replaced by a chlorine atom or another electrophile. The allylic position refers to the carbon atoms adjacent to the carbon-carbon double bond. Propene, for instance, has allylic hydrogens that are specifically targeted in chlorination reactions.

• In an allylic substitution, the reaction proceeds through the formation of an allylic radical, which is resonance-stabilized.
• This stability makes the reaction favorable, particularly under high temperatures and radical-forming conditions.
• Allylic substitution leads to products where the chlorine is bonded to an allylic carbon, such as \(\text{CH}_2\text{Cl}-\text{CH}=\text{CH}_2\).

Moreover, the formation of multiple isomeric products is possible due to the delocalization of electrons across the allylic radical. This contributes to the mixtures of substituted products often observed under these conditions.

Reaction Conditions

The reaction conditions greatly influence the outcome of chemical reactions. In the case of propene chlorination, the reaction takes place at a high temperature of around \(500^{\circ} \mathrm{C}\), which is essential for initiating free radical reactions. Generally, high temperatures are required to generate chlorine radicals by breaking the \(\mathrm{Cl}_2\) bond homolytically.

These conditions favor the formation of multiple types of products due to the activation of different reaction pathways:

• Mono-substitution at classical allylic sites, leading to products like \(\text{CH}_2\text{Cl}-\text{CH}=\text{CH}_2\).
• In certain conditions, especially prolonged exposure, di-substitution products such as \(\text{CH}_2\text{Cl}-\text{CHCl}-\text{CH}_2\text{Cl}\) may also form.

The competition between these pathways is dictated by the stability of intermediates and the energy input from reaction conditions. All three products considered in the original problem could be viable depending on these conditions, illustrating the complexity and diversity of outcomes in radical halogenation processes.