Question

\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{3}+\mathrm{Cl}_{2} \stackrel{800 \mathrm{~K}}{\longrightarrow} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}+\mathrm{HCl}\). This reaction takes place mainly by (a) \(\mathrm{S}_{\mathrm{N}} 1\) mechanism (b) \(\mathrm{S}_{\mathrm{N}} 2\) mechanism (c) free radical mechanism (d) E1 mechanism

Short answer

Answer: (c) free radical mechanism

Step-by-step solution

Understand the given reaction

In this reaction, toluene (\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CH}_{3}\)) reacts with chlorine (\(\mathrm{Cl}_{2}\)) at 800 K to form benzyl chloride (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\mathrm{Cl}\)) and hydrogen chloride (\(\mathrm{HCl}\)).

Examine the possibilities

We need to examine the characteristics of each mechanism and see which one is most suitable for this reaction. (a) \(\mathrm{S_N}1\) mechanism: This mechanism involves the formation of a carbocation intermediate where the nucleophile attacks the positively charged carbon atom. However, in this reaction, the intermediate formed would be a benzyl carbocation, which is not very stable. Thus, it is less likely for the reaction to proceed via this mechanism. (b) \(\mathrm{S_N}2\) mechanism: This mechanism involves a direct, concerted attack of the nucleophile on the substrate and does not involve the formation of intermediates. However, the reaction conditions (800 K) are very unfavorable for the \(\mathrm{S_N}2\) mechanism, and the benzyl group is not a good leaving group in this case. Thus, it is less likely for the reaction to proceed via this mechanism. (c) Free radical mechanism: This mechanism involves the formation of free radicals as intermediates. At high temperatures (such as 800 K), the homolytic cleavage of \(\mathrm{Cl}_{2}\) forms two highly reactive chlorine atoms, which can initiate the free radical mechanism. (d) E1 mechanism: This mechanism involves the formation of a carbocation intermediate and subsequent elimination of a leaving group in two separate steps. Since the formation of a benzyl carbocation is not favorable, it is less likely for the reaction to proceed via this mechanism.

Select the most plausible mechanism

Based on the analysis of each mechanism, the free radical mechanism seems to be the most plausible mechanism for this reaction. This is because of the high reaction temperature (800 K), which results in the formation of reactive chlorine atoms, capable of initiating the free radical pathway. Therefore, the correct answer is (c) free radical mechanism.

Key concepts

Free Radical Mechanism

Understanding the free radical mechanism is crucial to deciphering many organic reactions, especially those involving halogens. This mechanism involves the homolytic cleavage of bonds, where each atom retains one electron from the covalent bond. In the context of the given exercise, chlorine (\(\mathrm{Cl}_{2}\)) undergoes this process at high temperatures, producing two reactive chlorine radicals. These radicals are highly unstable and reactive due to their unpaired electrons. Here's how the process works:- **Initiation:** The bond between the two chlorine atoms in \(\mathrm{Cl}_{2}\) breaks, resulting in two chlorine radicals.- **Propagation:** One of these chlorine radicals reacts with toluene (\(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{3}\)), abstracting a hydrogen atom and forming hydrogen chloride (\(\mathrm{HCl}\)) and a benzyl radical.- **Termination:** The benzyl radical can react with another chlorine radical, forming benzyl chloride (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{Cl}\)).The main feature of a free radical mechanism is the chain reaction which continues until the reactants are consumed or the radicals are quenched.

Halogenation Reaction

Halogenation reactions involve the substitution of hydrogen atoms in hydrocarbons with halogen atoms. In our example, this process is demonstrated by the reaction of toluene (\(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{3}\)) with chlorine (\(\mathrm{Cl}_{2}\)) to form benzyl chloride (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{Cl}\)). The reaction typically proceeds via a radical mechanism, especially at elevated temperatures or in the presence of UV light, as they favor the formation of radical species. Here are some key points about halogenation:- **Selectivity:** Chlorination is less selective compared to bromination due to the higher reactivity of chlorine radicals. This means chlorination may result in a mixture of products, although in this specific scenario, the benzyl position is preferred.- **Regioselectivity:** In our case, the benzyl position (the CH3 group) is the most reactive site for chlorination due to the stability of the resultant benzyl radical.Halogenation is a crucial step in organic synthesis, altering the chemical properties of the hydrocarbon and allowing for further functionalization.

Reaction Conditions

The reaction conditions play a pivotal role in determining the mechanism and outcome of chemical reactions. In the provided exercise, the high temperature of 800 K is a decisive factor that steers the reaction through a free radical mechanism. Here's how temperature affects the reaction conditions:- **Energy Input:** High temperatures supply the required energy for bond dissociation, like the homolytic cleavage of \(\mathrm{Cl}_{2}\) into chlorine radicals.- **Reaction Rate:** Elevated temperatures typically increase the rate of reaction by increasing the kinetic energy of the molecules, facilitating more frequent and successful collisions.- **Mechanistic Influence:** The high temperature favors radical pathways over mechanisms like \(\mathrm{S_N}1\) or \(\mathrm{S_N}2\), which would require either stable carbocations or a good leaving group, conditions less favorable in this context.Overall, reaction conditions such as temperature are critical in dictating the pathway and efficiency of chemical reactions, ensuring the desired products.

Toluene Chemistry

Toluene (\(\mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{3}\)) is a simple aromatic hydrocarbon and a key player in organic synthesis. Understanding its chemistry helps in rationalizing its behavior in various reactions, such as the halogenation of toluene to form benzyl chloride. Here are some important aspects of toluene chemistry:- **Aromatic Stability:** The benzene ring in toluene provides stability due to resonance, but it is also a site for potential electrophilic aromatic substitution reactions.- **Side Chain Reactivity:** The methyl group in toluene is particularly reactive and more susceptible to free radical substitution than the aromatic ring, especially under conditions that favor radical formation.- **Industrial Relevance:** Toluene is a widely used starting material in the chemical industry, employed in the synthesis of numerous compounds due to its reactivity and simplicity.In reactions like the one provided, toluene's side chain reactivity facilitates the substitution with chlorine, creating valuable intermediates like benzyl chloride, which are useful in various industrial applications.