Question

When toluene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\right)\) reacts with chlorine gas in the presence of iron(III) catalyst, the product is a mixture of the ortho and para isomers of \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{ClCH}_{3} .\) However, when the reaction is light-catalyzed with no \(\mathrm{Fe}^{3+}\) catalyst present, the product is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}\). Explain.

Short answer

In conclusion, the different products formed in the reaction between toluene and chlorine gas under the two conditions are due to their respective mechanisms. In the presence of an iron(III) catalyst, the electrophilic aromatic substitution mechanism leads to the formation of a mixture of ortho and para isomers of C6H4ClCH3. In contrast, when the reaction is light-catalyzed with no Fe³⁺ catalyst, the radical halogenation mechanism occurs, leading to the formation of C6H5CH2Cl. These distinct mechanisms, determined by the presence or absence of an Fe³⁺ catalyst and light-catalysis, influence the ultimate substitution sites and products of the reaction.

Step-by-step solution

Understand the reactions under different conditions

When toluene reacts with chlorine gas in the presence of an iron(III) catalyst, the reaction is known as a halogenation reaction, specifically, electrophilic aromatic substitution. The iron(III) catalyst polarizes the chlorine molecules, turning them into electrophiles that can be more easily attacked by the electron-rich aromatic ring of toluene. This reaction favors the ortho and para positions on the aromatic ring due to the directing nature of the methyl or -CH3 group in toluene. The ortho and para isomers are favored over the meta isomer due to resonance structures and inductive effects. On the other hand, when the reaction is light-catalyzed and no iron(III) catalyst is present, the reaction is radical halogenation. This type of reaction replaces an aliphatic hydrogen with a halogen (chlorine, in this case) and involves the formation of radicals. The aromatic ring remains mostly unaffected in this reaction, and the product is formed through the homolytic cleavage of the C-H bond leading to the formation of a chlorine-substituted carbon atom in the benzyl (CH2) position.

Explain the differences in the products

The difference in the products of these two reactions can be attributed to their respective mechanisms: 1. Electrophilic Aromatic Substitution (Fe³⁺ catalyst present): In this reaction, the chlorine molecule is activated by the Fe³⁺ catalyst, making it an electrophile that is then attacked by the electron-rich aromatic ring of toluene. The electron flow proceeds through the ring, with the ortho and para positions favored due to resonance stabilization and inductive effects caused by the presence of the methyl group (-CH3) in toluene. This directing nature of the CH3 group leads to the formation of a mixture of ortho and para isomers of C6H4ClCH3. 2. Radical Halogenation (light-catalyzed, no Fe³⁺ catalyst): In this reaction, the absence of an iron(III) catalyst and the presence of light lead to a different mechanism. The reaction proceeds through a radical pathway, with the initiation step involving the homolytic cleavage of the chlorine molecule into two chlorine radicals. These chlorine radicals then react with toluene, replacing an aliphatic hydrogen in the -CH3 group, which forms a benzyl radical. This benzyl radical goes on to react with another chlorine atom, resulting in the formation of C6H5CH2Cl as the product. The aromatic ring remains mostly unaffected in this process, and the chlorine substitution occurs on the benzyl position. In conclusion, the different products seen in these reactions can be explained by the differing mechanisms of electrophilic aromatic substitution and radical halogenation, which are determined by the presence or absence of an iron(III) catalyst and light-catalysis.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a critical concept in organic chemistry. It's a type of reaction where an atom on an aromatic ring (often hydrogen) is replaced by an electrophile. This mechanism is widely seen in reactions involving benzene derivatives. Here, the aromatic ring acts as a nucleophile due to its high electron density.

In the case of toluene reacting with chlorine gas in the presence of an iron(III) catalyst, the process begins when chlorine molecules are polarized by the catalyst, forming a powerful electrophile. The methyl group in toluene is an ortho/para director because it donates electron density to the ring via hyperconjugation and inductive effects, stabilizing the formation of the intermediate sigma complex at ortho and para positions. Thus, the result is mainly the ortho and para isomers of chlorotoluene. Understanding how catalysts influence these reactions helps predict the positions where substitution will occur.

Key points to note:

• The electron-rich aromatic ring makes it susceptible to attack by electrophiles.
• The catalyst plays a crucial role in polarizing the halogen.
• The methyl group (-CH₃) directs the substitution to ortho and para positions.

Radical Halogenation

Radical halogenation is a process where a molecule undergoes a substitution involving free radicals. Free radicals are atoms or groups of atoms with unpaired electrons. These are highly reactive and initiate chain reactions.

In a light-induced reaction environment with toluene and chlorine, the mechanism is different. It involves homolytic cleavage, where light breaks the chlorine molecule into two chlorine radicals. These radicals are extremely reactive. The toluene, in the absence of a catalyst, doesn't undergo electrophilic substitution but rather radical substitution.

The reaction occurs at the methyl (-CH₃) group in toluene, where a hydrogen atom is replaced by chlorine through a series of radical chain reactions. This results in the formation of benzyl chloride ( C₆H₅CH₂Cl a). The aromatic ring stays mostly intact, showing that radical substitution often targets non-aromatic C-H bonds.

Highlights of the radical halogenation process:

• Initiated by light, causing homolytic fission of the chlorine molecule.
• Targets the hydrogen at the methyl group, not the aromatic ring.
• Results in the substitution product at the benzylic position.

Toluene Chlorination

Toluene chlorination is an excellent example of how different reaction conditions can lead to varied products. Depending on whether we use a catalyst or light alone, the approach and outcomes change.

With the presence of an iron(III) catalyst, the mechanism follows electrophilic aromatic substitution, yielding ortho and para isomers. The methyl group facilitates and directs the substitution due to its electron-donating properties.

When the reaction is catalyzed by light and without metal catalysts, the mechanism shifts to radical halogenation, targeting the benzylic position rather than influencing the aromatic ring. The light catalysis breaks chlorine into radicals that drive the reaction, illustrating the power of different conditions in directing chemical reactions.

Essentials of toluene chlorination:

• Different conditions result in entirely different products.
• The presence of a catalyst enriches substitution at the ring.
• Light catalysis targets different molecular positions due to the radical pathway.