Question

. Compare the reaction of benzene and toluene with chlorine (a) In presence of catalyst \(\mathrm{FeCl}_{3}\) (b) In presence of light

Short answer

Benzene undergoes electrophilic substitution to form chlorobenzene with FeCl3 and resists radical substitution under light. Toluene forms ortho-/para-chlorotoluene with FeCl3 but forms benzyl chloride under light.

Step-by-step solution

Understanding Benzene and Chlorine Reaction

When benzene reacts with chlorine, in the presence of a catalyst such as iron(III) chloride (FeCl3), it undergoes a substitution reaction. This is called electrophilic aromatic substitution. The chlorine substitutes one of the hydrogen atoms on the benzene ring, resulting in chlorobenzene and hydrochloric acid as a byproduct.

Benzene Reaction with Chlorine under Light

In the presence of light, benzene reacts differently with chlorine through a radical substitution reaction. This reaction isn't favored because benzene is aromatic and tends to resist substitution reactions that break the aromaticity. Chlorine radicals would disrupt the stable benzene ring, so this reaction rarely occurs without further activating reagents.

Understanding Toluene and Chlorine Reaction

Toluene, which is benzene with a methyl group, reacts with chlorine in the presence of FeCl3 through electrophilic aromatic substitution as well. However, the methyl group activates the ring and directs the incoming chlorine to the ortho and para positions, leading to mixtures of ortho- and para-chlorotoluene.

Toluene Reaction with Chlorine under Light

In the presence of light, toluene reacts with chlorine primarily by radical substitution, not on the benzene ring, but on the methyl group. This leads to the formation of benzyl chloride when chlorine replaces a hydrogen on the methyl group.

Key concepts

Benzene and Chlorine Reaction

Benzene is a remarkable organic compound known for its stable aromatic ring. When benzene encounters chlorine in the presence of a catalyst like iron(III) chloride (FeCl3), it undergoes an electrophilic aromatic substitution reaction. This means that the chlorine replaces a hydrogen atom on the benzene ring to form chlorobenzene, while hydrochloric acid is released as a byproduct. This reaction is typical because the catalyst helps to make the chlorine atom more electrophilic, allowing it to successfully attack the stable benzene ring and substitute hydrogen without compromising the aromaticity of benzene.
However, if benzene is exposed to chlorine under light, it follows a different path. Light can induce a radical substitution reaction, but benzene resists this because the stable aromatic ring does not easily undergo transformations that disrupt its structure. This kind of reaction requires harsh conditions or additional reagents, which is why it’s not commonly seen.

Toluene and Chlorine Reaction

Toluene, a derivative of benzene, has a methyl group attached to the benzene ring, making it more reactive than benzene. In the presence of a catalyst like FeCl3, toluene reacts with chlorine through electrophilic aromatic substitution. This substitution is particularly interesting, as the methyl group directs the incoming chlorine atoms to either the ortho or para positions relative to the methyl group. This results in a mix of ortho- and para-chlorotoluene products.
On the other hand, when toluene reacts with chlorine in the presence of light, the reaction follows a radical substitution mechanism. But instead of targeting the benzene ring, which would not favor stability, the substitution occurs on the methyl group. This leads to the formation of benzyl chloride, where a chlorine atom replaces one of the hydrogen atoms in the methyl group, showcasing the varied pathways toluene can take depending on conditions.

Radical Substitution Reaction

Radical substitution is a reaction type where radicals, highly reactive species with unpaired electrons, are involved. This reaction involves the substitution of atoms, often hydrogen, in a molecule through radical intermediates. It typically happens in the presence of light or heat, which provides the energy needed to form these radical species from molecules like chlorine.
In the context of toluene and chlorine, the radical substitution specifically occurs on the methyl side chain when exposed to light. This is in contrast to aromatic substitution, where the ring itself is modified. Radical substitutions are notable for their ability to break bonds and form new ones in a way that more stable reaction types, like electrophilic substitutions, may not proceed. This makes them quite versatile and powerful in organic chemistry.

Chemical Catalysts in Organic Reactions

Chemical catalysts play a crucial role in facilitating reactions in organic chemistry. They achieve this by lowering the activation energy necessary for a reaction, thereby increasing the reaction rate without being consumed in the process. In the case of benzene and toluene reactions with chlorine, catalysts like iron(III) chloride (FeCl3) are essential.

• Electrophilic aromatic substitution: The catalyst induces the formation of a more reactive chlorine species capable of attacking the aromatic ring.
• Regioselectivity: In toluene, the presence of the methyl group influences the direction of the catalyst-driven reaction, resulting in ortho- and para-product formation.

Catalysts not only enable reactions that might otherwise require harsh conditions but can also influence the pathways and products that result. Their strategic use is foundational in synthesizing a wide range of chemicals in both industrial and laboratory settings.