Question

In the given reaction, \(\mathrm{C}_{7} \mathrm{H}_{8} \stackrel{3 \mathrm{Cl} \Delta}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{Br}_{2} / \mathrm{Fe}}{\longrightarrow} \mathrm{B} \stackrel{\mathrm{Zn} / \mathrm{HCl}}{\longrightarrow} \mathrm{C}\) The compound \(\mathrm{C}\) is (a) o-bromotoluene (b) m-bromotoluene (c) p-bromotoluene (d) 3 -bromo-2, 2, 6-trichlorotoluene

Short answer

The compound C is (c) p-bromotoluene.

Step-by-step solution

Identify Starting Compound

The reaction starts with the compound \(\text{C}_7\text{H}_8\), which is commonly known as toluene. It is a benzene ring with a methyl group attached to it.

Analyze the First Reaction

The first step uses 3 moles of chlorine (Cl) in presence of heat (\(\Delta\)) to form compound A. This halogenation process generally occurs at the methyl group, which will transform to \(\text{C}_6\text{H}_5\text{CCl}_3\), trichloromethylbenzene or benzyl trichloride.

Analyze the Second Reaction

In the second step, compound A undergoes a bromination reaction with Bro 2 in presence of Fe, which typically affects the aromatic ring; it directs the bromine to the para position due to the steric effect of a bulky CClo 3 group, forming compound B: \(\text{p-bromo-}\text{trichloromethylbenzene}\).

Analyze the Third Reaction

The third reaction involves reducing the trichloromethyl group with Zn/HCl. This reduces the CClo 3 group back to a methyl group, resulting in the formation of compound C: \(p\text{-bromotoluene}\).

Determine the Final Compound

Given the transformations, compound C from the complete reaction sequence is \(p\text{-bromotoluene}\).

Key concepts

Halogenation

Halogenation is a fundamental reaction in organic chemistry that involves the introduction of halogen atoms into a compound. In the case of toluene (\(\text{C}_7\text{H}_8\)), the process involves replacing hydrogen atoms with halogen atoms. Specifically, when toluene is treated with chlorine (\(\text{Cl}_2\)) along with heat (\(\Delta\)), it undergoes a halogenation reaction.
The key step in this process is the substitution of the hydrogens in the methyl group. This type of reaction produces trichloromethylbenzene.

• Chlorination: A common method of halogenation that's particularly useful with aromatic compounds.
• Controlled Conditions: Temperature and presence of catalysts influence the degree and position of substitution in halogenation.

Understanding halogenation helps in predicting the structure and properties of the resulting compounds, making it easier for chemists to synthesize desired molecules.

Bromination

Bromination is another essential reaction used to introduce bromine atoms into organic molecules. In our given reaction sequence, compound A, known as trichloromethylbenzene, undergoes bromination to form compound B.
This process uses \(\text{Br}_2\) in the presence of iron (Fe) as a catalyst, which facilitates the substitution of hydrogen in the aromatic ring, particularly at the para position relative to the bulky trichloromethyl group.

• Para Position: This is favored because of the steric hindrance; it's the position opposite that of the existing bulky group on a benzene ring, leading to less crowding.
• Electrophilic Aromatic Substitution: Bromination typically follows this mechanism, where the aromatic ring acts as a nucleophile to attack the positively charged bromine ion.

Understanding how bromination works offers insights into the design and prediction of chemical structures based on aromatic compounds.

Reduction Reaction

A reduction reaction in organic chemistry involves the gain of electrons or a decrease in oxidation state. In the example provided, the final step of the reaction involves reducing trichloromethylbenzene to bromotoluene using zinc (Zn) and hydrochloric acid (HCl).
This reduction converts the trichloromethyl group back to a methyl group, effectively removing the chlorine atoms.

• Zinc as a Reducing Agent: Commonly used in organic reactions to facilitate the transfer of electrons or protons.
• Reversal of Halogenation: Reduction can undo certain aspects of prior halogenation, offering a strategic way to modify molecules after halogenation.

Reduction reactions are key in reshaping molecules through the removal or alteration of specific groups, leading to desired molecular structures.

Aromatic Compounds

Aromatic compounds are a class of compounds characterized by their stable ring structures and electron cloud systems. The most well-known aromatic compound is benzene, which serves as the basis for toluene and other related compounds.

Toluene, a simple aromatic compound, consists of a methyl group attached to a benzene ring.

• Stability: Owing to their resonance structures, aromatic compounds like benzene are notably stable.
• Reactivity: Despite their stability, aromatic compounds undergo specific reactions such as halogenation, allowing for modifications without losing their aromatic nature.

Aromatic compounds are foundational to organic chemistry and are central to many chemical processes, providing an essential backdrop for understanding reactions involving halogenation, bromination, and reductions.