Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following sequence of reagents can be used for the conversion of \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{3}\) into \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2} ?\) (a) \(\mathrm{SOCl}_{2} ; \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{SO}_{2} \mathrm{Cl}_{2} ;\) alc. \(\mathrm{KOH}\) (c) \(\mathrm{Cl}_{2} / \mathrm{hv} ; \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{SOCl}_{2} ;\) alc. \(\mathrm{KOH}\)

Step-by-Step Solution

Answer

Option (c) with \(\mathrm{Cl}_{2} / \mathrm{hv} \) and alc. KOH is correct.

Step 1: Analyze the Target Structure

The target molecule is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\), which is a styrene derivative. This alkene needs to be formed from an alkane.

Step 2: Identify Initial Functional Group Transformation

The starting molecule, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (ethylbenzene), can undergo a halogenation reaction at the benzylic position (the carbon next to an aromatic ring) when exposed to \(\mathrm{Cl}_{2}\) and light (\(hv\)). This forms \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\mathrm{CH}_2\mathrm{Cl}\).

Step 3: Introduce the Alkene via Dehydrohalogenation

The haloalkane \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2}\mathrm{CH}_2\mathrm{Cl}\) can undergo a dehydrohalogenation reaction in the presence of alcoholic potassium hydroxide (alc. KOH), which will remove the hydrogen and halide to form the double bond \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\).

Step 4: Verify the Correct Sequence

Review the reagent sequences given in the options. The sequence that first chlorinates with \(\mathrm{Cl}_{2}\) and \(hv\), followed by treatment with alc. KOH, is the correct mechanism. In this case, it is option (c), \(\mathrm{Cl}_{2} / \mathrm{hv} ; \mathrm{H}_{2} \mathrm{O}\).

Key Concepts

Styrene Formation

Styrene, a significant industrial chemical, is an important building block for polymers in products like rubber and plastic. The formation of styrene from simpler organic molecules involves various reactions, predominantly focusing on the introduction of a double bond, known as an alkene. In the context of organic synthesis, turning ethylbenzene into styrene requires a specific set of reactions that alter molecular structure without modifying the main benzene ring.

The double bond in styrene is added through a process called dehydrohalogenation, which removes elements of hydrogen and halogens from adjacent carbon atoms, allowing a double bond to form. Here, the end goal is to transform the single-bonded hydrocarbon chains into a compound with a characteristic vinyl group (-CH=CH2) next to the benzene ring. This requires precise control over reaction conditions to precisely guide the stepwise shifts in chemical structure, moving from a less reactive to a more reactive compound, culminating in styrene formation.

Halogenation

Halogenation is a fundamental organic chemistry reaction where one or more halogens (like chlorine or bromine) are introduced into an organic compound. It is a critical step in converting ethylbenzene to styrene. Through the process of radical halogenation, the ethyl group's hydrogen is substituted by a chlorine atom.

This reaction is initiated by light (usually ultraviolet light). The energy from light splits a chlorine molecule (Cl_{2}) into two highly reactive chlorine radicals. These radicals are then able to abstract hydrogen from the substrate, in this case, ethylbenzene (C_{6}H_{5}CH_{2}CH_{3}), creating a benzylic radical.

• Halogens like chlorine are chosen due to their high electronegativity and reactivity.
• Propagation of the radical chain ensures all hydrogen atoms in the target position are substituted to form the halide.

Through this mechanism, C_{6}H_{5}CH_{2}CH_{2}Cl is formed, a key intermediate precursor for further reactions towards forming styrene.

Dehydrohalogenation

Dehydrohalogenation is a chemical process that removes a hydrogen halide from an organic compound. This is crucial in converting a haloalkane to an alkene. After halogenation, the next step involves dehydrohalogenation, where C_{6}H_{5}CH_{2}CH_{2}Cl is transformed into styrene

In the context of this reaction:

• Alcoholic potassium hydroxide (alc. KOH) is often used to facilitate the process where it attacks the C_{6}H_{5}CH_{2}CH_{2}Cl molecule.
• The base abstracts a proton from the CH_{2} group, while simultaneously the chloride ion is displaced, forming a double bond.

Dehydrohalogenation helps stabilize the molecule, leaving a conjugated system of double bonds adjacent to the benzene ring in the newly formed styrene. This reaction not only creates the needed double bond but also enhances the reactivity of the compound for further synthetic applications.