Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Cl} \stackrel{\text { alc. } \mathrm{KOH}}{\longrightarrow}(\mathrm{X}) \stackrel{\mathrm{HBr}}{\longrightarrow}(\mathrm{Y}) \stackrel{\mathrm{Na}_{\text {ether }}}{\longrightarrow}(\mathrm{Z})\) In the above sequence of reaction, the product \((Z)\) is (a) propane (b) hexane (c) 2,3 -dimethylbutane (d) allyl bromide

Step-by-Step Solution

Answer

The product \((Z)\) is hexane.

Step 1: Identify the First Reaction

The first reaction involves the compound \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{Cl} \) with alcoholic KOH. This reaction typically leads to a dehydrohalogenation, which is an elimination reaction. It removes \( \text{HCl} \), resulting in the formation of propene \( \mathrm{CH}_{3}-\mathrm{CH} = \mathrm{CH}_{2} \).

Step 2: Understand the Second Reaction

The second step involves the compound propene \( \mathrm{CH}_{3}-\mathrm{CH} = \mathrm{CH}_{2} \) reacting with \( \text{HBr} \). This would lead to the addition of \( \text{HBr} \) across the double bond following Markovnikov's rule, resulting in 2-bromopropane \( \mathrm{CH}_{3}-\mathrm{CHBr}-\mathrm{CH}_{3} \).

Step 3: Determine the Third Reaction

The third reaction involves 2-bromopropane \( \mathrm{CH}_{3}-\mathrm{CHBr}-\mathrm{CH}_{3} \) reacting with sodium in ether, which is a typical setup for the Wurtz reaction. This reaction causes a coupling of two alkyl halide molecules, resulting in the formation of a larger alkane. Here, the coupling of two 2-bromopropane molecules leads to the formation of hexane \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}_{3} \).

Key Concepts

Dehydrohalogenation

Dehydrohalogenation is a type of elimination reaction found in organic chemistry. This reaction involves the removal of a hydrogen atom and a halogen atom from a molecule, typically leading to the formation of a double bond. In simpler terms, dehydrohalogenation turns alkyl halides, which are carbon chains with a halogen, into alkenes, which are carbon chains with a double bond.
This reaction is common when an alkyl halide reacts with a base. For instance, when alcoholic potassium hydroxide (KOH) is used, it facilitates the removal of hydrogen and chlorine from the molecule. Think of it as a way to 'unwrap' parts of the molecule to reveal a new structure, often making it more complex and reactive. In our exercise, 1-chloropropane reacts with alcoholic KOH to remove HCl, resulting in propene, a simple alkene.
Key factors affecting this reaction include the strength of the base and the structure of the starting alkyl halide. Primary alkyl halides, like the one in our example, typically react more readily under these conditions, leading to efficient double bond formation.

Markovnikov's Rule

Markovnikov's Rule is a principle in organic chemistry that helps predict the outcome of addition reactions to alkenes. When adding a hydrogen halide (like HBr) to an unsymmetrical alkene, the hydrogen atom from the adding molecule will attach to the carbon with more hydrogen atoms already present.
This rule can help simplify understanding how molecules behave in these reactions. In our example sequence, propene undergoes this kind of addition reaction. The double bond between carbon atoms in propene provides an attractive site for HBr. According to Markovnikov's Rule, the hydrogen from HBr will combine with the terminal carbon atom (with more hydrogens), while the bromine will attach to the central carbon atom. This results in the formation of 2-bromopropane.
Markovnikov’s Rule is incredibly useful when predicting the products of reactions involving unsymmetrical alkenes. It provides a straightforward explanation of where the atoms from the reagent will end up in the product, making it an essential tool for chemists.

Wurtz Reaction

The Wurtz reaction is a fascinating coupling reaction in organic chemistry. It involves the formation of longer carbon chains by linking alkyl halide molecules using sodium metal in an ether solvent. When the mixture is heated, the sodium facilitates the bonding of carbon atoms, effectively 'welding' two smaller molecules into a larger one.
This reaction is particularly useful for synthesizing larger alkanes from smaller units. In the exercise sequence, two molecules of 2-bromopropane are joined together through the Wurtz reaction. Sodium in dry ether allows these molecules to form new carbon-carbon bonds, ultimately resulting in the creation of hexane, a six-carbon alkane.
While the Wurtz reaction is a powerful technique, it is mainly restricted to symmetric coupling due to difficulties in controlling the reaction for different alkyl halides. Thus, it serves as a fundamental method in exploring the possibilities of organic synthesis through straightforward coupling processes.