Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the final product (C) in the following sequence of reactions. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I} \stackrel{\text { Alc. } \mathrm{KOH}, \Delta}{\longrightarrow}(\mathrm{A}) \stackrel{\mathrm{Br}_{2}}{\longrightarrow}(\mathrm{B}) \stackrel{\mathrm{KCN}}{\longrightarrow}(\mathrm{C})\) (a) \(\mathrm{NCCH}_{2} \mathrm{CH}_{2} \mathrm{CN}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) (d) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CN}\)

Step-by-Step Solution

Answer

The final product \(C\) is \(\mathrm{NCCH}_{2} \mathrm{CH}_{2} \mathrm{CN}\) (option a).

Step 1: Analyze the First Reaction

The first reaction involves treating ethyl iodide, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}\), with alcoholic KOH and heat (\(\Delta\)). This is an elimination reaction where the iodine is removed, resulting in the formation of an alkene. In this case, the product is ethylene, \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\), which we will call compound \(A\).

Step 2: Analyze the Second Reaction

The second reaction adds bromine (\(\mathrm{Br}_{2}\)) to the compound from Step 1. Ethylene undergoes an addition reaction with bromine to give 1,2-dibromoethane, \(\mathrm{BrCH}_{2} \mathrm{CH}_{2}\mathrm{Br}\). This product is compound \(B\).

Step 3: Analyze the Third Reaction

In the third reaction, 1,2-dibromoethane (\(B\)) is treated with potassium cyanide (\(\mathrm{KCN}\)). This reaction is a nucleophilic substitution where both bromine atoms are replaced by cyanide groups. The product is 1,2-dicyanoethane, \(\mathrm{NCCH}_{2} \mathrm{CH}_{2} \mathrm{CN}\), which is the compound \(C\).

Key Concepts

Elimination Reaction

An elimination reaction is a type of organic reaction where elements are removed from a molecule, resulting in the formation of a double bond. In the context of the given exercise, this occurs during the first step, when ethyl iodide (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I} \)) is treated with alcoholic potassium hydroxide (KOH) and heat.

This creates an environment conducive to the removal of a hydrogen atom and a halogen atom (iodine, in this case), leading to the formation of a double bond. The outcome is ethylene (\( \mathrm{CH}_{2}=\mathrm{CH}_{2} \)), an alkene.

Key characteristics of elimination reactions include:

• Involvement of a base (such as KOH) which abstracts a proton.
• Formation of alkenes as products.
• Often require heating to drive the reaction forward.

These reactions are contrasted with substitution reactions, as they result in the formation of a pi bond by removing atoms from the molecule.

Addition Reaction

Addition reactions are prevalent in the chemistry of alkenes. They typically involve the addition of atoms or groups of atoms to a carbon-carbon double bond.

In this exercise, ethylene (\( \mathrm{CH}_{2}=\mathrm{CH}_{2} \)) is formed after the elimination reaction and undergoes an addition reaction with bromine (\( \mathrm{Br}_{2} \)).

During this addition reaction, the double bond opens up, allowing each carbon atom to form a new bond with a bromine atom, thus forming 1,2-dibromoethane (\( \mathrm{BrCH}_{2} \mathrm{CH}_{2}\mathrm{Br} \)).

Important traits of addition reactions include:

• Double bonds converted to single bonds.
• Alkenes as common starting materials.
• No additional atoms lost in the process.

This reaction is characterized by the decrease in saturation of the molecule as a result of the additional bonds formed.

Nucleophilic Substitution

Nucleophilic substitution reactions are pivotal in organic chemistry, where a nucleophile replaces a leaving group in a molecule. In the third step of the given sequence of reactions, 1,2-dibromoethane (\( \mathrm{BrCH}_{2} \mathrm{CH}_{2}\mathrm{Br} \)) undergoes a nucleophilic substitution reaction with potassium cyanide (\( \mathrm{KCN} \)).

The bromine atoms, which serve as leaving groups, are replaced by the cyanide groups, yielding the product 1,2-dicyanoethane (\( \mathrm{NCCH}_{2} \mathrm{CH}_{2} \mathrm{CN} \)).

Typical properties of nucleophilic substitution reactions include:

• Involvement of a nucleophile, here the \( \mathrm{CN}^- \) ion from \( \mathrm{KCN}. \)
• Replacement of a leaving group, in this case, bromide ions.
• Often occur in polar solvents which stabilize the charged intermediates.

This type of reaction is crucial for the formation of a wide variety of organic compounds.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They are central to understanding organic chemistry because they determine the properties and reactivity of different organic compounds.

In the provided example, multiple functional groups are involved:

• Halides: Present in ethyl iodide and 1,2-dibromoethane, they make the molecules suitable substrates for elimination and substitution reactions.
• Cyanide group: Introduced in the final product, 1,2-dicyanoethane, by nucleophilic substitution. This group impacts the chemical behavior, solubility, and reactivity of the compound.
• Double bonds: Formed initially in ethylene as a result of the elimination reaction, indicating unsaturation.

These functional groups guide the transformation from starting materials to the final product, illustrating the diversity and intricacy of organic reactions.