Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Ethanol, on reacting with \(\mathrm{PCl}_{5}\) gives ' \(\mathrm{A}^{\prime}, \mathrm{POCl}_{3}\) and HCl. 'A' reacts with silver nitrate to form 'B' (major product) and \(\mathrm{AgCl}\). 'A' and ' \(\mathrm{B}\) ' respectively are(a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NO}_{2}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{6}\) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NO}_{2}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{6}\) and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\)

Step-by-Step Solution

Answer

(a) \(\text{C}_2\text{H}_5\text{Cl}\) and \(\text{C}_2\text{H}_5\text{NO}_2\).

Step 1: Understand the Reaction of Ethanol with PCl5

When ethanol (\(\text{C}_2\text{H}_5\text{OH}\)) reacts with phosphorus pentachloride (\(\text{PCl}_5\)), it substitutes the hydroxyl group (-OH) with a chlorine atom to form an alkyl chloride (\(\text{C}_2\text{H}_5\text{Cl}\)) denoted as 'A'. The byproducts of this reaction are \\(\text{POCl}_3\) and \(\text{HCl}\).

Step 2: Determine the Reaction of 'A' with Silver Nitrate

'Alkyl chlorides like \(\text{C}_2\text{H}_5\text{Cl}\), when reacted with silver nitrate (\(\text{AgNO}_3\), undergo a nucleophilic substitution reaction where chlorine atom is replaced by a nitro group. This forms the compound \(\text{C}_2\text{H}_5\text{NO}_2\), referred to as 'B', along with the precipitation of insoluble silver chloride (\(\text{AgCl}\)).

Step 3: Match with Given Options

After analyzing the reactions, 'A' is identified as\(\text{C}_2\text{H}_5\text{Cl}\), and 'B' is determined to be\(\text{C}_2\text{H}_5\text{NO}_2\). From the given choices, option (a) matches these findings, asserting that compounds A and B are\(\text{C}_2\text{H}_5\text{Cl}\) and \(\text{C}_2\text{H}_5\text{NO}_2\), respectively.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction type found in organic chemistry. It involves the replacement of a leaving group, typically a halide, by a nucleophile. A nucleophile is a chemical species that donates an electron pair to form a chemical bond in reaction. In the given exercise, the compound formed after the substitution of the hydroxyl group in ethanol by chlorine is ethyl chloride (\(\text{C}_2\text{H}_5\text{Cl}\)).

• In nucleophilic substitution, the leaving group and the substrate act like partners in a dance, with the substrate being the molecule that contains a reactive functional group.
• A successful reaction results in the nucleophile replacing the leaving group in the substrate.

Here, the chlorine atom is the nucleophile which replaces the hydroxyl group from ethanol to form the alkyl halide, ethyl chloride. Later, when ethyl chloride reacts with silver nitrate, the chlorine is replaced by a nitro group, showcasing another nucleophilic substitution process. This is characteristic of how versatile such reactions can be in building complex molecules from simpler ones.

Alkyl Halides

Alkyl halides, such as ethyl chloride (\(\text{C}_2\text{H}_5\text{Cl}\)), are compounds where a halogen atom is covalently bonded to an alkyl group. These compounds play a crucial role in various organic reactions due to the reactivity imparted by the halogen atoms.

• They are widely used in chemical syntheses and as intermediates in different industrial applications.
• Alkyl halides can undergo various reactions, such as nucleophilic substitutions, due to the presence of the polar carbon-halogen bond.

The bond between carbon and the halogen in alkyl halides is polar, with the carbon atom carrying a partial positive charge and the halogen a partial negative charge. This polarization makes alkyl halides susceptible to attack by nucleophiles. In our case, ethyl chloride acts as the substrate, and the chlorine atom serves as a leaving group that initiates the nucleophilic substitution reaction, leading to subsequent transformations.

Reaction Mechanisms

Understanding reaction mechanisms is pivotal in grasping how organic reactions proceed. A reaction mechanism is essentially a detailed step-by-step description of the transformation from reactants to products that outlines the movement of electrons.For the reactions in question, the mechanism involves identifying the main steps in the transformation of ethanol to ethyl chloride and then to the nitro compound with silver nitrate.

• The initial reaction of ethanol with \(\text{PCl}_5\) demonstrates the typical mechanism of hydroxyl group replacement by \(\text{Cl}^-\), producing an alkyl halide, ethyl chloride.
• In the next step, ethyl chloride's reaction with \(\text{AgNO}_3\) showcases another classic mechanism where \(\text{Cl}^-\) is replaced by a nitro group from \(\text{AgNO}_3\).

These mechanisms highlight critical interactions, such as the formation of intermediates and transition states that dictate the speed and outcome of the reaction. By breaking down each individual step, chemists can predict and control the formation of products in synthetic chemistry.