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

Question

Ethanol, on reacting with \(\mathrm{PCl}_{5}\) gives 'A', \(\mathrm{POCl}_{3}\) 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

The compounds 'A' and 'B' are \( C_2H_5Cl \) and \( C_2H_5NO_2 \), respectively (option a).

Step 1: Identify Reaction of Ethanol with PCl5

Ethanol (\( C_2H_5OH \)) reacts with \( PCl_5 \) to form a chloro compound. The known reaction of \( PCl_5 \) is to replace hydroxyl groups with chlorides in alcohols. Hence, the product \( 'A' \) formed is \( C_2H_5Cl \) (ethyl chloride). The by-products in this reaction are \( POCl_3 \) and \( HCl \).

Step 2: Identify Reaction of 'A' with Silver Nitrate

The compound \( 'A' \), which is \( C_2H_5Cl \) (ethyl chloride), reacts with silver nitrate (\( AgNO_3 \)) in a nucleophilic substitution reaction. This reaction results in the formation of \( 'B' \), along with \( AgCl \) as a precipitate. The reaction of haloalkanes with \( AgNO_3 \) generally produces alcohols or esters, depending on conditions. However, for simplicity, the likely major product \( 'B' \) is an ethyl nitrate, \( C_2H_5NO_2 \).

Step 3: Verify Product and Matches with Choices

From the steps, we determined \( 'A' \) is \( C_2H_5Cl \) and \( 'B' \) is \( C_2H_5NO_2 \). Comparing these products to the options given:(a) \( C_2H_5Cl \) and \( C_2H_5NO_2 \) matches our results exactly.(b), (c), and (d) include compounds that do not fit the logic of the above reactions.Thus, choice (a) is correct.

Key Concepts

Nucleophilic Substitution

In chemistry, nucleophilic substitution is an essential reaction mechanism. It involves the replacement of a substituent, often a leaving group, with a nucleophile. A nucleophile is an atom or molecule that donates an electron pair to form a new covalent bond. In nucleophilic substitution reactions, the leaving group departs with a pair of electrons, making room for the nucleophile.

There are two main types of nucleophilic substitution reactions:

• S_N1 (Unimolecular substitution): This involves a two-step mechanism where the leaving group first detaches to form a carbocation. The nucleophile then attacks this positively charged ion.
• S_N2 (Bimolecular substitution): This is a one-step mechanism where the nucleophile attacks the substrate as the leaving group departs. It often results in an inversion of configuration at the carbon center.

In our specific exercise, ethyl chloride (C_2H_5Cl) undergoes a nucleophilic substitution with silver nitrate (AgNO_3). Silver nitrate acts as the nucleophile, attacking the carbon and releasing the chloride ion in the form of AgCl precipitate. This interaction is a classic example of nucleophilic substitution in organic chemistry, resulting in the formation of ethyl nitrate (C_2H_5NO_2).

Organic Chemistry Reactions

Organic chemistry focuses on reactions involving carbon-containing compounds. Alcohols, such as ethanol, often participate in reactions that transform them into other types of organic molecules. When ethanol reacts with phosphorus pentachloride (PCl_5), it undergoes a transformation where the hydroxyl group (-OH) is replaced by a chlorine atom, resulting in ethyl chloride (C_2H_5Cl).

Reactions like this heavily rely on the study of functional groups. Functional groups are specific groups of atoms within molecules that dictate how that molecule will react. By understanding the behavior of these groups in various scenarios, chemists can predict and control reactions to produce desired products. The conversion of ethanol to ethyl chloride is an essential step in synthetic processes as ethyl chloride serves as an important building block in the chemical industry. It shows the interplay of organic chemistry reactions where alkanes, haloalkanes, and other organic compounds are formed or modified.

Ethyl Chloride Formation

Ethyl chloride is a simple haloalkane formed through the chlorination of ethanol. This occurs when ethanol (C_2H_5OH) reacts with phosphorus pentachloride (PCl_5). The reaction is straightforward in replacing the hydroxyl group (-OH) with a chlorine atom to produce ethyl chloride (C_2H_5Cl).

The process also generates byproducts like phosphoric oxychloride (POCl_3) and hydrochloric acid (HCl). Ethyl chloride is not only critical in laboratories but also in industrial settings where it's used as a precursor in producing more complex compounds. This reaction exemplifies the importance of understanding functional group transformations in organic synthesis and highlights how such reactions can be engineered to yield specific and valuable chemical products. The formation of ethyl chloride illustrates a practical instance of utilizing basic chemical principles to achieve new materials through simple transformations of organic molecules.