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

Question

Ethanol, on reacting with \(\mathrm{PCl}_{s}\) gives ' \(\mathrm{A}\) ', \(\mathrm{POCl}_{3}\) 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 \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{NO}_{2}\) (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) \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \) and \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NO}_{2} \)

Step 1: Analyze the reaction of Ethanol with \( \mathrm{PCl}_{5} \)

Ethanol reacts with phosphorus pentachloride (\( \mathrm{PCl}_{5} \)) to form a chloroalkane. The reaction is as follows: \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} + \mathrm{PCl}_5 \longrightarrow \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} + \mathrm{POCl}_3 + \mathrm{HCl} \). Thus, 'A' is \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \).

Step 2: Reaction of 'A' with Silver Nitrate

\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \) (ethanol chloride) reacts with silver nitrate (\( \mathrm{AgNO}_3 \)) in aqueous solution to form an ethyl alcohol (\( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \)), \( \mathrm{AgCl} \) as precipitate, and \( \mathrm{NO}_2^- \). The primary reaction forms \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{NO}_2 \). Thus, 'B' is \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{NO}_2 \).

Step 3: Confirm the correct pair of 'A' and 'B'

From Step 1, we found 'A' to be \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \) and from Step 2, 'B' is \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{NO}_2 \). This matches with option (a) in the given choices.

Key Concepts

Organic Chemistry

Organic Chemistry is the study of carbon-containing compounds and their reactions. In this particular scenario, we are focusing on the reaction of ethanol, a simple organic molecule, with phosphorus pentachloride (\( \mathrm{PCl}_{5} \)). This type of reaction is common in organic chemistry where alcohols are converted into alkyl halides. Understanding these transformations is crucial because they allow chemists to synthesize a variety of compounds from simpler starting materials. Ethanol, with the formula \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \), reacts with \( \mathrm{PCl}_{5} \) to replace the hydroxyl group (\( \mathrm{OH} \)) with a chlorine atom, forming \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \). This process is an example of substitution reaction, a type of reaction where one atom or group of atoms in a molecule is replaced by another atom or group of atoms. Having a solid grasp of these reactions can help students understand how different organic compounds are interrelated and can be transformed to synthesize new compounds.

Key aspects:

• Carbon is the central element in organic chemistry due to its ability to form four covalent bonds.
• Substitution reactions involve the replacement of one atom or group by another.
• Organic chemistry enables the synthesis of a vast array of compounds through simple starting materials.

Chloroalkanes

Chloroalkanes, also known as alkyl chlorides, are a group of organic compounds containing carbon, hydrogen, and chlorine. In this exercise, the compound \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \) is an example of a chloroalkane. These compounds are crucial in both industrial and laboratory syntheses due to their reactivity and ability to act as intermediates in the production of more complex molecules.
Chloroalkanes have a carbon-chlorine bond which is polar; this means that the chlorine atom, being more electronegative, pulls the electron density towards itself, making it susceptible to nucleophilic attacks. This property makes chloroalkanes pivotal in nucleophilic substitution reactions, where a nucleophile replaces the chlorine atom in the molecule.

Noteworthy points:

• Chloroalkanes can be formed by the substitution of hydroxyl groups in alcohols with chlorine.
• They are typically reactive due to the polar carbon-chlorine bond.
• They serve as important intermediates in organic synthesis.

Silver Nitrate Reaction

In the exercise, the reaction of \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \) with silver nitrate (\( \mathrm{AgNO}_3 \)) is highlighted. This reaction is emblematic in teaching about the behavior of chloroalkanes with silver salts, which are often used in the test tube reactions to determine the presence of halides.
Silver nitrate reacts with chloroalkanes to form a silver chloride (\( \mathrm{AgCl} \)) precipitate. As part of the reaction mechanism, the chlorine atom is displaced from the chloroalkane and interacts with \( \mathrm{AgNO}_3 \), forming \( \mathrm{AgCl} \) and producing a nitro compound as a major product. This reaction not only confirms the presence of chloride ions but also serves as a tool for synthesizing new compounds and studying reaction mechanisms.

Key points:

• Silver nitrate in aqueous solutions can precipitate silver halides, aiding in analytical chemistry to identify halide ions.
• The formation of a precipitate indicates a successful reaction and confirms the presence of the chlorine atom initially attached to the carbon skeleton.
• This type of reaction highlights the nuanced understanding necessary for predicting and manipulating chemical reactions in organic chemistry.