Problem 24 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A compound (X) containing C, H and \(\mathrm{O}\) is unreactive towards sodium. It does not add bromine. It also does not react with Schiff's reagent. On refluxing with an excess of hydriodic acid, (X) yields only one organic product (Y). On hydrolysis, (Y) yields a new compound (Z) which can be converted into (Y) by reaction with red phosphorus and iodine. The compound (Z) on oxidation with potassium permanganate gives a carboxylic acid. The equivalent weight of this acid is 60 . What are the compounds \((\mathrm{X}),(\mathrm{Y})\) and \((\mathrm{Z}) ?\) Write chemical equations leading to the conversion of \((\mathrm{X})\) to \((\mathrm{Y})\).

Step-by-Step Solution

Answer

X is ethyl ether, Y is ethyl iodide, Z is ethanol.

Step 1: Analyze Unreactive Behavior of X

Given that compound X is unreactive towards sodium and does not add bromine, X likely lacks acidic hydrogens and double bonds, suggesting it is a saturated alcohol or ether. Additionally, X does not react with Schiff's reagent, indicating it is not an aldehyde.

Step 2: Determine Compound Y from X

On reflux with an excess of hydriodic acid, compound X yields only one organic product Y, indicating X might be an ether (C-O-C), as hydriodic acid cleaves ethers to produce alcohols or iodides.

Step 3: Hydrolysis and Conversion of Y to Z

Compound Y undergoes hydrolysis to form a new compound Z, which can be converted back to Y using red phosphorus and iodine. This suggests Y could be an alkyl iodide, which can hydrolyze to form an alcohol.

Step 4: Identify Z from Oxidation Reaction

Compound Z, on oxidation with potassium permanganate, yields a carboxylic acid with an equivalent weight of 60. As the equivalent weight is the molecular weight for a monocarboxylic acid, this implies the acid could be formic acid (H-COOH, molecular weight = 46) or acetic acid (CH₃-COOH, molecular weight = 60). Therefore, Z is ethanol (C₂H₅OH) since oxidation of ethanol leads to acetic acid.

Step 5: Determine Compounds X and Y

Since Y is ethyl iodide (C₂H₅I) and converting Z (ethanol) back to Y involves red phosphorus and iodine, Z must be ethanol. This means X could be ethyl ether (C₂H₅OC₂H₅), cleaved by hydriodic acid to yield two molecules of ethyl iodide.

Step 6: Write the Chemical Equations

Given the above deductions, the equations for conversion are: 1. C₂H₅OC₂H₅ + 2 HI → 2 C₂H₅I (conversion of X to Y) 2. C₂H₅I + H₂O → C₂H₅OH + HI (hydrolysis of Y to Z)

Key Concepts

Ether Cleavage

In organic chemistry, ether cleavage is a reaction where an ether is broken down into smaller molecules, typically through the action of an acid. Ethers contain an oxygen atom bonded to two alkyl or aryl groups, and this arrangement can be disrupted by strong acids like hydriodic acid. The acid provides a proton that attacks the ether’s oxygen, leading to the breakage of the C-O bonds.
For example, when an ether such as diethyl ether \(C_2H_5OC_2H_5\) is treated with hydriodic acid \(HI\), it cleaves into two molecules of ethyl iodide \(C_2H_5I\). This reaction demonstrates the conversion of a stable ether into a more reactive alkyl halide.

• The protonation of the oxygen atom is the first step.
• The ether then splits, releasing an alcohol or an alkyl halide.
• The type of products formed depends on the type of ether and the cleaving agent used.

Understanding how ethers react under acidic conditions is crucial for predicting the outcomes of reactions involving these compounds.

Equivalent Weight Calculation

One of the fundamental concepts in chemistry is the calculation of equivalent weight, which helps chemists understand how substances react on the basis of stoichiometry. Equivalent weight can be defined as the mass of a compound that will combine with or displace 1 mole of hydrogen atoms. For acids, it is often equivalent to the molar mass divided by the number of protons the acid can donate.
In the exercise, we discovered that the carboxylic acid derived from oxidation has an equivalent weight of 60. This indicates the presence of only one replaceable hydrogen ion, typical of a monocarboxylic acid like acetic acid \(CH_3COOH\). By knowing this, students can better appreciate how equivalent weight is used to deduce molecular structures and identities in various chemical reactions.

Alcohol Oxidation

Alcohol oxidation is a key reaction in organic chemistry where an alcohol is converted to an aldehyde, ketone, or carboxylic acid. The type of product depends on the structure of the alcohol, as well as the oxidizing agent used. In this exercise, when ethanol \(C_2H_5OH\) is oxidized, it forms acetic acid \(CH_3COOH\).
The process of oxidation involves the removal of hydrogen atoms from the alcohol. A common oxidizing agent used is potassium permanganate \(KMnO_4\), which is very effective in transforming primary alcohols into carboxylic acids. Here’s a brief overview of typical alcohol oxidation:

• Primary alcohols first form aldehydes, which can further oxidize to carboxylic acids.
• Secondary alcohols form ketones, which generally do not oxidize further under normal conditions.
• Tertiary alcohols do not oxidize easily as they lack the necessary hydrogen atom on the hydroxyl-bearing carbon.

Understanding the oxidation process is important for the synthesis and manipulation of various chemical compounds.

Refluxing with Hydriodic Acid

Refluxing is a technique in chemistry where a mixture is heated to boiling and the vapor is condensed back to liquid, returning to the container. This allows the reaction to proceed at a warmer temperature without losing the solvent. In this context, refluxing with hydriodic acid transforms a stable ether into more reactive products through the process of ether cleavage.
The choice of hydriodic acid is crucial because it provides the strong acidic conditions necessary to cleave the ether bond. During reflux, the system maintains a constant temperature and concentration, facilitating the completion of the reaction. For example, through this method, diethyl ether is efficiently converted into two moles of ethyl iodide.

• Refluxing maintains the mixture at a constant temperature, necessary for the reaction to proceed efficiently.
• The process prevents the loss of volatile components, allowing for prolonged reaction times.
• It is particularly useful in reactions needing strong acidic conditions, as in ether cleavage.

Mastery of refluxing techniques equips students with valuable skills for executing various chemical reactions safely and effectively.