Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound is soluble in concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\). It does not decolourize bromine in carbon tetrachloride but is oxidized by chromic anhydride in aqueous sulphuric acid within two seconds, turning the orange solution to blue, then green and finally opaque. The original compound is (a) an alkane (b) an ether (c) a tertiary alcohol (d) a primary alcohol

Step-by-Step Solution

Answer

The original compound is a primary alcohol (d).

Step 1: Analyze the Solubility

The compound is soluble in concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \). Typically, ethers, alcohols, and some alkenes can dissolve in concentrated sulfuric acid due to potential hydrogen bonding or reactivity with the acid. Alkanes, being non-polar hydrocarbons, typically do not dissolve in \( \mathrm{H}_{2} \mathrm{SO}_{4} \).

Step 2: Consider the Reaction with Bromine

The compound does not decolourize bromine in carbon tetrachloride. This suggests that the compound does not contain a carbon-carbon double bond, as alkenes would typically react with bromine causing decolourization. This indicates that the compound is likely not an alkane or alkene.

Step 3: Examine the Oxidation by Chromic Anhydride

The compound is oxidized by chromic anhydride in aqueous sulfuric acid, changing colors from orange to blue, then green, and finally opaque. This reaction suggests a significant oxidation process typical of primary or secondary alcohols, which can be oxidized. Ethers and alkanes would not undergo such oxidation readily.

Step 4: Eliminate Options based on Reactions

Given the compound doesn't decolourize bromine (likely not an alkene) and it reacts quickly with chromic anhydride, we can rule out ethers since ethers generally don't oxidize easily in these conditions. Tertiary alcohols tend to resist oxidation compared to primary alcohols. This leaves a primary alcohol as the likely candidate.

Step 5: Conclusion

The compound's solubility in sulfuric acid, lack of reaction with bromine, and rapid oxidation by chromic anhydride strongly suggest that it is a primary alcohol.

Key Concepts

Understanding Chemical Reactions

Chemical reactions are processes where one or more substances are transformed into new substances. These reactions involve breaking and forming chemical bonds, leading to changes in the arrangement of atoms and molecules. In the context of the exercise, the compound undergoes an oxidation reaction, indicating a chemical change. Different reactions have unique characteristics:

• Oxidation: This is a reaction involving the loss of electrons from a substance. When a primary alcohol is oxidized, it typically leads to the formation of an aldehyde or a carboxylic acid, depending on the extent of oxidation.
• Reactivity with Bromine: Compounds with carbon-carbon double bonds, such as alkenes, react with bromine causing decolourization. Lack of this reaction indicates absence of such bonds.
• Solubility Reactions: These involve a substance dissolving in a solvent. Solubility in sulfuric acid suggests specific molecular interactions.

Breaking these reactions down helps in deducing the properties of the compound in question.

Exploring Compound Solubility

Compound solubility relates to a substance's ability to dissolve in a solvent, forming a homogeneous solution. It can be influenced by the polarity of the compound and the solvent, as well as the potential for hydrogen bonding. Here's how these factors play a role:

• Polar Solvents: Concentrated sulfuric acid (\( \mathrm{H}_{2} \mathrm{SO}_{4} \)) is highly polar, which means polar substances, such as many alcohols and some ethers, are likely to dissolve due to hydrogen bonding.
• Non-Polar Compounds: Alkanes, which are non-polar, usually do not dissolve in polar solvents like sulfuric acid because they lack the necessary intermolecular forces.
• Potential for Reaction: Some organic compounds might not just dissolve but also react with the solvent further dictating their solubility behavior.

Understanding solubility helps determine the nature of unknown compounds based on how they interact with different solvents.

Understanding Oxidation Reactions

Oxidation reactions are key in chemistry as they involve changes in the oxidation state of molecules. In the exercise, the compound undergoes rapid oxidation by chromic anhydride in an acid medium, showcasing a fundamental concept in organic chemistry. Here's a breakdown:

• Primary Alcohol Oxidation: When primary alcohols are oxidized, the alcohol group (\( -OH \)) usually converts into an aldehyde (\( -CHO \)) or further into a carboxylic acid (\( -COOH \)), depending on the conditions.
• Chromic Anhydride: This is a strong oxidizing agent often used in organic chemistry to oxidize alcohols. The rapid color change from orange to blue and then to green indicates different stages of the oxidation process.
• Resistance to Oxidation: Compounds like ethers and alkanes do not readily undergo oxidation under similar conditions, differentiating them from alcohols.

Comprehending these concepts aids in identifying and distinguishing between types of organic compounds based on their chemical behavior with oxidizing agents.