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 primarv alcohol

Step-by-Step Solution

Answer

The original compound is a primary alcohol (option d).

Step 1: Analyze Solubility

The compound is soluble in concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \). Concentrated sulfuric acid can dissolve many organic compounds, especially functionalized ones like alcohols and ethers. Simple alkanes (option a) are typically insoluble because they lack polar functional groups. This suggests that options involving alcohols and ethers are more likely.

Step 2: Examine Reaction with Bromine

The compound does not decolourize bromine in carbon tetrachloride. This indicates that the compound has no unsaturated double bonds, as those would react with bromine, leading to decolorization. This rules out the presence of alkenes or alkynes but does not directly eliminate any of the given options.

Step 3: Evaluate Oxidation Reaction

The compound is oxidized by chromic anhydride in aqueous sulfuric acid, quickly turning the solution from orange to blue, green, and finally opaque. Chromic anhydride (Jones reagent) is used to oxidize alcohols. Primary alcohols oxidize to aldehydes, secondary alcohols to ketones, and tertiary alcohols typically do not oxidize under these conditions, or slowly, due to the absence of a hydrogen atom bonded to the carbon with the hydroxyl group.

Step 4: Determine the Structure

The quick oxidation suggests that the compound is easily oxidized, ruling out tertiary alcohols that resist oxidation. Instead, primary alcohols are quickly oxidized to aldehydes, producing a notable color change due to intermediate chromate and reduced chromium species.

Key Concepts

Chemical Solubility

Chemical solubility refers to the ability of a compound to dissolve in a solvent. In organic chemistry, this often involves polar and non-polar interactions. For instance, compounds that are polar, like alcohols, are more likely to dissolve in polar solvents such as water or concentrated sulfuric acid \( \mathrm{H}_{2} \mathrm{SO}_{4} \).

In the given exercise, the organic compound's solubility in concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \) indicates the presence of functional groups that can interact with the solvent. Typically, alcohols and ethers exhibit this behavior due to their polar nature and hydrogen bonding capability with the \( \mathrm{H}_{2} \mathrm{SO}_{4} \) molecules.

This trait helps chemists infer potential functional groups in unknown compounds through solubility tests.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds. In organic chemistry, reactions often include oxidation-reduction or redox reactions, substitution, addition, and elimination reactions. These reactions are fundamental in understanding how compounds interact with each other.

In the context of the exercise, the reaction of the unknown compound with chromic anhydride in sulfuric acid highlights an oxidation process. The non-decolourization with bromine suggests the absence of double bonds like alkenes or alkynes. Such observations in reactivity provide crucial insights into the molecular structure.

Oxidation of Alcohols

Oxidation reactions in alcohols are characterized by the transformation of the hydroxyl group into either aldehydes or ketones. The type of oxidation product depends on whether the alcohol is primary, secondary, or tertiary.

A primary alcohol, when oxidized, forms an aldehyde, which can further oxidize to a carboxylic acid. Secondary alcohols form ketones, and tertiary alcohols generally do not oxidize under mild conditions because they lack a hydrogen atom attached to the hydroxyl-bearing carbon.

In the chromic anhydride test mentioned in the exercise, the rapid color change from orange to blue, green, and finally opaque signifies the oxidation of a primary alcohol, supporting its efficient reaction to form aldehydes.

Chromic Anhydride Test

The chromic anhydride test, also known as the Jones test, is a common method used to investigate alcohol oxidation. Chromic anhydride, \( \mathrm{CrO}_{3} \) in aqueous sulfuric acid acts as an oxidizing agent. This test is particularly effective at detecting primary and secondary alcohols due to their relative ease of oxidation.

In the presence of an oxidizable alcohol, the orange solution of chromic anhydride turns blue, then changes to green, and eventually opaque, resulting from intermediate chromate compounds. The final color change to opaque relates to the production of chromium(III) sulfate, completing the oxidation process. Such a test provides qualitative confirmation of the alcohol’s oxidation state.

Functional Groups in Chemistry

Functional groups are specific groupings of atoms within molecules that determine the chemical properties of those molecules. Familiar functional groups include alcohols, ethers, amines, carbonyls, and carboxylic acids, each having unique reactions.

Understanding functional groups is crucial for predicting reaction outcomes and solubility. In chemistry, functional groups can dictate a compound's acid-base properties, solubility in various solvents, and its overall reactivity.

In the original exercise, identifying the compound's solubility and its behavior in the oxidation test implies an underlying alcohol functional group. This functional group is key in understanding the compound's reactivity and explains its rapid oxidation in the presence of chromic anhydride.