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.

Step 1: Analyze the Solubility in H2SO4

The compound is soluble in concentrated \( \mathrm{H}_2\mathrm{SO}_4 \). Alcohols and ethers are generally soluble in \( \mathrm{H}_2\mathrm{SO}_4 \) due to their ability to donate electrons to the \( \mathrm{H}_2\mathrm{SO}_4 \) molecules. Alkanes are typically not soluble.

Step 2: Examine Reaction with Bromine in CCl4

The compound does not decolourize bromine in carbon tetrachloride, indicating that it does not have unsaturated bonds such as those found in alkenes or alkynes, which would react with bromine.

Step 3: Consider the Reaction with Chromic Anhydride

The compound is oxidized by chromic anhydride in aqueous sulfuric acid within two seconds, suggesting it can be oxidized quickly. Primary and secondary alcohols are typically oxidized by chromic anhydride, but tertiary alcohols are not oxidized as easily.

Step 4: Determine the Classification of Alcohol

Given that the compound changes the color of chromic anhydride from orange to blue to green and finally opaque, it indicates an oxidation process. Since tertiary alcohols generally resist oxidation under these conditions, the reaction is more consistent with a primary alcohol.

Step 5: Final Answer Selection

Combining all the evidence: Solubility in sulfuric acid excludes alkanes; no reaction with bromine excludes unsaturated compounds; easy oxidation aligns with primary alcohols. Thus, the original compound is a primary alcohol.

Key Concepts

Oxidation Reactions

In organic chemistry, oxidation reactions are processes in which electrons are removed from a molecule. They play a crucial role in transforming alcohols into aldehydes, ketones, or carboxylic acids, depending on the structure of the alcohol.
These reactions often involve oxidizing agents like chromic anhydride (CrO₃) dissolved in sulfuric acid.

• When a primary alcohol undergoes oxidation, it typically forms an aldehyde and can further oxidize to a carboxylic acid if the conditions allow.
• Secondary alcohols can be oxidized to ketones, but generally do not go further as there are no more hydrogen atoms to remove.
• Tertiary alcohols are resistant to oxidation since they lack the necessary hydrogen atom bonded to the carbon with the -OH group.

During an oxidation reaction, the color change typically observed, especially with potassium dichromate or chromic anhydride, is from orange to green as the dichromate ion is reduced.

Chromic Anhydride Test

The Chromic Anhydride Test is a common laboratory method used to determine whether a compound is a primary or secondary alcohol. This test exploits the different behaviors of alcohols under oxidizing conditions.
When chromic anhydride (CrO₃) is dissolved in aqueous sulfuric acid, it forms a potent oxidizing agent capable of converting alcohols:

• The test begins with an orange chromium(VI) solution.
• As the reaction proceeds, chromium(VI) is reduced to chromium(III), seen as a definitive color change to blue or green and eventually turning opaque.

A sudden color change signifies a rapid oxidation process, typically observed with primary and secondary alcohols. Tertiary alcohols do not exhibit this color change because they do not undergo oxidation.
This color transition is because of the transformation of chromium from the +6 oxidation state to the +3 oxidation state in the presence of alcohols.

Primary Alcohols

Primary alcohols are organic compounds characterized by the -OH (hydroxyl) group attached to a carbon atom, which is only bonded to one other carbon atom (making it a primary carbon).
This structure makes primary alcohols particularly susceptible to oxidation reactions.

• Upon exposure to an oxidizing agent, primary alcohols first transform into aldehydes.
• If oxidation continues, the aldehyde may further oxidize into a carboxylic acid.
• This reactivity distinguishes them from secondary and tertiary alcohols, which have different oxidation pathways or resist oxidation.

For example, ethanol (a primary alcohol) can be oxidized to form acetaldehyde, which can further oxidize to acetic acid.
Because primary alcohols can readily participate in such transformations, they are often used in chemical synthesis and industrial applications.