Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

A compound of the formula \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\) reacts with sodium and undergoes oxidation to give a carbonyl compound which does not reduce Tollen's reagent, the original compound is (a) sec-butyl alcohol (b) \(n\)-butyl alcohol (c) isobutyl alcohol (d) diethyl ether

Step-by-Step Solution

Answer

The original compound is sec-butyl alcohol.

Step 1: Understanding the problem

The question provides a compound with the formula \(\mathrm{C}_4 \mathrm{H}_{10} \mathrm{O}\) and indicates that it reacts with sodium and undergoes oxidation. The oxidation product does not react with Tollen's reagent.

Step 2: Identify possible alcohols

The given formula \(\mathrm{C}_4 \mathrm{H}_{10} \mathrm{O}\) suggests it could be an alcohol or an ether. We list the options: sec-butyl alcohol, \(n\)-butyl alcohol, and isobutyl alcohol. Diethyl ether is also a possibility, but ethers do not react with sodium in the same way alcohols do.

Step 3: Reaction with sodium

Alcohols react with sodium to produce hydrogen gas. \(n\)-butyl alcohol, sec-butyl alcohol, and isobutyl alcohol will react with sodium, but ethers, like diethyl ether, do not react in this way.

Step 4: Oxidation process of alcohols

Alcohols undergo oxidation to form carbonyl compounds. Primary alcohols oxidize to aldehydes and can further oxidize to carboxylic acids. Secondary alcohols oxidize to ketones.

Step 5: Analyze oxidation products

Tollen's reagent can oxidize aldehydes but not ketones. Thus, if the carbonyl product doesn't reduce Tollen's reagent, it must be a ketone.

Step 6: Identify secondary alcohol

Since the carbonyl compound does not react with Tollen's reagent, our original compound must be a secondary alcohol. Among the options, sec-butyl alcohol is the only secondary alcohol, which oxidizes to butanone, a ketone that does not react with Tollen's reagent.

Key Concepts

Alcohols

Alcohols are a class of organic compounds that feature one or more hydroxyl (-OH) groups bonded to a saturated carbon atom. This -OH group, known as a hydroxyl group, imparts unique properties to the alcohol molecules, such as polarity and hydrophilicity.
The simplest alcohol is methanol, which has just one carbon atom. However, alcohols can also have complex structures with multiple carbon atoms. They are generally categorized into three types depending on the connection of the carbon with the hydroxyl group: primary ( -primary, i.e., attached to a single carbon atom), secondary ( -secondary, i.e., connected to two carbon atoms), and tertiary ( -tertiary, i.e., connected to three carbon atoms) alcohols.

• **Primary alcohols** have their OH group attached to a carbon atom that is also connected to only one other carbon.
• **Secondary alcohols** have the OH group attached to a carbon connected to two other carbons.
• **Tertiary alcohols** have the OH group attached to a carbon connected to three other carbons.

A common reaction involving alcohols is their interaction with sodium, where they form alkoxides and hydrogen gas. For instance, in the presence of sodium, an alcohol reacts to produce sodium alkoxide and hydrogen gas. This characteristic reactivity with sodium allows alcohols to be distinguished from other compounds like ethers.

Oxidation Reactions

Oxidation reactions are fundamental processes in organic chemistry where an alcohol molecule undergoes a transformation due to the removal of hydrogen or the addition of oxygen. In the context of alcohols, oxidation is particularly important as it converts alcohols into carbonyl compounds, such as aldehydes, ketones, or even carboxylic acids.
Primary alcohols are the simplest to oxidize. They first convert into aldehydes when subjected to mild oxidation. With stronger oxidizing agents, these aldehydes can further oxidize to become carboxylic acids. Secondary alcohols, on the other hand, oxidize to form ketones without any further oxidation potential under similar conditions. Notably, tertiary alcohols usually do not oxidize because they lack the hydrogen atom needed for the reaction.

• **Primary alcohols (R-CH₂OH)**: Oxidation first produces an aldehyde, and further oxidation yields a carboxylic acid.
• **Secondary alcohols (R₂CHOH)**: Oxidation results in a ketone.
• **Tertiary alcohols (R₃COH)**: Generally resistant to oxidation barring extreme conditions.

These oxidation reactions are critical as they open up pathways for the preparation of numerous vital organic compounds. Understanding the oxidation states and reacts of alcohols paves the way for designing various synthetic routes in chemical processes.

Tollen's Reagent

Tollen's Reagent is a specific chemical test used primarily to identify aldehyde functional groups in organic compounds. Developed by Bernhard Tollens in the late 19th century, this reagent is a solution of ammoniacal silver nitrate that forms a complex ion, [ Ag(NH₃)₂⁺ ] , often referred to as the "Tollens' silver complex."
When introduced to an aldehyde, Tollens' reagent undergoes a redox reaction. The aldehyde is oxidized to a carboxylic acid while the silver ions in the reagent are reduced to metallic silver. This silver precipitates out, often creating a distinct and characteristic "silver mirror" on the inside of the reaction vessel.

• **Aldehydes**: React with Tollen's reagent yielding a silver mirror, indicating their presence.
• **Ketones**: Do not react with Tollen's reagent, as they do not oxidize in the same way aldehydes do.

This reaction's specificity for aldehydes makes Tollens' reagent a valuable analytical tool in organic synthesis and qualitative analysis, as it can differentiate between aldehydes and ketones effectively. Understanding its interaction with different compounds aids in identifying the presence of various functional groups and underscores the importance of choosing the correct reagent for chemical analysis.