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) \(\mathrm{n}\)-butyl alcohol (c) isobutyl alcohol (d) diethyl ether

Step-by-Step Solution

Answer

The original compound is sec-butyl alcohol.

Step 1: Understanding the Formula

The given compound has the formula \(\mathrm{C}_{4} \mathrm{H}_{10} \mathrm{O}\). This indicates it contains four carbon atoms, ten hydrogen atoms, and one oxygen atom. We know that the oxygen could be part of either an alcohol (\(\mathrm{C}\mathrm{H}_3\mathrm{CH_2}\mathrm{CH}_2\mathrm{OH}\)) or an ether (\(\mathrm{C}_2\mathrm{H}_5\mathrm{O}_2\)) structure.

Step 2: Determining the Alcohol or Ether

Given the options, only (a) sec-butyl alcohol, (b) \(\mathrm{n}\)-butyl alcohol, and (c) isobutyl alcohol are alcohols, while (d) diethyl ether is an ether. Alcohols can react with sodium to form an alkoxide, but ethers typically do not.

Step 3: Analyzing the Carbonyl Compound Product

Oxidation of an alcohol results in a carbonyl compound. Since the carbonyl compound does not reduce Tollen's reagent, it cannot be an aldehyde; it must be a ketone. This helps us focus on alcohols that yield ketones upon oxidation.

Step 4: Alcohol Oxidation Products

Upon oxidation, sec-butyl alcohol, which has the structure \(\mathrm{CH_3CH(OH)CH_2CH_3}\), forms 2-butanone (a ketone). Both \(\mathrm{n}\)-butyl alcohol and isobutyl alcohol would form aldehydes, not ketones, when oxidized.

Step 5: Concluding the Correct Option

The compound is sec-butyl alcohol because it can be oxidized to a ketone (2-butanone), which does not reduce Tollen's reagent. Thus, the correct answer is option (a).

Key Concepts

Alcohols and Ethers

In organic chemistry, alcohols and ethers are two fundamental types of compounds distinguished by the presence of an oxygen atom. An alcohol contains an oxygen atom connected with a hydrogen, forming a hydroxyl group (01OH)01 bonded to a carbon atom. This hydroxyl group makes alcohols polar and able to engage in hydrogen bonding, affecting their physical attributes.

Ethers, on the other hand, have an oxygen atom single-bonded to two carbon atoms, coming from various alkyl or aryl groups, represented as 01R0101O0101R'. Unlike alcohols, ethers cannot form hydrogen bonds with themselves due to the lack of a hydroxyl group, which results in lower boiling points.

• Alcohols react with sodium to release hydrogen gas, forming alkoxides; ethers generally do not.
• Due to the hydroxyl group, alcohols are more reactive compared to ethers.

This basic understanding helps in identifying and differentiating alcohols and ethers in various chemical reactions.

Oxidation Reactions

Oxidation reactions in organic chemistry involve either gaining oxygen, losing hydrogen, or losing electrons. When it comes to alcohols, oxidation is a core concept as it transforms alcohols into carbonyl compounds, either ketones or aldehydes depending on the alcohol's structure.

Primary alcohols, when oxidized, generally form aldehydes, which might further oxidize to form carboxylic acids if conditions are not controlled.

• Secondary alcohols, however, oxidize to form ketones.
• Tertiary alcohols are resistant to simple oxidation due to lack of hydrogen atoms on the carbon attached to the hydroxyl group.

Understanding the structure of the starting alcohol is crucial to predict the oxidation product correctly. This knowledge helps us determine the maturity and extent of the oxidation, which consequently informs us about the possible chemical reactions or products formed.

Carbonyl Compounds

Carbonyl compounds are characterized by the presence of a carbon atom double-bonded to an oxygen atom, known as the carbonyl group. This group is very reactive because it is polar, with the oxygen being more electronegative than the carbon.

There are two major types of carbonyl compounds:

• Aldehydes have at least one hydrogen atom attached to the carbonyl carbon; they are typically more reactive towards nucleophiles.
• Ketones, in contrast, have two carbon-containing groups attached to the carbonyl carbon; they are less reactive than aldehydes.

The structure of the starting alcohol determines whether oxidation will form an aldehyde or a ketone. In the case of sec-butyl alcohol, oxidation forms 2-butanone, a ketone. This plays a key role in distinguishing the type of reactions and reagents the compound will interact with, like Tollen's reagent.

Tollen's Reagent Test

Tollen's reagent is an important chemical test used to identify aldehydes. This reagent is a mild oxidizing agent, consisting of ammoniacal silver nitrate solution. When Tollen's reagent is added to an aldehyde, a silver mirror forms as the aldehyde is oxidized and silver ions are reduced to metallic silver.

The test is specific for aldehydes because ketones generally do not react, as they lack the necessary hydrogen atom on the carbonyl carbon to further oxidize. Understanding this reaction helps in determining the nature of a carbonyl compound post-oxidation.

• If a compound does not reduce Tollen's reagent, it confirms the presence of a ketone rather than an aldehyde.
• This knowledge is useful while identifying and differentiating carbonyl compounds in unknown samples.

Ultimately, Tollen's reagent test confirms that the original carbonyl product from our oxidation is indeed a ketone, aligning with the properties of sec-butyl alcohol oxidation product, 2-butanone.