Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{A} \frac{\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}}{\mathrm{H}_{2} \mathrm{SO}_{4}}-\mathrm{B} \underset{\text { vigrous oxidation }}{[\mathrm{O}]} \mathrm{CH}_{3} \mathrm{COOH}\) Here, \(\mathrm{B}\) is propanone so A will be (a) Ethyl alcohol (b) Isopropyl alcohol (c) n-propyl alcohol (d) tertiary butyl alcohol

Step-by-Step Solution

Answer

(b) Isopropyl alcohol

Step 1: Identify Reaction Type

The reaction involves oxidation, where organic compounds gain oxygen or lose hydrogen, typically using an oxidizing agent like \({\rm K_2Cr_2O_7}\) in presence of \({\rm H_2SO_4}\). As indicated, compound B undergoes vigorous oxidation to form acetic acid (\({\rm CH_3COOH}\)).

Step 2: Determine Initial Reactant B

Given that B is propanone (acetone, \({\rm CH_3COCH_3}\)), its vigorous oxidation results in acetic acid (\({\rm CH_3COOH}\)). The transformation suggests an initial step of forming B from an alcohol precursor (A) through a mild oxidation.

Step 3: Find Required Alcohol for B

Propanone can be derived through the oxidation of isopropyl alcohol (\({\rm CH_3CHOHCH_3}\)), which gets oxidized to form propanone via an intermediate ketone stage. Therefore, to produce propanone (B) from an alcohol (A), isopropyl alcohol must be A.

Step 4: Verify Compatibility with Choices

Check the given options: - (a) Ethyl alcohol requires full oxidation to acetic acid. - (b) Isopropyl alcohol forms propanone upon mild oxidation. - (c) n-propyl alcohol also produces an aldehyde. - (d) Tertiary butyl alcohol would not efficiently oxidize further in this context. Only option (b) aligns with the requirement to form propanone.

Key Concepts

Oxidizing Agents

Oxidizing agents are crucial in many reactions, especially in organic chemistry. They facilitate the process in which a molecule, ion, or atom loses electrons, often leading to a gain in oxygen or the loss of hydrogen.
In the context of alcohol oxidation, such agents include potassium dichromate ( {K_2Cr_2O_7} ) along with sulfuric acid ( {H_2SO_4} ). When used together, this mixture becomes a powerful oxidizing agent, capable of converting alcohols to ketones, aldehydes, or even acids depending on the conditions and the type of alcohol.

• Primary alcohols are often oxidized completely to carboxylic acids.
• Secondary alcohols typically stop at the ketone stage.
• Tertiary alcohols usually resist oxidation without breaking the carbon skeleton.

Understanding the role and strength of different oxidizing agents is crucial for predicting and controlling reaction outcomes in synthetic organic chemistry.

Organic Chemistry Reactions

Organic chemistry reactions involve the transformation of organic molecules through the breaking and forming of chemical bonds. These reactions are vital in creating complex molecules from simpler ones.
One key type of reaction in organic chemistry is oxidation, where a molecule loses electrons, often involving the addition of oxygen or removal of hydrogen.
In the scenario described, an alcohol undergoes oxidation with the assistance of {K_2Cr_2O_7} and {H_2SO_4} to form a ketone, which further oxidizes to an acid. These changes are representative of multiple steps in organic transformations:

• Mild oxidation can convert secondary alcohols to ketones.
• Further oxidation can lead ketones to become acids.

Each step in these reactions points to careful control over conditions to direct the alcohol to the desired product.

Ketone Formation

Ketones are formed through the oxidation of secondary alcohols, a common reaction in organic chemistry. This transformation is typically catalyzed by oxidizing agents such as potassium dichromate ( {K_2Cr_2O_7} ).
In this process, the secondary alcohol loses hydrogen atoms and forms a carbonyl group (C=O), resulting in a ketone.
An example is isopropyl alcohol ( {CH_3CHOHCH_3} ), which oxidizes to form propanone, also known as acetone ( {CH_3COCH_3} ).
Considerations during ketone formation include:

• Choice of oxidizing agents to control reaction progression.
• Reaction conditions to prevent over-oxidation.

Ketone formation is significant not only for its role in oxidation reactions but also as an intermediate step in further chemical transformations.

Acetic Acid Production

Acetic acid is a commonly encountered organic acid in both industrial and biological processes. It can be produced through the oxidation of alcohols or ketones, demonstrating further transformation pathways in organic chemistry.
In vigorous oxidation conditions, such as those involving potassium dichromate ( {K_2Cr_2O_7} ) and sulfuric acid ( {H_2SO_4} ), acetone can be oxidized to form acetic acid ( {CH_3COOH} ).
This conversion emphasizes the significant transformation potential of secondary alcohols into acids. The overall process includes:

• Initial oxidation to form a ketone intermediate.
• Further oxidation under more rigorous conditions to yield acetic acid.

Understanding the steps and conditions required to produce acetic acid from various organic precursors is vital for effectively planning and executing synthetic routes.