Problem 173 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following \begin{tabular}{ll} \hline Column-I & Column-II \\ \hline (a) Hydrocarboration of & (p) Primary alcohol \\ simplest unsaturated & \\ hydrocarbon followed & \\ by \(\mathrm{H}^{+} .\) & \\ \(\begin{array}{ll}\text { (b) Red } \mathrm{P} / \mathrm{HI} \text { reduces. } & \text { (q) Alkane } \\ \text { (c) Reductive ozonlysis } & \text { (r) Aldehyde } \\ \text { (d) Pyrolysis of xanthate } & \text { (s) Alkene } \\ & \text { (t) Reaction has syn } \\ \text { stereochemistry }\end{array}\) \end{tabular}

Step-by-Step Solution

Answer

(a) matches (p), (b) matches (q), (c) matches (r), (d) matches both (s) and (t).

Step 1: Understand Hydroboration-Oxidation

Hydroboration-oxidation of an unsaturated hydrocarbon (alkene) results in the formation of an alcohol. Thus, the correct match for (a) is (p) primary alcohol.

Step 2: Identify Reduction by Red P/HI

Red phosphorus and hydroiodic acid (HI) are used to reduce functional groups such as alcohols to alkanes. Hence, the match for (b) is (q) alkane.

Step 3: Determine Product of Reductive Ozonolysis

Reductive ozonolysis of alkenes results in the cleavage of the double bond forming aldehydes or ketones. Here it matches (r) aldehyde.

Step 4: Understand Pyrolysis of Xanthate

Pyrolysis of a xanthate ester is known to produce an alkene due to the elimination reaction. The reaction results in the formation of an alkene with syn stereochemistry, making it (s) alkene and (t) reaction has syn stereochemistry.

Key Concepts

Hydroboration-Oxidation

Hydroboration-oxidation is a two-step process that transforms alkenes into alcohols. First, in the hydroboration step, borane (BH₃) adds across the carbon-carbon double bond in the alkene. This step occurs in a concerted manner, which means it takes place in one smooth motion without intermediates.

A key feature of hydroboration is that it gives the anti-Markovnikov product. This means the hydrogen atom adds to the more substituted carbon, and the boron atom attaches to the less substituted one. Next, the oxidation step involves treating the organoborane intermediate with hydrogen peroxide (H₂O₂) in the presence of a base, typically sodium hydroxide (NaOH).

• The boron is replaced by an -OH group during this step.
• The overall result is the formation of a primary alcohol.

Hydroboration-oxidation is favored because it typically proceeds with high efficiency and gives products with predictable stereochemistry.

Reductive Ozonolysis

Reductive ozonolysis is an effective method to cleave alkenes, transforming them into smaller compounds, primarily aldehydes or ketones. In the first step, ozone (O₃) reacts with the alkene, breaking the double bond and forming an ozonide intermediate. This step occurs through a cyclic transition structure, thus ensuring the cleavage of the alkene into 1,2,3-trioxolane.

To prevent the formation of unwanted oxidized products such as carboxylic acids, the ozonide is reduced using a mild reducing agent, often zinc (Zn) or dimethyl sulfide (DMS). This reduction decomposes the ozonide into aldehydes or ketones.

• If Zn or DMS is used, reduction keeps the aldehyde form intact.
• The process avoids over-oxidation, making it suitable for sensitive molecules.

Reductive ozonolysis is particularly useful in structural elucidation and complex organic synthesis.

Xanthate Pyrolysis

Xanthate pyrolysis is a reaction that involves the thermal decomposition of xanthate esters. It is a powerful method for converting alcohols into alkenes through an elimination process. In this reaction, heating a xanthate leads to the expulsion of carbon disulfide (CS2) and the production of an alkene.

The pyrolysis process occurs through a six-membered cyclic transition state. This results in the elimination of the leaving group, forming the double bond of the alkene.

• Xanthate pyrolysis is characterized by syn stereochemistry, meaning that both the hydrogen and the xanthate leave from the same side of the molecule.

This stereochemical outcome is particularly valuable for the synthesis of specific alkene isomers. Thus, it plays a crucial role in organic synthesis, especially when stereochemistry is a concern.

Aldehyde Formation

Aldehydes are key organic compounds featuring a carbonyl group bonded to at least one hydrogen atom, depicted as \(R-CHO\). Reductive ozonolysis of alkenes is a common method to produce aldehydes, where the location of the original double bond determines the type formed.

In preparative organic chemistry, aldehydes are valued due to their reactivity, allowing them to participate in various further transformations:

• Aldehydes can be oxidized to carboxylic acids, adding functionality to the molecule.
• They are also susceptible to nucleophilic addition reactions, forming alcohols or other derivatives.

Because of these reactions, aldehydes serve as important intermediates. Understanding their formation and conversion is crucial for synthesis strategies and the design of complex molecules.