Problem 184 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(a) - (p), (b) - (q), (c) - (r), (d) - (s)

Step 1: Analyze Column-I

Let's first analyze the processes mentioned in Column-I:a) Hydroboration of a simplest unsaturated hydrocarbon followed by \( \mathrm{H}^+ \) is a typical method to form an alcohol. The general reaction involves the addition of borane \( (BH_3) \) to an alkene, which upon subsequent oxidation, produces a primary alcohol.b) Red P/HI (red phosphorus and hydroiodic acid) is used to reduce carboxylic acids and alcohols to hydrocarbons, typically converting them to alkanes.c) Reductive ozonolysis involves cleavage of alkenes using ozone \( (O_3) \) followed by a reducing agent to produce ketones or aldehydes.d) Pyrolysis of xanthate, known as the Chugaev reaction, leads to the formation of alkenes.

Step 2: Analyze Column-II

Now, let's understand the options in Column-II:p) Primary alcohols are a type of alcohol where the hydroxyl group \((OH)\) is attached to a primary carbon.q) Alkanes are saturated hydrocarbons with single bonds between carbon atoms.r) Aldehydes are organic compounds containing the formyl group \( (-CHO) \).s) Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond.t) A reaction having a 'syn' stereochemistry indicates that addends are added to the same face of the double bond.

Step 3: Match Processes to Products

Now, let's match the processes from Column-I to the corresponding products or properties in Column-II based on the analysis:(a) Hydroboration of the simplest unsaturated hydrocarbon followed by \( \mathrm{H}^{+} \) results in the formation of a primary alcohol. Thus, it matches with (p), 'Primary alcohol'.(b) Red P/HI reduces alcohols or carboxylic acids to alkanes, so it matches with (q), 'Alkane'.(c) Reductive ozonolysis of alkenes typically results in aldehydes or ketones depending on the structure of the starting material, so it matches with (r), 'Aldehyde'.(d) Pyrolysis of xanthate leads to the formation of alkenes; therefore, it matches with (s), 'Alkene'.

Key Concepts

Hydroboration

Hydroboration is a chemical reaction that involves the addition of borane (\( BH_3 \)) to an alkene. This reaction is significant in organic chemistry due to its ability to convert alkenes into alcohols. It proceeds via a syn-addition mechanism, meaning both the boron and the hydrogen atom add to the same side of the double bond. This stereochemistry is important as it influences the final structure of the alcohol.

• The reaction starts with borane adding across the alkene's double bond.
• Following the addition, the boron is replaced with a hydroxyl group through oxidation, commonly using hydrogen peroxide \( (H_2O_2) \) and sodium hydroxide \( (NaOH) \).
• The overall result is the formation of a primary alcohol.

This process is particularly useful for creating anti-Markovnikov products, where the less substituted carbon atom of the alkene receives the hydroxyl group.

Reductive Ozonolysis

Reductive ozonolysis is a significant reaction in organic chemistry used to cleave alkenes. It involves two main stages: ozone (\( O_3 \)) adds to the unsaturated bond to form an ozonide, followed by reduction.

• The initial reaction with ozone forms an unstable intermediate called an ozonide.
• The ozonide is then treated with a reducing agent, such as zinc (Zn) or dimethyl sulfide \( (DMS) \), to produce aldehydes or ketones.
• This reaction is utilized to determine the position of double bonds in complex molecules by breaking the alkene into smaller fragments.

Reductive ozonolysis efficiently transforms alkenes into useful carbonyl compounds, making it an essential tool in organic synthesis.

Chugaev Reaction

The Chugaev Reaction, named after the Russian chemist Lev Aleksandrovich Chugaev, is employed to transform alcohols into alkenes. It involves the thermal decomposition of xanthate esters (which are derivatives of carbon disulfide).

• Initially, an alcohol is converted into a xanthate ester by reacting with carbon disulfide \( (CS_2) \) and an alkyl halide.
• The xanthate ester undergoes pyrolysis upon heating, expelling carbon disulfide and leaving behind an alkene.

This reaction is particularly advantageous due to its ability to produce alkenes with high selectivity and characteristic cis- or trans-geometry, dictated by the stereochemistry of the starting alcohol.

Reaction Mechanism

Understanding reaction mechanisms is crucial for predicting the outcome of chemical reactions. A reaction mechanism essentially details the step-by-step process by which reactants convert into products.

• Each mechanism typically involves a series of elementary steps that describe the direction and rearrangement of electrons.
• They often illustrate the formation and breaking of chemical bonds.
• By understanding the sequence of intermediate species or structures formed, chemists can manipulate reaction conditions to favor desired products.

Mastering reaction mechanisms allows chemists to innovate and design new reactions, improving efficiency and selectivity in synthesis.

Stereochemistry

Stereochemistry is a critical aspect of organic chemistry that focuses on the spatial arrangement of atoms in molecules. It is essential since stereochemical properties can drastically affect a molecule's reactivity and interactions.

• Stereoisomers have the same molecular formula and sequence of bonded atoms but differ in three-dimensional orientation.
• This can lead to different physical, chemical, and biological properties.
• Reactions like hydroboration yield products with specific stereochemistry, crucial for applications such as drug development.

Understanding stereochemistry helps chemists control and predict the behavior of molecules in different environments, making it a cornerstone of molecular design and synthesis.