Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Given below are certain matching type questions, where two columns (each having 4 items) are given. Immediately after the columns the matching grid is given, where each item of Column I has to be matched with the items of Column II, by encircling the correct match(es). Note that an item of Column I can match with more than one item of Column II. All the items of Column II must be matched. Match the following : Column I (A) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CD}_{2} \mathrm{Br}\) on reaction with (B) \(\mathrm{PhCHBrCH}_{3}\) and \(\mathrm{PhCHBrCD}_{3}\), (C) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) on treatment with \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\)and \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OD}\) gives \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CD}=\mathrm{CH}_{2}\) (D) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) reacts faster than \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CD}_{2} \mathrm{CH}_{2} \mathrm{Br}\) on reaction with \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-} \quad\) in ethanol Column II (p) E1 reaction \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\)gives \(\mathrm{C}_{6} \mathrm{H}_{5}-\) \(\mathrm{CH}=\mathrm{CD}_{2}\) (q) E2 reaction both react with the same rate (r) E1cB reaction (s) First order reaction

Step-by-Step Solution

Answer

(A) matches (q), (B) matches (r), (C) matches (p), (D) matches (s).

Step 1: Analyze Column I

We will go through each item in Column I to understand the context. Each item represents a chemical reaction or property.- Item (A): This involves the compound \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CD}_{2} \mathrm{Br}\) reacting, which suggests possible isotope labeling in the chemical reaction.- Item (B): Reactants \( \mathrm{PhCHBrCH}_{3} \) and \( \mathrm{PhCHBrCD}_{3} \) likely point towards a scenario involving isomeric or isotope differences affecting reaction rates.- Item (C): This involves the compound \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) reacting with isotope-labeled ethanolate, leading to a final isotope-labeled alkene.- Item (D): This describes a comparative rate of reaction, focusing on kinetic isotope effect.

Step 2: Analyze Column II

Let's break down the definitions in Column II, each of which describes a type of reaction or general reaction property. - (p): E1 reaction, which is a unimolecular elimination reaction, often involving carbocation intermediate. - (q): E2 reaction involves bimolecular elimination, typically with simultaneous bond breaking/forming. - (r): E1cB reaction is a specific type of elimination reaction proceeding through a carbanion intermediate. - (s): First-order reaction depends linearly on the concentration of one reactant.

Step 3: Match Item A from Column I

The compound \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CD}_{2} \mathrm{Br}\) can undergo elimination. As it involves isotope labeling, it most likely forms a product reflecting a kinetic isotope effect, which suggests E2 reaction mechanisms are often sensitive to isotopes, aligning (A) with (q).

Step 4: Match Item B from Column I

Both \( \mathrm{PhCHBrCH}_{3} \) and \( \mathrm{PhCHBrCD}_{3} \) involve potential kinetic isotope effects. The E2 reaction rate can be affected by isotopes, but if both react at the same rate, it aligns best with (r) emphasizing an E1cB reaction, where isotope effects might not alter the rate much.

Step 5: Match Item C from Column I

The reaction \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) gives an alkene with isotopic substitution, fitting best with (p) as E1 reactions form alkenes and can rearrange atoms even when isotope labeling is involved.

Step 6: Match Item D from Column I

This involves a rate difference due to a kinetic isotope effect. Faster reaction of \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (lighter isotope reacts more rapidly) suggests (s) is a suitable match, as it involves first-order kinetic implications.

Key Concepts

Kinetic Isotope Effect

The kinetic isotope effect is a fascinating concept in chemistry that explores how different isotopes of an element can affect the rate of a chemical reaction. Isotopes are atoms with the same number of protons but different numbers of neutrons, resulting in different atomic masses. In reactions, bonds with heavier isotopes often require more energy to break because of their lower vibrational frequencies, leading to slower reaction rates compared to their lighter counterparts.
For example, consider a molecule where hydrogen (H) has been replaced with deuterium (D), a heavier isotope. Because bond strengths differ, the kinetic isotope effect means that reactions involving these isotopes will happen at different rates—often highlighted in elimination reactions.

• The primary kinetic isotope effect occurs when breaking bonds directly involving the labeled atom.
• The secondary effect is seen when isotopic substitution affects rates through changes other than bond breaking—like enzyme or catalyst interactions.

Understanding this effect is pivotal for determining reaction mechanisms and predicting reaction behavior when isotopes are involved.

Elimination Reactions

Elimination reactions are a type of chemical reaction in which a molecule loses atoms or groups of atoms, resulting in the formation of a double bond or, sometimes, a triple bond. These reactions are fundamental in organic chemistry and often lead to the synthesis of alkenes from alkyl halides. There are two main types of elimination reactions:

• E1 Reactions: This is a unimolecular process involving two distinct steps. First, a leaving group departs, forming a carbocation intermediate. Second, a base removes a proton, forming a double bond. It's typical in solutions with solvents that stabilize ions. These reactions usually observe a first-order kinetics, being rate-dependent on the concentration of the substrate only.
• E2 Reactions: This is a bimolecular process, occurring in one concerted step where the base removes a proton while the leaving group departs simultaneously. This mechanism requires a strong base and typically results in rapid reactions, especially with good leaving groups.

Elimination reactions are used extensively in organic synthesis to form complex carbon structures efficiently.

Isotope Labeling

Isotope labeling is a powerful technique used to track the movement of atoms through a chemical reaction. By replacing atoms in a molecule with isotopes—usually heavier ones like carbon-13 or deuterium—you get a clear picture of how these atoms are modified during reactions. This technique is invaluable in studying reaction mechanisms and metabolic pathways.
For instance, when investigating a reaction, you can isotopically label a hydrogen with deuterium. When the reaction proceeds, you can track where this atom goes and how it's bonded in the products. This data can highlight specific bonds involved in the reaction, clarifying the step-by-step transformation.
Isotope labeling helps chemists:

• Understand reaction mechanisms by identifying intermediate stages.
• Track complex pathways in biological systems.
• Determine molecular structure and dynamics, where nuclear magnetic resonance (NMR) spectroscopy often plays a role.

Therefore, isotope labeling serves as a key tool in both academic and industrial chemical research.

Reaction Mechanisms

Understanding reaction mechanisms is crucial in chemistry as they explain how and why chemical reactions occur. A reaction mechanism details each step of the transformation from reactants to products, including the breaking and forming of bonds. Knowing these details allows chemists to predict the conditions necessary for a reaction to occur and discover ways to optimize it.
In organic chemistry, reaction mechanisms often involve intermediates, which are distinct, short-lived entities formed along the reaction path. Mechanisms also describe how factors such as solvent, temperature, and presence of catalysts affect the reaction speed and outcome.

• Each step in a mechanism may involve distinct types of reactions such as substitution, addition, or elimination.
• By detailing a mechanism, one can deduce unexpected products or side reactions.
• Mechanisms help in identifying the rate-determining step, which is the slowest step of the reaction.

Reaction mechanisms are not just theoretic; they are essential to understanding real-world chemical processes and enhancing chemical reactions for industrial applications.