Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following Column-I (a) \(\mathrm{CH}_{3} \mathrm{CHDT}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{C}_{2} \mathrm{H}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{3}\) Column-II (p) No optical isomer (q) Two optical isomers (r) Four optical isomers (s) Three optical isomers (t) Two asymmetric carbon atoms.

Step-by-Step Solution

Answer

(a) matches to (q), (b) matches to (r) and (t), (c) matches to (r) and (t).

Step 1: Identify Asymmetric Carbons in Each Compound

An asymmetric carbon atom is bonded to four different groups. Let's examine the compounds in Column-I for asymmetric carbons. - (a) \( \mathrm{CH}_{3} \mathrm{CHDT} \): The carbon bonded to \( \mathrm{CH}_{3} \), \( \mathrm{H} \), \( \mathrm{D} \) (deuterium), and \( \mathrm{T} \) (tritium) is asymmetric because all four groups are different. - (b) \( \mathrm{CH}_{3} \mathrm{CH} (\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{C}_{2} \mathrm{H}_{3} \): Examine each carbon - the second and third carbons each bond to \( \mathrm{CH}_{3} \), \( \mathrm{OH} \), another carbon chain, and hydrogen, making them asymmetric. - (c) \( \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{3} \): Similarly, the second and third carbons lead to asymmetric centers.

Step 2: Determine Optical Isomers for Each Compound

Optical isomers exist if there are one or more asymmetric carbon atoms. The number of optical isomers is \( 2^n \) where \( n \) is the number of asymmetric carbons. - (a) \( \mathrm{CH}_{3} \mathrm{CHDT} \): There is one asymmetric carbon, giving \( 2^1 = 2 \) optical isomers. - (b) \( \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{C}_{2} \mathrm{H}_{3} \): Two asymmetric carbons, generating \( 2^2 = 4 \) optical isomers. - (c) \( \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{3} \): Also has two asymmetric carbons, producing \( 2^2 = 4 \) optical isomers.

Step 3: Match Column-I Compounds to Column-II Descriptions

Using the information about asymmetric carbons and optical isomers, match each compound from Column-I with descriptions from Column-II:- (a) \( \mathrm{CH}_{3} \mathrm{CHDT} \): Has 2 optical isomers, matching to (q).- (b) \( \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{C}_{2} \mathrm{H}_{3} \): Has 4 optical isomers and 2 asymmetric carbons, matching to (r) and (t).- (c) \( \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{3} \): Also has 4 optical isomers and 2 asymmetric carbons, matching to (r) and (t).

Key Concepts

Asymmetric Carbon Atoms

Understanding the concept of asymmetric carbon atoms is a fundamental aspect of organic chemistry. An asymmetric carbon atom, also known as a chiral center, is one that is connected to four different atoms or groups of atoms. This means no two groups attached to this carbon atom are the same. The presence of asymmetric carbons is a key indicator for the potential existence of optical isomers in a molecule. When you observe a molecule, check every carbon atom to see if it is attached to four distinct groups. If it is, you have an asymmetric carbon atom. This asymmetry is what provides the molecule its chirality—allowing it to have different spatial arrangements that are mirror images of each other, leading to optical isomerism.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms within molecules. This branch of chemistry predominantly focuses on how the three-dimensional configuration of these atoms affects the properties and reactivity of the molecules. One of the most compelling aspects of stereochemistry is its connection to optical isomers, which are molecules that can rotate plane-polarized light differently based on their spatial configuration. Optical isomers, also known as enantiomers, arise when a molecule contains chiral centers, typically asymmetric carbon atoms. The ability to generate optical isomers depends directly on the number of asymmetric carbon atoms within the compound. For instance, if a molecule has one asymmetric carbon atom, it can have two optical isomers. With two asymmetric carbons, the number of possible optical isomers rises to four. This exponential growth emphasizes how critical understanding stereochemistry is, as even small changes in a molecule's spatial arrangement can lead to drastically different chemical properties and behaviors.

Organic Chemistry

Organic chemistry is the science of carbon-containing compounds, essential to the study of life and the basis for many materials in our world. This branch of chemistry revolves around carbon's unique ability to form stable bonds with many elements, including itself, leading to complex and varied structures such as chains, rings, and branches. In organic chemistry, understanding the properties of compounds involves examining their structures, which often include identifying asymmetric carbon atoms and understanding their stereochemistry. This knowledge helps predict how molecules might interact with each other—a crucial insight in developing pharmaceuticals and materials. Organic chemistry also involves various reactions and mechanisms to synthesize new compounds, each influenced heavily by the compound's structure and spatial configuration. Mastery of organic chemistry provides the tools needed to innovate in fields such as drug development, petrochemicals, and plastics, among others.