Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

An optically active compound is (a) 1-bromobutane (b) \(\beta\)-bromobutyric acid (c) 2 -bromo-2-methylpropane (d) 1 -bromo-2-methylpropane

Step-by-Step Solution

Answer

β-Bromobutyric acid is optically active.

Step 1: Understanding Optical Activity

Optical activity refers to the ability of a compound to rotate the plane of polarized light. A compound is optically active if it has a chiral center. A chiral center is typically a carbon atom that is bonded to four different groups.

Step 2: Analyzing 1-Bromobutane for Chirality

1-bromobutane has the structure: CH₃-CH₂-CH₂-CH₂-Br. In this compound, there is no carbon atom bonded to four different groups. All carbon atoms are bonded in simple straight chains with hydrogens fulfilling the valence.

Step 3: Analyzing β-Bromobutyric Acid for Chirality

The structure for β-bromobutyric acid can be represented as Br-CH₂-CH₂-CH(COOH)-CH₃. The second carbon (from Br) is bonded to four different groups: H, CH₂Br, COOH, and CH₃, making it a chiral center. Therefore, this compound is optically active.

Step 4: Analyzing 2-Bromo-2-Methylpropane for Chirality

The structure for 2-bromo-2-methylpropane is (CH₃)₃-C-Br. There is no carbon atom bonded to four different groups. The central carbon is bonded to three methyl groups and a bromine atom, leading to no chiral center.

Step 5: Analyzing 1-Bromo-2-Methylpropane for Chirality

The structure of 1-bromo-2-methylpropane can be written as CH₃-CH(CH₃)-CH₂-Br. None of the carbon atoms in this compound are bonded to four different groups, lacking a chiral center.

Key Concepts

Chiral Center

A chiral center is a critical concept in the study of optical activity in chemistry. It typically refers to a carbon atom that is bonded to four different groups. These unique groups cause the molecule to be asymmetrical, which is a key feature for optical activity.
For a compound to exhibit chirality and hence be optically active, it must have at least one such chiral center. This asymmetry allows the compound to interact with polarized light in a unique way, rotating it either to the left or right.

• Chiral centers are the backbone of molecular chirality.
• Without a chiral center, a molecule generally cannot rotate polarized light.
• Checking for a chiral center involves analyzing the groups bonded to each carbon atom.

Understanding the presence of a chiral center is essential when determining if a molecule is optically active. In our exercise, the chiral center is found in β-bromobutyric acid, where one of the carbon atoms is bonded to four distinctly different groups.

Chirality

Chirality is a fascinating and essential concept in stereochemistry, referring to the geometric property of a structure being non-superimposable on its mirror image. It's like a pair of hands; though similar, the left hand is not superimposable onto the right. This phenomenon occurs in molecules due to the presence of a chiral center, which leads to two enantiomers: one that rotates light in one direction and one in the opposite direction.
Chirality has profound implications, especially in pharmaceuticals where it can impact drug efficacy.

• Without chirality, a compound cannot be optically active.
• Chirality results in two possible configurations of molecules called enantiomers.
• The biological activity of chiral molecules can dramatically differ between enantiomers.

This is important in many fields including chemistry and bioengineering. In the given problem, chirality is confirmed in β-bromobutyric acid, making it distinctly optically active compared to the other options that lack this asymmetric structure.

Polarized Light

Polarized light is a type of light beam in which the waves oscillate in a single plane, as opposed to ordinary light that vibrates in multiple planes. When polarized light passes through a solution of an optically active compound, such as one with chirality, the plane of polarization is rotated.
The rotation degree depends on several factors, including the nature of the compound and the concentration of the solution.

• Optical activity is directly observed by the degree of rotation of polarized light.
• The phenomenon is useful for identifying chiral compounds and determining their purity.
• Both the angle and direction of rotation provide valuable chemical information.

Understanding how polarized light interacts with optically active substances is valuable for many applications in chemical analysis. It highlights how structural aspects of a compound, like chirality, can influence physical properties.