Question

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

Short answer

The optically active compounds are (b) b2-bromobutyric acid and (d) 1-bromo-2-methylpropane.

Step-by-step solution

Understanding Optically Active Compounds

An optically active compound is one that can rotate the plane of polarized light due to the presence of a chiral center. A chiral center usually has four different substituents attached to it.

Analyzing 1-bromobutane

In 1-bromobutane, the bromine is attached to the first carbon of a straight chain butane. The structure is CH3-CH2-CH2-CH2-Br. Since the fourth carbon attached to the primary carbon (where the bromine is joined) has two hydrogens, it is not a chiral center and hence, not optically active.

Analyzing b2-bromobutyric Acid

b2-bromobutyric acid has the structure: CH3-CHBr-CH2-COOH. The second carbon (CHBr) is attached to four different groups: a hydrogen, a bromide, a methyl group, and a carboxylic acid group, making it a chiral center. Therefore, it is optically active.

Analyzing 2-bromo-2-methylpropane

2-bromo-2-methylpropane has the structure: (CH3)3CBr. The central carbon is attached to three identical methyl groups and one bromide, so there is no chiral center present, making it not optically active.

Analyzing 1-bromo-2-methylpropane

1-bromo-2-methylpropane has the structure: CH3-CH(CH3)-CH2-Br. Here, the carbon attached to the bromine is attached to a hydrogen, a methyl group, and an ethyl group, creating different substituents. Therefore, this compound has a chiral center and is optically active.

Key concepts

Chirality

Chirality is a fundamental concept in stereochemistry, describing a property where an object is not superimposable on its mirror image. Just like our hands, these objects, when mirrored, appear different from their original form. In chemistry, chirality often pertains to molecules, specifically when there is an asymmetry where the molecule and its mirror image are non-identical. This can significantly impact how a compound behaves chemically, particularly in reactions. To determine chirality in a molecule, it's essential to check whether it possesses a chiral center or more, which results in it being non-superimposable. The presence of chirality in compounds like drugs can lead to differences in how the compound interacts with biological systems, sometimes resulting in distinct physiological effects whether they are mirror images (enantiomers) or not.

Chiral Center

A chiral center, or a stereocenter, is the specific atom within a molecule to which different substituents are attached, making the molecule chiral. This center is typically a carbon atom bonded to four different atoms or groups. To identify a chiral center, inspect each carbon atom in a compound for four unique attachments. This uniqueness is vital because if two substituents are the same, the carbon cannot serve as a chiral center. In organic chemistry, discovering chiral centers is crucial as they are often the hot-spots for chemical activity, impacting how molecules interact and their optical activity. For instance, in the \[\beta\]-bromobutyric acid molecule, the presence of a carbon center bonded distinctly to a hydrogen, bromine, a methyl group, and a carboxyl group demonstrates a classic example of a chiral center. This distinct bonding allows such molecules to rotate plane-polarized light, contributing to their optical activity.

Optically Active Compounds

Optically active compounds are compounds capable of rotating the plane of polarized light they interact with, doing so through the action of their chiral centers. This ability stems from their structural asymmetry, which defines their interactions with light. When considering optical activity, it’s integral to recognize the difference between two enantiomers of the same compound. Each enantiomer will rotate light by the same magnitude but in opposite directions. This is often measured using polarimeters and can provide significant insight into the compound's structure and purity.Examples include compounds like \[\beta\]-bromobutyric acid and 1-bromo-2-methylpropane, each having at least one carbon atom as a chiral center leading to this optical property. These compounds' ability to rotate the plane of polarized light is due to the spatial arrangement of atoms around their chiral centers, making them essential in the study of stereochemistry.