Question

Which one of the following boron compounds is optically active? (a) Boron anhydride (b) Sodium tetraborate (c) Borosalicylic acid (d) Boron trifluoride

Short answer

Borosalicylic acid is optically active.

Step-by-step solution

Understanding the Problem

Optical activity is a property of compounds that can rotate plane-polarized light. For a compound to be optically active, it must be chiral, meaning it lacks symmetry and has a non-superimposable mirror image. As we examine the compounds given, we must identify the one that potentially lacks symmetry and has chiral centers.

Analyzing Each Compound

Let's analyze each compound: - **Boron anhydride (B₂O₃)**: This compound is symmetrical and does not have chiral centers, thus, it is not optically active. - **Sodium tetraborate (Na₂B₄O₇)**: Like boron anhydride, this compound is also symmetrical and lacks chiral centers, so it's not optically active. - **Borosalicylic acid**: This compound contains an aromatic ring substituted with boron and other functional groups which might break symmetry, potentially creating a chiral center depending on its structure. - **Boron trifluoride (BF₃)**: This is a planar molecule and inherently symmetrical, hence it is not optically active.

Final Deduction

After analyzing, borosalicylic acid is the compound with the potential for optical activity due to the presence of its aromatic ring and substituents that could result in chirality, unlike the other symmetrical compounds.

Key concepts

Chirality

Chirality is a fascinating concept in chemistry that describes a specific type of asymmetry. A molecule is considered chiral if it has a non-superimposable mirror image, much like how your left hand is a non-superimposable mirror image of your right hand. These mirror image molecules are referred to as enantiomers. For chirality to occur, a molecule must lack an internal plane of symmetry, allowing it to exist in two distinct forms that cannot be overlaid on one another.

Chirality is crucial in chemistry because chiral molecules can have vastly different biological activities. In nature, most molecules are chiral, and often only one enantiomer is biologically active. When studying optical activity, chemists are interested in how a molecule's chirality influences its ability to interact with plane-polarized light. Identifying chiral molecules is critical for applications in pharmaceuticals, where different enantiomers of a drug can have different therapeutic effects or side effects.

Symmetry in Molecules

Symmetry plays a significant role in determining whether a molecule is optically active. A molecule with perfect symmetry, like a sphere, will not be chiral. Symmetrical molecules have internal planes of symmetry or rotational axes. These elements of symmetry often cause the molecule to be superimposable on its mirror image, meaning it is not chiral.

In chemistry, identifying the type and degree of symmetry helps determine the molecular properties, such as polarity and optical activity. Asymmetric molecules, by contrast, lack these symmetry elements. This absence of symmetry is what makes a molecule chiral and thus potentially optically active. However, many molecules that appear asymmetrical at a glance can possess hidden symmetry that negates chirality.

• Planar molecules, like benzene, are usually symmetrical.
• Non-planar molecules lacking symmetry can be chiral.
• The presence of symmetry excludes optical activity.

Chiral Centers

A chiral center is a key feature in defining a molecule's chirality. Typically, a chiral center is a carbon atom bonded to four distinct atoms or groups. This results in a tetrahedral configuration that is non-superimposable on its mirror image, creating two enantiomers.

The presence of a chiral center is a direct indicator of potential optical activity. However, a molecule can have multiple chiral centers and still be non-chiral if the centers are symmetrically arranged, balancing the symmetry and resulting in a meso compound which is optically inactive. The identification of chiral centers in complex molecules is essential in anticipating their chemical behavior and interaction with polarized light.

The study of chiral centers helps chemists design and synthesize specific enantiomers for precise therapeutic uses, reflecting their importance in pharmaceutical research and development.

Plane-Polarized Light

Plane-polarized light is a type of light wave in which the vibrations occur in a single plane. It is used in experiments to determine the optical activity of a compound. When plane-polarized light passes through a solution of a chiral compound, the plane of polarization is rotated. This property is called optical rotation and is measured using a polarimeter.

The direction and magnitude of the rotation can help identify the compound and its concentration in a solution. Each enantiomer of a chiral compound will rotate the plane of polarized light in opposite directions but with the same magnitude. One enantiomer would be termed "dextrorotatory," rotating light to the right, whereas its mirror image would be "levorotatory," rotating light to the left.

• Light rotation is a critical method for studying chiral substances.
• Different enantiomers yield distinct rotation patterns.
• Polarimetry provides both qualitative and quantitative data about chiral molecules.

Understanding how chiral substances interact with plane-polarized light is essential in various scientific fields, particularly in the synthesis and analysis of pharmaceuticals.