Question

An organic molecule necessarily shows optical activity if it (a) Is non-superimposable on its mirror image (b) Is superimposable on its mirror image (c) Contains asymmetric carbon atoms (d) Is non-polar

Short answer

(a) Is non-superimposable on its mirror image.

Step-by-step solution

Understanding Optical Activity

Optical activity is a property of compounds that can rotate the plane of polarized light. To exhibit optical activity, a molecule must have chirality, which means it cannot be superimposed on its mirror image.

Determine Chirality

Chirality is often associated with the presence of an asymmetric carbon atom, that is, a carbon atom bonded to four different groups. However, simply having asymmetric carbons doesn't guarantee that a molecule will be optically active.

Analyze Given Options

Option (a) 'Is non-superimposable on its mirror image' describes a chiral molecule, which is generally optically active. Option (b) 'Is superimposable on its mirror image' is characteristic of an achiral molecule, which is not optically active. Option (c) 'Contains asymmetric carbon atoms' indicates potential chirality but not guaranteed optical activity by itself. Option (d) 'Is non-polar' doesn't relate directly to optical activity.

Identify the Correct Option

Because a molecule must be non-superimposable on its mirror image to be optically active, the correct choice is option (a). This is the definition of a chiral molecule, and such molecules are usually capable of rotating plane-polarized light.

Key concepts

Chirality

The term "chirality" comes from the Greek word for "hand," symbolizing the non-superimposibility of an object on its mirror image, similar to how your left hand cannot perfectly align with a right hand. In chemistry, chirality is a crucial feature that often influences the physical and chemical properties of compounds. A molecule is said to be chiral if it is not identical to its mirror image.

Chirality is important because it determines if a molecule can be optically active. Optical activity refers to the ability of a molecule to rotate the plane of polarized light. Chiral molecules, much like how one's left and right hands are opposites, have mirror images that cannot be matched, making them potential candidates for optical activity.

• Molecules that are chiral can cause the direction of light to shift.
• This rotational ability is measurable and a fundamental property in stereochemistry.
• It's crucial in industries like pharmaceuticals, where the direction and amount of optical rotation can determine the effectiveness or safety of a compound.

Asymmetric Carbon

An asymmetric carbon atom, also known as a stereocenter, is a pivotal point in the study of chirality and optical activity. To be considered asymmetric, a carbon must attach to four completely different atoms or groups. This uniqueness prevents the molecular structure from being superimposable on its mirror image, often resulting in chirality.

However, simply having an asymmetric carbon does not automatically endow a molecule with optical activity. There are cases where multiple asymmetric carbons are present in a molecule, yet the overall structure remains achiral due to symmetry considerations. This is referred to as a "meso" compound, where internal planes of symmetry counteract the potential optical activity.

• Asymmetric carbons are integral for molecular diversity, impacting molecular shape, reactivity, and how a molecule interacts with biological systems.
• While they often signify potential optical activity, additional analysis is necessary to confirm if the molecule is truly optically active.

Polarized Light

Polarized light is light waves oscillating in one plane, as opposed to ordinary light, where waves vibrate in multiple planes. Using a polarizing filter, scientists can create polarized light to study various molecular properties, including optical activity.

When polarized light passes through a chiral compound, it can be rotated. This property is a hallmark of optical activity and offers insight into the molecular structure. The angle and direction of rotation depend on the type of chiral compound and can be precisely measured to understand its behavior more deeply.

• Rotation can be either clockwise (dextrorotatory) or counterclockwise (levorotatory).
• Through this method, researchers can distinguish between enantiomers, which are mirror images of each other but differ in optical activity.
• Such techniques are foundational in many scientific fields, helping identify the characteristics of complex molecules.