Question

Stereogenic carbons can bring (A) Optical activity (B) Optical inactivity (C) Both (D) None of the above

Short answer

Stereogenic carbons can give rise to enantiomers, which can exhibit optical activity. However, if the stereogenic carbon is part of a meso compound, the overall molecule will be achiral and optically inactive. Therefore, the correct answer is (C) Both.

Step-by-step solution

Define Stereogenic Carbon

A stereogenic carbon is a carbon atom bonded to four different substituents, which results in stereoisomers – specifically, enantiomers – with non-superimposable mirror images. These stereoisomers have the same chemical connectivity but distinct spatial arrangements of the atoms.

Define Optical Activity

Optical activity refers to a substance's ability to rotate the plane of polarized light, which is determined by its molecular configuration. Enantiomers, which are a type of stereoisomer, can exhibit optical activity. One enantiomer will rotate polarized light in one direction (clockwise or counterclockwise), while its mirror image will rotate it in the opposite direction. Optical isomers, or enantiomers, are also called "chiral molecules".

Relation Between Stereogenic Carbons and Optical Activity

The presence of a stereogenic carbon in a molecule can give rise to enantiomers, which can exhibit optical activity. However, it is important to note that not all molecules with a stereogenic carbon will be optically active if it is part of a meso compound. Meso compounds are achiral, meaning they do not exhibit optical activity, even though they contain stereogenic carbons. Meso compounds have an internal plane of symmetry, which makes the overall molecule achiral.

Choose the Correct Option

Based on the information above, stereogenic carbons can lead to optical activity but can also be part of meso compounds, which are optically inactive. Therefore, the correct answer would be (C) Both.

Key concepts

Understanding Optical Activity

Optical activity is a fascinating phenomenon observed in chiral molecules. It describes the ability of such a molecule to rotate the plane of polarized light. This rotation depends heavily on the spatial arrangement of atoms within the molecule, which leads to a unique interaction with light. To delve deeper, when light waves are polarized, they oscillate in a single plane. If this plane of light passes through a chiral substance, the molecular structure will rotate the plane of the light to the right or left. This rotation is specific to the exact configuration of the molecule.

• Right-handed Rotation (Dextrorotatory): These substances rotate light clockwise and are denoted with a "+" sign.
• Left-handed Rotation (Levorotatory): These substances rotate light counterclockwise and are marked with a "-" sign.

Importantly, only chiral compounds exhibit optical activity. A molecule must not possess a plane of symmetry to qualify as chiral. This characteristic is primarily due to the presence of stereogenic carbon atoms, which establish non-superimposable arrangements.

Examining Enantiomers

Enantiomers are stereoisomers – specifically, they are pairs of molecules that are non-superimposable mirror images of each other. Think of how your left and right hands are mirror images but can't be perfectly aligned atop each other; enantiomers exhibit a similar relationship. Each enantiomer affects polarized light distinctly, one rotating it to the right and the other to the left, which is a foundational example of optical activity.

• D-enantiomer: This is the enantiomer that rotates light clockwise (dextrorotatory).
• L-enantiomer: This is the enantiomer that rotates light counterclockwise (levorotatory).

One crucial point is that enantiomers have identical physical properties, such as melting or boiling points, but differ in their interaction with polarized light and with other chiral environments. Therefore, understanding and identifying enantiomers are essential in stereochemistry and fields like pharmacology, where enantiomers can have drastically different biological effects.

Introduction to Meso Compounds

Meso compounds present an interesting exception in the study of optical activity. They contain multiple stereogenic centers, yet they are optically inactive. This happens because these compounds possess an internal plane of symmetry that cancels out the optical activity that individual stereogenic centers could otherwise induce. A meso compound can be envisioned as having a symmetrical mirror image within the same molecule. Even though it contains stereogenic carbons common in optically active substances, the molecular symmetry renders the entire compound achiral. For instance, if a meso compound has two identical halves, the rotation caused by one half is perfectly counteracted by the other, resulting in no net rotation of polarized light. This fascinating characteristic underscores the importance of both local and overall symmetry in determining the optical behavior of molecules.