Optical Isomers and Chirality

Optical Isomers

• Optical isomers are also called enantiomers.
• 'Enantio' in Greek means 'opposite', referring to mirror images.
• Enantiomers cannot be superimposed on each other.

Chiral Center

• A chiral center is an atom bonded to four different groups.
• This atom type will yield two optical isomers that are mirror images.
• A chiral center is also referred to as an asymmetric, stereochemical, or stereogenic center.

Propagation of Light Waves

• Light waves are a form of electromagnetic radiation.

Optical Activity

• A polarimeter is used to determine the optical activity of a sample

How a Polarimeter Functions:

• Unpolarized light has waves spanning 360 degrees.
• When unpolarized light passes through a polarizer, only one plane wave passes through, creating plane-polarized light.
• This plane-polarized light enters the sample tube containing a solution of a single optical isomer.
• As light passes through the solution, the plane angle changes, rotating either clockwise or counter-clockwise.
• The rotation direction depends on the specific optical isomer.
• To find the new angle, a second polarizer (analyzer) is used and rotated until the light passes through to the viewer.

Chiral Center Classifications

• A chiral center that rotates plane-polarized light clockwise is dextrorotatory and labeled D, d, or +.
• A chiral center that rotates plane-polarized light counter-clockwise is levorotatory and labeled L, l, or -.

Chiral Center Configurations

• To differentiate optical isomers, their three-dimensional orientation is designated as a configuration.
• The two configurations are R (rectus or right) and S (sinister or left).
• The R/S configuration does not define light rotation direction.

Configuration and Rotatory Direction

• Determining the configuration does not determine the direction of plane-polarized light rotation.
• Rotation must be determined by experiment.

Specifying R,S Configuration

• Rule 1: Look at the four atoms directly connected to the stereogenic carbon.
• Assign priorities in decreasing order of atomic number, ranking the atom with the highest atomic number first.
• Rule 2: If the first atom cannot determine priority, look at the second, third, etc., atoms outwards until a difference is found.
• Rule 3: Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.

Fischer Projections

• R or S stereochemical designation can be assigned through three steps.
• Step 1: Assign priorities to the four groups.
• Step 2: Place the lowest priority group at the top, rotating on paper by 180° or holding one group steady while rotating the other three clockwise or counterclockwise.
• Fischer projections cannot be rotated by 90° or 270°.
• Step 3: Determine the rotation of groups in order of priority from 1 → 2 → 3.

Relationship of Optical Isomers

• Enantiomers have opposite configurations at all chiral centers.
• Diastereomers have one configuration the same and one different.
• Meso compounds have an internal plane of symmetry and no net optical activity.

Key Points

• An optical, chiral, or stereogenic center contains an atom (usually carbon) connected to four different groups, requiring an sp³ hybridized atom without lone pairs.
• A molecule with a chiral center can have two mirror images (enantiomers) with differing three-dimensional arrangements.
• If there are "n" chiral centers, there will be 2^n possible optical isomers; however, often only one is found in nature.
• Optically active molecules rotate plane-polarized light either clockwise (dextrorotatory) or counter-clockwise (levorotatory).
• Cahn-Ingold-Prelog rules determine group priorities in Fischer projections.
• Configuration does not determine the direction of rotation of plane-polarized light; the direction must be found experimentally.
• Multiple chiral centers in a molecule result in enantiomers (opposite configurations at all chiral centers), diastereomers (not all chiral centers have the same configurations), or meso compounds (center of symmetry cancels total optical activity).