Chirality, Enantiomers, and Stereochemistry in Organic Molecules

Chirality

A property of a molecule that is not superimposable on its mirror image.

Chiral center (stereogenic center)

An sp³-hybridized carbon atom bonded to four different substituents.

Enantiomers

Stereoisomers that are non-superimposable mirror images of each other.

Properties of enantiomers

Same melting point, boiling point, density, and refractive index; rotate plane-polarized light in equal amounts but opposite directions; interact differently with other chiral environments (e.g., enzymes, receptors, taste, smell).

Optical activity

The ability of a chiral compound to rotate plane-polarized light.

Dextrorotatory (+)

An optically active compound that rotates plane-polarized light clockwise.

Levorotatory (-)

An optically active compound that rotates plane-polarized light counterclockwise.

Racemic mixture (racemate)

A 1:1 mixture of two enantiomers; optically inactive due to equal but opposite rotations canceling out.

Polarimeter

An instrument used to measure the rotation of plane-polarized light by a chiral compound.

Specific rotation

A standardized measure of optical rotation that accounts for path length and concentration.

Conformations and chirality

If two conformations interconvert rapidly (e.g., cyclohexane chairs), the molecule is not optically active overall.

Fischer projection

A 2D drawing method for 3D molecules where horizontal bonds project out of the plane (towards the viewer) and vertical bonds project into the plane (away from the viewer).

Fischer projection rules

The carbon chain is drawn vertically with the most oxidized carbon at the top; only 180° rotations are allowed; 90° rotations change the stereochemistry and are not valid.

Assigning R/S from Fischer projections

The lowest priority group (often H) points forward, so the normal R/S assignment must be reversed.

Diastereomers

Stereoisomers that are not mirror images of each other.

Properties of diastereomers

They have different physical properties (melting point, boiling point, solubility, etc.) and are easier to separate than enantiomers.

Cis/trans isomers

Diastereomers that arise from restricted rotation in double bonds or rings.

Meso compound

A molecule that contains stereocenters but also has a plane of symmetry, making it achiral and optically inactive.

2ⁿ rule

The maximum number of stereoisomers possible for a molecule with n stereocenters is 2ⁿ. The actual number may be reduced if meso compounds exist.