Stereochemistry Lecture 1

Optical Activity and Stereochemistry

• Light can be described as a wave vibrating perpendicular to its direction of propagation, and can be unpolarized or plane polarized.
• Specific rotation is a physical property of an optically active compound, calculated by dividing the observed rotation by the length of the sample tube and the concentration of the sample.
• Stereoisomers with two chiral carbon atoms can have enantiomers and diastereomers.

Diastereomers

• Diastereomers are stereoisomers that are not mirror images of each other.
• They differ in all physical properties and can have opposite configurations at some, but not all, chiral centers.

Meso Compounds

• Meso compounds have two or more chiral centers and are optically inactive due to internal compensation.
• They have a plane of symmetry and can have identical substitution on both chiral centers.

Optical Isomerism in Cyclic Compounds

• Cyclic compounds can exhibit enantiomerism and geometric isomerism.
• A cyclic compound with two dissimilar chiral carbon atoms can have two enantiomeric pairs.

Stereochemistry

• Stereochemistry refers to the 3-dimensional properties and reactions of molecules.
• Isomers are compounds with identical molecular formulas but different structural formulas.

Optical Isomerism

• Optical isomerism occurs when molecules differ only in the spatial orientation of atoms, not in their arrangement.

Chiral Carbon Atoms, Enantiomers, and Racemic Mixtures

• A carbon with four different bonded groups is called a chiral carbon atom, chirality center, or stereocenter.
• A chiral carbon atom can exist in either of two three-dimensional arrangements that are non-superimposable mirror images (enantiomers).

Measurement of Optical Activity

• Optical activity can be measured using a polarimeter, which operates on the principle of rotating plane-polarized light.
• Specific rotation is defined as the observed rotation of an optically active compound in a cell with a 10 cm path using light from sodium metal vapor (589 nm).

Specification of Configuration

• When naming compounds containing multiple chiral atoms, the configuration around each chiral atom must be specified in numerical order, separated by commas, all in parentheses at the start.
• For geometric isomers, the configuration is determined by the priority of the groups attached to the double bond, with the highest priority group on the same side being termed Z (zusammen) and on opposite sides being termed E (entgegen).