This section introduces the concept of stereoisomers, and covers how to recognize and name the stereoisomers that result in alkenes from restricted rotation about π-bonds.
- Molecules can rotate around σ-bonds but not π-bonds.
- Two molecules that differ only in how they are rotated about their σ-bonds are identical molecules (in different conformations).
- Stereoisomers have the same molecular formula and the same connectivity, but differ in the arrangement of their atoms in space.
- Simple alkenes with one substituent off each carbon in the double bond can be named using the cis/trans system.
- If both substituents are on the same side of the double bond, it is a cis-alkene.
- If the two substituents are on opposite sides of the double bond, it is a trans-alkene.
- CIP rules systematically prioritize each substituent, where higher atomic number corresponds to higher priority.
- The E/Z system of nomenclature can be used to name any alkene with stereoisomers.
- An (E)-alkene has the two higher priority substituents on opposite sides of the double bond.
- A (Z)-alkene has the two higher priority substituents on the same side of the double bond.
This section explores stereoisomers arising from a carbon atom bonded to four different groups, and how this affects the properties of molecules.
- Constitutional isomers have the same molecular formula, but different connectivity.
- Stereoisomers have the same connectivity, but different spatial arrangements.
- Enantiomers and diastereomers are two types of stereoisomers.
- An atom is a stereocentre if switching the positions of two of its substituents leads to a different stereoisomer.
- An asymmetric centre is a specific type of stereocentre in which an atom has four different substituents.
- The mirror image of a molecule with one asymmetric centre is non-superimposable with (i.e. not identical to) the original molecule.
- (R) and (S) are labels that describe the configuration (i.e. 3D arrangement in space) of an asymmetric centre.
- To assign a configuration as (R) or (S), give each substituent a priority using the CIP rules, then draw a Newman projection with the #4 priority in the back.
- If the #2 → #3 → #4 priorities in the Newman projection proceed clockwise, it is assigned (R) configuration.
- If the #2 → #3 → #4 priorities in the Newman projection proceed counterclockwise, it is assigned (S) configuration.
- Diastereomers are stereoisomers that are not mirror images of each other.
- If a molecule has n stereocentres, then there are up to 2n possible stereoisomers.
- Meso compounds have asymmetric centres, but are not chiral. This is because the mirror image of a meso compound is identical to the original molecule.
- All meso compounds have an internal plane of symmetry.
- Fischer projections can efficiently depict molecules with many stereocentres.
- Vertical lines indicate groups going into the page (i.e. dashes) and horizontal lines represent groups coming out of the page (i.e. wedges).
- Constitutional isomers have the same molecular formula, but different connectivities.
- Stereoisomers have the same connectivity, but different arrangements of their atoms in 3D space.
- Enantiomers are stereoisomers that are mirror images of each other.
- Diasteriomers are stereoisomers that are not mirror images of each other.
- Enantiomers have exactly same physical properties.
- Enantiomers can be distinguished from each other only when they interact with other chiral objects.
- Using a polarimeter, enantiomers can be distinguished by observing which direction the polarized light rotates.
- A sample with only one enantiomer is optically pure.
- A sample with more of one enantiomer than the other is optically active.
- A sample with equal amounts of each enantiomer (racemic) is optically inactive.