Stereoisomers and Chirality

• Isomers are distinct compounds sharing the same molecular formula.
• Isomers fall into two primary categories including constitutional isomers and stereoisomers.

Isomers

• Constitutional isomers, also known as structural isomers, exhibit differences in IUPAC names, functional groups, physical properties, and chemical properties.
• Stereoisomers are isomers that vary solely in the spatial arrangement of atoms.
• Stereoisomers share identical IUPAC names, save for prefixes like cis or trans, and possess the same functional group(s).
• Configuration refers to a molecule's specific three-dimensional arrangement.
• Stereoisomers diverge in configuration.
• Stereoisomers can be either geometric (cis/trans) or optical.
• Optical isomers are chiral and display optical activity.

Chiral and Achiral Molecules

• While all objects possess a mirror image, these images may or may not be superimposable.
• A molecule or object matching its mirror image is termed achiral, indicating a lack of chirality.
• Conversely, a molecule or object not matching its mirror image is termed chiral.
• A carbon atom bonded to four distinct groups is known as a chiral center.
• Enantiomers are non-superimposable mirror image stereoisomers, such as isomers A and B of 2-butanol.
• Molecules with only one stereogenic center are invariably chiral.
• Molecules with two or more stereogenic centers may or may not be chiral.
• Achiral molecules typically contain a plane of symmetry, whereas chiral molecules do not.
• A plane of symmetry is a mirror plane bisecting a molecule such that one half reflects the other.

Stereogenic Centers

• To identify a stereogenic center, each tetrahedral carbon atom in a molecule must be inspected, focusing on four groups bonded to tetrahedral carbons.
• Carbons that do not form tetrahedral stereogenic centers should be omitted from consideration.
• CH2 and CH3 groups should be omitted from consideration, as well as sp or sp² hybridized carbons.
• Stereogenic centers can also appear at carbon atoms within a ring structure.
• To locate stereogenic centers on ring carbons, rings should always be drawn as flat polygons, and look for tetrahedral carbons bonded to four different groups.

Drawing Stereogenic Centers

• When drawing enantiomers of a chiral compound, use the standard convention for tetrahedrons.
• Arrange two bonds in the plane, one in front (wedge), and one behind (dash).
• Position the four groups (H, OH, CH₃, CH₂CH₃) on any bond to the stereogenic center to create the first enantiomer, and reflect the other, while drawing the molecule.

Labeling Stereogenic Centers

• The three dimensional arrangement about a tetrahedral carbon atom refers to a configuration.
• Distinguishing enantiomers requires designating them by name by adding the prefix R or S to the IUPAC name.
• Assigning the prefixes R or S utilizes the Cahn-Ingold-Prelog system.
• To designate enantiomers as R or S as priorities must be assigned
• Each group bonded to the stereogenic center must be assigned priorities in decreasing order of atomic number with the atom with the highest atomic number ranked highest at (1).
• When two atoms on a stereogenic center are identical, priority is assigned based on the atomic number of atoms bonded to them by focusing on an atom of higher atomic number.
• When two isotopes are bonded to the stereogenic center, prioritise those with higher mass number.
• To assign priority to an atom within a multiple bond, consider a multiply bonded atom as an equivalent number of singly bonded atoms.

Diastereomers

• A molecule containing n stereogenic centers has a maximum of 2^n stereoisomers.
• Switching the H and Br positions leads to a new stereoisomer, different from A and B and is labeled C. Its mirror image is labeled Das enantiomers.
• Diastereomers refers to stereoisomers that are not mirror images.
• Stereoisomers: A and B.
• Enantiomers: A and B; C and D.
• Diastereomers: A and C; A and D; B and C; B and D.

Meso Compounds

• Consider stereoisomers of 2,3-dibromobutane.
• It has two stereogenic centers with a maximum of 4 stereoisomers.
• Add the H, Br, and CH3 groups to the stereogenic centers, forming one stereoisomer A, and then draw its mirror image В.
• Switching only two groups on one stereogenic center finds the other stereoisomers. Switching the positions of H and Br on A forms C, which is different from both A and B.
• A meso compound is an achiral compound that has tetrahedral stereogenic centers.
• Compound contains a plane of symmetry and is achiral.
• Meso compounds generally contain a plane of symmetry and possess two mirror image halves.
• One stereoisomer of 2,3-dibromobutane that is superimposable on its mirror image means that there only three stereoisomers, not four.
• To determine whether there is a relationship between two non-identical molecules, assess if they have the same molecular formula.
• Meso compounds is an achiral compound that contains tetrahedral stereogenic centers, this is a meso compound.

R and S Assignments in Compounds with Two or More Stereogenic Centers.

• For a compound having more than one stereogenic center, R and S configurations must be assigned.
• Identical compounds have the same R,S designations at every tetrahedral stereogenic center.
• Enantiomers have exactly opposite R,S designations.
• Diastereomers have the same R,S designation for at least one stereogenic center and the opposite for at least one of the other stereogenic centers.

Disubstituted Cyclicalkanes

• 1,3-dibromocyclopentane has two stereogenic centers and a maximum of four stereoisomers.
• Disubstituted cycloalkanes can have two substituents on the same side of the ring (cis isomer, A) or on opposite sides of the ring (trans isomer, B).
• A disubstituted Cycloalkane A and B are stereoisomers but not mirror images.
• To find the other stereoisomers draw the mirror images and test for superimposability to determine if compound exists.
• The cis isomer is superimposable on its mirror image and therefore the images are identical, A is an achiral meso compound.
• The tans isomer is not superimposable on its mirror image and labeled C makes B and C different compounds.
• Isomers, B and C are enantiomers.
• Because one stereoisomer of 1,3-dibromocyclopentane is superimposable on its mirror image then there are only there only three stereoisomers.