Isomers Isomers: different compounds with the same molecular formula Constitutional isomers: isomers with a different connectivity Stereoisomers: isomers with the same molecular formula, the same connectivity but a different orientation of their atoms in space that cannot be interconverted by rotation about a single bond

Isomers u Relationship among isomers

Chirality Mirror image: the reflection of an object in a mirror Objects that are not superposable on their mirror images are said to be chiral, that is, they show handedness Objects that are superposable on their mirror images are said to be achiral, that is, they do not show handedness. An achiral object has at least one element of symmetry

Elements of Symmetry Plane of symmetry: an imaginary plane passing through an object dividing it such that one half is the mirror image of the other half

Elements of Symmetry Plane of symmetry (contd.)

Elements of Symmetry Center of symmetry: a point so situated that identical components of the object are located equidistant on opposite sides and equidistant from the point along any axis passing through the point

Stereocenter The most common (but not the only ) cause of chirality in organic molecules is a carbon atom bonded to four different groups A carbon with four different groups bonded to it is an example of a stereocenter Stereocenter: an atom in a molecule at which interchange of two atoms or groups of atoms bonded to it gives a stereoisomer

Enantiomers Enantiomers: stereoisomers that are nonsuperposable mirror images; refers to the relationship between pairs of objects On the three following screens are examples chiral molecules. Each has one stereocenter and can exist as a pair of enantiomers.

Enantiomers Lactic acid

Enantiomers 1,2-propanediol

Enantiomers 3-Chlorocyclohexene

R,S Convention Priority rules 1. Each atom bonded to the stereocenter is assigned a priority, based on atomic number. The higher the atomic number, the higher the priority

R,S Convention 2. If priority cannot be assigned on the basis of the atoms bonded to the stereocenter, look to the next set of atoms. Priority is assigned at the first point of difference.

R,S Convention 3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds

Naming Enantiomers 1. Locate the stereocenter 2. Assign a priority to each substituent from 1 (highest) to 4 (lowest) 3. Orient the molecule so that the group of lowest priority (4) is directed away from you 4. Read the three groups projecting toward you in order from highest (1) to lowest priority (3) 5. If reading is clockwise, configuration is R (from the Latin rectus). If it is counterclockwise, configuration is S (from the Latin sinister).

Naming Enantiomers (S)-1,2-Propanediol

R,S Configuration (R)-3-Chlorocyclohexene

Fischer Projections Fischer projection: a two- dimensional representation showing the configuration of a stereocenter –horizontal lines represent bonds projecting forward –vertical lines represent bonds projecting to the rear –the only atom in the plane of the paper is the stereocenter

Fischer Projections 1. Orient the stereocenter so that bonds projecting away from you are vertical and bonds projecting toward you are horizontal 2. Flatten it to two dimensions

Fischer Projections Example: convert each three-dimensional representation to a Fischer projection with the carbon chain vertical. Assign an R,S configuration to each stereocenter