Chapter 5 outline stereochemistry study of the 3D structure of molecules it may seem irrelevant but it s not because stereoisomers have very different physical chemical and biological properties constitutional structural isomers differ in their bonding sequence stereoisomers same bonding sequence but differ in their arrangement in space chiral different from its mirror image achiral not chiral not differing from its mirror image superimposable identical in all respects the 3D positions of all atoms coincide wen the molecules are placed on top of each other enantiomers a pair of non superimposable mirror image molecules mirror image isomers asymmetric carbon atom or chiral carbon atom a carbon atom that is bonded to 4 different groups stereocenter stereogenic atom any atom at which the interchange of two groups gives a stereoisomer chirality test 1 If a compound has no asymmetric carbon atom it is usually achiral there are some exceptions 2 If a compound has just on asymmetric carbon atom it must be chiral 3 If a compound has more than one asymmetric carbon it may or may not be chiral internal mirror plane a plane of symmetry through the middle of a molecule dividing the molecule into two mirror image halves A molecule with an internal mirror plane of symmetry cannot be chiral any molecule that has an internal mirror plane of symmetry cannot be chiral even though it may contain asymmetric carbon atoms configurations the two possible spatial arrangements around a chirality center or other stereocenter Cahn Ingold Prelog convention most widely accepted system for naming the configurations of chirality centers 1 Assign a relative priority to each group bonded to the asymmetric carbon a atoms with higher atomic numbers receive higher priorities b in case of ties use the next atoms along the chain of each group as tiebreakers c treat double and triple bonds as if each were a bond to a separate atom 2 Using a 3D drawing or model put the fourth priority group away from you and view the molecule with the first second and third priority groups radiating toward you like the spokes of a steering wheel counterclockwise is S clockwise is R polarimetry common method used to distinguish between enantiomers based on their ability to rotate the plane of polarized light in opposite directions plane polarized light is composed of waves that vibrate in only one plane rotation of the plane of polarized light is called optical activity therefore substances that rotate the plane of polarized light are said to be optically active optical isomers compounds with identical properties except for the direction in which they rotate polarized light polarimeter measures the rotation of polarized light compounds that rotate the plane of polarized light to the right clockwise are dextrorotatory and compounds that rotate the plane toward the left counterclockwise are called levorotatory specific rotation a measure of a compound s ability to rotate the plane of polarized light given by a specific equation chiral probe a molecule or an object that is chiral and can use its own chirality to differentiate between mirror images racemic mixture also called racemeate a d l pair or a weird equals sign pair a mixture of equal quantities of enantiomers such that the mixture is optically inactive A reaction that uses optically inactive reactants and catalysts cannot produce a product that is optically active Any chiral product must be formed as a racemic mixture optical purity o p the specific rotation of a mixture of two enantiomers expressed as a percentage of the specific rotation of one of the pure enantiomers Similar to enantiomeric excess enantiomeric excess e e the excess of one enantiomer in a mixture of enantiomers expressed as a percentage of the mixture Similar to optical purity o p e e excess of one over the other x 100 entire mixture A molecule cannot be optically active if its chiral conformations are in equilibrium with their mirror images We consider such a molecule to be achiral To determine whether a conformationally mobile molecule can be optically active consider its most symmetric conformation Allenes compounds having two C C double bonds that meet as a signle carbon atom C C C The two outer carbon atoms are trigonal planar with their planes perpendicular to each other Fischer projection a method for drawing an asymmetric carbon atom as a cross The carbon chain is kept along the vertical with the IUPAC numbering from top to bottom Vertical bonds project away from the viewer and horizontal bonds project toward the viewer 2n rule a molecule with n chiral carbon atoms might have as many as 2n stereoisomers meso compound an achiral compounds that contains chiral centers usually asymmetric carbon atoms originally an achiral compounds that has chiral diasteromers absolute configuration the detailed stereochemical picture of a molecule including how the atoms are arranged in space Alternatively the R or S configuration at each symmetric carbon atom relative configuration the experimentally determined relationship between the configurations of two molecules even though the absolute configuration of either may not be known D L configurations D has the same relative configuration as glyceraldehyde L has the same relative configuration as glyceraldehyde resolution the process of separating a racemic mixture into the pure enantiomers resolution requires a chiral resolving agent resolving agent a chiral compound or chiral material on a chromatographic column used for separating enantiomers