Chapter 6 Stereochemistry
Enantiomersand the Tetrahedral Carbon Stereoisomers: • Have the same molecular formula and the same order of attachment of their atoms (the same connectivity), but different three-dimensional orientations of their atoms in space. • example: the cis-trans isomers of cycloalkanes
Isomers • Different compounds with the same molecular formula Constitutional isomers • Isomers with a different order of attachment of atoms in their molecules Stereoisomers • Isomers with the same order of attachment of atoms in their molecules, but a different orientation of their atoms in space Enantiomers • Stereoisomers whose molecules are mirror images of each other Diastereomers • Stereoisomers whose molecules are not mirror images of each other
Molecules of the type CH3X and CH2XY are identical to their mirror images, but a molecule of the type CHXYZ is not.
The Reason for Handedness in Molecules:Chirality • Chiral: • molecules that are not identical to their mirror forms • objects that are not superposable on their mirror images are said to be chiral; that is, they show handedness • Achiral: • objects that are superposable on their mirror images are said to be achiral; that is, they do not show handedness. • an object is achiral if it possesses a plane of symmetry
Chirality centers (stereocenters): • The most common (but not the only ) cause of chirality in an organic molecules is the presence of a stereocenter • The most common type of stereocenter is a tetrahedral carbon atom with four different groups bonded to it.
Propanoic acid has a plane of symmetry that makes one side of the molecule a mirror image of the other side. Lactic acid has no such symmetry plane.
Optical Activity • Ordinary light: consists of waves vibrating in all planes perpendicular to its direction of propagation • Plane polarized light: consists of waves vibrating only in parallel planes • Polarimeter: a device for measuring the extent of rotation of plane polarized light • Observed rotation: the number of degrees, a, through which a compound rotates the plane of polarized light • Dextrorotatory (+): rotation of the plane of polarized light to the right • Levorotatory (-): rotation of the plane of polarized light to the left
FIGURE 6.5Schematic representation of a polarimeter. Plane-polarized light passes through a solution of optically active molecules, which rotate plane of polarization.
Specific Rotation • Specific rotation, [a]D: observed rotation of the plane of polarized light when a sample is placed in a tube 1.0 dm (=10 cm) in length and at a concentration of 1 g/mL observed rotation, a (degrees) Path length, l(dm) x concentration, c (g/mL) [a]D =
OH HO C C H H CH H C CH CH CH CH 3 3 3 2 2 3 (S)-(+)-2-Butanol (R)-(-)-2-Butanol 25 25 = +13.52 = -13.52 [ a ] [ a ] D D
FIGURE 6.6Crystals of sodium ammonium tartrate, taken from Pasteur’s original sketches. One of the crystals is dextrorotatory in solution, and other is levorotatory.
Sequence Rules for Specifying Configuration • The configuration of a stereocenter: • R configuration • S configuration • R,S - Priority Rules • Each atom bonded to the stereocenter is assigned a priority • Priority is based on atomic number; the higher the atomic number, the higher the priority • If a decision can’t be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms until priority is assigned
Assignment of configuration to a chirality center 1. Look at the four atoms directly attached to the chirality center, and rank them according to atomic number, and assign a priority from 1st (highest) to 4th (lowest) to each substituent 2. Orient the molecule so that the group of lowest priority (4) is directed away from you 3. Read the three groups projecting toward you in order from highest (1) to lowest priority (3) 4. If reading is clockwise (right turn), configuration is R (from the Latin rectus); if it is counterclockwise (left turn), configuration is S (from the Latin sinister)
FIGURE 6.8Assignment of configuration to (a) (R)-(－)-lactic acid and (b) (S)-(＋)-lactic acid.
Enantiomers and Diastereomers • For a molecule with n stereocenters, a maximum of 2nstereoisomers are possible • with 1 stereocenter, 21 = 2 stereoisomers are possible • with 2 stereocenters, a maximum of 22 = 4 stereoisomers are possible • Diastereomers are stereoisomers that are not mirror images
Molecules with More Than Two Stereocenters 28 =256 stereoisomers 28-1=128 pairs enantiomers Only one is produced in nature
Meso Compounds • Meso compounds • has a plane of symmetry A symmetry plane cutting through the C2-C3 bond of meso-tartaric acid makes the molecule achiral.
Physical Properties of Stereomers • Enantiomers • have identical physical and chemical properties in achiral environments • Differ only in specific rotation • Diastereomers • are different compounds and have different physical and chemical properties • Meso compound • has different physical and chemical properties from its diastereomers
Racemic Mixtures and the Resolution of Enantiomers • Racemicmixture (racemate): an equimolar mixture of two enantiomers • because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific activity is zero (optical inactive, )
Resolution of enantiomers • One means of resolution is to convert the pair of enantiomers into two diastereomers • diastereomers are different compounds and have different physical properties, which can be used to separate them • A common reaction for chemical resolution is salt formation • after separation of the diastereomers, the enantiomer are recovered
FIGURE 6.13Reaction of a racemic lactic acid with optically pure (R)-1-phenylethanamine leads to a mixture of diastereomeric salts, which have different properties and can, in principle, be separated.
Constitutional isomers are compounds whose atoms are connected differently • Stereomers are compounds whose atoms are connected in the same way but with a different spatial arrangement
Chirality in Nature • Although the different enantiomers of a chiral molecule have the same physical properties, they almost always have different biological properties
Although these molecules can exist as a number of stereoisomers, generally only one is produced and used in a given biological system
(a) One enantiomer fits easily into a chiral receptor site to exert its biological effect, but(b) the other enantiomer can’t fit into the same receptor
The S enantiomer of ibuprofen soothes the aches and pains of athletic injuries. The R enantiomer has no effect.