Chapter 6. Stereochemistry. Enantiomers and 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.
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Molecules of the type CH3X and CH2XY are identical to their mirror images, but a molecule of the type CHXYZ is not.
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.
FIGURE 6.5Schematic representation of a polarimeter. Plane-polarized light passes through a solution of optically active molecules, which rotate plane of polarization.
observed rotation, a (degrees)
Path length, l(dm) x concentration, c (g/mL)
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.
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.
28 =256 stereoisomers
28-1=128 pairs enantiomers
Only one is produced in nature
A symmetry plane cutting through the C2-C3 bond of meso-tartaric acid makes the molecule achiral.
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
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.