Isomers -Compound with the same molecular formula but different structures Constitutional Isomer -differ in the way their atoms are connected. Stereoisomer(Configurational isomer)-differ in the way their atom are arranged in space ISOMERS
CONSTITUTIONAL ISOMERS Example: C2H6OC2H6O C2H6OC2H6O C 4 H 9 Cl
CONFIGURATIONAL ISOMER Cis-Trans Isomers Cis–trans isomers (also called geometric isomers) result from restricted rotation. Restricted rotation can be caused either by a double bond or by a cyclic structure. Cis isomer has the substituent on the same side of the double bond Trans isomer has the substituent on opposite sides of the double bond.
Drawing Enantiomers Perspective formula Solid lines – bonds in the plane of the paper Solid wedge- coming out of the paper toward you Hatched wedge - going back into space behind the paper The solid wedge and hatched wedge must be adjacent to one another.
Horizontal line – The bonds project out of the plane toward the viewer Vertical line- The bonds extend back from the plane away from viewer The carbon chain usually drawn vertically Fischer projection Drawing Enantiomers
CLASS EXERCISE 1 Draw enantiomers for each of the following compounds using perspective formulas and Fischer projections;
Naming Enantiomers 1.Rank the groups (atoms) bonded to the asymmetric center in order to priority The higher the atomic no. the higher the priority The R,S system of nomenclature
2.Orient the molecules Lowest priority (4) directed away from you. 3.Draw imaginary arrow from the group of highest priority to the next highest priority If the arrow point clockwise (R-Configuration) Counterclockwise (S-Configuration) Naming Enantiomers The R,S system of nomenclature
Naming from the Perspective Formula 1. Rank the groups bonded to the asymmetric center with the lowest priority group in the back.
2. If the group (or atom) with the lowest priority bonded by a hatched wedge; Clockwise-R;Anticlockwise-S 3. If the group (or atom) with the lowest priority IS NOT bonded by a hatched wedge; then switch two groups so group 4 is bonded by a hatched wedge
17 Configuration from the Fischer Projection 2. Rank the groups (or atoms) that are bonded to the asymmetric center and then draw an arrow from the highest priority (1) to the next highest priority (2). 1. Consider the 3-dimensional equivalent to the Fischer Projection.
3. If the lowest priority is on a vertical bond; clockwise (R- Configuration),Counterclockwise (S-Configuration) 4. If the lowest priority is on a horizontal bond, the configuration is opposite to the direction of the arrow.
ISOMERS WITH MORE THAN ONE ASYMMETRIC CENTER Max no stereoisomer = 2 n n= no. of asymmetric center
Diastereoisomers Stereoisomers (1) and (2) Stereoisomers (3) and (4) Stereoisomers (1) and (3),(1) and (4),(2) and (3), (2) and (4) ENANTIOMERS DIASTEREOISOMERS Diastereoisomers are stereoisomers that are not enantiomers
21 Identification of Asymmetric Carbons in Cyclic Compounds Identification of Asymmetric Carbons in Cyclic Compounds
22 Meso Compounds Have two or more asymmetric centers and an internal plane of symmetry ( -plane) Meso compounds are achiral molecules with asymmetric centers
23 If a compound with two asymmetric centers has the same four groups bonded to each of the centers, one of its stereoisomers will be a meso compound