For cis-trans isomers to exist

(stereogenic atoms)

stereocenter: atom at which interchange of two groups produces a stereoisomer

Designating the Configuration of Cis-Trans Isomers

configuration: the three-dimensional arrangement of groupsabout a stereocenter

Z(zusammen – together)

E(entgegen – opposite)

Cahn-Ingold-Prelog (CIP) Priority System

1) Of the two atoms attached to ONE carbon of the doublebond, the atom with the higher atomic number has thehighest priority.

2) For isotopes, the higher atomic weight gets higher priority(ie., D has higher priority than H).

3) If rule 1 is not suff icient, look at the groups connected tothe first atom and apply rule 1 to the atom of next higheratomic number.

4) Multiple bonds are treated as multiplies single bonds.

Cahn-Ingold-Prelog Sequence Rules

Rule 1: Of the two atoms attached to one C of the C=C, the one with the higher atomic number has the higher priority.

C has higherpriority than H

Cl has higherpriority than F

(Z)-1-chloro-1-fluoro-1-propene

Rule 2: If the two atoms attached to the carbon are the same, compare the atoms attached to them in order of decreasing priority. The decision is made at the firstpoint of difference.

C bonded to 3 H’s

C bonded to C and 2 H’s

C bonded to O and 2 H’s

C bonded to 2 C’sand one H

Rule 3: Double bonds and triple bonds in the groups attachedto the C=C are treated as though they are constructedfrom two or three single bonds, respectively.

C O

H

C

O

O

C

H

C C H C

C

C

C

C

C

H

This is the Z-stereoisomer.

Fig. 6-5, p. 188

Conformations: the various shapes that a molecule can assumeby rotations about single bonds

Newman projections

torsional strain: the destabilization caused by eclipsed bonds

less stable more stable by 2.9 kcal/mol

Fig. 6-7, p. 191

Fig. 6-8a, p. 195

Table 6-1, p. 194

Cyclobutane

90o less angle strain than cyclopropane

lots of torsional strain (if planar)

angle strain + torsional strain = 26.0 kcal/mol

lowest energy conformation is nonplanar

Cyclobutane rings are less common.

axial hydrogens

equatorial hydrogens

Methylcyclohexane

CH3 is equatorial CH3 is axial

axial methyl is destabilized by1.7 kcal/mol (axial strain energy)

95% 5%

G = -RTlnKo

K = 18