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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 groups about a stereocenter. Z - PowerPoint PPT Presentation
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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