Transcript Rule 1

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
(zusammen – together)
E
(entgegen – opposite)
Cahn-Ingold-Prelog (CIP) Prio rity System
1) Of the two atoms attached to ONE carbon of the double
bond, the atom with the highe r atomic numbe r has the
highest priority.
2) For isotopes, the highe r atomic weight gets highe r priority
(ie., D ha s higher priority than H).
3) If rule 1 is not sufficient, look at the g roups connected to
the first atom and apply rule 1 to the atom of next higher
atomic number.
4) Multiple bond s 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 o
with the higher atomic number has the higher priority.
C has higher
priority than H
Cl has higher
priority 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 first
point of difference.
C bonded to O and 2 H’s
C bonded to 3 H’s
C bonded to C and 2
H’s
C bonded to 2 C’s
and one H
Rule 3:
Double bonds and triple bonds in the groups attac
to the C=C are treated as though they are constructed
from two or three single bonds, respectively.
O C
C O
H
C O
H
C
C H
C
C
C
C H
C
C
This is the Z-stereoisomer.
Fig. 6-5, p. 188
Conformations: the various shapes that a molecule can assume
by rotations about single bonds
Newman projections
less stable
torsional strain:
more stable by 2.9 kcal/mol
the destabilization caused by
Fig. 6-7, p. 191
Fig. 6-8a, p. 195
Table 6-1, p. 194
Cyclobutane
o
90
less angle strain than cyclopropane
lots of torsional strain (if planar)
lowest energy conformation is nonplanar
angle strain + torsional strain = 26.0 kcal/mol
Cyclobutane rings are less common.
equatorial hydrogens
axial hydrogens
Methylcyclohexane
axial methyl is destabilized by
1.7 kcal/mol
(axial strain energy)
o
G = -RTlnK K = 18
95%
CH3 is equatorial
5%
CH3 is axial