Sect 4.6: Monosubstituted
cyclohexane rings
Methylcyclohexane
conformations
CH 3
H
CH 3
H
Equitorial methyl
Axial methyl
Energy difference between an axial
and an equitorial methyl group
CH 3
E
N
E
R
G
Y
1.7 kcal/mol
H
0 kcal/mol
CH3
H
1,3-Diaxial interactions on the top of the ring
1,3-diaxial interactions
CH3
H
H
STERIC REPULSION RAISES THE ENERGY
OF THE AXIAL CONFORMATION
1,3-Diaxial interactions
1,3-diaxial interactions
CH3
H
CH
H
H
H
H
H
H
H
1,3-Diaxial interactions:
Newman projection view
Monosubstituted cyclohexanes: gauche steric interactions
GAUCHE STERIC
INTERACTIONS
gauche steric nteractions
60o CH3
(like gauche butane)
CH3
CH2
CH3
Axial
CH2
180o
CH2
CH3
CH2
No gauche steric
problem when the
group is equitorial
Equitorial
General rule
Large groups will generally prefer to occupy an
equatorial position where there is an absence of
1,3-diaxial (steric) interactions
G
axial
conformation
equatorial
conformation
Keep in mind, however, that the axial conformation will
also be present, but in smaller amount.
Table 4.5: Conformational energy differences
for substituents attached to a cyclohexane ring
X
H
X
Equitorial preferred
H
DGo for group in the axial position
Group X
CH3CH3CH2CH3-CHCH3
CH3
CH3-CCH3
kcal/mol
kJ/mol
Group
ClBr-
2.1
7.1
7.5
8.8
>5
>21
1.7
1.8
kcal/mol
kJ/mol
1.7
2.1
HOC6H5-
0.4
0.5
0.7
3.1
CH3-C-O-
0.7
2.9
O
2.9
13
t -BUTYLCYCLOHEXANE
Too big a group to
go into the axial
position - must go
equatorial.
H
H
C
H
C
H
C
H
C
H
H
H
H
Basically “locks” the ring in a chair with the
tert -butyl group in the equatorial position.
The axial value for this group in Table 4-5 ( >5 Kcal/mole)
indicates a minimum value because there is so little axial
that it is difficult to measure any real value.
tert-Butylcyclohexane with
the group axial
HUGE steric strain
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Sect 4.7: cis and trans
isomerization in cycloalkanes
cis-trans isomerism
• Different spatial arrangements
• The arrangements cannot be converted into
one another by rotation
• cis Both substituents on same side of plane
• trans Substituents on opposite sides of plane
cis and trans isomers
applies to substituents on a ring or (later) double bond
Cl
Cl
Cl
Cl
cis
both substituents are on
the same side of the ring
trans
the substituents are on
opposite sides of the ring
These two compounds are geometric isomers
Naming cis /trans isomers
Cl
Cl
place designation
in front of name
cis-1,2-dichlorocyclopropane
notice
italics
Cl
trans-1,2-dichlorocyclopropane
Cl
How many different dimethylcyclobutanes are there?
CH3
CH3
CH3
CH3
Constitutional
1,1
isomers
cis /trans isomers
(geometric)
no cis/trans here
CH3
1,2CH3
CH3
cis
CH3
1,3CH3
CH3
cis
CH3
CH3
CH3
CH3
trans
CH3
CH3
trans
Planar ring approximation
Notice that it is OK to use planar rings
when figuring out cis / trans isomers.
CH3
CH3
Use planar structures on tests!
You only need to use puckered rings
when you are dealing with conformations.
CH3
CH3
CH
CH3
CH
Sect 4.8: disubstituted
cyclohexanes:
cis/trans isomerism
Use chair structures!!
trans-1,2-dimethylcyclohexane
has two possible conformers
trans-1,2-dimethylcyclohexane
CHAIR-1
CHAIR-2
CH3
Methyl
above
H
CH 3
Methyl
below
H H
CH 3
H
e,e
a,a
CH3
Which conformer is more stable?
The trans e,e one!
Calculating the energy difference using values from Table 4.5
trans-(a,a)-1,2-dimethylcyclohexane
3.4 kcal/mol higher
(two axial methyls)
2 x 1.7 kcal
trans-(e,e)-1,2-dimethylcyclohexane
Reference = 0 kcal/mol
DGo = (2)(1.7) – 0 = 3.4 kcal/mol
Group
CH3CH3CH2CH3-CHCH3
CH3
CH3-CCH3
kcal/mol kJ/mol
1.7
1.8
2.1
7.1
7.5
8.8
>5
>21
Group
kcal/mol
ClBr-
kJ/mol
1.7
2.1
HOC6H5-
0.4
0.5
0.7
3.1
CH3-C-O-
0.7
2.9
O
2.9
13
1,3-Diaxial interactions (steric)
on top and bottom of ring
No diaxial interactions
lots of room
H
H
H
H
H
H
H
H
H
H
H
H
C
H
H
C
H H
C
H
C
H
H
H
H
H
Two axial-axial problems
@ 1.7 kcal/mol each
Equatorial groups are
assumed to be 0 kcal/mol
What about cis-1,2-dimethylcyclohexane?
Class exercise!!
What about cis / trans isomers in disubstituted
rings other than 1,2-dimethylcyclohexane?
1,1-dimethylcyclohexane: no cis/ trans isomers
1,3-dimethylcyclohexane: 4 chair structures
1,4-dimethylcyclohexane: 4 chair structures
Which conformer has the higher energy?
Both are trans!
CH 3
H
H
CH 3
H
HO
OH
H
This one!
CH3 axial
= 1.7 kcal/mol
OH equatorial = 0 kcal/mol
CH3 equatorial = 0 kcal/mol
OH axial
1.7 kcal/mol
DG = (1.7 - 0.7) = 1.0 kcal/mol
= 0.7 kcal/mol
0.7 kcal/mol
Guideline
In substituted cyclohexane rings, the best
(lowest energy) conformation will have the
largest groups in equatorial positions
whenever possible.
Sect 4.9: decalin
skip this section, winter 07
cis and trans ring fusions
H
H
H
H
trans-ring fusion
cis-ring fusion
H
H
H
bonds are cis
H
bonds are trans
trans-decalin
cis-decalin: less stable
other representations
trans
H
H
H
Drawing
Conventions
solid wedge =
towards you
dashed wedge =
away from you
A dot implies
the hydrogen
is towards you
(on top).
cis
H
H
H
H
bottom
H
top
Sect 4.10: read this
section; no lectures
Skip this section, winter 07
Sect 4.11: cis/trans
isomerism in alkenes
Alkene geometry: planar
p bond
sp2 p bond
R
R
C
R
sp2
C
R
R
R
R
R
R
R
s bond
s bond
SIDE VIEW
planar
END VIEW
R
R
ROTATION BREAKS THE
p BOND
Unlike s bonds, p bonds do not rotate.
NO!
R
R
R
R
It requires about 50-60 kcal/mole ( ~ 240 kJ/mole )
to break the p bond - this does not happen at
reasonable temperatures.
cis / trans isomers (geometric isomers)
Because there is no rotation about a
carbon-carbon bond, isomers are possible.
R
R
C
C
H
H
R
C
H
cis
substituents on
the same side of
main chain
C
R
H
trans
substituents on
opposite sides of
main chain
Compare cis / trans isomers in ring compounds to alkenes
R
R
C
C
H
C
H
C
R
H
cis
R
H
R
trans
R
R
R
cis / trans isomers are also called geometric isomers
Two identical substituents
If an alkene has two identical substituents on one of
the double bond carbons, cis / trans isomers are
not possible.
H
CH 3
C
H
H3C
C
H
C
CH 2-CH 3
CH 3
CH 2
H
CH 2-CH 3
C
C
H
H
all of these compounds are identical
no cis / trans isomers
C
CH 3
Some other compounds with no cis / trans isomers
H
CH3
CH3CH2
CH3
H
CH3
CH3
H3C
no cis / trans isomers
CH3
Naming cis /
trans isomers of alkenes
main chain stays
on same side of
double bond = cis
CH3CH2
H
CH2CH3
main chain crosses
to other side of
double bond = trans
CH3CH2
H
cis-3-hexene
notice that these
prefixes are in
italics
H
H
CH2CH3
trans-3-hexene
Rings with double bonds
trans double bonds are not possible until the ring
has at least eight carbon atoms
if C<8 then the
chain is too short
to join together
CH2
cis
C=5
CH2
cis
CH2
trans
CH2
C=8
cis
smallest ring that
can have a trans
double bond
C=6
trans
Note that both cis and trans exist for C8.
Be Careful !!!
The main chain determines cis / trans in the IUPAC name
CH3
H
CH2CH3
CH3
CH3
CH3
H
CH2CH3
cis-3-methyl-2-pentene
trans-3-methyl-2-pentene
This compound is cis
but the two methyl
groups are
….trans to each other.
This compound is trans
but the two methyl
groups are
….cis to each other.
but the terms cis and trans are also used to designate
the relative position of two groups: a new system is needed!
Sect 4.12: E/Z
nomenclature
E/Z system of nomenclature
To avoid the confusion between what the main
chain is doing and the relationship of two similar
groups ….. the IUPAC invented the E/Z system.
Cl
I
F
H
cis ?
trans ?
This system also allows alkenes like the
one above to be classified …..
an impossibility with cis / trans.
E / Z Nomenclature
In this system the two groups attached to each carbon
are assigned a priority ( 1 or 2 ).
If priority 1 groups are both on same side of double bond:
Z isomer = zusammen = together (in German)
same
side
1
1
2
2
Z
1
2
2
opposite
sides
1
E
If priority 1 groups on opposite sides of double bond:
E isomer = entgegen = opposite (in German)
Assigning priorities
1. Look at the atoms attached to each carbon of
the double bond.
2. The atom of higher atomic number has higher (1)
priority.
example
1
1
F
F>H
2
H
I
Br
I > Br
2
Since the 1’s are on the same side, this compound is Z
(Z)-1-bromo-2-fluoro-1-iodoethene
notice use of parentheses
Priorities in the E-Z
Nomenclature system
1
1
C
C
1
C
C
1
(Z)
(E)
3. If you can’t decide using the first atoms attached,
go out to the next atoms attached. If there are
non-equivalent paths, always follow the path with
atoms of higher atomic number.
Once you find a difference, you can stop.
1
1
CH3
H
2
C
CH2F
path goes to
F not to H
H
F
H
CH2CH3
H
2
comparison
stops here
C C
H
path goes to
C not to H
This molecule has Z configuration.
Let’s give this compound a cis/trans name
and an E/Z name
CH3
H
CH2F
CH2CH3
trans-3-fluoromethyl-2-pentene (longest chain)
(Z)-3-fluoromethyl-2-pentene (priorities)
4. C=C double bond: equivalent to having two carbons.
C=O double bond: equivalent to having two oxygens.
C
CH CH2
C
2
CH CH2
O C
C O
C O
H
H
1
1
2
O
CH2
Br
CH2
C
C
H2N
CH3
CH2
1
CH2
2
(E)
OH
2
2
O
CH2
Br
CH2
C
C
H2N
CH3
CH2
1
OH
1
(Z)
More than one double bond: dienes
DIENES AND POLYENES
Hexadiene
trans, trans
trans, cis
E,E
E,Z
(2E,4E)-2,4-hexadiene
(2E,4Z)-2,4-hexadiene
(2Z,4Z)-2,4-hexadiene
(2Z,4E)-2,4-hexadiene
identical
cis, cis
cis, trans
Z,Z
Z,E
(E) structure
no E/Z
4
2
6
1
3
5
(E)-1,3-hexadiene
cis and Z are not always the same for a given ring
2
1
CH2OH
H
1
2
bonds in the ring
are cis
but this compound
is E
Sect 4.13: Relative
stabilities of alkenes:
hydrogenation
Hydrogenation of Alkenes
C
C
+
H
H
catalyst
C
C
H
H
an addition
reaction
The catalyst is Pt, PtO2, Pd, or Ni
Examples
Pt
CH3 + H2
CH3 CH CH CH3 + H2
CH3
Pt
CH3 CH2 CH2 CH3
CH3
CH2
+ H2
Pt
Both hydrogen atoms add to the same side
of the double bond
CH3
not
H
observed
anti
addition
X
CH3
CH3
H2 /
Pt
H2 / Pt
H
CH3
CH3
CH3
H
stereospecific
H
syn
addition
Hydrogenation is exothermic
C C
+ H2
C C
H H
DH = approx. -30 kcal/mol
Exothermic reaction!
-28.6
-27.6
-28.6
-30.3
+
heat
Butene isomers --- Heats of hydrogenation
Higher energy
(less stable)
DH
+H2
-30.3
Lower energy
(more stable)
+H2
-28.6
+H2
-27.6
kcal/mol
CH3CH2CH2CH3
All are hydrogenated to the
same product (butane) therefore
their energies may be compared.
different positions
of the double bond
Alkene isomers
stability
R
H
1,1-
R
R
H
H
H
monosubstituted
H
R
R
H
H
R
H
less stable
cis
1,2-
H
R
trans
R
R
R
R
R
H
R
R
trisubstituted
1,2-
disubstituted
increasing substitution
tetrasubstituted
more stable
Steric repulsion is responsible for energy differences
among the disubstituted alkenes
steric
repulsion
R
R
1,1-
H
steric
repulsion
H
R
R
H
H
cis-1,2(Z)
R
H
H
R
trans-1,2(E)
Some examples of stabilities of isomers
EXAMPLE ONE
disubstituted
has lower energy than
(more stable)
ISOMERS
monosubstituted
EXAMPLE TWO
has lower energy than
(more stable)
trisubstituted
ISOMERS
disubstituted
Sect 4.14, 4.15, 4.16
Bicyclic compounds and spiro
compounds
Naming a bicyclic compound
1 carbon
bridgeheads
3 carbons
2 carbons
bicyclo[3.2.1]octane
number of
rings
total number
sizes of bridges,
of carbon atoms
largest first
Bicyclic ring compounds
bicyclo[1.1.1]pentane
bicyclo[1.1.0]butane
bicyclo[2.1.1]hexane
bicyclo[2.2.2]octane
bicyclo[2.2.1]heptane
bicyclo[3.1.1]heptane
bicyclo[4.4.1]undecane
Rings in nature
Many examples of the trans ring fusion are found
CH3
in nature.
transH
trans
trans
eq
CH3
H
CH3
H
CH3
CH3
H
HO
H
H
cholestanol
(a close relative of cholesterol)
The cis ring fusion is not found nearly as often as trans.
NATURAL PRODUCTS : compounds that occur in living sytems,
such as plants and animals.
CH3 OH
CH3
O
TESTOSTERONE
O
CH3
CH3
CH3
PROGESTERONE
O
ESTROGEN
Some bicyclic natural products
CH3
CAMPHOR
TREE
CH3
CH3
CH3
CH3
TURPENTINE
O
camphor
CH3
CH3
a-pinene
CH3
EUCALYPTUS O
CH3
cineole
CH3
CH3
TURPENTINE
b-pinene
Spiranes
Here the smaller ring comes
first in the name.
Spiro ring junctions always
involve two rings, so bi- and
tricyclo, etc. are not needed.
The prefix “spiro” is used
instead.
spiro[2.4]heptane
Polycyclic compounds
These have been made synthetically.
basketane
adamantane
cubane
propellane
“bucky ball”
buckminsterfullerene