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Sect 4.6: Monosubstituted

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Sect 4.7: cis and trans isomerization in cycloalkanes. cis-trans isomerism ... Sect 4.12: E/Z nomenclature. To avoid the confusion between what the main ... – PowerPoint PPT presentation

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Title: Sect 4.6: Monosubstituted


1
Sect 4.6 Monosubstituted
cyclohexane rings
2
Methylcyclohexane conformations
Axial methyl
Equitorial methyl
3
Energy difference between an axial and an
equitorial methyl group
E N E R G Y
1.7 kcal/mol
0 kcal/mol
4
1,3-Diaxial interactions on the top of the ring
1,3-diaxial interactions
STERIC REPULSION RAISES THE ENERGY OF
THE AXIAL CONFORMATION
5
1,3-Diaxial interactions
1,3-diaxial interactions
CH3
H
H
H
H
CH
H
H
H
H
1,3-Diaxial interactions Newman projection view
6
Monosubstituted cyclohexanes gauche steric
interactions
GAUCHE STERIC INTERACTIONS
gauche steric nteractions
CH3
60o
(like gauche butane)
CH3
CH2
CH3
Axial
CH2
180o
CH3
CH2
Equitorial
No gauche steric problem when the group is
equitorial
CH2
7
General rule
Large groups will generally prefer to occupy an
equatorial position where there is an absence of
1,3-diaxial (steric) interactions
axial conformation
equatorial conformation
Keep in mind, however, that the axial
conformation will also be present, but in smaller
amount.
8
Table 4.5 Conformational energy differences for
substituents attached to a cyclohexane ring

Equitorial preferred
DGo for group in the axial position
Group X
kcal/mol
kJ/mol
Group
kcal/mol
kJ/mol
CH3-
1.7
7.1
Cl-
0.4
1.7
Br-
CH3CH2-
1.8
7.5
0.5
2.1
CH3-CH-
2.1
8.8
0.7
2.9
HO-
CH3
C6H5-
3.1
13
CH3
CH3-C-O-
0.7
2.9
gt5
gt21
CH3-C-
O
CH3
9
t -BUTYLCYCLOHEXANE
Too big a group to go into the axial position -
must go equatorial.
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 ( gt5
Kcal/mole) indicates a minimum value because
there is so little axial that it is difficult to
measure any real value.
10
tert-Butylcyclohexane with the group axial
HUGE steric strain
11
Molecules viewed with Chime
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12
Sect 4.7 cis and trans isomerization in
cycloalkanes
13
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

14
cis and trans isomers
applies to substituents on a ring or (later)
double bond
Cl
Cl
Cl
Cl
cis
trans
both substituents are on the same side of the
ring
the substituents are on opposite sides of the ring
These two compounds are geometric isomers
15
Naming cis /trans isomers
place designation in front of name
Cl
Cl
cis-1,2-dichlorocyclopropane
notice italics
Cl
trans-1,2-dichlorocyclopropane
Cl
16
How many different dimethylcyclobutanes are
there?
Constitutional isomers
1,1-
1,2-
1,3-
cis /trans isomers (geometric)
no cis/trans here
17
Planar ring approximation
Notice that it is OK to use planar rings when
figuring out cis / trans isomers.
Use planar structures on tests!
You only need to use puckered rings when you are
dealing with conformations.
18
Sect 4.8 disubstituted cyclohexanes
cis/trans isomerism Use chair structures!!
19
trans-1,2-dimethylcyclohexane
has two possible conformers
trans-1,2-dimethylcyclohexane
CHAIR-1
CHAIR-2
Methyl above
Methyl below
e,e
a,a
Which conformer is more stable?
The trans e,e one!
20
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
D
G
(2)(1.7) 0 3.4 kcal/mol
o
Group
kcal/mol
kJ/mol
Group
kcal/mol
kJ/mol
CH
-
1.7
7.1
Cl-
0.4
1.7
3
Br-
CH
CH
-
1.8
7.5
0.5
2.1
3
2
CH
-CH-
8.8
0.7
2.1
2.9
HO-
3
CH
C
H
-
3.1
13
3
6
5
CH
3
CH
-C-O-
0.7
2.9
3
gt5
gt21
CH
-C-
3
O
CH
3
21
1,3-Diaxial interactions (steric) on top and
bottom of ring
No diaxial interactions lots of room
Two axial-axial problems _at_ 1.7 kcal/mol each
Equatorial groups are assumed to be 0 kcal/mol
22
What about cis-1,2-dimethylcyclohexane?
Class exercise!!
23
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
24
Which conformer has the higher energy?
Both are trans!
This one!
CH3
CH3
axial 1.7 kcal/mol
equatorial 0 kcal/mol
OH
OH
axial 0.7 kcal/mol
equatorial 0 kcal/mol
1.7 kcal/mol
0.7 kcal/mol
25
Guideline
In substituted cyclohexane rings, the best
(lowest energy) conformation will have the
largest groups in equatorial positions whenever
possible.
26
Sect 4.9 decalin skip this section, winter 07
27
cis and trans ring fusions
trans-ring fusion
cis-ring fusion
bonds are cis
bonds are trans
cis-decalin less stable
trans-decalin
28
other representations
Drawing Conventions
top
solid wedge towards you
dashed wedge away from you
bottom
A dot implies the hydrogen is towards you (on
top).
29
Sect 4.10 read this section no lectures Skip
this section, winter 07
30
Sect 4.11 cis/trans isomerism in alkenes
31
Alkene geometry planar
p bond
sp2
p bond
sp2
s bond
planar
s bond
END VIEW
SIDE VIEW
32
ROTATION BREAKS THE p BOND
Unlike s bonds, p bonds do not rotate.
NO!
It requires about 50-60 kcal/mole ( 240 kJ/mole
) to break the p bond - this does not happen
at reasonable temperatures.
33
cis / trans isomers (geometric isomers)
Because there is no rotation about a
carbon-carbon bond, isomers are possible.
cis
trans
substituents on the same side of main chain
substituents on opposite sides of main chain
34
Compare cis / trans isomers in ring compounds
to alkenes
cis
trans
cis / trans isomers are also called geometric
isomers
35
Two identical substituents
If an alkene has two identical substituents on
one of the double bond carbons, cis / trans
isomers are not possible.
all of these compounds are identical
no cis / trans isomers
36
Some other compounds with no cis / trans isomers
no cis / trans isomers
37
Naming cis / trans isomers of alkenes
main chain stays on same side of double bond cis
main chain crosses to other side of double bond
trans
cis-3-hexene
trans-3-hexene
notice that these prefixes are in italics
38
Rings with double bonds
trans double bonds are not possible until the
ring has at least eight carbon atoms
if Clt8 then the chain is too short to join
together
trans
cis
cis
C 5
C 8
smallest ring that can have a trans double bond
cis
C 6
trans
Note that both cis and trans exist for C8.
39
Be Careful !!!
The main chain determines cis / trans in the
IUPAC name
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!
40
Sect 4.12 E/Z nomenclature
41
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.
cis ?
F
Cl
trans ?
I
H
This system also allows alkenes like the one
above to be classified .. an
impossibility with cis / trans.
42
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
opposite sides
Z
E
If priority 1 groups on opposite sides of double
bond
E isomer entgegen opposite (in German)
43
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.
1
1
example
I gt Br
F gt H
2
2
Since the 1s are on the same side, this compound
is Z
(Z)-1-bromo-2-fluoro-1-iodoethene
notice use of parentheses
44
Priorities in the E-Z Nomenclature system
45
3. If you cant 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.
path goes to F not to H
1
1
comparison stops here
2
2
path goes to C not to H
This molecule has Z configuration.
46
Lets give this compound a cis/trans name
and an E/Z name
C
H
F
C
H
2
3
H
C
H
C
H
2
3
trans-3-fluoromethyl-2-pentene (longest
chain) (Z)-3-fluoromethyl-2-pentene (priorities)
47
4. CC double bond equivalent to having two
carbons. CO double bond equivalent to
having two oxygens.
2
C
1
48
1
2
1
2
(E)
49
2
2
1
1
(Z)
50
More than one double bond dienes
51
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
52
(E) structure
no E/Z
4
2
6
1
5
3
(E)-1,3-hexadiene
53
cis and Z are not always the same for a given ring
1
2
bonds in the ring are cis
H
1
2
but this compound is E
54
Sect 4.13 Relative stabilities of alkenes
hydrogenation
55
Hydrogenation of Alkenes
catalyst

C
C
H
H
C
C
H
H
an addition reaction
The catalyst is Pt, PtO2, Pd, or Ni
56
Examples
57
Both hydrogen atoms add to the same
side of the
double bond
not observed
anti addition
X
H2 / Pt
H2 / Pt
syn addition
stereospecific
58
Hydrogenation is exothermic
DH approx. -30 kcal/mol
Exothermic reaction!
-27.6
-28.6
-30.3
-28.6
59
Butene isomers --- Heats of hydrogenation
Higher energy
(less stable)
Lower energy (more stable)
H2
H2
H2
DH
-27.6
kcal/mol
-28.6
-30.3
CH3CH2CH2CH3
All are hydrogenated to the same product
(butane) therefore their energies may be compared.
60
different positions of the double bond
Alkene isomers
stability
1,1-
cis
1,2-
monosubstituted
trisubstituted
trans
1,2-
tetrasubstituted
less stable
disubstituted
more stable
increasing substitution
61
Steric repulsion is responsible for energy
differences among the
disubstituted alkenes
steric repulsion
steric repulsion
1,1-
cis-1,2-
(Z)
trans-1,2-
(E)
62
Some examples of stabilities of isomers
EXAMPLE ONE
has lower energy than (more stable)
ISOMERS
disubstituted
monosubstituted
EXAMPLE TWO
has lower energy than (more stable)
ISOMERS
trisubstituted
disubstituted
63
Sect 4.14, 4.15, 4.16
  • Bicyclic compounds and spiro compounds

64
Naming a bicyclic compound
bridgeheads
1 carbon
3 carbons
2 carbons
bicyclo3.2.1octane
total number of carbon atoms
sizes of bridges, largest first
number of rings
65
Bicyclic ring compounds
bicyclo1.1.1pentane
bicyclo1.1.0butane
bicyclo2.1.1hexane
bicyclo2.2.2octane
bicyclo2.2.1heptane
bicyclo3.1.1heptane
bicyclo4.4.1undecane
66
Rings in nature
Many examples of the trans ring fusion are
found in nature.
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.
67
(No Transcript)
68
O
C
H
3
C
H
3
C
H
3
PROGESTERONE
O
ESTROGEN
69
Some bicyclic natural products
TURPENTINE
CAMPHOR TREE
a-pinene
camphor
EUCALYPTUS
TURPENTINE
b-pinene
cineole
70
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.
spiro2.4heptane
71
Polycyclic compounds
These have been made synthetically.
basketane
adamantane
cubane
propellane
bucky ball
buckminsterfullerene
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