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Chapter 03
Alkanes and Cycloalkanes:
Conformations and cis-trans
Stereoisomers
CHEM 341: Spring 2012
Prof. Greg Cook
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©2012 Gregory R Cook
Sunday, January 29, 12
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Conformations
•
All single bonds freely rotate at room
temperature (unless constrained by a ring).
•
•
Thus, linear alkanes are in constant motion.
If the molecules were frozen to absolute zero you
could see different arrangements of the groups
depending on the state of the bond rotations.
•
Conformers: Different rotational isomers
(conformations) of a molecule.
•
Conformational Analysis: Study of how
conformations affect a molecule.
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Representation of 3D Structures
Sawhorse Projection: A view of a molecule showing
wedges and dashes for bonds coming out or
going into the plane of the paper - resembles a
sawhorse.
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•
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Representation of 3D Structures
Newman Projection: A view of a molecule looking
straight down one C-C single bond (see below).
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•
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Ethane Conformations
Ethane Conformations
HH
C
H
H
H
C
H
H
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sawhorse
view down this axis
to see Newman
Projection
60°
H
H
H
H
0°
rotate front C
by 60°
H
H
HH
Newman Projection
eclipsed
higher in energy by
2.9 kcal/mol
H
Newman Projection
staggered
Eclipsed
H
H
Eclipsed
Eclipsed
E
2.9 kcal/mol
Staggered
0°
Staggered
60°
120°
Staggered
180°
240°
Staggered
300°
360°
rotation
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A few more terms
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•
•
60°
H
Staggered Conformation: one in which
H
H
the relationship of the groups on
H
H
one carbon versus an adjacent
H
carbon (front and back on a
Newman Projection
staggered
Newman projection) are aligned 60°
apart.
Eclipsed Conformation: one in which
the relationship of the groups on
one carbon versus an adjacent
carbon are aligned 0° apart.
0°
H
H
H
H
HH
Newman Projection
eclipsed
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A few more terms
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•
•
60°
H
Torsional Strain: The strain
H
H
introduced by electron repulsion of
H
H
bonds on adjacent carbons. This is
H
highest when the bonds are eclipsed Newman Projection
staggered
and lowest when staggered.
Steric Strain: The strain introduced
when atoms are forced to become
close to each other (they bump into
each other).
0°
H
H
H
H
HH
Newman Projection
eclipsed
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anti Butane Conformations
Butane Conformations
HH
H3C
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H
C
H
H
C
CH3
sawhorse
view down this axis
to see Newman
Projection
CH3
H
anti-methyls - 180°
H
H
CH3
Newman Projection
anti-staggered
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anti-Butane
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eclipsed Butane Conformations
Butane Conformations
H
CH3
H
H
H
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CH3
Newman Projection
anti-staggered
CH3 and H
rotate front C
by 60°
CH
H3
H3C
CH3 and H
H
H
H
Newman Projection
eclipsed
higher in energy by 3.8 kcal/mol
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Eclipsed Butane
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gauche Butane Conformations
Butane Conformations
CH
H3
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H3C
H
rotate front C
by 60°
H
H
Newman Projection
eclipsed
60°
H3C
CH3
H
H
H
H
0.9 kcal/mol higher in
energy than the antistaggered (lowest
energy) conformation
Newman Projection
gauche-staggered
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Gauche Butane
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eclipsed Butane Conformations
Butane Conformations
H3C
CH3
H
H
rotate front C
by 60°
H
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H
Newman Projection
gauche-staggered
CH3 and CH3
CH
CH33
H
H
H
H
4.5 kcal/mol higher
in energy than the
anti-staggered
conformation
Newman Projection
eclipsed
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syn-eclipsed Butane
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Butane Energy Profile
H
CH3
H
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CH3
60°
CH
H3
H
H3C
H H3C
H
H
H
CH3
H
CH
CH33
H
H
H
60°
CH3
H3C
H
H
H
H
H
CH
H3
H
H
H
H3C
H
H
CH3
H
H
H
H
H
CH3
Eclipsed
Methyls Aligned
Eclipsed
Eclipsed
E
3.8 kcal/mol
Staggered
anti
0°
4.5 kcal/mol
Staggered
gauche
60°
120°
Staggered
gauche
180°
rotation
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0.9 kcal/mol
240°
Staggered
anti
300°
360°
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A few more terms
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•
•
Anti Conformation: one in which the
two groups on adjacent carbons are
as far apart as possible (180°) in a
staggered conformation.
Gauche Conformation: one in which
the two groups on adjacent carbons
are still staggered, but closer (60°
apart).
H
CH3
H
H
H
CH3
Newman Projection
anti-staggered
60°
H3C
CH3
H
H
H
H
Newman Projection
gauche-staggered
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Higher Alkanes
•
The most stable conformation in longer chain
alkanes is the all-anti conformation
H
H
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H
H
C
H
H
C
C
H
C
C
H
C
H
H
H
H
H
H
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Cyclic Compounds
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•
Ring Strain
•
Angle Strain: the strain due to bond angles being
forced to expand or contract from their ideal.
•
Torsional Strain: the strain due to electron
repulsion of eclipsing bonds.
•
Steric Strain: the strain due to atoms coming
too close.
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Cyclic Compounds
Heat of Combustion: the amount of heat
(energy) released when a molecule burns
completely with oxygen.
Ring Strain
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•
3
4
5
6
7
8
Ring Size
9
10
11
12
13
14
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Cyclopropane
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•
Highest amount of angle strain
60°
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Cyclopropane
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Cyclobutane
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Cyclopentane
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Cyclohexane
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Axial and Equatorial Positions of
Cyclohexane
Pink - Axial
Blue - Equatorial
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Ring Flips in Cyclohexane
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Energy of Chair Flip Conformations
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Cyclohexane Axial Positions More
Crowded
1,3-diaxial interaction
axial methyl
H
CH3
ring flip
H
H
H
H
equatorial methyl
H
CH3
H
H
H
H
1.8 kcal/mol more
stable conformation
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axial Methyl Cyclohexane
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equatorial Methyl Cyclohexane
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methylcyclohexane Ring Flips
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A few more terms
•
Stereoisomers: Isomers (different compounds)
that have all the same number and kind of atoms
that are all connected the same, but differ in
their arrangement in three dimensions.
•
Due to the restricted rotation in cycloalkanes,
molecules with more than one substituent could
have the groups either on the same side (cis) or
opposite sides (trans) of the plane of the ring.
These are stereoisomers.
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cis and trans Cycloalkanes
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Br
Br
H
H
Br
trans-1,2-dibromocyclopropane
Br
Br
H
Br
H
cis-1,2-dibromocyclopropane
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cis and trans dimethylcyclopentane
H3C
H3C
CH3
CH3
=
H
H
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cis-1,3-dimethylcyclopentane
H3C
H3C
CH3
H
=
H
CH3
trans-1,3-dimethylcyclopentane
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dimethylcyclohexane
ax
CH3
cis-1,2-dimethylcyclohexane
1
2
CH3
2
CH3
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1
H
CH3
1
2
CH3
2
H
CH3 ax
eq
H
H
ring flip
H
1 CH3
H
same interactions in both conformations -equal in energy
ax
CH3
trans-1,2-dimethylcyclohexane
1
2
H
CH3
eq
ring flip
ax
CH3
2
1 CH3
eq
CH3
eq
H
lower in
different interactions in both
energy
conformations -- NOT equal in
energy
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cis-1,2-dimethylcyclohexane
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cis-1,2-dimethylcyclohexane
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trans-1,2-dimethylcyclohexane
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trans-1,2-dimethylcyclohexane
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trans-1,2-dimethylcyclohexane
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cis-1,3-dimethylcyclohexane
cis-1,3-dimethylcyclohexane
1
ax
CH3
CH3
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H
3
CH3
trans-1,3-dimethylcyclohexane
CH3
H
1
CH3
1
ring flip
H
3
3
ax
CH3
3
3
H
different interactions in both
conformations -- NOT equal in
energy
1 CH3
H
eq
lower in
energy
ax
CH3
ring flip
1
H
eq H3C
eq
H3C
H
3
ax CH3
1 CH3
eq
H
same interactions in both conformations -equal in energy
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cis-1,3-dimethylcyclohexane
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1,4-dimethylcyclohexane
cis-1,4-dimethylcyclohexane
1
CH3
4
1
H3C
eq
H3C
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ax
CH3
4
H3C
4
H
1 CH3
eq
same interactions in both conformations -- H
equal in energy
H
ax
CH3
CH3
1
H
ring flip
H
4
trans-1,4-dimethylcyclohexane
1
ax CH
3
4
ax CH3
H
H
ring flip
H3C
eq
different interactions in both
conformations -- NOT equal in
energy
4
1 CH3
lower in
energy
eq
H
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1,4-dimethylcyclohexane
cis-1-isopropyl-4-methylcyclohexane
CH3
CH
1
4
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H3C
H3C
ax
CH3
CH
ax CH
3
ring flip
1
CH3
4
H3C
eq
H
CH3
4
H
1 CH
H
The larger group would prefer to
be in the equatorial position -NOT equal in energy
H
lower in
energy
eq
CH3
H
Y
1,3-diaxial interaction
Y
kcal/mol strain
Y
kcal/mol strain
-F
0.12
-CH3
0.9
-Cl
0.25
-CH2CH3
0.95
-Br
0.25
-CH(CH3)2
1.1
-OH
0.5
-C(CH3)3
2.7
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Boat Conformations
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boat cyclohexane
H
H
H
H
H
H
H
H
norbornane
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Sugar Structure
•
Glucose
H
O
H
HO
OH
H
H
OH
H
OH
H OH
H O
HO
HO
H
H
OH
H
OH
CH2OH
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Polysaccharides
Starch
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•
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Polycyclic Molecules
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fused
spiro
bridged
CO2R
O
H
H
R
H
OR
H
H
H
R
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Heterocyclic Molecules
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ethylene
oxide
aziridine
O
furan
O
pyrrolidine
tetrahydropyran
piperidine
O
H
N
O
H
N
H
N
tetrahydrofuran
H
N
pyrrole
O
pyran
N
H
N
pyridine
O
morpholine
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