Post on 30-Aug-2020
Alkane and CycloalkaneConformations
Chem 341 - Fall 2007
2
Exam 1 - Friday, September 21
‣ Begin at 7:45 am
‣ Last Name A-N - Steven’s Auditorium
‣ Last Name O-Z - Sudro 24
‣ Carla’s Review - Tuesday, 6:00 pm, Steven’s Auditorium
‣ Dr. Cook’s Review - Wednesday, 4:00 pm, Sudro 24
Ethane Conformations
3
C CH
H H
HH
H
Ethane Conformations
sawhorse
view down this axis to see Newman Projection
H
H
H
H
H
H
Newman Projectionstaggered
60° rotate front C
by 60° H
H
HHH
HNewman Projection
eclipsed
0°
higher in energy by 2.9 kcal/mol
E
rotation0° 60° 120° 180° 240° 300° 360°
Staggered Staggered Staggered Staggered
Eclipsed Eclipsed Eclipsed
2.9 kcal/mol
4
anti Butane Conformations
CCH3C
CH3H
HH
H
Butane Conformations
sawhorse
view down this axis to see Newman Projection
H
CH3
H
H
CH3
H
Newman Projectionanti-staggered
anti-methyls - 180°
anti-Butane
5
6
eclipsed Butane Conformations
Butane Conformations
H
CH3
H
H
CH3
H
Newman Projectionanti-staggered
rotate front Cby 60°
H
CH3
HH3CH
HCH3 and H
higher in energy by 3.8 kcal/mol
CH3 and H
Newman Projectioneclipsed
Eclipsed Butane
7
8
gauche Butane Conformations
Butane Conformations
H
CH3
H
H3C
H
H
Newman Projectiongauche-staggered
rotate front Cby 60°
H
CH3
HH3CH
H 0.9 kcal/mol higher in energy than the anti-staggered (lowest energy) conformation
Newman Projectioneclipsed
60°
Gauche Butane
9
10
eclipsed Butane Conformations
Butane Conformations
H
CH3
H
H3C
H
H
Newman Projectiongauche-staggered
rotate front Cby 60°
CH3 and CH3
4.5 kcal/mol higher in energy than the anti-staggered conformationH
CH3
HH H
CH3
Newman Projectioneclipsed
syn-eclipsed Butane
11
Butane Energy Profile
12
E
rotation0° 60° 120° 180° 240° 300° 360°
Staggeredanti
Staggeredgauche
Staggeredgauche
Staggeredanti
Eclipsed
EclipsedMethyls Aligned
Eclipsed
3.8 kcal/mol 4.5 kcal/mol 0.9 kcal/mol
H
CH3
H
H
CH3
H
H
CH3
HH3CH
H
H
CH3
H
H3C
H
H
60°
H
CH3
HH H
CH3
H
CH3
H
H3C
H
H
60°
H
CH3
HH3CH
H
H
CH3
H
H
CH3
H
Cyclic Compounds
‣ 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.
13
Cyclic Compounds
‣ Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen.
14
3 4 5 6 7 8 9 10 11 12 13 14Ring Size
Ring
Stra
in
Cyclopropane
‣ Highest amount of angle strain
15
60°
Cyclopropane
16
Cyclobutane
17
Cyclopentane
18
Cyclohexane
19
Axial and Equatorial Positions of Cyclohexane
20
Pink - AxialBlue - Equatorial
Ring Flips in Cyclohexane
21
Cyclohexane Axial Positions More Crowded
22
H
CH3
H
HH
HH CH3
H
H
H
Haxial methyl equatorial methyl
1,3-diaxial interaction
1.8 kcal/mol more stable conformation
ring flip
axial Methyl Cyclohexane
23
equatorial Methyl Cyclohexane
24
methylcyclohexane Ring Flips
25
dimethylcyclohexane
26
CH3
HCH3
H
ax
eq
cis-1,2-dimethylcyclohexane
same interactions in both conformations -- equal in energy
ring flip
CH3
H
CH3
Heq
ax
ring flip
1
21
221
CH3
CH3
CH3
H CH3
H
ax
eq
trans-1,2-dimethylcyclohexane
different interactions in both conformations -- NOT equal in energy
H
CH3
H
CH3eqax
1
21
221
CH3
CH3lower in energy
cis-1,2-dimethylcyclohexane
27
cis-1,2-dimethylcyclohexane
28
trans-1,2-dimethylcyclohexane
29
trans-1,2-dimethylcyclohexane
30
trans-1,2-dimethylcyclohexane
31
cis-1,3-dimethylcyclohexane
32
CH3
H CH3
H
ax
eq
cis-1,3-dimethylcyclohexane
same interactions in both conformations -- equal in energy
ring flip
axring flip1 1
3
1CH3
CH3different interactions in both conformations -- NOT equal in energy
lower in energy
3
CH3
H
H3C
H
3eq
CH3
H CH3
H
ax
eq
trans-1,3-dimethylcyclohexane
ax
1 1
3
1CH3
CH3
3
H
H3C
H
CH3
3
eq
cis-1,3-dimethylcyclohexane
33
1,4-dimethylcyclohexane
34
CH3
H CH3
H
ax
eq
cis-1,4-dimethylcyclohexane
same interactions in both conformations -- equal in energy
ring flip
ax
ring flip
1 11CH3
different interactions in both conformations -- NOT equal in energy
lower in energy
eq
CH3
H CH3
H
ax
eq
trans-1,4-dimethylcyclohexane
ax
1 14
1CH3
eq
H3C
H3C
4 4
4
4
4
H3C
H
H
CH3
H
H3C
CH3
H
1,4-dimethylcyclohexane
35
ax
ring flip
The larger group would prefer to be in the equatorial position -- NOT equal in energy
lower in energy
eq
CH
HCH
H
ax
eq
cis-1-isopropyl-4-methylcyclohexane
1 14
1CH
H3C4
4
H3C
H
CH3
H
H3C CH3
CH3
CH3CH3
CH3
YH1,3-diaxial interaction
Y kcal/mol strain
-F 0.12
-Cl 0.25
-Br 0.25
-OH 0.5
Y kcal/mol strain
-CH3 0.9
-CH2CH3 0.95
-CH(CH3)2 1.1
-C(CH3)3 2.7
Boat Conformations
36
boat cyclohexane
H
H
H
HH H
H H
norbornane
37
Sugar Structure
‣ Glucose
OHHHHOOHHOHH
CH2OH
OH
HO
H
HO
H
HOHH OH
OH
OH
38
Polysaccharides
‣ Starch
Stereoids
39
O
OR
R
CO2R
RH
H
H HH H