Conformation Lecture
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Transcript of Conformation Lecture
1
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°
2
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
3
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°
4
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
Butane Energy Profile
5
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.
6
Cyclic Compounds
‣ Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen.
7
3 4 5 6 7 8 9 10 11 12 13 14Ring Size
Ring
Stra
in
Cyclopropane
‣ Highest amount of angle strain
8
60°
Cyclopropane
9
Cyclobutane
10
Cyclopentane
11
Cyclohexane
12
Axial and Equatorial Positions of Cyclohexane
13
Pink - AxialBlue - Equatorial
Cyclohexane Axial Positions More Crowded
14
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
15
equatorial Methyl Cyclohexane
16