conformation lecture

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1 anti Butane Conformations C C H 3 C CH 3 H H H H Butane Conformations sawhorse view down this axis to see Newman Projection H CH 3 H H CH 3 H Newman Projection anti-staggered anti-methyls - 180°

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Page 1: 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°

Page 2: Conformation Lecture

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

Page 3: Conformation Lecture

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°

Page 4: Conformation Lecture

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

Page 5: Conformation Lecture

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

Page 6: Conformation Lecture

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.

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Page 7: Conformation Lecture

Cyclic Compounds

‣ Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen.

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3 4 5 6 7 8 9 10 11 12 13 14Ring Size

Ring

Stra

in

Page 8: Conformation Lecture

Cyclopropane

‣ Highest amount of angle strain

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60°

Page 9: Conformation Lecture

Cyclopropane

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Page 10: Conformation Lecture

Cyclobutane

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Page 11: Conformation Lecture

Cyclopentane

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Page 12: Conformation Lecture

Cyclohexane

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Page 13: Conformation Lecture

Axial and Equatorial Positions of Cyclohexane

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Pink - AxialBlue - Equatorial

Page 14: Conformation Lecture

Cyclohexane Axial Positions More Crowded

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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

Page 15: Conformation Lecture

axial Methyl Cyclohexane

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Page 16: Conformation Lecture

equatorial Methyl Cyclohexane

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