H3C

Conformations: 3-D arrangements of atoms in a molecule from rotation/twisting about single bonds

Newman projection: conformational view along a carbon-carbon bond

H3CH3C

H3CH3C

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ConformationsStaggered conformation: atoms/groups on 1 carbon are as far apart as possible from the atoms/groups on adjacent carbon

H

H H

H H

H

= 60o

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ConformationsEclipsed conformation: atoms/groups on 1-C in

closet approach to atoms/groups on adjacent C

H

H H

H

HH

same plane= 0o

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ConformationsTorsional strain or eclipsed interaction strainstrain (E) arises from nonbonded atoms rotate from

staggered to eclipsed conformations

+12.6 kJ/mol

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Conformations

Ethane as a function of dihedral angle

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Conformations

anti conformation– a conformation about a single bond in which the

groups lie at a dihedral angle of 180°

CH3

H H

H H

CH3

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ConformationsSteric strain (nonbonded interaction strain):

when atoms separated by 4 (more) bonds are forced closer

Angle strain:when a bond angle must compressed or expanded from 109.5o

calculate total strain - determine the lowest energy conformation ( molecular mechanics )

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Conformations

– conformations of butane as a function of dihedral angle

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Cyclopropane

angle strain: bond angles compressed (109.5° 60°)torsional strain: all 6Hs eclipsed

strain energy highH

H

H

H

H

H

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Cyclobutane

puckering torsional strain slight angle strain

H

H

H

H

H H

H H

H H

HH

H

H

HH

net - puckered (butterfly) minimum energy conformation

but strain energy highcp qz

Cyclopentane

“envelope” - torsional strain slightly angle strain

envelope ~ 1/4th strain energy of cyclopropane

H

H

H

H

H H

H

H HH

H H

H

HH

H

H

H

H

H

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Cyclohexane - chair conformationBond bond angles - 110.9° StaggeredNo angle or torsional strain

six Hs are axial and six equatorial

H

H

H

H

H

H

HHH

H H H

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cyclohexane 2 equivalent chair conformations

Flip converts

6 equatorial 6 axial

6 axial Hs in one chair 6 equatorial Hs in other

H

H

H

H

H

H

HHH

H HH

H

HH

H

H

H

H H

H H H

H?

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Equatorial and axial methyl conformations

Methylcyclohexane

CH3H

HH

H

H

H

HH H

HH

H

H

HH

H

H

HHH

HCH3

H +7.28 kJ/mol

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