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
cp qz
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
cp qz
ConformationsEclipsed conformation: atoms/groups on 1-C in
closet approach to atoms/groups on adjacent C
H
H H
H
HH
same plane= 0o
cp qz
ConformationsTorsional strain or eclipsed interaction strainstrain (E) arises from nonbonded atoms rotate from
staggered to eclipsed conformations
+12.6 kJ/mol
cp qz
Conformations
Ethane as a function of dihedral angle
cp qz
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
cp qz
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 )
cp qz
Conformations
– conformations of butane as a function of dihedral angle
cp qz
Cyclopropane
angle strain: bond angles compressed (109.5° 60°)torsional strain: all 6Hs eclipsed
strain energy highH
H
H
H
H
H
cp qz
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
cp qz
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
cp qz
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?
cp qz
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
94 / 6
cp qz