H
H
H
H
H
H
H
H
H
H
H
H
Eclipsed
Staggered
Wedge and broken line method
H
H
H
H
H
H
H
H
H
H
H
H
Eclipsed
Staggered
Newman projection method
Staggered and eclipsed conformers In the staggered conformation, the
H atoms are as far apart as possible. This reduces repulsive forces
between them. This is why staggered conformers are stable. In the eclipsed
conformation, H atoms are closest together. This gives higher repulsive forces
between them. As a result, eclipsed conformers are unstable. At any
moment, more molecules will be in staggered form than any other
conformation.
Torsional energy and torsional strain Torsional energy is the energy
required for rotating about the C–C s bond. In ethane, this is very low (only
3 kcal). Torsional strain is the strain observed when a conformer rotates
away from the most stable conformation (i.e. the staggered form). Torsional
strain is due to the slight repulsion between electron clouds in the C–H
bonds as they pass close by each other in the eclipsed conformer. In ethane,
this is also low.
Conformational isomerism in propane Propane is a three-carbon- (sp
3
-
hybridized) atom-containing linear alkane. All are tetrahedrally arranged.
When a hydrogen atom of ethane is replaced by a methyl (CH 3 ) group, we
have propane. There is rotation about two C–C s bonds.
H
H H
H H
H
H
H
C
H 3
H
H
CH 3
Propane
H
H
H
H
H
CH 3
CH 3
H
H
H
H
H
Eclipsed
Staggered
Newman projection of propane conformers
In the eclipsed conformation of propane, we now have a larger CH 3 close to
H atom. This results in increased repulsive force or increased steric strain.
The energy difference between the eclipsed and staggered forms of propane
is greater than that of ethane.
Conformational isomerism in butane Butane is a four-carbon- (sp
3
-
hybridized) atom-containing linear alkane. All are tetrahedrally arranged.
When a hydrogen atom of propane is replaced by a methyl (CH 3 ) group, we
have butane. There is rotation about two C–C s bonds, but the rotation
about C 2 –C 3 is the most important.
38
CH3 STEREOCHEMISTRY
H
H
H
H
H
H
H
H
H
H
H
Eclipsed
Staggered
Wedge and broken line method
H
H
H
H
H
H
H
H
H
H
H
H
Eclipsed
Staggered
Newman projection method
Staggered and eclipsed conformers In the staggered conformation, the
H atoms are as far apart as possible. This reduces repulsive forces
between them. This is why staggered conformers are stable. In the eclipsed
conformation, H atoms are closest together. This gives higher repulsive forces
between them. As a result, eclipsed conformers are unstable. At any
moment, more molecules will be in staggered form than any other
conformation.
Torsional energy and torsional strain Torsional energy is the energy
required for rotating about the C–C s bond. In ethane, this is very low (only
3 kcal). Torsional strain is the strain observed when a conformer rotates
away from the most stable conformation (i.e. the staggered form). Torsional
strain is due to the slight repulsion between electron clouds in the C–H
bonds as they pass close by each other in the eclipsed conformer. In ethane,
this is also low.
Conformational isomerism in propane Propane is a three-carbon- (sp
3
-
hybridized) atom-containing linear alkane. All are tetrahedrally arranged.
When a hydrogen atom of ethane is replaced by a methyl (CH 3 ) group, we
have propane. There is rotation about two C–C s bonds.
H
H H
H H
H
H
H
C
H 3
H
H
CH 3
Propane
H
H
H
H
H
CH 3
CH 3
H
H
H
H
H
Eclipsed
Staggered
Newman projection of propane conformers
In the eclipsed conformation of propane, we now have a larger CH 3 close to
H atom. This results in increased repulsive force or increased steric strain.
The energy difference between the eclipsed and staggered forms of propane
is greater than that of ethane.
Conformational isomerism in butane Butane is a four-carbon- (sp
3
-
hybridized) atom-containing linear alkane. All are tetrahedrally arranged.
When a hydrogen atom of propane is replaced by a methyl (CH 3 ) group, we
have butane. There is rotation about two C–C s bonds, but the rotation
about C 2 –C 3 is the most important.
38
CH3 STEREOCHEMISTRY