60
STEREOCHEMISTRY
low-energy staggered anti conformer and the highest
energy eclipsed conformer is about 18.8 kJ mol
−1 .
There will still be free rotation about C–C bonds
in butane at room temperature, but the larger energy
barrier compared with that for ethane means that the
staggered conformers are preferred, and calculations
show that, at room temperature, about 70% of
molecules will be in the anti conformer and about
15% in each gauche conformer.
3.3.2 Conformations of cyclic compounds
Cyclopropane, cyclobutane, cyclopentane,
cyclohexane
The practical consequences of conformational isomerism become much more significant when we consider cyclic compounds. The smallest ring system will
contain three atoms; in the case of hydrocarbons this
will be cyclopropane.
Now, simple geometry tells us that the inside angle
in cyclopropane must be 60
◦ . This is considerably
less than the 109.5
◦ of tetrahedral carbon, and the
consequences are that the amount of overlap of
the sp
3 orbitals in forming the C–C bonds must
be considerably less than in an acyclic system like
ethane. With poorer overlap, we get a potentially
weaker bond that can be broken more easily. We term
this ring strain, and although three-membered rings
exist and are quite stable, they are frequently subject
to ring-opening reactions (see Section 6.3.2).
• angle 60˚
• highest ring strain
• must be planar
• bonds eclipsed
cyclopropane
H
H
H
CH 2
H
H
H
H
H
H
H
poor orbital overlap
in C C bonds
C
C
maximum
orbital overlap
C
C
poor orbital
overlap
A further feature of three-membered rings is that
they must be planar, and a consequence of this is
that, in cyclopropane, all C–H bonds are in the highenergy eclipsed state. There can be no conformational
mobility to overcome this.
In cyclobutane, the internal angle is 90
◦ . Consequently, there is high ring strain, but this is not so
great as in cyclopropane.
If cyclobutane were planar, all C–H bonds would
be in the high-energy eclipsed state. It transpires
that cyclobutane is not planar, since it can adopt a
H
H
H
H
H
H
H
H
cyclobutane
• angle 90°
• high ring strain
• non-planar conformer minimizes eclipsing
H
H
H
H
H
H
H
H
interconversion of equal energy
non-planar conformers through
planar intermediate
H
H
H
H
H
H
H
H
H
H
CH 2
CH 2
H
H
note how we can show
perspective by having a full
bond at the front, and an
incomplete bond at the rear
equilibrium arrow is used to indicate
interconversion of structures
STEREOCHEMISTRY
low-energy staggered anti conformer and the highest
energy eclipsed conformer is about 18.8 kJ mol
−1 .
There will still be free rotation about C–C bonds
in butane at room temperature, but the larger energy
barrier compared with that for ethane means that the
staggered conformers are preferred, and calculations
show that, at room temperature, about 70% of
molecules will be in the anti conformer and about
15% in each gauche conformer.
3.3.2 Conformations of cyclic compounds
Cyclopropane, cyclobutane, cyclopentane,
cyclohexane
The practical consequences of conformational isomerism become much more significant when we consider cyclic compounds. The smallest ring system will
contain three atoms; in the case of hydrocarbons this
will be cyclopropane.
Now, simple geometry tells us that the inside angle
in cyclopropane must be 60
◦ . This is considerably
less than the 109.5
◦ of tetrahedral carbon, and the
consequences are that the amount of overlap of
the sp
3 orbitals in forming the C–C bonds must
be considerably less than in an acyclic system like
ethane. With poorer overlap, we get a potentially
weaker bond that can be broken more easily. We term
this ring strain, and although three-membered rings
exist and are quite stable, they are frequently subject
to ring-opening reactions (see Section 6.3.2).
• angle 60˚
• highest ring strain
• must be planar
• bonds eclipsed
cyclopropane
H
H
H
CH 2
H
H
H
H
H
H
H
poor orbital overlap
in C C bonds
C
C
maximum
orbital overlap
C
C
poor orbital
overlap
A further feature of three-membered rings is that
they must be planar, and a consequence of this is
that, in cyclopropane, all C–H bonds are in the highenergy eclipsed state. There can be no conformational
mobility to overcome this.
In cyclobutane, the internal angle is 90
◦ . Consequently, there is high ring strain, but this is not so
great as in cyclopropane.
If cyclobutane were planar, all C–H bonds would
be in the high-energy eclipsed state. It transpires
that cyclobutane is not planar, since it can adopt a
H
H
H
H
H
H
H
H
cyclobutane
• angle 90°
• high ring strain
• non-planar conformer minimizes eclipsing
H
H
H
H
H
H
H
H
interconversion of equal energy
non-planar conformers through
planar intermediate
H
H
H
H
H
H
H
H
H
H
CH 2
CH 2
H
H
note how we can show
perspective by having a full
bond at the front, and an
incomplete bond at the rear
equilibrium arrow is used to indicate
interconversion of structures