Rings and Strain: Angle Strain
• The sp3 orbitals about a
carbon atom have a
tetrahedral geometry with
idealized interorbital angles of
109.5°.
• Constraining carbon atoms in a
ring can lead to a forced
internuclear bond angle.
Section 5.2
1
Rings and Strain: Angle Strain (cont.)
• When the internuclear angle is smaller than the
interorbital angle, overlap is poor and bonds are weaker:
Section 5.2
2
Rings and Strain: Torsional Strain
• The constrained ring also
prevents bond rotation, so
eclipsing interactions be locked
in place:
Section 5.2
3
Rings and Strain: Torsional Strain (cont.)
• Rings of >3 carbon atoms are more flexible; they adopt
conformations that minimize torsional strain.
• An example is “puckering” of cyclobutane:
Section 5.2
4
Chair Cyclohexane
• When the ring size reaches six carbons, there is enough
flexibility to attain a conformation—chair cyclohexane—
in which all eclipsing interactions are eliminated.
Section 5.2
5
Chair Cyclohexane (cont.)
• The angle and torsional strain
in chair cyclohexane are
minimal. There are three pairs
of parallel C-C bonds:
Section 5.2
6
Chair Cyclohexane (cont.)
• Groups attached to chair cyclohexane may be positioned
in one of two distinct type of sites: axial or equatorial.
Section 5.2
7
Chair Cyclohexane (cont.)
• Each ring carbon has one equatorial bond and one axial
bond. One group points up and one down on each C,
alternating around the ring:
Section 5.2
8
Chair Cyclohexane: Ring Flip
• There are two chair conformations of cyclohexane. Going
from one chair conformation to the other causes axial
and equatorial positions to interconvert:
Section 5.2
9
Chair Cyclohexane: Ring Flip (cont.)
• This ring flip leads to equatorial substituents becoming
axial and vice versa. This will be an increasingly
important concept as we learn about substituted
cyclohexanes.
Section 5.2
10
Monosubstituted Cyclohexanes (cont.)
• The one with methyl equatorial is more stable by 1.74
kcal/mol because an axial methyl has two gauche
interactions with the ring (one on each side of the
methyl):
Section 5.5
11
Monosubstituted Cyclohexanes (cont.)
Section 5.5
12
Monosubstituted Cyclohexanes (cont.)
• The bigger the substituent, the more favored is the chair
form with the substituent in the equatorial position:
Section 5.5
13
1,2-Disubstituted Cyclohexane (cont.)
• Trans-1,2-dimethylcyclohexane has two possible
conformations:
Section 5.6
14
1,2-Disubstituted Cyclohexane (cont.)
• The diequatorial isomer is the more stable:
Section 5.6
15
1,2-Disubstituted Cyclohexane (cont.)
• If the substituents are different, four stereoisomers
result:
Section 5.6
16
1,2-Disubstituted Cyclohexane (cont.)
• Concerning stability, larger groups produce more steric
strain when they occupy axial positions.
The more stable conformation of cis-1-isopropyl2-methylcyclohexane has an equatorial isopropyl group:
Section 5.6
17
Bicyclic Compounds
• A compound can have more than one ring. We will
consider three classes of bicyclic compounds
in which one or more carbon is shared between rings:
Section 5.7
18
Bicyclic Compounds (cont.)
• The three-dimensional shape of spirocyclic compounds is
not adequately reflected by the 2D drawing:
Section 5.7
19
Bicyclic Compounds (cont.)
• Bicyclics sharing two or more carbons are more prevalent
than spiro compounds. The fusion positions or
bridgeheads are highlighted:
Section 5.7
20
Bicyclic Compounds (cont.)
• The second ring may fuse onto the first at two positions,
either cis- or trans- to one another:
Section 5.7
21
Bicyclic Compounds (cont.)
• Consider cis- and trans-decalin:
Section 5.7
22
Bicyclic Compounds (cont.)
• When fusing the second ring onto cyclohexane,
it is important to recall that axial/equatorial sites
are different:
Trans-diaxial fusion
impossible!
Trans-diequatorial
fusion possible
Section 5.7
23
Adamantane
• Adamantane is a polycyclic compound that can
be thought of as an expanded cyclohexane:
Section 5.9
24
Adamantane (cont.)
• The adamantane framework is composed entirely of
fused chair cyclohexanes, and is therefore quite a stable
unit:
Section 5.9
25
Adamantane (cont.)
• The stability of this building block is clear considering
that diamonds are an infinite network of adamantane
units!
Section 5.9
26