Organic Chemistry I:
Cycloalkane Stability and
Conformations
October 31, 2008
Dr. Milkevitch
Chapter 3
1
Cycloalkane Stability
• 5- and 6-membered rings
• Most commonly found
• Most stable
• But Why?
• Adolf Von Baeyer-1905
• Attempted to explain relative stabilities of cyclic
alkanes
• Reasoned that non-cyclic (acyclic) alkanes had
bond angles of 109.5°
• Bond angles of 109.5° puts electrons in bonds as
far apart as possible
» Most stable
Chapter 3
2
Cycloalkane Stability
•However, if a cyclic alkane has bond angles other than 109.5 °:
•Orbitals of carbon-carbon bonds cannot:
•Achieve optimum overlap
•There must be some angle strain
•Example: Cyclobutane
•90° bond angles
Chapter 3
3
Cycloalkane Stability
•Also notice:
•Newman projection: bonds are eclipsed
•Gives rise to torsional strain
•It all adds up:
•Angle strain + torsional strain = Ring strain.
Chapter 3
4
How Ring Strain is Measured
•Heats of combustion
•Amount of heat released
•When a compound is burned in an excess of O2
•Produce a table of molar heats of combustion
•Divide heat of combustion by the number of CH2 groups
•Energy per CH2 groups
Chapter 3
5
Heats of Combustion/CH2
Alkane + O2 CO2 + H2O
697.1 686.1
664.0
658.6 kJ
Ring
Strain
per CH2
Long-chain
38.5
27.5
5.4
663.6 kJ/mol
662.4
658.6
0
3.8
5.0
=>
Chapter 3
6
Focus on Cyclohexane
•
Combustion data shows no ring strain present
• Must have bond angles near 109.5° (no angle strain)
• Must have no eclipsing of bonds (no torsional strain)
•
What if cyclohexane were planar?
• Makes sense, doesn’t it?
• Bond angles would be 120°, but should be 109.5°
• implies some angle strain
• Planar ring would have eclipsed bonds on CH2 groups
• Implies some torsional strain
•
Conclusion:
• Cyclohexane ring cannot be planar
• Cyclohexane must have some type of alternate
confirmation(s)
• There are two major conformations of cyclohexane
=>
Chapter 3
7
Chair Conformer
Bond angles = 109.5°
Dr. Milkevitch
in his office
Staggered conformation
=>
Chapter 3
8
Boat Conformer
Bond angles = 109.5°
Adopts “twist” boat,
to relieve torsional
=>
strain
Chapter 3
9
Cyclohexane Conformations
•At any instant:
•Most molecules in chair confirmation
•Energy barrier between chair & boat sufficiently low
•Conformations interconvert several times per second
•Interconversion:
•“Footrest” of chair flips up, planar with sides of molecule
•Forms “half-chair” conformation
•High energy process
Chapter 3
10
Conformational Energy
=>
Chapter 3
11
Axial and Equatorial Positions
•Examine cyclohexane in chair confirmation
•2 different kinds of hydrogen
•First kind:
•6 bonds (one on each carbon)
•Directed up and down
•Parallel to axis of ring
•Called axial hydrogens
•Second Kind:
•6 other bonds (one on each carbon)
•Point out from ring
•Called Equatorial hydrogens
Chapter 3
12
Axial and Equatorial Positions
=>
Chapter 3
13
Monosubstituted Cyclohexanes
=>
Chapter 3
14
Monosubstituted Cyclohexanes
•A substituent on a cyclohexane ring (chair conformation)
•Can occupy:
•An axial position
•An equatorial position
•However, at RT there are 2 chair conformations
•In equilibrium
•Called a “chair-chair” interconversion
•“Chair-chair” interconversion:
•Changes axial to equatorial, and vice-versa
Chapter 3
15
Monosubstituted Cyclohexanes
•But which is the lowest energy?
•Substituent in equatorial position
Chapter 3
16
Substituent in Axial Position
•2 gauche interactions
Chapter 3
17
Substituent in Equatorial
Position
•No gauche interactions
Chapter 3
18
1,3-Diaxial Interactions
•Gauche interactions of axial substituent
•Places electron clouds near to each other
=>
•Form of steric hinderance
Chapter 3
19
Disubstituted Cyclohexanes
•Very severe interaction when large groups are axial
•More stable interaction: both groups equatorial
=>
Chapter 3
20
Disubstituted Cyclohexanes
•What happens when both groups cannot be equatorial?
•In other words, one group axial, one group equatorial
•Interconversion, get the same thing
Chapter 3
21
Disubstituted Cyclohexanes
•Result: More stable conformation
•Larger group equatorial
•Smaller group axial
Chapter 3
22
Recognizing Cis-Trans Isomers
•Each ring carbon has 2 available bonds
•One up, one down
H
Up
CH3
CH3
CH3
down
=
Up
down
CH3
H
Trans-1,2 dimethylcyclohexane
Chapter 3
23
Bulky Groups
• Groups like t-butyl cause a large energy
difference between the axial and equatorial
conformer.
• Most stable conformer puts t-butyl equatorial
regardless of other substituents.
=>
Chapter 3
24
Bicyclic Alkanes
• Fused rings share two adjacent carbons.
• Bridged rings share two nonadjacent C’s.
bicyclo[3.1.0]hexane
Chapter 3
bicyclo[2.2.1]heptane
=>
25
Naming Bicyclic Alkanes
•Based on the name of the alkane
•Having the same number of carbons in the ring system
•Name follows prefix bicyclo
•Set of brackets with 3 numbers
•Corresponds to the number of carbons in the bridges
Chapter 3
26
Naming Bicyclic Alkanes
•All fused and bridged cyclic systems have 3 bridges connecting
the 2 bridgehead atoms
•Numbers in brackets give the number of carbon atoms
•In each of the 3 bridges connecting the bridgehead carbons
•In order of decreasing size
Bridgehead carbons
Chapter 3
27
End of Chapter 3
Chapter 3
28