CH 4: Organic Compounds:
Cycloalkanes and their
Stereochemistry
Renee Y. Becker
CHM 2210
Valencia Community College
1
Cycloalkanes
• Rings of carbon atoms (CH2 groups)
• Formula: CnH2n
• Nonpolar, insoluble in water
• Compact shape
• Melting and boiling points similar to branched
alkanes with same number of carbons
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Naming Cycloalkanes
• Cycloalkane usually base compound
– May be cycloalkyl attachment to chain
• It is off of a chain that has a longer carbon chain
• Number carbons in ring if >1 substituent.
• Number so that sub. have lowest numbers
– Give first in alphabet lowest number if possible
CH2CH3
CH2CH3
CH3
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Naming Cycloalkanes
• Find the parent. # of carbons in the ring.
• Number the substituents
4
Example 1
Give IUPAC names
CH2CHCH2CH3
CH3
H3C
CH3
CH3
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Example 2
Draw the structure
a) propylcyclohexane
b) cyclopropylcyclopentane
c) 3-ethyl-1,1-dimethylcyclohexane
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Stereoisomerism
• Compounds which have their atoms connected
in the same order but differ in 3-D orientation
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Cis-Trans Isomerism
• Cis: like groups on same face of ring
• Trans: like groups on opposite face of ring
• Sub. Do not have to be on adjacent carbons of
ring
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Cycloalkane Stability
• 5- and 6-membered rings most stable
• Bond angle closest to 109.5
• Angle (Baeyer) strain
• Measured by heats of combustion
per -CH2 – The more strain, the higher the heat of combustion,
per CH2 group
– The energy released as heat when one mole of a
compound undergoes complete combustion with
oxygen.
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Stability of Cycloalkanes: The Baeyer Strain Theory
• Baeyer (1885):
since carbon
prefers to have
bond angles of
approximately
109°, ring sizes
other than five and
six may be too
strained to exist
• Rings from 3 to 30
C’s do exist but are
strained due to
bond bending
distortions and
steric interactions
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Summary: Types of Strain
• Angle strain - expansion or compression of
bond angles away from most stable
• Torsional strain - eclipsing of bonds on
neighboring atoms
• Steric strain - repulsive interactions between
nonbonded atoms in close proximity
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Heats of Combustion (per CH2 group) Alkane + O2  CO2 + H2O
166.6 164.0
157.4
Long-chain
158.7
158.6
158.3
157.4
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Cyclopropane
• Large ring strain due to angle compression
• Very reactive, weak bonds
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Cyclopropane
Torsional strain because of eclipsed hydrogens
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Cyclobutane
• Angle strain due to compression
• Torsional strain partially relieved by ringpuckering
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Cyclopentane
• If planar, angles would be 108, but all hydrogens
would be eclipsed.
• Puckered conformer reduces torsional strain.
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Cyclohexane
• Combustion data shows it’s unstrained.
• Angles would be 120, if planar.
• The chair conformer has 109.5 bond angles
and all hydrogens are staggered.
• No angle strain and no torsional strain.
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Chair Conformer
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Boat Conformer
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Conformational Energy
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Axial and Equatorial Positions
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Drawing the Axial and Equatorial Hydrogens
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Monosubstituted Cyclohexanes
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1,3-Diaxial Interactions
• Difference between axial and equatorial conformers
is due to steric strain caused by 1,3-diaxial
interactions
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Hydrogen atoms of the axial methyl group on C1 are
too close to the axial hydrogens three carbons away
on C3 and C5, resulting in 7.6 kJ/mol of steric strain
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Disubstituted Cyclohexanes
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Conformational Analysis of Disubstituted Cyclohexanes
• In disubstituted cyclohexanes the steric effects of both
substituents must be taken into account in both
conformations
• There are two isomers of 1,2-dimethylcyclohexane. cis
and trans
• In the cis isomer, both methyl groups are on the same
face of the ring, and compound can exist in two chair
conformations
• Consider the sum of all interactions
• In cis-1,2, both conformations are equal in energy
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Conformational Analysis of Disubstituted Cyclohexanes
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Trans-1,2-Dimethylcyclohexane
• Methyl groups are on opposite faces of the ring
• One trans conformation has both methyl groups
equatorial and only a gauche butane interaction between
methyls (3.8 kJ/mol) and no 1,3-diaxial interactions
• The ring-flipped conformation has both methyl groups
axial with four 1,3-diaxial interactions
• Steric strain of 4  3.8 kJ/mol = 15.2 kJ/mol makes the
diaxial conformation 11.4 kJ/mol less favorable than the
diequatorial conformation
• trans-1,2-dimethylcyclohexane will exist almost
exclusively (>99%) in the diequatorial conformation
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Trans-1,2-Dimethylcyclohexane
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Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial
around the ring.
CH3
CH3
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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.
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Example 3
Draw the most stable conformation
a) ethylcyclohexane
b) isopropylcyclohexane
c) t-butylcyclohexane
d) cis-1-t-butyl-3-ethylcyclohexane
e) trans-1-t-butyl-2-methylcyclohexane
f) trans-1-t-butyl-3-(1,1-dimethylpropyl)cyclohexane
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Example 4
Which of the following is the most strained
ring? Least strained? Why?
a)
c)
b)
d)
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Table 4.2 Axial and Equatorial Relationship in Cis and trans Disub
Cyclohexanes
Cis/trans pattern
Axial/Equatorial Relationship
1,2–Cis
a,e
e,a
1,2-trans
a,a
e,e
1,3-cis
a,a
e,e
1,3-trans
a,e
e,a
1,4-cis
a,e
e,a
1,4-trans
a,a
e,e
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