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John E. McMurry
www.cengage.com/chemistry/mcmurry
Chapter 4
Organic Compounds:
Cycloalkanes and theirStereochemistry
Modified by Dr. Daniela R. Radu
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! Because cyclic molecules are commonly encountered in allclasses of biomolecules:
- Proteins, Lipids, Carbohydrates, Nucleic acids
It is important to understand the nature of these molecules as
you will encounter them in the next phases of your academictraining
Why this Chapter?
! Many organic compounds contain rings of carbonatoms. Examples:
-
Prostaglandins
- Steroids
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! Cycloalkanes are saturated cyclic hydrocarbons
! Have the general formula (CnH2n)
4.1 Naming Cycloalkanes
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1) Find the parent. # of carbons in the ring. Ex. 6 C" Parent: Cyclohexane
2) Number the substituents
Naming Cycloalkanes
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! Cycloalkanes are less flexible than open-
chain alkanes
" Much less conformational freedom in
cycloalkanes
4.2 Cis-Trans Isomerism in
Cycloalkanes
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! Because of their cyclic structure, cycloalkanes have 2 faces as
viewed edge-on
“top” face “bottom” face
- Therefore, isomerism is possible in substituted cycloalkanes
- There are two different 1,2-dimethylcyclopropane isomers
Cis-Trans Isomerism in
Cycloalkanes (Continued)
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! Stereoisomerism
!
Compounds which have their atoms connected
in the same order but differ in 3-D orientation
Cis-Trans Isomerism in
Cycloalkanes (Continued)
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! Rings larger than 3 atoms are not flat
! Cyclic molecules can assume nonplanarconformations to minimize angle strain and torsionalstrain by ring-puckering
! Larger rings have many more possible conformations
than smaller rings and are more difficult to analyze
4.3 Stability of Cycloalkanes:
Ring Strain
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! Baeyer (1885): since carbon prefers to have bondangles 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 tobond bending distortions and steric interactions
Stability of Cycloalkanes: The
Baeyer Strain Theory
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! Angle strain - expansion or compression of bondangles away from most stable
! Torsional strain - eclipsing of bonds on neighboring
atoms
! Steric strain - repulsive interactions between
nonbonded atoms in close proximity
Types of Strain That Contribute to
Overall Energy of a Cycloalkane
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Cyclopropane ! 3-membered ring must have planar structure
! Symmetrical with C–C–C bond angles of 60°
! Requires that sp3 based bonds are bent (and
weakened)
! All C-H bonds are eclipsed
4.4 Conformations of
Cycloalkanes
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! In cyclopropane, the C-C bond is displaced
outward from internuclear axis
Bent Bonds of Cyclopropane
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! Cyclobutane has less angle strain than cyclopropane but moretorsional strain because of its larger number of ring hydrogens, andtheir proximity to each other
! Cyclobutane is slightly bent out of plane - one carbon atom is about25° above
! The bend increases angle strain but decreases torsional strain
Cyclobutane
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! Planar cyclopentane would have no angle strain butvery high torsional strain
! Actual conformations of cyclopentane are nonplanar,reducing torsional strain
! Four carbon atoms are in a plane
! The fifth carbon atom is above or below the plane – lookslike an envelope
Cyclopentane
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! Substituted cyclohexanes occur widely in nature
! The cyclohexane ring is free of angle strain andtorsional strain
! The conformation has alternating atoms in a common
plane and tetrahedral angles between all carbons! This is called a chair conformation
4.5 Conformations of
Cyclohexane
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Step 1 Draw two parallel lines, slanted downwardand slightly offset from each other. This
means that four of the cyclohexane carbons
lie in a plane.
Step 2 Place the topmost carbon atom above and to
the right of the plane of the other four, and
connect bonds.
Step 3 Place the bottommost carbon atom below
and to the left of the plane of the middle four,
and connect the bonds. Note that the bonds
to the bottommost carbon atom a parallel to
the bonds to the topmost carbon.
How to Draw Cyclohexane
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Steric Strain
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! The chair conformationhas two kinds ofpositions forsubstituents on thering: axial positions
and equatorial positions
! Chair cyclohexane hassix axial hydrogensperpendicular to the
ring (parallel to the ringaxis) and sixequatorial hydrogensnear the plane of thering
4.6 Axial and Equatorial Bonds in
Cyclohexane
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! Each carbon atom in cyclohexane has one axial andone equatorial hydrogen
! Each face of the ring has three axial and three
equatorial hydrogens in an alternating arrangement
Axial and Equatorial Positions
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Drawing the Axial and Equatorial
Hydrogens
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! Chair conformations readily interconvert,
resulting in the exchange of axial and equatorial
positions by a ring-flip
Conformational Mobility of
Cyclohexane
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! Cyclohexane ring rapidly flips between chairconformations at room temp.
! Two conformations of monosubstituted cyclohexanearen’t equally stable.
! The equatorial conformer of methyl cyclohexane is more
stable than the axial by 7.6 kJ/mol
4.7 Conformations of
Monosubstituted Cyclohexanes
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! Difference between axial and equatorial conformers isdue to steric strain caused by 1,3-diaxial interactions
! 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
1,3-Diaxial Interactions
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Steric Strain in Monosubstituted
Cyclohexanes
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! Gauche butane is less stable than anti butane by 3.8 kJ/mol
because of steric interference between hydrogen atoms on the two
methyl groups
! The four-carbon fragment of axial methylcyclohexane and gauchebutane have the same steric interaction
! In general, equatorial positions give the more stable isomer
Relationship to Gauche Butane
Interactions
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! In disubstituted cyclohexanes the steric effects of both substituents must betaken 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, andthe compound can exist in two chair conformations
! Consider the sum of all interactions
! In cis-1,2, both conformations are equal in energy
4.8 Conformations of
Disubstituted Cylcohexanes
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! 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-diaxialinteractions
! 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 conformation11.4 kJ/mol less favorable than the diequatorial conformation
! trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in thediequatorial conformation
Trans-1,2-Dimethylcyclohexane
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Axial and Equatorial Relationships
in Disubstituted Cyclohexanes
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! Decalin consists of two cyclohexane rings joined toshare two carbon atoms (the bridgehead carbons, C1and C6) and a common bond
! Two isomeric forms of decalin: trans fused or cis fused! In cis-decalin hydrogen atoms at the bridgehead
carbons are on the same face of the rings! In trans-decalin, the bridgehead hydrogens are on
opposite faces
! Both compounds can be represented using chaircyclohexane conformations
! Flips and rotations do not interconvert cis and trans
4.9 Conformations of Polycyclic
Molecules
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Conformations of Polycyclic
Molecules (Continued)
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Draw two constitutional isomers of cis-1,2-
dibromocyclopentane?
Let’s Work a Problem
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Answer
First, we need to understand what constitutional isomer
means!the #’s of atoms, and types of atoms are the
same, just the arrangement may be different. We have
a 5 Carbon cyclic alkane, so we can only have a casewhen we have a 1,2- or a 1,3- dibromo linkage, as
these links will be symmetrical with respect to middle
carbon.
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! Many organic compounds contain rings of carbon atoms
e.g.
- Prostaglandins
-
Steroids
Organic Compounds can be
Open-Chained or Cyclic