chapter 3 alkanes and cycloalkanes: conformations and cis-trans stereoisomers
TRANSCRIPT
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Chapter 3Chapter 3Alkanes and Cycloalkanes: Alkanes and Cycloalkanes:
Conformations and cis-trans Conformations and cis-trans StereoisomersStereoisomers
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Conformational AnalysisConformational Analysis
Conformations are different spatial arrangements of a
molecule that are generated by rotation about single
bonds.
Conformational analysis is the study of how conformational
factors affect the structure of a molecule and its properties.
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Representing ConformationsRepresenting Conformations
These are common ways to show conformations.
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Newman ProjectionsNewman Projections
In Newman projections we sight down a C C bond. ⎯The front carbon by a point and the back carbon by a circle.
Each carbon has three other bonds that are placed
symmetrically around it.
The bonds on the back carbon are shown sticking out from
the circle.
H
H
H
H
HH
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Newman Projections of EthaneNewman Projections of Ethane
Newman projections differ with respect to the rotation
of the front and back carbon atoms relative to each
other. The angles H-C-C-H angle is the torsional or
dihedral angle.
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An Important Point:
The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.
The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.
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Relative Stability of Newman ProjectionsRelative Stability of Newman Projections
The eclipsed conformation of ethane is the highest
energy conformation. Repulsion between bonds
destabilizes the eclipsed conformation.
The staggered conformation is the most stable. Better
electron delocalization stabilizes the staggered
conformation.
Conformations that are not staggered are said to have
torsional strain.
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Relative Stability of Ethane ConformationsRelative Stability of Ethane Conformations
Ethane has infinite conformations corresponding to changes
in the H-C-C-H torsional angle. Follow the “red” hydrogen
atoms.
eclipsed
staggered
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Relative Stability of Newman ProjectionsRelative Stability of Newman Projections
At any instant, almost all of the molecules are in
staggered conformations; hardly any are in eclipsed
conformations.
The difference between these two conformations is 12
kJ/mol.
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Propane Conformations
•Propane is shown here as a perspective drawing and as Propane is shown here as a perspective drawing and as
a Newman projection looking down the C1a Newman projection looking down the C1—C2 bond.—C2 bond.
•Chapter 3Chapter 3 •1010
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•The staggered conformations of propane is lower in energy than the eclipsed The staggered conformations of propane is lower in energy than the eclipsed
conformations. Since the methyl group occupies more space than a hydrogen, the conformations. Since the methyl group occupies more space than a hydrogen, the
torsional strain will be 0.3 kcal/mol higher for propane than for ethane.torsional strain will be 0.3 kcal/mol higher for propane than for ethane.
Propane Conformations
•Chapter 3Chapter 3 •1111
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Conformations of ButaneConformations of Butane
There are two different staggered conformations for butane.
The anti conformation is the most stable and the gauche
conformation is higher in energy because the larger CH3
groups are closer. This is called steric strain.
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Strain in Newman ProjectionsStrain in Newman Projections
Torsional strain is the strain that results from eclipsed
bonds.
Steric hindrance results when two atoms are too close
together. Also called van der Waals strain.
Steric strain is the combination of both of these.
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Newman Projections of ButaneNewman Projections of Butane
The eclipsed conformation with the CH3 groups eclipsed
has the most steric strain and is the highest energy
conformation.
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Conformations of Higher AlkanesConformations of Higher Alkanes
The lowest energy conformation of alkanes has all bonds
staggered. With simple alkanes this has a zig-zag
arrangement of carbon atoms.
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The Shapes of Cycloalkanes:The Shapes of Cycloalkanes:Planar or Nonplanar?Planar or Nonplanar?
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© 2013 Pearson Education, Inc.
Stabilities of Cycloalkanes
• Five- and six-membered rings are the most common in nature.
• Carbons of cycloalkanes are sp3 hybridized and thus require an angle of 109.5º.
• When a cycloalkane carbon has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain.
• Angle strain is sometimes called Baeyer strain in honor of Adolf von Baeyer, who first explained this phenomenon.
• Torsional strain arises when all the bonds are eclipsed.
• Chapter 3Chapter 3 • 1717
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• Torsional strain strain that results from eclipsed bonds
• van der Waals strain (steric strain)strain that results from atoms being too close together
• angle strainstrain that results from distortion of bondangles from normal values
Types of Strain
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Angle strain in CycloalkanesAngle strain in Cycloalkanes
Tetrahedral carbons prefer bond angles of 109.5o and
angle strain refers to the strain molecules have when
this bond angle cannot be matched.
Cyclopropane has the highest angle strain since its
bond angles are about 60o.
Other cycloalkanes try to minimize the angle strain and
are therefore not planar.
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Angle strain in CycloalkanesAngle strain in Cycloalkanes
Heat of combustion gives a way to measure the relative
stability of cycloalkanes. The lowest heat of combustion per CH2 group corresponds to the most stable cycloalkane.
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CyclopropaneCyclopropane
Strong sp3-sp3 s-bonds cannot be formed because of
60o bond angle.
The bonds are called bent bonds.
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CyclopropaneCyclopropane
There is also high torsional strain because the C-H
bonds are all eclipsed.
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© 2013 Pearson Education, Inc.
Nonplanar Cyclobutane
• Cyclic compound with four carbons or more adopt nonplanar conformations to relieve ring strain.
• Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.
• Chapter 3Chapter 3 • 2323
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CyclopentaneCyclopentane
Planar cyclopentane has low angle strain since the natural
bond angle is 108o. Torsional strain is significant because
the C-H bonds are eclipsed.
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CyclopentaneCyclopentane
The envelope and half-chair conformations have similar
lower energy and rapidly interconvert. They relieve some,
but not all, torsional strain.
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ConformationsConformations of Cyclohexane of Cyclohexane
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CyclohexaneCyclohexane
The most stable conformation of cyclohexane is known as
the chair conformation.
The side view shows the bonds across from each other
are parallel. One end carbon (1) is up and the other (4) is
down.
Solid wedges show bonds projecting towards the viewer.
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CyclohexaneCyclohexane
The Newman projection shows that all bonds are
staggered minimizing torsional strain.
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CyclohexaneCyclohexane
Other conformations include the skew boat.
Most molecules are in the chair conformation and less
than 5 molecules in 100,000 are in the skew boat
conformation at any point in time at 25 oC.
Therefore we concentrate on the chair conformation
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Axial and Equatorial Bonds Axial and Equatorial Bonds in Cyclohexanein Cyclohexane
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DefinitionsDefinitions
The 12 hydrogens of cyclohexane can be divided into two
groups: axial and equatorial.
Equatorial hydrogens lie around the equator of the molecule.
Axial hydrogens are directed alternately up and down.
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Conformational InversionConformational Inversion in Cyclohexane in Cyclohexane
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Ring InversionRing Inversion
There are two chair conformations of cyclohexane that
rapidly interconvert.
An axial group in the original chair conformation becomes
equatorial in the ring-inverted form and vice versa.
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Ring InversionRing Inversion
Ring inversion procedes through highest energy half-chair
conformations and the twist boat and boat conformations.
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Conformational Analysis of Conformational Analysis of Monosubstituted CyclohexanesMonosubstituted Cyclohexanes
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Interconversion between the two chair conformations occurs rapidly and the methyl group is either equatorial or axial. The conformation with an equatorial methyl is favored.
MethylcyclohexaneMethylcyclohexane
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Van der Waals strain between the methyl and the axial hydrogen atoms on the same side of the molecule destabilize the conformation with an axial methyl.
MethylcyclohexaneMethylcyclohexane
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Crowding is less pronounced with a "small" substituent such as fluorine so the difference in energy is lower than that observed for the methyl substitutent.
Chair Inversion of FluorocyclohexaneChair Inversion of Fluorocyclohexane
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Crowding is more pronounced with a “larger" substituent such as the tertiary butyl group so the difference in energy is much higher than that observed for the methyl substitutent.
Chair Inversion of t-butylcyclohexaneChair Inversion of t-butylcyclohexane
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DisubstitutedDisubstituted Cycloalkanes: Cycloalkanes:cis-trans Stereoisomerscis-trans Stereoisomers
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Isomers are different compounds that have the same molecular formula.
Constitutional isomers differ in connectivity.
Stereoisomers have the same connectivity but a different arrangement of the atoms in space.
Definitions of IsomersDefinitions of Isomers
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A cycloalkane with two substituents on different carbons in the ring may have two orientations.
If the substituents are on the same side of the ring we say they are cis to each other.
If the substituents are on the opposite sides of the ring we say they are trans to each other.
ciscis and and transtrans Substituents Substituents
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The cis-stereoisomer is higher in energy due to van der Waals strain. The difference in energy is determined by measuring the heat of combustion.
Relative Energies of StereoisomersRelative Energies of Stereoisomers
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Conformational AnalysisConformational Analysisof Disubstituted Cyclohexanesof Disubstituted Cyclohexanes
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cis trans
CH3
5219 kJ/mol 5212 kJ/mol
less stable more stable
Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other.
H3C
H H
H3C
CH3H
H
1,4-Dimethylcyclohexane Stereoisomers
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CH3H3C
H H
Two equivalent conformations; each has one axial methyl group and one equatorial methyl group
H
CH3
HCH3
H
H3C
H
CH3
Conformational analysis ofcis-1,4-
dimethylcyclohexane
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CH3
H3C
H
H
Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.
H
H3C
H
CH3
H
H3C
H
CH3
Conformational analysis oftrans-1,4-
dimethylcyclohexane
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cis trans
5223 kJ/mol 5217 kJ/mol
less stable more stable
Analogous to 1,4 in that trans is more stablethan cis.
CH3
CH3H
HH3C
CH3
H
H
1,2-Dimethylcyclohexane Stereoisomers
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CH3
CH3H
H
Two equivalent conformations; each has one axial methyl group and one equatorial methyl group
HCH3
H
CH3 H
CH3
H
CH3
Conformational analysis ofcis-1,2-
dimethylcyclohexane
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CH3
H3C H
H
Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.
H
CH3
H
CH3
H
H3C
H
CH3
Conformational analysis oftrans-1,2-
dimethylcyclohexane
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cis trans
5212 kJ/mol 5219 kJ/mol
more stable less stable
Unlike 1,2 and 1,4; cis-1,3 is more stable than trans.
H3C
CH3
H
H
CH3
H3C
H H
1,3-Dimethylcyclohexane Stereoisomers
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CH3
H3C
H H
Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.
H3C
HH
CH3
H
CH3
H
CH3
Conformational analysis ofcis-1,3-
dimethylcyclohexane
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Two equivalent conformations; each has one axialand one equatorial methyl group.
H3C H
H CH3
H
H3C
HCH3
H3C
CH3
H
H
Conformational analysis oftrans-1,3-
dimethylcyclohexane
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Compound Orientation -H° (kJ/mol)
cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-eq 5217*
cis-1,3-dimethyl eq-eq 5212*trans-1,3-dimethyl ax-eq 5219
cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-eq 5212*
*more stable stereoisomer of pair
Table 3.2 Heats of Combustion ofIsomeric Dimethylcyclohexanes
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With two different substituents the lowest conformation of
a particular isomer will have the larger substituent in the
equatorial position. Consider for example the two chair
conformations of cis-1-tert-butyl-2-methylcyclohexane
Other Disubstituted CyclohexanesOther Disubstituted Cyclohexanes
More stable
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Most stable conformation minimizes total strain.
This is more complicated than cyclohexane because
there are many conformations.
Furthermore several conformations may be of similar
energy.
Cycloheptane and Larger RingsCycloheptane and Larger Rings
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Polycyclic Ring SystemsPolycyclic Ring Systems
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Spirocyclic SystemsSpirocyclic Systems
Spirocyclic compounds have two rings with one common atom.
Named as spiro[number.number]alkane. The alkane suffix corresponds to the number of carbons in the two rings.The numbers of carbons in each ring not including the common atom are given in increasing order. This is spiro[3.4]octane.
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Two nonadjacent atoms common to two, or more, rings.
Bridged CompoundsBridged Compounds
Named as bicyclo[number.number.number]alkane. The parent alkane corresponds to the total number of carbons in the bicyclic skeleton. The numbers correspond to the number of carbon atoms between the bridgehead atoms in descending order.
This is bicyclo[3.2.1]octane.
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Compounds where two rings share a common side. Named as bridged bicyclic systems with one bridge with 0 carbons.
Fused Ring CompoundsFused Ring Compounds
cis-Bicyclo[4.4.0]decane
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Steroids have a tetracyclic carbon skeleton with
fused rings. SteroidsSteroids
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Cyclic compounds that contains an atom other than
carbon in the ring (these are called heteroatoms).
Oxygen containing heterocycles:
Heterocyclic CompoundsHeterocyclic Compounds
ethylene oxide and tetrahydrofuran
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Nitrogen containing heterocycles:
Heterocyclic CompoundsHeterocyclic Compounds
pyrrolidine and piperidine