Stereochemistry 4. of Alkanes and Cycloalkanes
Based on McMurrys Organic Chemistry, 6th edition, Chapter 4 2003 Ronald Kluger Department of Chemistry University of Toronto
The Shapes of Molecules
The three-dimensional shapes of molecules result
from many forces A molecule may assume different shapes, called conformations, that are in equilibrium at room temperature (the conformational isomers are called conformers, emphasis on the first syllable) The systematic study of the shapes molecules and properties from these shapes is stereochemistry The field of stereochemistry is one of the central parts of organic chemistry and includes many important topics
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4.1 Conformations of Ethane
Conformers interconvert
rapidly and a structure is an average of conformers Molecular models are three dimensional objects that enable us to visualize conformers Representing three dimensional conformers in two dimensions is done with standard types of drawings
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Representing Conformations
Sawhorse representations
show molecules at an angle, showing a molecular model C-C bonds are at an angle to the edge of the page and all C-H bonds are shown Newman projections show how the C-C bond would project end-on onto the paper Bonds to front carbon are lines going to the center Bonds to rear carbon are lines going to the edge of the circle
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Ethanes Conformations
There barrier to rotation between conformations is small (12 kJ/
mol; 2.9 kcal/mol) The most stable conformation of ethane has all six CH bonds away from each other (staggered) The least stable conformation has all six CH bonds as close as possible (eclipsed) in a Newman projection energy due to torsional strain
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4.2 Conformations of Propane
Propane (C3H8)
torsional barrier around the carbon carbon bonds 14 kJ/ mol Eclipsed conformer of propane has two ethane-type HH interactions and an interaction between CH and CC bond
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4.3 Conformations of Butane
anti conformation has two methyl groups 180 away from each
other Rotation around the C2C3 gives eclipsed conformation Staggered conformation with methyl groups 60 apart is gauche conformation
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4.4 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 Cs do exist but are strained due to bond bending distortions and steric interactions
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4.5 The Nature of Ring Strain
Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanar
conformations to minimize angle strain and torsional strain by ring-puckering Larger rings have many more possible conformations than smaller rings and are more difficult to analyze
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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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4.6 Cyclopropane: An Orbital View
3-membered ring must have planar structure Symmetrical with CCC bond angles of 60 Requires that sp3 based bonds are bent (and
weakened) All C-H bonds are eclipsed
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Bent Bonds of Cyclopropane
Structural analysis of cyclopropane shows
that electron density of C-C bond is displaced outward from internuclear axis
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4.7 Conformations of Cyclobutane and Cyclopentane
Cyclobutane has less angle strain than cyclopropane
but more torsional strain because of its larger number of ring hydrogens Cyclobutane is slightly bent out of plane - one carbon atom is about 25 above
The bend increases angle strain but decreases torsional strain
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Cyclopentane
Planar cyclopentane would have no angle strain but
very 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 looks like an envelope
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4.8 Conformations of Cyclohexane
Substituted cyclohexanes occur widely in nature The cyclohexane ring is free of angle strain and
torsional strain The conformation is has alternating atoms in a common plane and tetrahedral angles between all This carbons is called a chair conformation
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How to Draw Cyclohexane
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4.9 Axial and Equatorial Bonds in Cyclohexane
The chair conformation
has two kinds of positions for substituents on the ring: axial positions and equatorial positions Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring
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Axial and Equatorial Positions
Each carbon atom in cyclohexane has one
axial and one equatorial hydrogen Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement
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Drawing the Axial and Equatorial Hydrogens
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4.10 Conformational Mobility of Cyclohexane
Chair conformations readily interconvert,
resulting in the exchange of axial and equatorial positions by a ring-flip
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Bromocyclohexane
When bromocyclohexane ring-flips the bromines
position goes from equatorial to axial and so on At room temperature the ring-flip is very fast and the structure is seen as the weighted average
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4.11 Conformations of Monosubstituted Cyclohexanes
The two conformers of a monosubstituted
cyclohexane are not equal in energy The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol
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Energy and Equilibrium
The relative
amounts of the two conformers depend on their difference in energy E = RT ln K R is the gas constant [8.315 J/ (Kmol)], T is the Kelvin temperature, and K is the equilibrium constant between isomers
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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 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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Relationship to Gauche Butane Interactions
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 gauche butane have the same steric interaction In general, equatorial positions give more stable isomer
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4.12 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 1,2dimethylcyclohexane. cis and trans In the cis isomer, both methyl groups 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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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 1,3diaxial 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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4.13 Boat Cyclohexane
Cyclohexane can also be in
a boat conformation Less stable than chair cyclohexane due to steric and torsional strain C-2, 3, 5, 6 are in a plane H on C-1 and C-4 approach each other closely enough to produce considerable steric strain Four eclipsed H-pairs on C2, 3, 5, 6 produce torsional strain ~29 kJ/mol (7.0 kcal/mol) less stable than chair
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4.14 Conformations of Polycyclic Molecules
Decalin consists of two cyclohexane rings joined to share two carbon
atoms (the bridgehead carbons, C1 and 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 chair cyclohexane conformations Flips and rotations do not interconvert cis and trans
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