#3: Conformations and cis-trans Stereoisomers Flashcards
Conformations
Different spatial arrangements of a molecule that are generated by rotation about single bonds.
Conformational Analysis
Study of how conformational factors affect the structure of a molecule and its properties.
Staggered Conformation
Conformation of the type shown, in which the bonds on adjacent carbons are as far away from one another as possible.
Eclipsed Conformation
Conformation in which bonds on adjacent atoms are aligned with one another.
Wedge and Dash
A way to show conformations. Wedges between atoms represent atoms pointing away from you, and dashes represent atoms pointing towards you.
Sawhorse
Shows the conformation of a molecule without having to resort to different styles of bonds.
Newman Projection
We write down the C-C bond, and represent 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.
Torsion or Dihedral Angle
The angle between C-H bonds of adjacent carbons.
Eclipsed bonds are characterized by a torsion angle of 0 degrees. When 60 degrees, the spatial relationship is gauche. When it is 180, it is anti. Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms.
Ethane Conformation
Of the two conformations of ethane, the staggered is 12 kJ/mol (2.9 kcal/mol) more stable than the eclipsed. The staggered conformation is the most stable conformation, the eclipsed is the least stable conformation.
Staggered vs. Eclipsed Conformation
Staggered is more stable than eclipsed.
One reason is that the repulsions between bonds on adjacent atoms destabilze the eclipsed conformation.
Another reason is that better electron delocalization stabilizes the staggered conformation.
Only the second one is truly believed to be correct.
Torsional Strain
Conformations in which the torsion angles between adjacent bonds are other than 60 degrees are said to have torsional strain.
Eclipsed bonds produce the most torsional strain; staggered bonds none.
Steric Strain
Additional sources of strain in molecules combined with torsional strain.
When Molecules Are in Staggered/Eclipsed Conformation
At any instant, almost all of the molecules are in staggered conformations; hardly any are in eclipsed conformations.
Staggered vs. Eclipsed in terms of Potential Energy
Eclipsed conformations occur when the potential energy is at a maximum, staggered when potential energy is minimum.
Activation Energy
The conversion of one staggered conformation of ethane at 60 degrees to another at 180 requires energy to pass through the eclipsed conformation at 120 degrees. This amount of energy is the activation energy.
Molecules must become energized in order to undergo a chemical reaction, or in this case, to undergo rotation around a carbon-carbon bond.
Transition State
The point of maximum potential energy encountered by the reactants as they proceed to products.
Half-Life
Length of time it takes for one half of the molecules to have reacted. It takes less than 10^-6 seconds for half of the molecules in a sample of ethane to have gone from one staggered conformation to another at 25 degrees Celsius.
Rate of Rotation and Temperature
The rate of rotation about the carbon-carbon bond increases with temperature.
Conformational Analysis of Butane
Consider conformations related by rotation about the bond between the middle two carbons (CH3CH2-CH2CH3). Unlike ethane, in which the staggered conformations are equivalent, two different staggered conformations occur in butane. The methyl groups are gauche to each other in one, anti in the other. Both conformations are staggered, so are free of torsional strain, but two of the methyl hydrogens of the gauche conformation lie within 210 pm of each other. This distance is less than the sum of their van der Waals radii, and there is a repulsive force between them (Steric Hindrance).
At any instant, almost all the molecules exist in staggered conformations, and more are present in the anti conformation than in the gauche. The total strain in this structure is approximately equally divided between the torsional strain associated with three pairs of eclipsed bonds and the van der Waals strain between the eclipsed methyl groups.
Steric Hindrance
An effect on the structure or reactivity that depends on van der Waals repulsive forces.
Also called Van der Waals strain. Destabilization that results when two atoms or groups approach each other at distances less than the sum of their van der Waals radii.
Conformations of Higher Alkanes
Higher alkanes having unbranched carbon chains are, like butane, most stable in their all-anti conformations. The energy difference between gauche and anti conformations is similar to that of butane, and appreciable quantities of the gauche conformation are present in liquid alkanes at 25 degrees Celsius.
In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane have their carbon “backbones” arranged in a zigzag fashion. All bonds are staggered, and the chains are characterized by anti arrangements of C-C-C-C units.
Angle Strain
The strain a molecule has because one or more of its bond angles deviate from the ideal value; in the case of alkanes, the ideal value is 109.5 degrees.
Cycloalkanes: Planar of Nonplanar?
During the 19th century it was widely believed, incorrectly, that cycloalkane rings are planar. A leading advocate of this view was Adolf von Baeyer. He noted that compounds containing rings other than those based on cyclopentane and cyclohexane were rarely encountered naturally and were difficult to synthesize. Baeyer connected both observations with cycloalkane stability, which he suggested was related to how closely the internal angles of planar rings matched the tetrahedeal value of 109.5 For example, the 60 degree bond angle of cyclopropane and the 90 degree bond angles of a planar cyclobutane ring are much smaller than the tetrahedeal angle of 109.5. Baeyer suggested that 3- and 4-membered rings suffer from angle strain.
According to Baeyer, cyclopentane should be the most stable of all cycloalkanes because the ring angles of a planar pentagon, 108, are closer to the tetrahedeal angle than those of any other cycloalkane. A prediction of the Baeyer strain theory is that the cycloalkanes beyond cyclopentane should become increasingly strained and correspondingly less stable. The angles of a regular hexagon are 120, and the angles of larger polygons deviate more and more from the ideal tetrahedral angle.
Problems with the theory become apparent when we use heats of combustion to probe the relative energies of cycloalkanes. The most important thing is the heat of combustion per methylene group. Because all of the cycloalaknes have molecular formulas of the type CnH2n, dividing the heat of combustion by n allows direct comparison of ring size and potential energy. Cyclopropane has the highest heat of combustion per methylene group, which is consistent with the idea that its potential energy is raised by angle strain. Cyclobutane has less angle strain at each of its carbon atoms and a lower heat of combustion per methylene group. Cyclopentane, as expected, has a lower value still. Notice, however, that contrary to the prediction of the Baeyer strain theory, cyclohexane has a smaller heat of combustion per methylene group than cyclopentane. If angle strain were greater in cyclohexane than in cyclopentane, the opposite would have been observed.
Furthermore, the heats of combustion per methylene group of the very large rings are all about the same and similar to that of cyclohexane. Rather than rising because of increasing angle strain in large rings, the heat of combustion per methylene group remains constant at approximately 653 kJ/mole (156 kcal/mol), the difference between successive members of a homologous series of alkanes. We conclude, therefore, that the bond angles of large cycloalkanes aren’t much different from the bond angles of alkanes themselves. The prediction of the Baeyer strain theory that angle strain increases steadily with ring size is contradicted by experimental fact.
The Baeyer strain theory is useful to us in identifying angle strain as a destabilizing effect. Its fundamental flaw is its assumption that the rings of cycloalkanes are planar. With the exception of cyclopropane, cycloalkanes are nonplanar.
Axial
One of the most important findings to come from conformational studies of cyclohexane is that its 12 hydrogen atoms can be divided into two groups.
Six of them form the axial hydrogens. They have their bonds parallel to a vertical axis that passes through the ring’s center. These axial bonds alternately are directed up and down on adjacent carbons.
Equatorial
One of the most important findings to come from conformational studies of cyclohexane is that its 12 hydrogen atoms can be divided into two groups.
Six of them are equatorial hydrogens. They’re located approximately along the equator of the molecule. The four bonds to each carbon are arranged tetrahedrally, consistent with an sp3 hybridization of carbon.
Ring Inversion (Chair-Chair Interconversion)
Alkanes are not locked into a single conformation. Rotation around the central carbon-carbon bond in butane occurs rapidly, interconverting anti and gauche conformations. Cyclohexane, too, is conformationally mobile. Through a process known as ring inversion, or chair-chair interconversion, one chair conversion is converted to another chair.
In the first step, the chair conformation is converted to a skew boat. In this step, cyclohexane passes through a higher-energy half-chair conformation. The skew boat is converted to an alternate skew boat, via the boat conformation. The second skew boat then proceeds to the inverted chair via another half-chair conformation. The skew boat conformations are intermediates in the process of ring inversion. The half chair conformations are highest in energy because they have the most eclipsing interactions. The difference in energy between the chair and half-chair conformations is the activation energy for the chair-chair interconversion, which is 45 kJ/mol (10.8 kcal/mol).. It’s a very rapid process with a half-life of 10^-5 sec at 25 degrees Celsius.
The most important result of ring inversion is that any substituent that is axial in the original chair conformation becomes equatorial in the ring-inverted form and vice versa.
Intermediate
A local minimum on the potential energy profile.
Conformational Analysis of Monosubstituted Cyclohexanes
Ring inversion in methylcyclohexane differs from that of cyclohexane in that the two chair conformations are not equivalent. In one chair the methyl group is axial; in the other it is equatorial. At room temperature 95% of the molecules of methylcyclohexane are in the chair conformation that has an equatorial methyl group, whereas only 5% of the molecules have an axial methyl group.
When two conformations of a molecule are in equilibrium with each other, the one with the lower energy predominates. Why is equatorial methylcyclohexane more stable than axial methylcyclohexane?
A methyl group is less crowded equatorial than axial. The distance between hydrogens in axial methyl group is less than the sum of the van der Waals radii of two hydrogens, causing strain. Nothing of the sort occurs in equatorial. An example of a steric effect. An axial substituent is said to be crowded because of 1,3-diaxial repulsions between itself and the other two axial substituents located on the same side of the ring.
cis-trans Steroisomers
When a cycloalkane bears two substituents on different carbons–methyl groups, for example—these substituents may be on the same or on opposite sides of the ring. When substituents are on the same side, we say they are cis to each other; if they’re on opposite sides, they’re trans to each other.
Stereoisomers
Isomers that have their atoms bonded in the same order–that is, they have the same constitution, but they differ in the arrangement of atoms in space. Stereoisomers of the cis–trans type are sometimes referred to as geometric isomers.
cis-trans Stereoisomers Stability with Cyclohexane
trans still comes out as more stable, but it’s unrealistic to believe it is due to van der Waals strain between cis substituents. This is due to the fact that the methyl groups are too far away from each other. To understand why trans is more stable, we need to examine each sterioisomer in its most stable comformation.
cis can adopt either of two equivalent chair conformations, each having one axial methyl group and one equatorial methyl group. The two are in rapid equilibrium with each other by ring interconversion. The equatorial methyl group becomes axial and the axial methyl group becomes equatorial.
The methyl groups are described as cis because both are up relative to the hydrogen present at each carbon. If both methyl groups were down, they would still be cis to each other. Notice that ring inversion does not alter the cis relationship between the methyl groups. Nor does it alter their up vs. down quality; substituents that are up in one conformation remain up in the ring inverted form.
The most stable conformations of trans has both methyl groups in equatorial orientations. The two chair conformations of trans are not equivalent. One has two equatorial methyl groups; the other, two axial methyl groups.
The more stable chair is the one with both equatorial methyl groups.
There is sometimes exceptions that puts cis as the more stable conformation. 1,3-dimethylhexane specifically. The most stable conformation of it has both methyl groups equatorial. Both trans conformations have one axial and one equatorial methyl group.
If a distributed cyclohexane has two different substituents, then the most stable conformation is the chair that has the larger substituent in an equatorial orientation. This is most apparent when one of the substituents is a bulky group such as tert-butyl. Thus, the most stable conformation of cis-1-tert-butyl-2-methylcyclohexane has an equatorial tery-butyl group and an axial methyl group.
