#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.