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Define conformers and it's uses and the stability factor in breif .
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Answers
Explanation:
Conformations of alkanes
Alkanes are capable of several different conformational isomers; isomers where the atoms and connectivity remains the same, but can be interconverted to one another by rotation around a single bond.
For our first example, let us look at the most simple example, ethane.
Side View
Side View Web MO
Newman Newman WebMO
Staggered
(0.00 kcal/mol)
Eclipsed
(2.66 kcal/mol)
Calculations done at B3LYP/6-311+G(2d,p). Click any image above to see the optimized structure.
IMPORTANT: Before moving on, take some time and click on one of the pictures above to go to the appropriate WebMO output. While there, rotate the 3D model around till you can line it up in both the side view and newman projection viewpoint. Being able to mentally picture these rotations in your head can make stereochemistry in later chapters much, much easier. So before moving on, make sure you understand how each of the four images in each row are the same isomer drawn in different ways. If done correctly, you should end up doing something like this:
Shown in the table above, for ethane there are two different isomers, staggered and eclipsed. In the staggered conformation, the hydrogen atoms are lined up to be equally spaced apart in the newman projection. For the eclipsed conformation, each hydrogen on the front carbon overlaps with the hydrogens on the back carbon.
In terms of stability, the staggered conformation is more stable than the eclipses. This is for two reasons:
1) Steric hindrance. In the eclipsed conformation, the positioning of the atoms forces them closer together, increasing the amount of steric strain in the molecule.
2) Stabilization of the staggered conformation by hyperconjugation.
Using WebMO, we can have the program perform a scan along a coordinate. This essentially picks one parameter in the molecule and changes it in a series of steps, and changes how the energy of the molecule changes as a result. In this case we will scan the dihedral angle between two hydrogens on different carbons.
Coordinate scan done at B3LYP/6-311+G(2d,p). Click the image above to see the WebMO output.
The result of the scan is the plot shown above. Starting as a dihedral angle of 0 in the middle of the plot represents an eclipsed conformation. Here we can see that the eclipsed form is a maximum on the potential energy surface, a peak among the hills and valleys of the plot. This means that the eclipses formation is a transition state, meaning ethane never actually exists in this conformation. The minima on the plot at 60 and 180 degrees are staggered conformation. As discussed earlier, the staggered conformation is most stable conformation, so it appears lower on the potential energy surface. Remember, lower energy (a more negative value on the y-axis) correlates to greater stability.
Conformations of butane
With the basics down, let us look at a slightly more complex example, butane. For this exercise, you can think of butane as ethane with two methyl groups attached the end. These additions give rise two new types of conformations not seen in ethane.
Side View Side View WebMO Newman Newman WebMO
Eclipsed Syn
(5.68 kcal/mol)
Staggered Gauche
(0.95 kcal/mol)
Eclipsed
(3.23 kcal/mol)
Staggered Anti
(0.00 kcal/mol)
Calculations done at B3LYP/6-311+G(2d,p). Click any image above to see the optimized structure.
Using WebMO, the dihedral angle between the terminal carbons can be scanned and the plot of the potential energy surface can be made.
Coordinate scan done at B3LYP/6-311+G(2d,p). Click the image above to see the WebMO output.
Starting at a dihedral angle of zero, the two methyl groups are overlapping in the syn conformation. This is the highest energy conformation the methyl groups are larger than hydrogen, so overlapping them causes more steric strain than any other conformation.
From out computational results, rotating one of the methyl groups 65.738 degrees moves us right along the x-axis to the gauche conformation. Here nothing is overlapping which makes it much more stable in comparison to eclipsed syn, however the methyl groups are still close enough to cause a steric interaction. This steric strain forces the gauche conformation slightly past sixty degrees like in our ethane example, as pushing those bulkier groups just a little bit farther apart stabilizes the molecule.
One more approximately sixty degree increment yields the eclipsed conformation. The overlapping groups causes steric strain and makes it higher in energy, though it is not as high in energy as the syn conformation.
Finally one more rotation to 180 degrees gives the lowest energy conformation, the anti conformation. Placing the largest groups as far apart as possible minimizing the steric strain.
