The purpose of this exercise is to investigate the observed predominance of the

Question

The purpose of this exercise is to investigate the observed predominance of the anti-periplanar el of the trans alkene product using molecular-modeling techniques lanar elimination and COMMENTS The E2 reaction is considered to be controlled by kinetics rather than by thermodynamics, because the reaction is irreversible under standard conditions. Kinetic control means that the products that are formed fastest wil be present in the greatest amount. Product stability (a thermodynamic property) will have no direct bearing on the product distribution. Instead, the differences in activation energies for the various reaction pathways would allow us to predict the product distribution. Basic molecular-modeling techniques do not allow us to estimate the energy of the transition states; however, we can estimate the relative amounts of the "starting materials (the different conformations of the starting material). If there is little difference in activation energy of the various pathways, then the relative amounts of the various conformations will give us a good ap of the product distribution. If, on the other hand, there is a large diflerence in activation energy of the various pathways, then the energy differences of the conformers may not accurately reflect the ratio of products. INSTRUCTIONS l. Draw the three alkenes (1-butene and cis/trans 2-butene) in the ChemDraw Window, and minimize Chem3D. Record the minimized energies. Dipoie/Dipoie o.19 4 using Chem3D. Record the minimized energies. Dipoie /Dipoie o0.19 4 2. Draw 2-bromobutane in the ChemDraw Window, ar ane inths ChemDrw All atonetime- i nhema eorgthe miendte mec 2brono ti gesrnecuea uth 2oond t tie nl fal3d0h pased. Draw all of the Newman projections (note that ChemDraw includes templates for both staggered and 3. Rotate your 2-bromobutane structure about the C2-C3 pond, 60P at a time, until a eclipsed Newmans), and record the corresponding energies How to Draw in the ChemDraw Window Click on the Solid Bond tool in the toolbar (it looks like a straight line). Then click in the drawing space. You will get a single bond. To lengthen the chain, put the Solid Bond tool at the end of that single bond, you should see a "hit box" (on my computer it shows up as green). With the hit box active, click another bond off of the first one, giving you three carbons total. Do that one more time, and you have (on my computer it shows up as green). With the hit box active, click again. You should sprout butane. To go from a single to a double bond, put the Solid Bond tool over the middle of the bond in question. You should see a rectangular hit box appear. Click once, and it will turn into a double bond. Use this method to draw your three alkenes. To draw 2-bromobutane, use the same method to draw butane, and then sprout a branch off of carbon 2 using the Solid Bond tool. To convert the carbon to bromine, put the Solid Bond tool over the end of the bond (hit box active), and press the B key. This is the shortcut for putting in bromine. (There are other shortcuts: O, P.S, L are some of the common ones. Feel free to experiment.)

Explanation / Answer

1. Butene isomers formed in the reaction by the E2 elimination of 2-bromobutane are drawn below. We get three isomers cis-butene, trans-butene and terminal 1-butene products.

2. The structure of 2-bromobutane is drawn below

3. The newmann projections for all possible rotations are shown below

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