a) Use Chem3D to compare the bond lengths of the C1-C2 double bond with the C3-C4 single bond. How do you account for this difference?

Transcription

1 Copyright 2002 Prentice-all, Inc. All rights reserved. Chapter 7 - Structure and Synthesis of Alkenes 1. In Chem3D, draw the structure of 1-pentene. a) Use Chem3D to compare the bond lengths of the C1-C2 double bond with the C3-C4 single bond. ow do you account for this difference? b) Use Chem3D to compare the bond lengths of the C3-C4 bond with the C2-C3 bond. ow do you account for this difference? (see Sect 9-5) c) Use Chem3D to compare the bond lengths of a vinyl C- bond with an alkyl C- bond in 1- pentene. ow do you account for this difference? (see Sect 9-5) 2. Problem 7-3 ChemDraw can perhaps help you in this problem, by allowing you the sequential formation of bonds that might result in isomers. Refer to Solved Problem 7-1: draw the carbon skeleton for the first structure shown an amide. The Cl can be placed at the 1st carbon or 3rd carbon from the end of the "lefthand" end of the chain. In the second structure, the Cl can be placed at any of the other carbon atoms (or even the nitrogen). Similarly, isomers can be generated from the third structure and fourth structure by replacing a with a Cl. Other skeletons to work with are C C-C-C-N, C-C C-C-N, C-C- C C-N, C-C-C-C N among many others. An O and a Cl can in principle replace any hydrogen in the chain. 3. Problem 7-8 (h) Draw Z- and E-cyclodecene in ChemDraw. Select the Cyclohexane Ring tool. Draw a six-membered ring by clicking in the document window. Point to the right hand vertical bond and click to deposit a second ring. Use the eraser tool to remove the bond common to the two rings. Use the Solid Bond tool to introduce a double bond. If the second bond is placed outside of the ring, click on it to place it on the other side of the existing bond. Make sure that the position of the ring bonds off of the double bond are appropriate for the stereoisomer that you are trying to draw. Save the drawings as cycloalkenes or an identifiable file. 4. Sect. 7-7C a) Use Chem3D to verify the steric energy differences between cis and trans-2-butene. Select the Text Building tool, click in the model window, and type "cis-2-butene" and press Enter or Return. From the MM2 menu, choose Compute Properties and click Run. Note the steric energy for the frame. Repeat for trans-2-butene. Which is more stable, cis or trans-2-butene? b) Compare the steric energies of: Z-2-pentene, Z-4-methyl-2-pentene, and Z-4,4-dimethyl-2-pentene. A fast way to build these models is to build from your cis or trans-2-butene models. Replace an atom using the Text Building tool. Click the hydrogen atom, type "C" in the text box, and press Enter

2 or Return. Additional methyl groups can be added by double-clicking on the appropriate hydrogen atom. Minimize the energy and examine the results. c) Compare the steric interactions of a methyl group with: another methyl group, an ethyl group, an isopropyl group, and a tert-butyl group (see a and b). 5. Examine the steric interactions in cis- and trans-2-butene (see problem 4) using space-filling models. Build the model as in 4 (a). From the View menu, choose Settings and Model Display. Under Model Type select Space Filling. Close the Settings window. Examine the model from various perspectives. Chem3D will show you the steric hindrance with the Show Close Contacts command in the Analyze menu. 6. Sect 7-7D In ChemDraw, draw cyclobutene. Use the Cyclobutane Ring tool to draw a four-membered ring and the Solid Bond tool to add a double bond. Select the cyclobutene, Copy and Paste into Chem3D. Minimize the energy. Select the four carbon atoms of the ring by shift-clicking. Use the Deviation from Plane command under the Analyze menu to verify that Chem3D has determined a planar structure. Use the X-axis and Y-axis Rotation Bars and the Trackball tool to view the model from various perspectives. What kinds of strain are present in the planar cyclobutene? 7. Sect 7-7D trans cycloalkenes a) Draw cis-cyclooctene in Chem3D. Determine the steric energy. Minimize the energy from the MM2 menu. The steric energy appears in the message window. b) Draw trans-cyclooctene in Chem3D. What is the steric energy? c) According to Chem3D, which is more stable, cis- or trans-cyclooctene? 8. a) Import your ChemDraw drawing of (E)-cyclodecene (see problem 3 above) into Chem3D. Determine the steric energy. Minimize the energy from the MM2 menu. The steric energy appears in the message window. b) Draw (Z)-cyclodecene (see problem 3 above). What is the steric energy? c) According to Chem3D, which is more stable, cis- or trans-cyclodecene?

3 9. Sect 7-7E Confirm that the "Bredt's Rule violation" structure shown is an unstable arrangement. In ChemDraw, make sure that Fixed Lengths is checked in the Tools menu. Use the Template tool. Click on the tool and a menu of templates appears. Click on the BICYCLIC template. The bicyclo[2.2.1]heptane ring system is in the upper right corner. Click on that square and the templates will disappear. Locate the crosshairs cursor where you want the drawing and click the mouse. The drawing will appear. The drawing may be selected, moved, resized, or duplicated as usual. Duplicate the bicyclo[2.2.1]heptane drawing by selecting and dragging while pressing the Ctrl key. Use the Solid Bond tool to insert a double bond at the bridgehead of one structure. Insert a double bond at the 2-position in the other structure. c b a d bicyclo[2.2.1]hept-1-ene bicyclo[2.2.1]hept-2-ene Import the structure with the bridgehead double bond (bicyclo[2.2.1]hept-1-ene) into Chem3D by selecting, copying, and pasting. Minimize the energy. Determine the steric energy.

4 Examine the substituent pattern around the double bond. Verify that the atoms in the sixmembered ring are trans to each other by measuring the distances between carbon atoms a and b compared to carbons c and b. Import the bicyclo[2.2.1]hept-2-ene into Chem3D, minimize, and determine the steric energy. Which structure is the more stable, and how do these results compare with "Bredt's rule"? 10. Problem 7-11 (d) Draw the molecule in ChemDraw, import into Chem3D, and minimize. Describe the shape. Is the molecule planar? 11. Sect 7-9A Bulky Base Draw the t-butoxide ion in Chem3D. From the View menu, choose Settings and Model Display. Under Model Type select Space Filling. Open a new window. Draw the methoxide ion. Use the Shrink Button to make the oxygen about the size of the oxygen in the t-butoxide model. (Use the Grow Button to adjust in the other direction). With the two model windows open side-by-side, rotate each to observe the steric bulk of the t- butoxide ion in comparison to the methoxide ion. Do you see what is meant by the term "bulky base"? Perform similar comparisons with diisopropylamine, triethylamine, 2,6-dimethylpyridine, and 2,2,6,6- tetramethylpipiridine. N 2,2,6,6-tetramethylpipiridine This last example is not in Wade, but provides a dramatically sterically hindered base. The lithium salts of diisopropylamine and 2,2,6,6-tetramethylpipiridine (formed by deprotonation with methyllithium or butyllithium) are examples of commonly used sterically hindered strong bases (see Wade, Sect 22-2B).

5 12. Problem 7-15 (a) Draw the reactant in ChemDraw. There is a template for the Fischer projection in the Template tool, CONFORM.CTP palette. Add the atom labels to complete the Fischer projection. Copy and Paste into Chem3D. Check the stereochemistry by rotating the model with the Trackball tool. Select the bond between the carbons bearing the bromines and look down this bond by choosing Move to Z-Axis in the Tools menu. Make sure that the bromine atoms are anti and coplanar. As the bromine atoms are eliminated, the two carbons flatten out until they are sp 2 hybridized. Note the relationship between the ethyl and methyl groups as the leaving groups leave. 13. Problem 7-18 (b) Draw the two bromodecalins in ChemDraw and paste into Chem3D. Recall that in an E2 elimination, the hydrogen atom and the leaving group must be in a trans-diaxial relationship. 14. Problem 7-19 Draw the structures in ChemDraw and paste into Chem3D. In (b) you will have to replace the hydrogen atom trans to the bromine with a D using the Text Building tool. Examine the models for a hydrogen trans to the leaving group; then verify that the hydrogen atom and the leaving group can become diaxial. 15. By recourse to reasonable mechanisms, predict the product(s) of the following reactions: Br (C 3 ) 3 C-O - C 3 3 C C 3 2 SO 4 O heat (4 products)