Class Activity 5A. Conformations of Alkanes Part A: Acyclic Compounds

Transcription

1 Class Activity 5a Conformations of Alkanes Part A: Acyclic Compounds 1 Model 1: Isomers Class Activity 5A Conformations of Alkanes Part A: Acyclic Compounds C C O C C C C C C C O O A B C wedge, bond coming out of page (up) dash, bond going into page (back) Constitutional Isomers: atoms are connected differently Conformational Isomers: atoms are connected the same, but differ in the rotation around single bonds. 1. (a). For the structures in Model 1, write down the order of atom connectivity for compounds B and C (ignore the hydrogens). Compound A is given as an example. A: C C C O B: C: (b). Is the order of connectivity the same for structures A and B? (circle) yes / no (c). Is the order of connectivity the same for structures B and C? (circle) yes / no (d). Is the order of connectivity the same for structures A and C? (circle) yes / no (e). Using the definition of constitutional isomers, indicate which of the pairs below are constitutional isomers. (circle) A and B B and C A and C 2. (a). In Model 1, what does a wedge represent? What does a dash represent? (b). Compare structures A and C. Are the atoms connected in the same order? (c). What is different between structures A and C? (d). Based on the definitions in Model 1, would A and C be constitutional or conformational isomers? Make sure everyone in your group agrees before moving on.

2 Class Activity 5a Conformations of Alkanes Part A: Acyclic Compounds 2 Model 2: Conformations of Ethane 3. In the wedge dash representation of ethane, how are the hydrogens drawn to show their spatial orientation? 4. Envision the wedge dash model where the C-C bond is in the plane of your paper (use a pen to simulate the bond). Visually rotate the whole molecule by 90 o, so that the C-C bond is now perpendicular to the paper. This is the sawhorse representation. On the sawhorse model above, write an F near the front carbon and a B near the back carbon. Circle the bond that was rotated by The Newman projection is the same as a sawhorse projection that has just been compressed along the C-C bond. ow is the carbon in the front represented in a Newman projection? ow is the carbon in the back represented? 6. (a). From Model 2, compare the two conformations of ethane, staggered and eclipsed. What is different between these conformations? (b). Once your group agrees on the above questions, discuss which conformation would be the most stable. Explain your reasoning. (c). Describe why the names staggered and eclipsed represent the forms shown.

3 Class Activity 5a Conformations of Alkanes Part A: Acyclic Compounds 3 Model 3: Newman Projections Newman projections clearly show the dihedral angle,, which defines the difference between the staggered and eclipsed conformations. = a b = a b = 0 o c staggered gauche = 60 o anti = 180 o eclipsed = 0 o C C dihedral angle, 7. Using the information in Model 3, identify the dihedral angle for the following: Staggered gauche = Staggered anti = Eclipsed = 8. In the staggered form of Model 3: (a). What is the dihedral angle between a and b (60 o or 180 o )? Is this gauche or anti? (b). What is the dihedral angle between a and c (60 o or 180 o )? Is this gauche or anti? 9. (a). A Newman projection is draw by looking straight down a particular C C bond. The carbon furthest away is drawn as a large circle, while the carbon closest is represented as a small circle or the center of the Y shape. Draw a Newman projection of propane as viewed along the C1-C2 bond. (INT: draw C1 in front, C2 in back, then add all substituents). (b). Once your group agrees on the Newman projection drawing of propane, draw both the staggered and eclipsed forms of the Newman projection above. Circle the form that is most stable.

4 Class Activity 5a Conformations of Alkanes Part A: Acyclic Compounds 4 Model 4: Potential Energy diagram for Rotation about the C C bond of Ethane Different conformations of ethane have different energy, as shown in the energy diagram below. 10. Consider the energy diagram shown in Model 4. (a). What is the dihedral angle between the two hydrogens (in blue) shown in the Newman projection on the bottom left of the diagram? Looking at the second structure, which carbon was rotated to form the dihedral angle of 60 o (circle) front / back. As you continue to rotate to change the dihedral angle, what do you notice about the conformations? (b). Do the Newman projections with a dihedral angle of 0 and 120 o have the same energy? If all hydrogens are equivalent, would you be able to distinguish between these two forms? (c). Do the Newman projections with a dihedral angle of 120 o and 180 o have the same energy? If all hydrogens are equivalent, would you be able to distinguish between these two forms? 11. Draw a staggered Newman projection for butane (four carbons) as viewed along the C1-C2 bond AND a staggered projection as viewed along the C2-C3 bond (refer to 9a for drawing Newman projections). Do these two structures look the same? 12. (a). Draw Newman projections for butane as viewed along the C2-C3 bond with dihedral angles between the two methyl groups of 0 o, 60 o, 120 o and 180 o. (b). Identify which of the above Newman projections are staggered and which are eclipsed. (c). Once your group has reached agreement on the Newman projections, discuss whether you would expect all the staggered forms to have the same energy? Explain.

5 Class Activity 5a Conformations of Alkanes Part A: Acyclic Compounds 5 (d). Would you expect all the eclipsed forms to have the same energy? Explain. (e). Which Newman projection would be the most stable, if any? Explain. Reflection: on a separate sheet of paper. As a group, describe three concepts your group has learned from this activity and the one most important unanswered question about this activity that remains with your group. Place this in your group folder before leaving class. Additional 13. Draw a Newman projection of the most stable conformational isomer of 2,4-dimethylhexane as viewed along the C3-C4 bond. 14. Convert the following compound into a Newman projection as viewed along the C2-C3 bond. Draw the most stable conformation. 15. Convert the following Newman projections into line-angle formulas. (a). C 3 C 3 3 C C 2 C 2 C 3 (b). C 3 (c). 3 C C 2 C 3 C 3 C 3 C 2 C 3 C 3