Exam Questions in Organic Chemistry: Cyclopentadiene and Cyclohexanone, Exams of Chemistry

Download Exam Questions in Organic Chemistry: Cyclopentadiene and Cyclohexanone and more Exams Chemistry in PDF only on Docsity! Page 1 id#___________________ CHE538 Exam 2, Principles of Organic Chemistry Fall 2005 Warning: write/print neatly! If I can’t read it you don’t get credit. 1. Think about the cyclopentadieyl anion in this problem. a. (10 pts.) What are the energies in terms of resonance stabilization (beta) of the five Huckel MOs of this molecule? Go with Frost. b. (10 pts.) The pKa of cyclopentadiene is 15 whereas the pKa of 1,4- pentadiene is 30. Please explain this observation. How is pKa related to energy? _______________________________________________________________________ ____________________________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ c. (15 pts.) Use reflection plane symmetry to simplify the 5x5 secular determinant of C5H5(-) by writing out a symmetric and a dissymmetric secular determinant. Redraw the structure and number the atoms so I can follow your work. You don’t need to solve the determinants. _____________________________________________________________________ __________________________________________________________________________________ C H Page 2 id#___________________ d. (10 pts.) Guess at what the HOMO(s) and the lowest-energy pi MO of C5H5(-) look like. I would like you to draw 3 MOs in the space below. Let’s look at them from above; just draw circles on a pentagon instead of 2-lobed p orbitals. 2. Consider cyclohexanone below. In solution phase the molecule undergoes chair to chair conformational change. The energy barrier between the two chair conformers is about 10.5 Kcal/ mol. O O O a. (10 pts.) What is the point group of the chair cyclohexanone? What is the point group of C5H5(-)? ___________________________________ ________________________________ b. (10 pts.) The absorbance of the carbonyl is at 1716 cm-1. How many times does the carbonyl vibrate before the molecule changes conformation? _______________________________________________________________________ __________________________________________________________________________________ _______________________________________________________________________ c. (10 pts.) Even though the structure is dynamically flat, is it reasonable to think that the transition state of the conformational change is flat. Why or why not? Hint: the twisted boat structure at right is a stationary state on the path of conformational change. ______________________________________________________________________ __________________________________________________________________________________ _______________________________________________________________________ O