EES 4200 Worksheet 9 February 26, 2002 TOPICS: Conformations of Alkanes (continued), Nomenclature of Cycloalkanes, and Conformations of Cycloalkanes 1. Draw the sawhorse representations for the following compounds: a. hexane b. isohexane c. 3-tert-butylhexane a. hexane H H H H H H H H H H H H H H b. isohexane H3C H H H H H H H H + H H C H H c. 3-tert-butylhexane H H H H H H H H3C H H H3C CH3 H H H H 2. Which of the pairs of compounds given below has the higher steric strain? a. pentane/hexane b. cyclopropane/propane c. 1,2-dimethylcyclopropane/cyclopropane a. Hexane (bulkier compound) b. Cyclopropane (atoms are closer together in the cyclical form) c. 1,2-dimethylcyclopropane (bulkier compound) 3. Cycloalkanes have an additional strain that alkanes do not have. What is this called and what causes it? Angular strain-the bond angle is smaller than that of the normal sp3 hybridization state (109.5º) 4. True/False There is only one conformation of cyclopropane. True 5. Cyclobutane prefers a conformation where 1 C atom(s) is/are out of plane of the other 3 C atom(s). This relieves torsional strain but compromises the angular strain. 6. Cyclopentane prefers the envelope shape conformation. The dynamic movement of C atoms when they respond to the movement of other atoms in the ring is known as pseudorotation. 7. There are two conformations of lower energy for cyclohexane. Name them. Which has the lower energy and why? Explain your answer using drawings. “Chair” conformation “Boat” conformation The chair conformation is much more stable than the boat conformation because the non-planar carbons (the ones on each end) are in different planes from each other decreasing torsional and steric strains. 8. The action of cyclohexane’s carbons twisting up and down spontaneously is known as ring inversion, chair-chair interconversion, or ring flipping. An axial carbon becomes an equatorial carbon after this occurs. 9. Which of the following substituted cyclohexanes is more stable and why? CH3 CH3 The second substituted cyclohexane is more stable because the ethyl group is in the equatorial position (less torsional strain).
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