Exam_1__Key - CHEM 12ATrego Fall 2008 Exam 1 Name 1 Which...

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CHEM 12A—Trego Fall 2008 Exam 1 Name: ___________________________________ 1. Which of the following carbanions is more stable? Justify your response with resonance structures, and identify major and minor contributors (if appropriate). N N O O O O Equal Contributors Minor Major Contributor (charge delocalized on to O) pK a =43 pK a =8.6 The nitroethane carbanion is more stable. It has resonance stability from delocalizing charge on to a more electronegative element. 2. The cyclohexyl cation forms 50,000 times faster than the cyclopropyl cation. Explain this remarkable difference in rate. (Hint: a carbocation, like a carbon radical is sp 2 hybridized.) OTs OTs Acetic Acid Acetic Acid Formation of the carbocation entails a change in hybridization from sp 3 to sp 2 , and change in bond angle from 109.5° to 120°. The cyclopropane ring is highly strained with all sp 3 carbons—the ideal bond angle is constrained towards 60°. The need for the carbocation to expand the bond angle toward 120° only leads to more ring strain and increases the energy of an already endothermic step.. The cyclohexane ring can readily expand a bond angle to 120°.
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