CHM 203 - Exam 1 - 2005 Key

CHM 203 - Exam 1 - 2005 Key - Chemistry 203, 2005...

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- 1 - Chemistry 203, 2005 Preliminary Exam #1 Answers 1. (25 points) Ammonium cyanate and ammonium isocyanate played an important role in the discovery of isomerism by chemists Justus Liebig and Friedrich Wöhler, who demonstrated that the two salts shared the same elemental formula but had quite different chemical properties. The salts both contain anions with only three atoms (C, N, and O). It was subsequently shown that the cyanate anion has the connectivity O-C-N and that the isocyanate anion has the connectivity O-N-C. (a) (10 points) Draw Kekulé structures for the cyanate and isocyanate anions below. Clearly indicate all nonbonding pairs of electrons and all formal atomic charges in your structures. O C N O C N OR O N C ______________________________ ______________________________ cyanate anion isocyanate anion (b) (10 points) The cyanate anion has two relatively important resonance structures. Using Kekulé structures, draw the two resonance structures below. Clearly indicate all nonbonding pairs of electrons and all formal atomic charges in your structures. O C N O C N (c) (5 points) Draw a circle around the resonance structures shown in part (b) that you expect to be the more important resonance contributor (i.e. more stable structure) to the overall electronic structure of the cyanate anion. Clearly explain the chemical reasoning behind your choice. Both resonance structures have the same number of bonds (four). A main difference between the resonance structures is the localization of one of the nonbonding pairs of electrons on oxygen vs. nitrogen. The left-hand resonance structure places the
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- 2 - electron pair on the more electronegative oxygen atom, resulting in its greater stability and, therefore, making it the more important resonance structure. 2. (33 points) Two eclipsed conformational isomers of n -butane ( 1 and 2) , which are transition states for interconversion of staggered ground states, are shown below. Experiments show that 2 is more stable than 1 by 1.3 kcal/mol. CH
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CHM 203 - Exam 1 - 2005 Key - Chemistry 203, 2005...

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