This Year's Exam #1

C3443x EXAM 1
Professor Horne
September 29, 1997
50 minutes

1. (5 pts. ea.) Provide IUPAC names for the following:

2. (5 pts. ea.) Modify the structures below to show multiple bonds, unshared pair of electrons (indicated by dots), and locations of formal charge. Draw only the major resonance form with complete octets.

3. (5 pts. ea.) In each of the following pairs of Kekulé structures indicate (by circling yes or no) whether or not they are valid resonance forms.

4. (5 pts. ea.) In each of the following groups, circle the molecule that is most acidic.

5. Consider butyric acid, CH3CH2CH2CO2H. The gauche conformer is destabilized by 0.7 kcal·mol-1 over the anti which is free of torsional and steric strain.

(a) (10 pts) Draw the relevant Newman projections of the gauche and anti conformations and label each conformer. The destabilization in gauche butyric acid is less than that for gauche butane. Explain (i.e. don't just say something is sterically larger/smaller you must explain why using no more than 15 words).

(b) (10 pts) By extending the conformational analysis of butanes to cyclohexanes as done in class, calculate the percent of each chair conformer of cyclohexanecarboxylic acid at room temperature (298 K). R = 1.98 cal mol-1 deg-1.

6. Cyclopropane possesses unusual reactivity due to strain. One such reaction is its hydrogenation to propane which is exothermic. Note that in this reaction 2H from H2 are added to two adjacent ring carbons while breaking the C-C bond.

(a) (15 pts) Calculate the dissociation enthalpy (DH°) for C-C bond homolysis in cyclopropane as shown below.

BOND DISSOCIATION ENERGIES (DH°)

(b) (10 pts) Hydrogenation of 1,2-dimethylcyclopropane (cis and trans) gives two C5H12 isomers, n-pentane and 2-methylbutane as products. Of the two isomers, predict which one would predominate and whether the corresponding reaction would proceed faster, slower, or the same compared to the hydrogenation of cyclopropane to propane. Explain using structures and no more than a few sentences.