EXAM 1

Comprehensive Organic Chemistry

CHEMISTRY 225

Friday, February 15, 2008

NAME (print clearly): ___________________________________________________

TA:_________________ Sect. Day:______________ Sect. Time:_________________

Take a few moments to look over the exam. Answer each question on the exam paper.

No calculators, texts, notes, or laptops. Important clues and structures are in bold.

Do all preliminary drawing or computations on the work sheets at the end of the exam. The work sheets will not be graded.

The exam is 55 minutes.

STOP writing and hand in your exam when you are asked to do so.

Remember: Neatness is to your advantage.

1. (20pts) Conformation I ______

2. (20 pts) Potpourri II ______

3. (20 pts) Resonance (Do 1 of 3) ______

4. (20 pts) Conformation II ______

5. (20 pts) Thermochemistry ______

______________________________________

Total (100 pts)

1. Conformation I: (20 pts) Draw the most stable Newman projection conformations of 2,3-dimethylbutane viewed along the C2-C3 sigma bond. Place the appropriate energies in the Newman projections below. [Use the circles as templates for the Newman projections.] Calculate the energy (kcal/mol) of both conformations. Place your answers in the appropriate boxes. Show work. [H/H, eclipsed, 1.0 kcal/mol; CH3/H eclipsed, 1.3 kcal/mol; C2H5/H, eclipsed, 1.4 kcal/mol; CH3/CH3, eclipsed, 3.0 kcal/mol; CH3/CH3, gauche, 0.9 kcal/mol; CH3/C2H5, gauche, 1.0 kcal/mol.] Show your work!

2. Potpourri: (30 pts.; equal weight) Circle the best answer(s) in each of the following:

a) 2-Methylpentane and 2,3-dimethylbutane have a difference of 0.8 kcal/mol in their heats of formation. What is the difference in their heats of combustion in kcal/mol?

0 0.8 -5 5 157 -157

b) The heat of formation of 2-methylpentane is -41.7 kcal/mol (previous question). What is the expected heat of formation of 2-methylhexane? Show work.

c) Circle the acids that are readily deprotonated by NaOCH₃.

NH₃ cyclopropane HCCH NH₄⁺ CH₃CH₂CO₂H

d) Circle the species with sp² hybridization

NH₄⁺ ethylene BF₄^- ethane (CH₃)₂CO

e) Circle the compounds with net dipole moments.

3. Resonance: (20 pts.) The concept of resonance plays an important role in organic chemistry. Explain and illustrate the role of resonance in one of the following cases. Use orbitals in your explanations.

a) The higher pKa value of peracid RCO₃H vs. carboxylic acid RCO₂H

b) The lower bond dissociation energy of the sp³ C-H bond in propene compared with the same bond in propane.

c) Relative stabilization of the carbocation RCHOCH₃ vs. RCHCl

4. Conformation II: (20 pts.) Vinyl compound A, C₁₀H₁₈, absorbs a maximum of one mole of hydrogen to form a 1,4-disubstituted cyclohexane B that has K_eq=0 kcal/mol for the equilibrium between its chair conformations.

a) (12 pts.) What are the structures of A and B? Explain briefly.

b) (8 pts.) Place the substituents for A in their appropriate locations in the chair equilibrium shown below.

5. Thermochemistry: (20 pts) Alkane A, C₆H₁₂ gives a single free radical monochlorination product B. No other monochlorinated products are possible. The overall heat of the reaction is DH^o_rxn = -30 kcal/mol. BDEs: Cl₂ = 58 kcal/mol; HCl = 103 kcal/mol; R-H: (primary) = 98 kcal/mol, (secondary) = 95 kcal/mol, (tertiary) = 91 kcal/mol.

a) What are the structures and names of alkanes A and B?

b) Show the propagation steps for this reaction.

c) Calculate the heat of each propagation step and the BDE of R-Cl. Illustrate and show work.
Work Sheets
Work Sheets
Work Sheets