EXAM 1

CHEMISTRY 220a

Friday, September 25, 2009

NAME (print): ________________________________________________________

TA:_________________ Sect. Day:_________________ Sect. Time:_____________

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

No calculators. You may use molecular models. 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. (20 pts) Conformation I ______

2. (25 pts) Potpourri ______

3. (20 pts) Thermochemistry ______

4. (15 pts) Bonding/Orbitals ______

5. (20 pts) Conformation II ______

______________________________________

Total (100 pts)

1. Conformation I: (20 pts) For the eclipsed and staggered conformations of 2,3-dimethybutane viewed along the C2-C3 sigma bond, draw a Newman projection of the most stable eclipsed and staggered conformations. Place the energies for each interaction in the Newman projections below. [Use the circles as templates for the Newman projections.] Calculate the total energy (kcal/mol) of both conformations. Place your answer in the appropriate box. [H/H, eclipsed, 1.0 kcal/mol; CH3/H eclipsed, 1.3 kcal/mol; CH3/CH3, eclipsed, 3.0 kcal/mol; CH3/CH3, gauche, 0.9 kcal/mol.] Show your work!

2. Potpourri: (25 pts.; equal weight) Circle the best answer(s) where applicable in each of the following:

a) The heat of formation (DH_f^o) of 2-methylpentane is -41.7 kcal/mole. Estimate the heat of formation of 2-methylhexane. Explain briefly.

b) Circle the compound C₈H₁₈ that is expected to have the fewest number of degrees between its melting and boiling points. [Remember shapes of molecules.]

n-octane 2,2,3,3-tetramethylbutane 2,2-dimethylhexane

2,3-dimethylhexane 2,3,4-trimethylpentane

c) Circle the “acids” that are readily deprotonated by potassium hydroxide (KOH).

n-octane CH₃CO₂H NH₃ NH₄⁺ acetylene

d) Circle the species with sp hybrided atoms

HCN ethylene CS₂ CH₂=C=CH₂ BeCl₂

e) Briefly explain and illustrate why BrCH₂CH₂Br has a net dipole while

trans-BrCH=CHBr does not. [Newman projections of the most stable conformations of 1,2-dibromoethane might help.]

3. Thermochemistry: (20 pts.) n-Butane (DH_f^o = -30.0 kcal/mol) and 2-methylpropane (isobutane; DH_f^o = -32.1 kcal/mol) both have the formula C₄H₁₀.

a) (5 pts.) What type of isomers are they?

b) (5 pts.) What is the difference in their heats of combustion?

c) (10 pts.) Prove part b) with a “Standard State” illustration of the combustion of the two isomers. [Your diagram should show the products of combustion and the number of moles of each.]

4. Bonding/Orbitals: (15 pts) Allene 1 (1,2-propadiene) is often drawn as shown below. However, the picture is not an accurate representation of allene.

a) (10 pts) Use p-orbitals/p-bonds to draw a meaningful representation of allene.

b) (5 pts) Redraw allene in the same style as picture 1 but in a meaningful way.

5. Conformation II: (20 pts) Consider trans-1-chloro-3-ethylcyclohexane.

a) (10 pts) Draw the chair conformations below so that the equilibrium arrows are correct. Be sure groups are clearly axial or equatorial.

chair A chair B

b) (10 pts) What is the value of DGo for the above equilibrium given the following A- values for the monosubstituted (X) cyclohexane: X = Cl = 0.5 kcal/mol; X = C2H5 = 1.9 kcal/mol? Show calculations.

Chair A energy = ___________

Chair B energy= ___________

Work Sheet
Work Sheet

Work Sheet