EXAM 1

CHEMISTRY 220a

Friday, September 27, 2002

NAME (print): ___________________________________________________

TA:_____________________ Day:_________________ Time:__________

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

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. There is a Periodic Table on the last page of the exam.

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. (24 pts) ______

2. (20 pts) ______

3. (25 pts) ______

4. (16 pts) ______

5. (15 pts) ______

_______________________________

Total (100 pts)

1. (24 pts; equal weight) Compound A (C₁₀H₂₀), which is a 1,4-disubstituted cyclohexane, has DG⁰ = 0 kcal/mol for the difference in energy between its two chair conformations. [DGo (kcal/mol) values for axial and equatorial substituents in mono-substituted cyclohexanes: -CN, 0.2; -COOH, 1.4; -CH3, 1.7; -CH2CH3, 1.8; -CH(CH₃) ₂, 2.1; -C(CH₃)₃, 5.4.]

a) Name alkane A.

b) Draw the structure of alkane A in each of the templates shown below. Be clear about equatorial and axial bonds where applicable.

c) Draw the chair conformations of the stereoisomer of compound A in each of the appropriate templates shown below. Be clear about equatorial and axial bonds and pay attention to the arrows.

d) What is the difference in energy between the two chair conformations of this stereoisomer? Explain briefly.

2. (20 pts) For the eclipsed and staggered conformations of 2-methylpentane viewed along the C2-C3 sigma bond, draw a Newman projection of the most stable eclipsed and least stable staggered conformations. 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 answer in the appropriate box. 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.]

3. (25 pts.; equal weight) Circle the best answer(s) in each of the following:

a) The C₁-C₄ dihedral angle in the most stable conformation of n-butane.

0 30 60 120 180

b) The compound C₈H₁₈ expected to have the fewest number of degrees between its melting and boiling points.

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

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

c) The acids that are readily deprotonated by NaOH

Cyclohexane HCN NH₃ NH₄⁺ CH₃CO₂H

d) Species with sp³ hybridization

NH₄⁺ ethylene BF₄^- ethane BeH₂

e) Compounds with net dipole moments

BrCH₂CH₂Br CH₂Br₂ trans- BrCH=CHBr CBr₄ HCBr₃

4. (16 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:

a) The relative acidity of methanol vs. acetic acid

b) The enhanced ability of CH₂=CHCH₂Br to ionize to R+ Br-

c) Stabilization of the carbocation RCHOCH₃

5. (15 pts) Provide a molecular orbital representation of one of the following compounds. Include pi-bonds, p-orbitals, etc. Provide necessary commentary.

a) allene (1,2-propadiene) CH₂=C=CH₂

b) methylacetylene (1-propyne) CH₃CCH

Work Sheets
Work Sheets
Work Sheets
Periodic Table