EXAM 1
CHEMISTRY 220a
Friday, September 24, 2004
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.
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. Structure/Conformation (30 pts) ______
2. Conformation (20 pts) ______
3. Potpourri (30 pts) ______
4. Orbitals (20 pts) ______
_____________________________________
Total (100 pts)
1. Structure/Conformation (30 pts): Limonene, which is isolated from lemon grass, is a C10 hydrocarbon that contains a 6-membered ring and two double bonds.
a) (5 pts) In addition to carbon, what other atoms are present in limonene and how many are there of each kind? Show your reasoning.
Addition of hydrogen to the double bonds of limonene (alkene ˆ alkane) produces two 1,4-disubstituted cyclohexane stereoisomers, A (C10H20) and B.
b) (5 pts) What is the formula of B? Why?
Continued É
Compound A has an energy difference of 3.8 kcal/mol between
its two chair conformations Amajor and Aminor.
Compound B has an energy
difference of 0.4 kcal/mol between
its chair conformations Bmajor
and Bminor.
c) (20 pts) Using the relevant data below, determine the structures of Amajor, Aminor, Bmajor, and Bminor . Place the substituents on the chair templates in their correct positions and configurations (pay attention to the equilibria), enter the energy in each box, and the designations (Amajor, Aminor, Bmajor, and Bminor ) in the circles. Values [DGo (kcal/mol) ] for energy differences between axial and equatorial isomers of mono-substituted cyclohexanes are as follows: -CN, 0.2; -COOH, 1.4; -CH3, 1.7; -CH2CH3, 1.8; -CH(CH3) 2, 2.1; -C(CH3)3, 5.4.] Show work.
Compound A:
Compound
B:
2. Conformation (20 pts): Draw Newman projections of the three most stable conformations of 2,3-dimethylpentane viewed along the C2-C3 sigma bond. Use the circles as templates for the Newman projections. Calculate the energy (kcal/mol) of each conformation. 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. Potpourri (6 x 5 pts = 30 pts.; equal weight):
a) Circle the C1-C4 dihedral (torsional) angle in the highest energy conformation of n-butane.
0o 60o` 120o 180o 270o
b) Circle the compound C8H18 having the most negative heat of formation and the smallest difference between its mp and bp (5.6 oC).
n-octane 2,2,3,3-tetramethylbutane 2,2-dimethylhexane
2,3-dimethylhexane 2,3,4-trimethylpentane
c) Circle the acid that is ranked third in relative pKa.
CH4 CH3OH NH3 NH4+ CH3CO2H
d) Circle the compounds that contain atoms with sp hybridization.
acetylene ethyne CO2 HCN BeH2
e) Circle the compounds with net dipole moments.
BrCH2CH2Br cyclohexane BrHC=C=CHBr propane HCBr3
f) Circle the species in which resonance plays a role.
RCO2- CH3ONa CH2=CHCH2+ CH3CH2CH2+ +CH2CH2N(CH3)2
4. Orbitals (20 pts): Provide a molecular orbital representation of acrylonitrile (CH2=CHCN). Include pi-bonds, p-orbitals, and non-bonding electrons. Identify the hybridization of each carbon. You may use lines for sigma bonds. Provide necessary commentary.