Chem2O6 - 1997/98

Problem Set #4 ANSWERS October 17, 1997

USE YOUR MODELS !!

1. Give IUPAC names for the following structures:

Structures PS4 #1

Answer:

(i) trans-4-(1-methylethyl)-1-propylcyclohexane
(ii) 6-bromo-2-fluoro-3,3,5-trimethylheptane
(iii) 2,3,6-trimethylheptane

2. Draw structures for the following compounds:

    (a) 1-chloro-2,2-dimethylpentane
    (b) 3-(1,1-dimethylethyl)hexane
    (c) 4-propylheptane
    (d) 2-bromo-3-chloro-5-(2-methylpropyl)dodecane

Answer:

Structures PS4 #2

3. Draw Newman projections of 2,3-butanediol looking down the 2,3-bond. Which is the most stable conformation?

If a gauche OH-OH interaction is worth 1.6 kJ/mol, a gauche CH3-CH3 interaction 3.5 kJ/mol, and a gauche CH3-OH interaction 2.0 kJ/mol, how much more stable is your selected conformation above than the least stable conformer?

Answer:

Structures PS4 #3

The double-headed arrows indicate gauche interactions.

Conformer 1 is the most stable because there are only two (CH3-OH) gauche interactions present, whereas there are three gauche interactions present in conformers 2 and 3 (one CH3-CH3, one CH3-OH, and one OH-OH). Conformer 1 will be 3.2 kJ/mol more stable than 2 and 3 because conformer 1 has 2 X (CH3-OH) = 2 X 2.0 = 4.0 kJ/mol worth of gauche interactions, whereas 2 and 3 have (CH3-CH3) + (CH3-OH) + (OH-OH) = 3.6 + 2.0 + 1.6 = 7.2 kJ/mol.

4. Construct a model of trans1,4-dibromocyclohexane in its lower energy conformation. What is the orientation of the two C-Br bonds (i.e., axial or equatiorial)?

Invert the chair into the alternate chair. What is the orientation of the two C-Br bonds now? Identify any 1,3-diaxial interactions present in both conformers.

Answer:

Structures PS4 #4

5. The equatorial conformer of ethylcyclohexane is 7.4 kJ/mol more stable than the conformer in which the ethyl group is axial. Calculate the percentage of ethylcyclohexane molecules which are in the equatorial conformation at 25oC.

Answer:

Keq = exp(-Delta-Go/RT)
      = exp(-7.4/{(.00831)(298)})
      = exp(-2.99)
      = 0.0504

Keq = [axial]/[equatorial] = 0.0504
[equatorial] = 1 / 1.0504 = 0.95
95% of ethylcyclohexane molecules are in the equatorial conformation at 25oC.

6. Use the following stereostructural formulas to determine if the compounds are cis or trans geometric isomers. Indicate if the more stable conformer has been drawn for each case. Convert the structures to planar projection formulas using the dash-wedge method for depicting stereochemistry.

Structures PS4 #6

Answer:

Structures PS4 #6

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17oct97; wjl