Problem Set 3
Chapter 4
Due: Monday, February 6, 2006
(This photograph is in the hallway
across from 110 SCL) In 1885, as an addendum to a paper on
acetylenic compounds, Baeyer proposed that cyclopentane
was the least
strained of the cycloalkanes.
While he accepted the idea that the carbon atoms in
cycloalkanes were tetrahedral, he treated the
cycloalkanes as though they were flat. He argued that
there is only one cyclohexane carboxylic acid, not two
(axial and equatorial) as was predicted by a chair
cyclohexane.
Reading and Enrichment
Assignments:
a. Work through How to Draw Cyclohexanes (PowerPoint)
b. Visit the Conformation Module in the Study Aids for cycloalkanes
c. Cultural Enrichment: The Evolution of Formulas and Structure in Organic Chemistry During the 19th Century (PowerPoint).
1. Using the heats
of formation tables,
explain the difference in the heats of formation of cis- and
trans-1,4-dimethylcyclohexane. What are the heats of combustion of
these stereoisomers? Show work. Draw a diagram of the heats of
formation and combustion for these stereoisomers relative to the
standard state. Put your values on the chart.
2. Compound A (MW=142.19), a 1,4-disubstituted cyclohexane,
has the following composition: C, 67.57%; H, 9.92%. The difference in
conformational energy for the two chair conformations of A is
0.3 kcal/mol. Using the data in Table 4-3, page 142, determine the
structure of A. Illustrate and explain. What is the
conformational energy difference for the stereoisomer of A,
---namely A'. Explain and illustrate. Show the chair
comformations of A and A' with the appropriate
equilibrium arrows to illustrate the major and minor conformations.
Label each conformation with its energy.
3. There are seven dimethyl cyclohexanes. Draw them as an equilibrium
of both their chair conformations. Calculate the energy between the
chair conformations for the seven equilibria.
4. cis-Decalin is ~2.6 kcal/mol less stable than
trans-decalin (pg. 146) where a gauche butane interaction is
worth 0.85 kcal/mol (one-half the value of CH3 in Table
4-3).
a) Explain and illustrate.
b) What is cis-decalin capable of doing that the trans-decalin cannot do.
5. Consider the structure of cholic acid (pg. 150). It contains both
a cis- and trans-decalin.
a) Redraw the structure on pg. 150 and label them.
b) Why is the cis-decalin not capable of conformational inversion? (Make a stripped down version of the A-B-C ring system with your models to help yourself.)
c) Label all the methyls, hydroxyls and hydrogens attached to 6-membered rings in the picture as equatorial or axial.
d) Draw cholic acid as a 3D-structure (vide supra, pg. 150). Use the letter "R" for the carboxylic acid side chain.
2.