Experiment 3:
STEREOCHEMISTRY AND MOLECULAR
MODELING OF CYCLOALKANES
THE LAB REPORT…

This experiment is NOT in the current edition of the lab
manual.

Each student should print a copy of the FINAL LAB
REPORT sheet accessible on the Blackboard Learn
course website and bring it to lab to complete.

Molecular model kits will be provided for assistance, or
students can bring their own to lab.

There is no other PRE lab or POST lab assignment for
this experiment, just the completion of the FINAL LAB
REPORT sheet.
OBJECTIVES

To practice drawing cyclohexane rings in chair
conformations, and learn how to recognize axial and
equatorial positions.

To practice drawing monosubstituted and
disubstituted cyclohexane rings in both chair
conformations.

To learn how to compare the stability of chair
conformations, and identify which conformer is
favored at equilibrium.
THE LAB REPORT…
Fill in the blanks using a
single term from the list at
the top. Use textbook for
assistance if necessary.
How to Draw Cyclohexane
1. Draw a wide V.
2. Draw a line going down at a
60o angle, ending just before the
center of the V.
4. Draw a line parallel to the
line from step 2, going down
exactly as low as that line.
3. Draw a line parallel to the left
side of the V ending just before
the left side of the V.
5. Connect the dots.
THE LAB REPORT…
Follow instructions given for all
problems.
How to Draw the
Axial and Equatorial Hydrogens
1. Draw all axial positions as parallel lines, alternating in
directions.
SUMMARY: All substituents are
drawn like this:
a
a
e
a
e
e e
e
e
2. Draw all equatorial positions as pairs of parallel lines.
parallel with
this side!
a
a
a
THE LAB REPORT…
Follow instructions given for all
problems.
Conformational Mobility of
Cyclohexane-Ring Flips
Conformations of
Monosubstituted Cyclohexanes

Although ring-flip occurs rapidly, the two conformers
are not EQUAL!
This conformer has more
diaxial interactions, therefore
is higher in energy!
1,3-Diaxial Interactions

Q: What causes the difference in energy between
the conformers?


Steric strain due to 1,3-diaxial interactions.
Q: What is a 1,3-diaxial interaction?

Atoms on C1 are too close to those on C3 and C5!
Drawing Both Chair Conformations:
Monosubstituted rings
1. Draw a chair conformation.
2. Place the substituent in an axial
position.
Br
1
axial
3. Draw the ring flip and the axial group becomes equatorial.
Br
1
2
axial
2
equatorial
1
Br
2
THE LAB REPORT…
Follow instructions given for all
problems.
Drawing Both Chair Conformations:
Disubstituted rings
1. Using a numbering system, determine the
location and configuration of each substituent.
Draw the structure using solid and dashed lines to
indicate relative (cis vs. trans) stereochemistry.
Br
2. Place the substituents on the first chair
using the info from step 1.
Br
H
Bromine is at C-1
and is UP
H
1
3
1
2
CH2CH3
2
3
CH2CH3
Ethyl is at C-3
and is DOWN
3. Draw the second chair skeleton, and place
the substituents using the info from step 1.
2
H
1
3
CH2CH3 H
Br
THE LAB REPORT…
Follow instructions given for all
problems.
Conformational Analysis of
Disubstituted Cyclohexanes

Q: What is conformational analysis?


Assessing energy of cycloalkane by summing all steric
interactions.
Q: Why is it important?

Can help predict which conformations are more favorable
and more likely to exist.
THE LAB REPORT…
Follow instructions given for all
problems.
THE LAB REPORT…
This table is given on the last
page of the lab report…
OVERVIEW

Review concepts.

Draw a chair conformation of cyclohexane.

Draw and label axial and equatorial positions on a
chair conformation of cyclohexane.

Draw chair conformation of monosubstituted
cyclohexane before and after ring flip.

Draw chair conformations of disubstituted
cyclohexanes before and after ring flip.

Compare stability of chair conformations and predict
major conformer.
Just a suggestion…
BRING YOUR
TEXTBOOK WITH YOU
TO LAB!
(IT MAY HELP !)