Louisiana State University Elimination Reaction Lab Report

5. Answer the following questions about gas chromatography. You may consult your Mohrig textbook(chapter 20). (4 points)
Assume that your GC trace shows signal for only the two product alkenes and heptane. In what
order will they come off the column? Which has the lowest retention time?
a.
1.
2.
3.
b. How does the flow rate of the carrier gas affect the retention time of a compound on a GC column?
C.
How does the column temperature affect the retention time of a compound on a GC column?
d. An example GC trace is shown below. In addition to retention time, what other key information can
be obtained? Why is this information important for our lab?
SFID1
1500000
1000000
500000
1.876
3.871
4.465
089+
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.0
4.5
Pre-Lab Assignment for Elimination Reactions (18 points)
Name:
Answer the following questions in the space provided. Turn in as a hard copy upon entering the lab to perform
this experiment.
1. Write out the two main reactions you (and a partner) will conduct. Draw structures for the two alkenes
you could form in each of these reactions. (2 points)
2. What is the most likely mechanism for alkene formation in each elimination reaction? List the
mechanism (E1 or E2) and draw it out in each case. Be sure to use equilibrium arrows or unidirectional
arrows between intermediates, where appropriate. (6 points)
3. Write out a stoichiometry table for both reactions in your notebook. What is the limiting reagent in
each case? What is the theoretical yield of alkene mix? (Since the alkenes are isomers, you can
determine the theoretical total mass of product by using the molecular mass of either compound.) (4
points, in notebook)
4. What are the boiling points of each alkene formed? What is the boiling point of heptane? (List a
reference – Aldrich catalog is fine) (2 points)
Question: Is the Zaitsev or Hofmann product favored under acid or base elimination conditions? How can the
outcome be explained by the mechanism?
Procedure: You will work with a partner to accomplish the following two elimination reactions. Both partners
must stay in the lab until the entire project is done, so consider proper teamwork principles to accomplish an
overall efficient use of time.
Elimination of 2-methyl-2-butanol under acidic conditions: Place 2.4 grams of 2-methyl-2-butanol in a 25 mL
round bottom flask with a stir bar. Carefully add 15 mL of 6 M sulfuric acid to the flask. Add an air condenser to
the flask and complete the fractional distillation apparatus shown in Figure 12.17 (except the air condenser will
replace the fractionation column.) Place the receiving flask in an ice water bath. Gently heat the reaction in a
water bath at 85-90 °C. Collect the distillate that comes over below 45 °C. Make a GC sample as directed below.
Elimination of 2-chloro-2-methylbutane under basic conditions: Put 23 mL of basic 1-propanol solution in a 50
mL round bottom flask. (There are 3.0 g of KOH per 23 mL of the solution.) Slowly pour 3.0 mL of 2-chloro-2-
methylbutane into the basic solution. Add an air condenser to the flask and complete the fractional distillation
apparatus shown in Figure 12.17 (except the air condenser will replace the fractionation column.) Use a tared,
25 mL round bottom flask in ice as a collecting flask. Heat and stir the reaction for one hour in a 75-80 °C water
bath, monitoring the temperature closely so that it does not exceed 80 °C during the reflux period. Add
additional ice to the receiving flask as necessary. A white precipitate will form in the reaction flask. After one
hour, increase the water bath temperature to 90-95 °C and distill the reaction mixture. Collect all the distillate
boiling below 45 °C. Make a GC sample as directed below.
Gas Chromatography: Prepare a GC sample for the autosampler. (You will leave your sample to be analyzed by
an automated GC sampler.)
1. Obtain a GC vial and cap.
2. Place 2-3 drops of your distillate in the vial, then add heptane until the vial is half full.
3. screw on the cap, making sure it is on TIGHTLY!
4. Your Al will run your GC for you, so ask them how they would like the vial labeled. One option is to label
the vial with a Sharpie with your three initials, the day (R or F) and the time (1:20 or 2:30) of your lab.
For example: LCB, R 12:15
5. Place in the GC vial collection box.
6. Be sure to have your Al plain how you will receive the data.
GC parameters:
Column Type: HP-55% Phenylmethyl Siloxane
Oven: 65 °C, isothermal for 5 min
He flow: Flow rate 7.5 mL/min
Injector: Auto-injection, volume 1.0 uL
Heater 150 °C
FID Detector: Heater 250 °C