Download Octenes from E1 versus E2 Eliminations

Exp't 74
Synthesis of Octene Isomers: E1 vs. E2 Elimination
(Adapted by R. Minard (Penn State Univ.) from A.I. Vogel's "Elementary Practical Organic Chemistry", Longman
Group Ltd., London (1966) and Gilow, H. M. Microscale Elimination Reactions, J. Chem. Educ. 1992 69 A265) [Rev 3/16/02]
Introduction:
Alkene isomers can be formed by either E1 or E2 eliminations. The dehydration of 1-octanol
is an example of an E1 elimination. The alcohol is protonated by a strong acid, in this case
H2SO4. Elimination of water yields a carbocation in a slow, rate -limiting step. The loss of a
neighboring proton to form the C=C can then occur or hydrogens can move from adjacent
carbons to the + charged carbon via hydride (H ) shifts to give more stable 2° carbocations.
H2SO4
H
HO +
HO
slow
Δ
1-octanol
-H+
+
+
fast
??????
1-octene
In an E2 reaction, the base abstraction of a proton and the expulsion of the leaving group
occurs simultaneously to form the C=C. The dehydrobromination of 1-bromooctane with the
strong base, potassium tert-butoxide, in triethylene glycol demonstrates the E2 elimination
reaction:
H
CH3
Br
CH3
C
O K+
CH3
H
_
B:
1-bromooctane
+
??????
HO CH2CH2O H
triethylene glycol
1-octene
Δ
This lab is designed to demonstrate the difference in the mechanisms of these two reactions
as evidenced by the product or products formed. Two reactions are carried out. Yields are not
calculated since only a small amount needs to be collected in order to do the GC/GC-MS analysis.
Read the description of the microscale instant distillation procedure in your Lab Guide (Chapter
1).
Prelaboratory Exercise:
1. Give 3 isomers that could be obtained from the E1 reaction?
2. Why is tert-butoxide anion used as the base in this particular E2 reaction?
3. Use arrow-pushing to show the mechanism of the dehydration of 2-octanol.
Cautions:
Concentrated sulfuric acid is corrosive. Potassium t-butoxide is a strong base. Avoid contact
with skin and handle with care.
TLC
2
You are required to run a TLC to monitor the progress of the reaction. Plates should have three
spots (or lanes) on the origin: one for the main organic starting material that is being transformed,
one for a cospot (starting material and the reaction mixture), and one for the reaction mixture.
Syntheses:
E1 Dehydration of 1-Octanol
First set up a sand bath using your heating mantle half-filled with sand and a Varistat setting of
45.
Fill a 10 x 100 mm reaction tube to the 0.5 mL mark with 1-octanol (n-octyl alcohol) and insert a
1/2-inch stir bar. Add 5 drops of conc. sulfuric acid. While stirring, heat the reaction for 20 to 30
minutes. At first you will see water droplets and a cloudy liquid condensing on the walls of the
reaction tube, but after another 10 min, you just see a clear liquid condensing about halfway up
the tube. (You can start setting up the E2 reaction below while this is heating.) After heating for
20 to 30 min, insert a clean Pasteur pipet into the reaction vapors about 1 cm above the boiling
liquid and draw up the vapors into the Pasteur pipet. The vapors will condense and the small
drop of liquid can be transferred to a tared shorty vial. Draw up vapors another 3 or 4 times,
transferring each droplet to the shorty vial so that you have about one or two full drops in the
shorty vial. Weigh the shorty vial to calculate a yield of the product.
Add a spatula-tip-full of anhydrous potassium carbonate (K2CO3 – Hooded Shelf) to the liquid
followed by enough CH2Cl2 to almost fill the shorty vial. Label the vial E1. Analyze this solution
first by GC and if desired GC-MS.
E2 Dehydrohalogenation of 1-Bromooctane
Obtain the potassium t-butoxide reagent which is kept in vial in a dessicator jar on the Hooded
Shelf. Using your scoopula, quickly add small quantities of this white fluffy solid to a reaction
tube to a volume of between 0.5 and 0.75 mL when tapped down. Do this quickly as this strong
base readily picks up moisture from the atmosphere and turns to a sticky gooey mess. Do not
leave the lid off the reagent bottle for more than a few seconds at a time and put it back in the
dessicator jar as soon as you are finished with the reagent.
Add triethylene glycol to a volume level of 1.5 mL in the reaction tube. Add about 0.3 mL of 1bromooctane using your 1-mL pipet with the 2.5-mL plastic syringe and syringe-pipet connector
(Common Shelf). Add a 1/2" stir bar and heat the mixture while stirring gently until a small amount
of clear liquid can be seen condensing or refluxing on the upper walls of the reaction tube.
Reach into these vapors with a Pasteur pipet and draw them up. They will condense in the
pipet. Transfer this small drop of liquid to a tared shorty vial. Draw up more vapors until the
equivalent of 1 or 2 drops of product has been collected in the shorty vial. Weigh shorty vial to
calculate a yield for the product. Add 1 mL CH2Cl2 to the vial and then enough solid anhydrous
Na2SO4 to just cover the bottom of the vial and dry the CH2Cl2 solution. Dry for 5 min and then
analyze by GC and, if desired, GC-MS.
Cleaning up:
The small amount of reaction residues can be flushed down the drain. The product GC solutions
should be disposed of in the Halogenated Organics container.
Analysis:
In addition to TLC analysis, you may be instructed to analyze your final product by IR, NMR or GC.
Analyze your sample according to your Assignment sheet and the instructions on Sample
Preparation in Lab Guide.
3
Both products should be analyzed by injecting 1 µL of the CH2Cl2 solution into the GC and, if
necessary, GC-MS. Identify the 1-butene peak in both chromatograms by matching retention
times for this reaction product in both.
Final report:
Show the mechanisms of dehydration of 1-octanol and the dehydrobromination of 1bromooctane. Using data from the GC and your chemical knowledge, describe the composition
of both distillates. Give relative amounts of each isomer, and assign isomers to all GC peaks on
the printout based on what would be reasonable based on the fact that trans is more stable than
cis and more highly substituted double bonds are more stable than less substituted ones
(Zaitzev’s rule).
Answer the following:
1. Cocentrated sulfuric acid has a great affinity for water. Why not dry 2-octene with
sulfuric acid instead of anhydrous calcium chloride?
2. Is it possible that some 3-octene could form? Propose a possible sequence of reactions
that could form 3-octene. Given that the process of hydration/dehydration is reversible,
how would the percent composition of 3-octene change if you were to stir the reaction
for 24 hours before analyzing the mixture?
3. Show the mechanism for the reaction of trans-3-octene and Br2.
References:
1. Williamson, Kenneth. "Macroscale and Microscale Organic Experiments"; 2nd Edition, D.C.
Heath and Company: Lexington, Massachusetts, 1994; p.87.