Investigation of the Distribution of Alkene Products from E1 and E2 Reaction Pathways
Adapted from Chemistry 344 Lab Manual Spring 2006 Edition
University of Wisconsin – Madison
Allen D. Clauss, Jason K. Pontrello, and T. Andrew Tseng
The following experiments investigate the distribution of alkene elimination products from both E1 and E2
reaction pathways. 2-Methyl-2-butanol will be the starting material for the E1 reaction, and 2-chloro-2methylbutane will be the starting material to for the E2 reaction. You will work with a partner during this
laboratory period. One student will carry out the E1 procedure and the other will carry out the E2 procedure.
You will then share your data and results with respect to the distribution of the alkene isomers. Note: the
products of these reactions are low-boiling liquids. The flask used to collect and store these liquids must
be kept cold in an ice-water bath at all times to avoid losing the product by evaporation.
Pre-lab Questions
1. Draw structures of the elimination products most likely to form in the following reaction. Circle the
product that will form in the highest yield and provide a rational.
NaOEt
Cl
HOEt
2. Consider the cis and trans isomers of 1-tert-butyl-4-chlorocylcohexane.
Cl
Cl
a) One of these molecules will not react with methoxide ion (CH3O–) in methanol via an E2 pathway.
Which one?
b) For the isomer that will not undergo the E2 reaction, what is the most likely product formed when
the E2 reaction is attempted?
3. In what region of the 1H NMR will the vinyl protons (protons on the sp2 hybridized carbons) on 2methyl-2-butene and 2-methyl-1-butene show up? What would expect the splitting pattern and
integration to be for each of the two isomers?
E1: Dehydration of 2-methyl-2-butanol
HO
H
H2SO4
H
+
H
Scheme 1. Synthetic scheme for the E1 elimination reaction of 2-methyl-2-butanol with sulfuric acid to give 2methyl-2-butene and 2-methyl-1-butene.
In this experiment, the product is removed from the reaction mixture by fractional distillation as the reaction
proceeds. The flask containing the reaction mixture is connected to a fractional distillation setup.
Figure 1. Fractional distillation apparatus.
Procedure
Place 2.0 mL (1.6 g) of 2-methyl-2-butanol in a 25 mL round-bottomed flask containing a magnetic stir bar.
Carefully add 10 mL of 6 M sulfuric acid to the material in the flask. Gently heat the mixture and when
distilling begins, regulate the heating so that the temperature of the vapor distilling over does not exceed 45 °C.
Cover the air condenser and distilling adapter with foil to help facilitate a timely distillation. Immerse the
receiving flask in an ice-water bath. The distillation rate should be about 1 to 2 drops per second. Collect all
distillate boiling below 45 °C (2-methyl-1-butene, bp 31 °C; 2-methyl-2-butene, bp 38 °C). Analyze your
product by gas chromatography and 1H NMR. How many compounds are indicated by gas chromatography
or by 1H NMR? What are the relative amounts of each compound present in the distillate?
E2: Dehydrochlorination of 2-Chloro-2-methylbutane
Cl
H
KOH
H
+
H
Scheme 2. Synthetic scheme for the E2 elimination reaction of 2-chloro-2-methylbutane with potassium
hydroxide to give 2-methyl-2-butene and 2-methyl-1-butene.
In this experiment, the reaction mixture needs to be refluxed for 1 hour and then the product is removed from
the reaction mixture by fractional distillation. To facilitate these operations, the setup for the reflux is
constructed by attaching an air condenser to a 25 mL round-bottom flask and then attaching a reflux condenser
to the top of the air condenser. This can be seen on the next page (Figure 2). After the period of heating at
reflux, the apparatus is fitted for distillation, seen in Figure 1.
Procedure
Place 1.8 g (excess) of potassium hydroxide and 15 mL of n-propanol in a 25 mL round-bottomed flask
containing a magnetic stir bar. Warm and stir the mixture until the potassium hydroxide has dissolved. Cool
the flask to room temperature (allow to sit in air for a few minutes), then further cool using an ice-water bath,
and cautiously add 1.3 g (12 mmol, 1.5 mL) of 2-chloro-2-methylbutane. Heat the mixture for one hour,
maintaining a gentle reflux. During the period of heating a white solid, potassium chloride, precipitates from
solution.
Use care in the following manipulations, glassware will be hot!
Remove the heating source from the reaction flask and allow the mixture to cool to room temperature. Remove
the reflux condenser from the top of the air condenser and replace it with a distillation head fitted with a
thermometer. Attach the reflux condenser to the outlet of the distillation head. Equip the flask for fractional
distillation as seen in Figure 1. Cover the air condenser and distilling adapter with foil to help facilitate a timely
distillation. Immerse the receiving flask in an ice-water bath. Distil the product mixture (sand bath ~140
°C), collecting all distillate boiling below 45 °C (2-methyl-1-butene, bp 31 °C; 2-methyl-2-butene, bp 38 °C).
Analyze your product by gas chromatography and 1H NMR (be careful when transferring the product, it
may shoot out of your pipette). How many compounds are indicated by gas chromatography or by 1H NMR?
What are the relative amounts of each compound present in the distillate?
Figure 2. Reflux apparatus.