Experiment 12:
BASE PROMOTED
ELIMINATION OF HBR FROM
AN ALKYL HALIDE
Br
Base
ROH
+
+
Objectives

To synthesize an isomeric mixture of
alkenes from “E2” base promoted
elimination of HBr from 2-bromoheptane
under reflux.

To purify the products through simple
distillation.

To study the effect of base size on
product distribution using GC analysis.

To identify characteristic absorptions in
the IR spectra of reactants and products.
CHEMICAL EQUATION
Br
2-Bromoheptane
MF: C7H15Br
MW: 179.1
o
bp: 180 C
Base
ROH
+
+
MF: C7H14
MF: C7H14
MF: C7H14
MW: 98.2
MW: 98.2
MW: 98.2
bp: 94 oC
bp: 98 oC
bp: 98-99 oC
d: 0.697 g/mL
d: 0.701 g/mL
d: 0.708 g/mL
d: 1.14 g/mL
Dehydrohalogenation of 2-bromoheptane.
MECHANISM
1. Base oxygen attacks and
removes a proton from
carbon adjacent to bromine…
Br
Ha Ha
Hb
C H
b +
Hb
b
OH
ALL STEPS OCCUR IN A
CONCERTED FASHION!
ROH
+
a
+
a
b
a
2. …C=C bond forms…
3. …bromine is eliminated from the
opposite side of the molecule.
MECHANISM
(MOST SUBSTITUTED ALKENE ISOMER)
Br
C
H H
H
H
Base
H
+
HBase
+
• Removal of more hindered proton, leading to
more stable, more highly substituted alkene.
• MAJOR product for small base, MINOR
product for large base!
Br
MECHANISM
(LEAST SUBSTITUTED ALKENE ISOMER)
Br
C
H H
H
H
Base
H
+
HBase
+
Br
• Removal of least hindered proton, leading to
a less substituted alkene product.
• MAJOR product for large base, MINOR
product for small base.
STERIC HINDRANCE

The reason that the
bulkier (larger) base
gives more of the less
substituted alkene is
that steric hindrance
prevents it from
approaching a
hydrogen on a more
highly substituted
carbon.
OVERVIEW




Heat alkyl bromide and assigned base
in solvent under reflux to synthesize
products.
Purify products by simple distillation to
remove unreacted starting materials.
Prepare GC sample.
Analyze and determine product ratio
using GC results.
EXPERIMENTAL PROCEDURE
(SYNTHESIS)
CaSO4 drying
tube in
thermometer
adapter
• Add 2-bromoheptane and alcohol solvent
to 25 mL flask w/3 boiling chips.
• Clamp flask to ring stand.
• Add solid base to flask using powder
funnel.
water out
•Place the condenser on the flask.
•Using a thermometer adapter, place a
CaSO4 drying tube in the top of the
condenser.
water in
• Begin water flow, apply heat.
heating mantle
• Reflux the solution for 60 minutes.
to voltage regulator
iron ring
• Cool flask using a beaker of tap water.
EXPERIMENTAL PROCEDURE
(PURIFICATION)
• Once cooled, arrange a simple
distillation apparatus.
Keck clips here!
• Apply water flow and heat.
water out
water in
Heating Mantle
iron ring
to
voltage
regulator
• Remember to clamp both
flasks to ring stand!
• When the first drop reaches
the receiving flask, mark the
temperature (Ti).
• Collect ~5 mL in the receiving
flask.
• Read the temperature again
(Tf). Remove the heat.
•
Prepare a GC sample.
Table 12.1
GC Retention Times (min) and Adjusted Area Percent
Compound
Standard
Rt
SMALL BASE
(methanol/sodium methoxide)
Sample
Rt
Area
Percent
Adjusted
Area
Percent
LARGE BASE
(t-butanol/potassium t-butoxide)
Sample
Rt
Area
Percent
Adjusted
Area
Percent
methanol
t-butanol
---
1-heptene
trans-2heptene
cis-2-heptene
• Remember to get data from the opposite alcohol/base pair!
EXPERIMENTAL PROCEDURE
(IR Analysis)
Br
IR spectra on page 103 of lab manual!
Table 12.2
Functional
Group
C-Br stretch
sp3
sp2
CH stretch
CH stretch
Alkene C=C stretch
Base
Values
(cm-1)
500-700
2850-3000
3000-3100
1600-1680
2bromoheptane
1heptene
cis-2heptene
trans-2heptene
Frequency
(cm-1)
Frequency
(cm-1)
Frequency
(cm-1)
Frequency
(cm-1)
SAFETY CONCERNS


Sodium methoxide and potassium tbutoxide are strong bases and
corrosive! Use gloves when handling!
Methanol and t-butanol are flammable!
Wear safety goggles at all times and
use extreme caution when heating!
WASTE MANAGEMENT

After adding water to the reaction flask,
pour this and product solution into
container labeled “LIQUID ORGANIC
WASTE”.
CLEANING…



Clean round bottom flasks with
soap/water/wash acetone.
Clean all remaining reflux/distillation
glassware with wash acetone ONLY while in
your lab hood! DO NOT REMOVE THIS
GLASSWARE FROM YOUR HOOD!
Funnel and beaker can be cleaned with
soap/water/wash acetone.
LABORATORY NOTEBOOK
(Pre-lab)
• OBJECTIVE
(Must clearly state…)
•What compounds you will make and how
• How you will purify the compound
• How you will determine the purity of your compound
• CHEMICAL EQUATION
 Include the general chemical equation from page 99.
•TABLE OF PHYSICAL DATA
(Complete the following table using MSDS
sheets from a site on WWW Links ONLY. Wikipedia is unacceptable)
Compound
2-bromoheptane
cis-2-heptene
trans-2-heptene
1-heptene
methanol
t-butanol
sodium methoxide
potassium t-butoxide
MW (g/mol)
• REFERENCE TO PROCEDURE
bp (Co)
180
d (g/mL)
1.14
XXX
XXX
XXX
XXX
(Must include…)
•full title, including edition and authors
•page numbers where actual procedure can be found
HAZARDS
Flammable, irritant
LABORATORY NOTEBOOK
(In-lab)
• DATA/CALCULATIONS
•
•
•
•
•
•
•
Initial weight of 2-bromoheptane used
Initial weight and identity of base used
Theoretical yield calculation (not just the value!)
Physical state and color of product
GC vial slot #
GC sample solvent identity
Give an example of an adjusted area % calculation
• EXPERIMENTAL PROCEDURE
•
•
In paragraph form, briefly describe the procedure that you actually
followed during the lab.
Paragraph must be written in PAST TENSE, PASSIVE VOICE.
•
Include ACTUAL volumes or weights of chemicals used during the experiment.
•
Include any mistakes, accidents, or observations if necessary.