13
modular
publisher:
laboratory.
H.A. Neidig
SYNT
program
in
chemistry
719
organic editor: Joe Jeffers
Brominating Alkenes
prepared by Carl T. Wigal, Lebanon Valley College
PURPOSE OF THE
EXPERIMENT
EXPERIMENTAL OPTIONS
BACKGROUND REQUIRED
BACKGROUND
INFORMATION
Synthesize vicinal dihalides by brominating alkenes. Characterize vicinal dihalides by using the silver nitrate test and by using melting point
measurement to determine the relative stereochemistry.
Using Semi-Microscale Techniques to Brominate Alkenes
Cinnamic Acid
cis-Stilbene
trans-Stilbene
Using Microscale Techniques to Brominate Alkenes
Cinnamic Acid
cis-Stilbene
trans-Stilbene
You should consult your textbook for the Cahn-Ingold-Prelog System
for assigning the configuration of a chiral center. You should be familiar with techniques for reflux, for vacuum filtration, and for melting
point measurement.
The halogenation of alkenes is an important reaction in the chemical industry. For example, over 8 million tons of 1,2-dichloroethane per year are
produced by the addition of chlorine (Cl2)to ethylene. This product is
used both as a solvent and in the preparation of polyvinyl chloride, PVC,a
common organic polymer used in household plumbing. The products obtained from alkene halogenation are called vicinal dihalides because the
two halogen substituents are attached to adjacent carbon atoms.
When the halogen used is either bromine (Br2) or chlorine (Cl2),
halogenation of alkenes occurs rapidly at room temperature, and the resulting vicinal dihalides are stable. Fluorination is a violent reaction that
is difficult to control and is accompanied by several side reactions. Iodination is an endothermic process, resulting in vicinal diiodides that tend
to revert to alkenes. Consequently, the most common applications of
alkene halogenation are chlorination and bromination.
Typically,alkenes undergo reactions through electrophilic addition,
a process in which the alkene pi (1t)bond is replaced with two sigma (0)
bonds. The general mechanism of electrophilic addition involves two
steps, as shown in Figure 1 on the next page.
Copyright @ 1998 by Chemical Education Resources, Inc., P.O. Box 357, 220 South Railroad, Palmyra, Pennsylvania 17078
No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-
copying, recording, or any informationstorage and retrievalsystem, without permission in writingfromthe publisher. Printed in the United
States of America
I
168
SYNT: 719 BrominatingAlkenes
Figure 1 Electrophilic addition to an
alkene
carbocation
The first step involves reaction of an electron deficientspecies,called an
electrophile (E+),with the electron-rich 1tbond of the alkene. The two electrons of the 1tbond shift toward the electrophile, forming a new carbonelectrophile crbond.This step results in formation ofa positively charged intermediate. In most instances, the positive charge centers on a carbon atom,
so the electron-deficient intermediate is called a carbocation.
In the second step, the electrophilic carbocation reacts with an
electron-rich species called a nucleophile (Nu-). The nucleophile donates an electron pair to the positively charged intermediate forming a
carbon-nucleophile crbond.
Alkene bromination follows the same general mechanism with a few
important modifications, as shown in Figure 2. In the first step, the proximity of the 1telectrons of the alkene to Br2polarizes the bromine-bromine
bond. This polarization induces a bond dipole that allows Br2to act as an
electrophile. Electrons flow from the 1tbond to the polarized Br2'forming
a carbon-bromine bond and breaking the bromine-bromine bond, producing a bromide ion (Br-). The positively ch~rged intermediate in
bromination is not a carbocation, but a bromonium ion. This cyclicintermediate results from a nonbonding electron pair from bromine that stabilizes the positive charge on carbon. The bromonium ion is more stable
than a simple carbocation because all atoms of the bromonium ion have
an octet of electrons.
Figure 2
Bromination of cyclohexene
~
"
.8+
B
..0...r-Br:
V
bromonium ion
+
.
..
~
..
:Br:
:Br:
In the second step, Br- acts as a nucleophile, attacking the electron-deficient bromonium ion. The second carbon-bromine crbond of the vicinal
dihalide forms in this step. An important feature of this step is the resulting
stereochemistry. Bromide ion adds to the side opposite the carbon-bromine
bonds of the bromonium ion. This process is called anti-addition.
Anti-addition of Br- occurs because Br- is blocked from one face of the
bromonium ion by the bromine atom. The consequence of anti-addition of
Br2to cyclic alkenes is the formation of trans vicinal dihalides.
Chiral carbon atoms, or chiral centers, are generated in many organic reactions. A chiral carbon is a carbon atom that is bonded to four
different substituents. As a consequence, two different configurations
are possible for a chiral center: rectus(R) or sinister (5).
@ 1998Chemical Education Resources
SYNT: 719 BrominatingAlkenes
169
In the case of alkene bromination, the formation of the two carbonbromine crbonds could result in two new chiral centers in the vicinal dihalide. Using the 2n rule, where n is the number of chiral centers in a
molecule, a maximum of four stereoisomers could result from alkene
bromination. However, not all stereoisomers are formed during a single
bromination. Bromonium ion formation preserves the stereochemical
integrity of the starting material. Therefore, trans alkenes form transbromonium ions and cis alkenes form cis bromonium ions.
Consider the bromination of maleic and fumaric acids, as shown in
Figure 3. When maleic acid, the cis isomer, is brominated, the bromonium ion formed has cis configuration. The product is a mixture of
two stereoisomers. These stereoisomers are enantiomers having the absolute configurations of 2R,3R and 25,35. Enantiomers are molecules
that contain chiral centers and are non-superimposable mirror images.
maleicacid
fumaric acid
Figure 3
@ 1998Chemical Education Resources
Bromination of maleic acid and fumaric acid
Anti-addition of Br- to the cis bromonium ion can take place by either path a or path b in Figure 3. Addition by path a results in 2R,3R,
while addition via path b results in 25,35. Addition occurs at the same
rate by either path; therefore, the two enantiomers are produced in equal
amounts. A mixture containing equal amounts of a pair of enantiomers
is called a racemic mixture. A racemic mixture is often designated by
placing (.:t)at the front of the name.
Unlike bromination of maleic acid, bromination of fumaric acid results in a single stereoisomer. The bromination of fumaric acid results in
a trans bromonium ion intermediate. Addition of Br- to the trans bromonium ion by either path a or path b yields the same compound. The absolute stereochemistry of the product is 2R,35, which is identical to
25,3R. This compound is an example of a meso compound, which is a
170
SYNT: 719 BrominatingAlkenes
molecule that contains chiral centers, but also contains an internal plane
of symmetry. As a result, meso compounds have superimposable mirror
images. Meso dibromides result from the bromination of sYmmetrically
disubstituted trans alkenes.
If the alkene is not sYmmetrically substituted, a pair of enantiomers
will result. The relative stereochemistry of the enantiomers could have
both chiral centers having the same configuration (R, R or 5,5) or the opposite configuration (R,5 or 5,R). The prefixes erythro and threo are used
to differentiate these stereoisomers. Erythro refers to a pair of enantiomers having a configuration similar to the sugar erythrose. The erythro
form is often described as "meso-like" because the molecule would have
a plane of symmetry if the two dissimilargroups wereequivalent. Threo
refers to a pair of enantiomers having a configuration similar to the
sugar threose. These configurations are shown in Figure 4.
Figure 4 Configurations of (a) erythrose
and (b) threose
(R,R)
(S,S)
(R,S)
(S,R)
(:t )-erythrose
(:t)-threose
(a)
(b)
In this experiment, you will brominate an alkene using pyridinium
tribromide, a comparatively safe, convenient source of bromine. You
will characterize your product by measuring its melting point and by
conducting a silver nitrate test. Vicinal dihalides react with alcoholic silver nitrate within five minutes to form a precipitate of the corresponding
silver halide. This reaction can serve as a simple test for the presence of
bromine or chlorine atoms. You will determine the relative stereochemistry of your product by melting point measurement because the stereoisomers' melting points differ significantly.
Using Semi-Microscale
Techniques to Brominate Alkenes
Equipment
250-mL beaker
25-mL filter flask,
with vacuum tubing
filter paper
lO-mL graduated cylinder
Hirsch funnel, with adapter
magnetic stir bar
magnetic wand
melting point capillary tubes
micropipet, 100 to lOOO-IlL
2 Pasteur pipets, with latex bulb
reflux assembly
condenser, with tubing
25-mL round-bottom flask
thermometer, -10 to 260 0 C
sand bath*
spatula
2 support stands
13 x 100-mm test tube
2 utility clamps
'stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electric
flask heater filled with sand
@ 1998 Chemical Education Resources
171
SYNT: 719 BrominatingAlkenes
Reagents
-
clamp
clamp
substance
quantity
water out
condenser
1111
,.,
-
water in
roundbottom
flask
0
thermometer--IfI
and Properties
molar
mass
bp
(0C)
mp
(0C)
d
(g/mL)
(g/mol)
acetic acid*, glacial
cinnamic acid
1,2-dibromo1,2-diphenylethane
6mL
60.05
450 mg
148.16
1.049
133
340.07
t
2,3-dibromo-3-phenylpropanoic acidt
ethanol*,95%
pyridinium
tribromide*
118
307.97
0.5 mL
0.6-1.155 g 319.84
Figure 5 Semi-microscale reflux silver nitrate*,
apparatus
2% in ethanol
0.5mL
cis-stilbene
300 J1L
180.25
trans-stilbene
300 mg
180.25
14513mm
1.011
124
"amount for one bromination
tproduct
Preview
· Assemble the reflux apparatus
·
to aspirator
-
25-mLfilterflask
Figure 6 Vacuum filtration apparatus
PROCEDURE
·
·
·
Add the alkene,pyridinium tribromide,and aceticacid
·
·
Use vacuum filtration to isolate the product
Reflux the reaction mixture
Allow the reaction mixture to cool to room temperature
· Removethe stir bar
Add water and cool the reaction mixture in an ice-water bath
·
Dry and weigh the product
·
Test the product with silver nitrate reagent
Measure the melting point of the product
Chemical Alert
acetic acid-corrosive
cinnamic acid-irritant
ethanol-flammable
and irritant
pyridinium tribromide-corrosive
silver nitrate-toxic and oxidizer
and lachrymator
Caution: Wear departmentally approved safety goggles at all times
while in the chemistry laboratory.
@ 1998 Chemical EducatiQn Resources
1/1.
SYNT: 719 BrominatingAlkenes
1. Assembling the Apparatus
Assemble the reflux apparatus shown in Figure 5 on the previous page.
2. Brominating the Alkenes
Caution: Acetic acid is corrosive. Pyridinium tribromide is corrosive and a lachrymator. Prevent eye, skin, and clothing contact.
Avoid inhaling or ingesting these compounds. Use a fume hood to
dispense these reagents.
[NOTE 1]
NOTE 1: Your laboratory instructor
will designate which alkenes you will
brominate.
NOTE 2: The solid materials are not
soluble at room temperature in acetic
acid.
Cinnamic Acid
Caution: Cinnamic acid is irritating. Prevent eye, skin, and clothing
contact.
Remove the 25-mL round-bottom flask from the apparatus. Place 450
mg of cinnamic acid, 6.0 mL of acetic acid, and 1.155 g of pyridinium
tribromide in the round-bottom flask. Add a magnetic stir bar.
[NOTE
2] Proceed to Part 3.
cis-Stilbene
Remove the 25-mL round-bottom flask from the apparatus. Place 300 p.L
of cis-stilbene, 6.0 mL of acetic acid, and 600 mg of pyridinium
tribromide in the round-bottom flask. Add a magnetic stir bar. [NOTE
2]
Proceed to Part 3.
trans-Stilbene
Remove the 25-mL round-bottom flask from th~ apparatus. Place 300
mg of trans-stilbene, 6.0 mL of acetic acid, and 600 mg of pyridinium
tribromide in the round -bottom flask. Add a magnetic stir bar. [NOTE
2]
Proceed to Part 3.
3. Refluxing the Reaction
Reattach the round-bottom flask to the reflux apparatus. Start the flow of
water through the condenser. Heat the reaction mixture to reflux while
stirring. Reflux for 20 min. After 20 min, remove the flask from the heat.
Allow the reaction mixture to cool for 5 min. Turn off the water.
Remove the condenser and use a magnetic wand to remove the stir
bar. Add 8.0 mL of distilled or deionized water to the flask. Prepare an
ice-water bath by half filling a 250-mL beaker with equal volumes of ice
and water. Place the flask in the ice-water bath for 15 min.
4. Collecting, Washing, and
Drying the Crystals
While the reaction mixture is cooling in the ice bath, assemble a vacuum
filtration apparatus, as shown in Figure 6 on the previous page. Turn on
the water to the aspirator and moisten the filter paper with a few drops
of water. Filter the crystalline solid using the vacuum filtration apparatus. Wash the crystals with 3.0 mL of water. Allow the crystals to dry in
the Hirsch funnel by pulling air through the funnel for 15 min. Weigh
your dried product and record its mass.
5. Identifying the Product
Caution: Ethanol is flammable and irritating. Keep away from
flames or other heat sources. Silver nitrate is toxic and oxidizing.
Prevent eye, skin, and clothing contact. Avoid inhaling fumes and
ingesting these compounds.
@ 1998 Chemical Education Resources
~
173
SYNT: 719 Brominating Alkenes
Measure and record the melting point of the product. Using a small test
tube, dissolve approximately 10 mg of the product in 0.5 mL of 95%
ethanol. To this test tube, add 0.5 mL of 2% ethanolic silver nitrate.
Allow the test tube to stand for 5 min. Record the presence or absence
of a precipitate.
6. Cleaning Up
Place your recovered materials in the appropriate labeled collection containers as directed by your laboratory instructor. Clean your glassware
with soap or detergent.
Caution: Wash your hands thoroughly with soap or detergent before leaving the laboratory.
Using Microscale Techniques to Brominate Alkenes
Equipment
250-mL beaker
conical vial reflux assembly*
condenser, with tubing
5.0-mL conical vial
elastomeric connector
reflux assembly*
condenser, with tubing
elastomeric connector
5.0-mL round-bottom flask
25-mL filter flask,
with vacuum tubing
filter paper
forceps t
10-mL graduated cylinder
Hirsch funnel, with adapter
magnetic stir bar or spin vane
melting point capillary tubes
100-I1Lmicropipet
2 Pasteur pipets, with latex bulb
sanctbatht
spatula
2 support stands
13 x100-mm test tube
thermometer, -10 to 260°C
2 utility clamps
'use reflux assembly indicated by your laboratory instructor
tor a magnetic wand
~stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electric
flask heater filled with sand
Reagents and Properties
substance
acetic acid*, glacial
cinnamic acid
--
1,2-dibromo1,2-diphenylethane t
2,3-dibromo-3-phenylpropanoic acidt
ethanol*, 95%
pyridinium
tribromide*
molar
bp
mp
d
mass
(g/mol)
(0 C)
(0 C)
(g/mL)
2mL
60.05
118
150 mg
148.16
quantity
I.
t
'II
i
1.049
,
133
_i
-I
340.07
307.97
0.5 mL
200-385 mg 319.84
@ 1998 Chemical Education Resources
J
174
SYNT: 719 Brominating Alkenes
silver nitrate*,
2% in ethanol
0.5 mL
cis-stilbene
180.25
100 ilL
100 mg
trans-stilbene
14513mm
180.25
1.011
124
"amounts for one bromination
tproduct
Preview
·
·
·
·
·
·
·
·
·
·
PROCEDURE
Assemble the reflux apparatus
Add the alkene, pyridinium tribromide, and acetic acid
Reflux the reaction mixture
Allow the reaction mixture to cool to room temperature
Remove the stir bar
Add water and cool the reaction mixture in an ice-water bath
Use vacuum filtration to isolate the product
Dry and weigh the product
Measure the melting point of the product
Test the product with silver nitrate reagent
Chemical Alert
acetic acid-corrosive
cinnamic acid-irritant
ethanol-flammable
and irritant
pyridinium tribromide-corrosive
silver nitrate-toxic and oxidizer
and lachrymator
Caution: Wear departmentally approved safety goggles at all times
while in the chemistry laboratory.
1. Assembling the Apparatus
2. Brominating the Alkenes
[NOTE1]
NOTE 1: Your laboratory instructor
will designate which alkenes you will
brominate.
NOTE 2: The solid materials are not
soluble at room temperature in acetic
acid.
Depending upon your glassware, assemble the reflux apparatus shown
in Figure 7(a) or 7(b).
Caution: Acetic acid is corrosive. Pyridinium tribromide is corrosive and a lachrymator. Prevent eye, skin, and clothing contact.
Avoid inhaling or ingesting these compounds. Use a fume hood to
dispense these reagents.
Cinnamic Acid
Caution: Cinnamic acid is irritating. Prevent eye, skin, and clothing
contact.
Remove the 5.0-mL conical vial (or round-bottom flask) from the
apparatus. Place 150mg of cinnamic acid, 2.0 mL of acetic acid, and 385
mg of pyridinium tribromide in the conical vial (flask). Add a magnetic
stir bar. [NOTE
21 Proceed to Part 3.
@ 1998 Chemical Education Resources
175
SYNT: 719 Brominating Alkenes
Figure 7 Microscale reflux apparatus
with (a) conical vial or (b) round-bottom
flask and elastomeric connectors
water out
condenser
water in
round-bottom
flask
conical vial
stir bar
(a)
(b)
cis-Stilbene
_
-
to aspirator
25-mLfilterflask
Remove the 5.0-mL conical vial (or round-bottom flask) from the
apparatus. Place 100 }.lLof cis-stilbene, 2.0 mL of acetic acid, and 200 mg
of pyridinium tribromide in the conical v~al(flask). Add a magnetic stir
bar. [NOTE
2) Proceed to Part 3.
trans-Stilbene
Figure 8 Vacuum filtration apparatus
Remove the 5.0-mL conical vial (or round-bottom flask) from the
apparatus. Place 100 mg of trans-stilbene, 2.0 mL of acetic acid, and 200
mg of pyridinium tribromide in the conical vial (flask). Add a magnetic
stir bar. [NOTE
2) Proceed to Part 3.
3. Refluxing the Reaction Reattach the conical vial (flask) to the reflux apparatus. Start the flow of
water through the condenser. Heat the reaction mixture to reflux while
stirring. Reflux for 15 min. After 15 min, remove the vial (flask) from the
heat. Allow the reaction mixture to cool for 5 min.
Remove the condenser and use forceps or a magnetic wand to remove the stir bar. Add 2.5 mL of distilled or deionized water to the conical vial (flask). Prepare an ice-water bath by half-filling a 250-mLbeaker
with equal volumes of ice and water. Place the vial (flask) in the icewater bath for 15 min.
4. Collecting, Washing, and While the reaction mixture is cooling in the ice bath, assemble a vacuum
Drying the Crystals filtration apparatus, as shown in Figure 8.
Turn on the water to the aspirator and moisten the filter paper with a
few drops of water. Filter the crystalline solid using the vacuum filtration apparatus. Wash the crystals with 3 mL of water.
Allow the crystals to dry in the Hirsch funnel by pulling air through
the funnel for 15 min. Weigh your dried product and record its mass.
@ 1998 Chemical Education Resources
176
SYNT:719 BrominaringAlkenes
5. Identifying the Product
Caution: Ethanol is flammable and irritating. Keep away from
flames or other heat sources. Silver nitrate is toxic and oxidizing.
Prevent eye, skin, and clothing contact. Avoid inhaling fumes and
ingesting these compounds.
Measure and record the melting point of the product. Using a small test
tube, dissolve approximately 10 mg of the product in 0.5 mL of 95%
ethanol. To this test tube, add 0.5 mL of 2% ethanolic silver nitrate.
Allow the test tube to stand for 5 min. Record the presence or absence of
a precipitate.
6. Cleaning Up
Place your recovered materials in the appropriate labeled collection containers as directed by your laboratory instructor. Clean your glassware
with soap or detergent.
Caution: Wash your hands thoroughly with soap or detergent before leaving the laboratory.
Post-Laboratory Questions
1. (a) Compare the melting point of your product(s) to the data provided. In each case, identify the product you produced.
compound
2,3-dibromo-3-phenylpropanoic
mp (0C)
acid
(~)-1,2-dibromo-1,2-diphenylethane
94
203
110
meso-1,2-dibromo-1,2-diphenylethane
238
(~)-threo
(~)-erythro
(b) Draw your product in its correct stereochemical configuration.
(c) Compare your results with your predictions for Pre-Laboratory
Assignment question 4.
2. Calculate the percent yield that you obtained from your alkene
brominations.
3. (a) When silver nitrate solution was added to your product, what
did you observe?
(b) Explain your observations.
4. Write reactions for the brominations you performed, in each case
showing the intermediate bromonium ion that formed.
@ 1998 Chemical Education Resources