Experiment 5. The C
Chemistry of Alkenes: Cis-Trans
Trans
Isomerization, Addition and Oxidation Reactions.
References:
Brown & Foote, Chapters 2,
2,and 6-8
INTRODUCTION:
In this experiment you will investigate some of the common alkene reactions. In Part A, you will
carry out a cis-trans
trans isomerization. In Part B, you will examine some addition reactions and Part
C will involve an oxidation.
BACKGROUND:
Figure 1: Reactions of Alkenes
Part A: Cis-Trans
Trans Isomerization of Maleic Acid.
Maleic acid and fumaric acid have the same molecular formula, C4H4O4, but different structures.
They are known as the cis and trans isomers respectively, and have widely different physical,
physi
chemical and biochemical properties. As shown below, the carboxylic groups in maleic acid lie
on the same side of the double bond [[cis- or (Z)-isomers];
isomers]; the carboxylic groups in fumaric acid
lie on opposite sides of the double bond [[trans- or (E)-isomers].
ers]. Using a strong acid catalyst
coupled with a strong nucleophile (such as HCl), maleic acid can be converted to fumaric acid,
as shown below. The reaction proceeds via a conjugate 1,4 addition followed by a rotation and
then the loss of HCl (reverse of the 1st step).
Part B: Addition to Alkenes.
There are a number of reagents that may be added to an alkene, some with different mechanisms
(see Solomons & Fryhle, Ch.8). However, the final products can fall into two categories, syn and
anti additions, as shown in Figure 1. You should make sure you understand the addition
reactions.
In Part B, you will briefly examine the Radical Halogenation of alkanes (see Solomons & Fryhle,
Ch.10), so you should also make yourself familiar with Radical Hal
Halogenation.
In today’s experiment we will use cyclohexene as a typical alkene, ligroin (also known as
petroleum ether which is mostly hexanes) boiling at 66
66-75 oC as a typical mixture of alkanes, and
unpurified ligroin as an impure alkane.
Part C: Reduction/Oxidation of Alkenes
Alkenes.
Alkenes can either be reduced to an alkane or oxidized. In this experiment we will examine an
oxidation reaction. Our oxidant in this experiment will be potassium permanganate and it will
follow this general equation:
There
re are also oxidation reactions which result in the cleavage of the C=C and the products are
some type of carbonyl or carboxyl functional group (depending on the oxidant and reactant), but
we will not be examining these O
Oxidative Cleavages in this experiment.
PRE-LAB
LAB PREPARATION
PREPARATION:
Read the experimental procedure so that you are prepared for the lab and you understand the
safety and disposal information for the chemicals you are using in this experiment.
1. Which would you expect to be more thermodynamic
thermodynamically
ally stable, fumaric acid or maleic acid?
Why?
2. Write general equations for reactions used in Parts B & C (synthesis not mechanisms). Use
generic functional groups (R1, R2 etc)) for these reactions, like in the background section.
3. Draw the products of bromination of Z- and E-2-butene. Specify which (if any) of these
products is optically active. Are any of the structures identical?
EXPERIMENTAL PROCEDURE:
Start with Part A and perform Parts B, C and D as time permits during the lab (ie. during the 30
min reflux). You should finish all Parts by the end of the lab. All work MUST be done
individually.
Safety and Disposal Data for Compounds used in Experiment 4.
Compound
Mol. Wt. (g/mol)
Safety and Disposal Data
Bromine
159.80
Very toxic. Corrosive. Causes severe burns. Use gloves
when handling. Dispose in Halogenated Organic Waste.
Chloresterol
386.66
Avoid contact with skin and eyes.
Chlorofrom
119.37
Harmful if swallowed. Irritating to skin. Dispose in
Halogenated Organic Waste (Contains Br2).
Cyclohexane
84.16
Highly flammable. Harmful. Irritating to skin. Dispose in
Halogenated Organic Waste (Contains Br2).
Cyclohexene
82.14
Highly flammable. Harmful in contact with skin and if
swallowed. Dispose in Halogenated Organic Waste (Br2).
Fumaric Acid
116.07
Irritant. Dispose in Solid Waste.
Gasoline
2M & 6M (HCl)
Hydrochloric Acid
Ligroin
---
Highly Flammable. Irritant. Harmful if swallowed. Dispose
in Halogenated Organic Waste (Contains Br2).
36.46
Irritating to eyes, respiratory system and skin. Use gloves,
especially when handling 6M HCl. Dispose HCl in
‘Inorganic Acids and Salts’.
---
Highly Flammable. Dispose in Halogenated Organic Waste.
Maleic Acid
116.07
Irritant. Dispose in Solid Waste.
Pinene
136.23
Flammable. Irritating to eyes, respiratory system and skin.
Dispose in Halogenated Organic Waste (Contains Br2).
Potassium
Permanganate
(in 10% H2SO4)
158.03
Strongly Corrosive!! Oxidizing. Harmful if swallowed.
Dispose in ‘Inorganic Acids and Salts’
Equipment Used
All the equipment needed to perform this experiment should be in one of the three drawers in the
top row of your workstation, one of the drawers underneath your fumehood or the common
counter for your TAs group.
However the microkit (microscale glassware) should be obtained directly from your TA. At the
end of the experiment, return your complete CLEAN! microkit and get the TA to initial the
return of your microkit.
Part A: Cis-Trans Isomerization of Maleic Acid.
1. Refluxing Maleic Acid
WEAR GLOVES!!!
a) You should begin the experiment by washing the vial, the
condenser AND the blue connector. If the previous person to use the
equipment didn’t rinse it, there may be concentrated HCl on the
equipment.
b) Dissolve 1g of maleic acid in 1mL of warm water in a 10 mL
cylindrical vial. Add 2 mL of conc. HCl and a boiling chip.
c) Attach a micro-condenser to the vial, and then attach the
condenser to a water supply (water goes in through the bottom, out
through the top). Turn on the water supply, so that water is trickling
out the exit tubing.
d) Commence heating using the aluminum block, placed directly on
top of the heater-stirrer.
e) Once the solution starts to boil, the vapour will condense and fall
back into the vial, known as refluxing. The reaction should reflux
for 30 minutes. Once a good stable reflux is established, take the time to start the other parts (just
keep an eye on the time!)
2. Collecting and Purifying the Fumaric Acid
a) Cool the mixture to room temperature and then in an ice-water bath.
b) Vacuum filter the crystals using 2 pieces of filter paper on a Hirsch funnel. Use the filtrate to
rinse the cylindrical vial. BE CAREFUL! You are filtering ACID! If you don’t use more than
one piece of filter paper, the acid eats through it and you will not collect any crystals.
c) Recrystallize your Fumaric acid from 2M HCl (heat ~25 mL). If you forget how to
recrystallize, refer to Exp. #3. Do not heat the solution very hot during the recrystallization!
d) Dry the product as thoroughly as possible and weigh it.
e) Calculate the % yield of your purified product.
f) Determine the melting point of the starting Maleic Acid and your Fumaric Acid product. The
melting point of one of the isomers is ~150°C above the melting point of the other isomer. So
once your first isomer melts you may want use the Heat Booster on the Melting Point Apparatus.
g) Run an IR of your Fumaric Acid to verify that you still have carboxylic acid groups and that
you have not reacted and modified some of the functional groups or added new ones.
SAFETY NOTE: Since you were using concentrated HCl, please rinse the glassware and
connectors, so you don’t burn the next person to use the equipment.
Part B: Addition to Alkenes.
Record your results in a table format. Make good/brief observations!
1. Addition of Bromine (in Chloroform)
a) Put two 1 mL portions of purified ligroin in two test tubes, 1 mL of unpurified ligroin in a
third and 1 mL of cyclohexene in a fourth.
b) Add 5-6 drops of a 3% solution of bromine in chloroform into each test tube (one at a time).
c) If decolourization occurs, test for the evolution of hydrogen bromide (due to Radical
Halogenation) with wet litmus.
d) Place one of the purified ligroin-containing tubes in your locker out of the light (AS SOON
AS you add the bromine!) and expose the other to bright sunlight or hold it close to a light bulb.
If decolourization occurs, test for hydrogen bromide, as before. When a change is noted in the
sample exposed to light, compare its appearance with that of the mixture kept in the dark.
2. Addition of Bromine-Water
a) Measure 3 mL of a 3% aqueous solution of bromine into each of three test tubes.
b) Add 1 mL portions of purified ligroin to the first test tube, 1 mL of unpurified ligroin to the
second and 1 mL of cyclohexene to the third.
c) Stopper each test tube and shake gently to transfer some bromine-water to the organic phase.
Part C: Reduction/Oxidation of Alkenes.
1. Oxidation using Acid Permanganate
a) Put 1 mL portions of purified ligroin in a test tube, 1 mL of unpurified ligroin in a second and
1 mL of cyclohexene in a third.
b) Add permanganate solution (1% potassium permanganate in 10% sulfuric acid) dropwise until
a purple color persists. Do you observe a difference in behaviour among the substances under
test?
Part D: Test for Unsaturation.
The above, simple reactions (Parts B1, B2 & C1) can be used as a ‘test’ to differentiate alkanes
from alkenes or to detect the presence of alkenes in alkanes. Use one of these ‘tests’ to determine
which of the following hydrocarbons are saturated and which contain unsaturated material. Use
approx. 1 mL of each trial compound and rank them from most to least saturated.
Pinene, the principal constituent of turpentine oil
Gasoline
Cyclohexane
Cholesterol (a 10% solution in CHCl3 was made for the tests).
Note: Since this is a problem based lab, you will be required to include a brief conclusion of
your findings.