Question

Both compounds prepared in this experiment must be distinguishable from one another. Determine 3 physical tests that can be done to distinguish the two isomers. Then, write simple procedures to carry out the actual tests using available lab equipment.

compunds: Maleic acid and Fumeric acid

Answer 1

This experiment deals with the topic of isomerism. Isomers are chemical compounds that have the
same chemical formula, but different chemical structure. A special type of isomerism, called
cis/trans isomerism, may occur in molecules which possess a double bond. Cis/trans isomers are
also called geometric isomers.
Why is geometric isomerism important? In many instances, the shift from one geometric structure to
another turns out to have profound chemical or biochemical significance. The chemistry of vision
provides an interesting example. As you may know from biology, the retina is our light detector at
the back of the eye. Photoreceptor cells in the retina (rod cells) contain 11-cis-retinal, a compound
derived from vitamin A. When light reaches the retina, the cis-retinal compound rapidly flips into the
trans-isomer, and the process gives out a signal to nerve cells and hence to the brain. Then, an
enzyme catalyzes the reverse process by flipping trans-retinal back to the cis-isomer, ready for the
next light signal. This is the role of cis-trans isomerism in vision.
Criteria for a compound to exhibit geometric isomerism
Geometric (cis-trans) isomers are compounds which have the same chemical formula but differ in
the spatial arrangement of the groups that are attached to a double bond or to a ring. How does this
come about?
 The molecule must have a double bond or a ring structure. There is free rotation around a
single (sigma) bond, and restricted rotation around a double (1 sigma + 1 pi) bond. Therefore,
groups which are attached to the carbon atoms of a double bond are held fixed on one side of
the double bond or the other. A similar argument applies to a cyclic (ring) compound.
 The two groups attached to the first carbon atom involved (in a double bond or ring) must be
different from each other. The other two groups attached to the other carbon atom must also
be different from each other. Otherwise the two isomers will be indistinguishable!

Notice that the two molecules have identical chemical formulas—they are both C4H4O4. The
difference is in the way the atoms and groups are arranged around the double bond. In Maleic Acid,
the cis isomer, the two carboxylic acid groups, —COOH, are on the same side of the double bond.
In fumaric Acid, the trans isomer, the carboxylic acid groups are on opposite sides of the double
bond and are connected to different carbon atoms.
Despite their similar chemical structure, maleic and fumaric acid are different compounds, with
different chemical and physical properties.The molecules do not normally interconvert between the two structures because there is no free
rotation about the double bond. Additionally, one of the two structures is usually more stable because
there is less steric hindrance (i.e. less crowding) between the functional groups of atoms. Which
isomer would you predict to be the more stable molecule, maleic or fumaric acid? Although the
double bond is fairly strong, the -bond can be broken with the input of energy—we will use heat as
the source of energy and enlist the help of a catalyst to speed up the reaction. When the -bond
reforms, it will do so in a way that gives the most stable geometric isomer that minimizes steric
hindrance.
Procedure (Work in Teams of Two)
Part 1. Observing the Physical Properties of the Unknowns A and B
1. Test the relative melting points of unknowns A and B by placing approximately equal amounts
of each unknown (about the size of a pea) in two separate test tubes The unknowns are maleic
and fumaric acid.
2. Place each test tube in a test tube holder and warm both samples over a Bunsen burner. Be sure
to heat gently (the flame of a properly adjusted Bunsen burner has a temperature of about 800
oC!!), by waving each test tube over the flame—i.e. Do not heat either of them directly in the
flame. Record your observations in table 1 noting which sample melts first. After this test, you
should be able to predict which compound is which.
3. Empty the contents of each tube into the waste container under the hood.
4. Test the solubility of each unknown by placing approximately equal amounts of each unknown
(about the size of a pea) in two separate test tubes. Add about 2 mL of DI water to each tube and
shake vigorously. Record your observations in table 1 and note the relative solubilities of each
unknown. At this point you should know the identity of each unknown!Part 2. Breaking the Double Bond in an Isomerization Reaction
1. Set up an hot water bath by adding 200 mL water to a 400 mL beaker and placing on an electric
hot plate. Allow the water to come to a boil. Meanwhile, while the water bath is warming up…
2. Accurately weigh out about 1.0 g of the more soluble compound (record the mass in table 2 to ±
0.001g) and add it to a clean 125 mL Erlenmeyer flask (it’s O.K. if the flask is a little wet).
3. Use a small graduated cylinder to add 10 mL of DI water to the flask. Gently swirl the flask to
dissolve as much of the solid as possible. Any undissolved solid will dissolve when the solution
is heated.
4. Take the flask to the HCl dispenser under the hood and use the pipette pump to add 10 mL
concentrated HCl (i.e. 12 M HCl). HCl acts as a catalyst in the isomerization reaction.

5. Set up the isomerization apparatus as shown in figure 1.
 Insert into the flask a one-hole stopper with a long glass tube
to act as a reflux condenser to trap and limit the escape of
gaseous HCl while heating.
 Caution! After the isomerization reaction the reflux
condenser will be contaminated with concentrated HCl.
 Rinse the reflux condenser with water in the sink
immediately after removing it from the flask at the end
of the isomerization reaction.
 Clamp the neck of the flask to a ring stand
 Submerge the flask slightly in the boiling water.
6. Allow gentle heating for 10 minutes. Then turn off the hot plate.
Record your observations during the course of the reaction in
table 2.
7. Remove the flask from boiling water and allow it to cool for 10
minutes.
8. While you wait for the flask to cool…
 Set up an ice bath in a 400 mL beaker and begin cooling 20
to 25 mL of DI water.
 Carry out steps 2- 4 above with the other unknown—stopper
the flask t to prevent the release of HCl gas into the room.

After the flask (in step 7) has cooled for 10 minutes place it in the ice bath.

While the flask containing the reaction product is cooling in
the ice bath, prepare the apparatus for suction filtration as
shown in figure 2. Connect the hose to a vacuum line. You
must clamp the flask, and keep it clamped throughout
(otherwise the funnel or the flask, or both, will tip over and
break!).
11. Carry out suction filtration as follows:
 Weigh a piece of filter paper of correct size (record the
mass in table 2) and place it in the Büchner funnel. Use a
squirt bottle to wet the filter paper with distilled water
and make it snugly fit over the holes.
 Turn on the vacuum.
 Hold the cooled reaction flask and swirl it before rapidly
pouring its contents onto the filter paper in the Büchner
funnel. Your isomerization product is the solid that
collects on the filter paper.
 Add 5 to 10 mL of ice cold DI water to the 125 mL
flask, swirl to dislodge any remaining solid product and
then pour into the Büchner funnel.
12. Wash the solid on the filter paper with another 5 to 10 mL of
ice cold DI water—why use ice water? (Hint: see the
solubility data in the table on page 2.) Continue suction for
another five minutes to dry the solid. While the solid is
drying…
 Repeat the isomerization reaction with the other
unknown—i.e. repeat steps 5 – 13 with the unknown
having the lowest solubility in water.
13. After at least 5 minutes of drying, remove the filter paper
and product from the funnel. Weigh the filter paper and
product in a plastic weighing boat of known mass. After
weighing, label the boat so that you remember which sample
is which. Record the mass and your observations about the
appearance of the product in table 2.
14. Test the solubility of both samples by placing approximately
equal amounts of each product in two separate test tubes.
Add 2 mL of deionized water to each tube and shake
vigorously. Record your observations in table 2.
15. Clean-up: Place your unknown samples and the filtrate from
suction filtration in the waste containers under the hood.
 Caution!! The filtrate and the reflux condenser each
contain concentrated HCl!! After emptying the filtrate
into the waste container, rinse the Büchner flask and the
reflux condenser with copious amounts of tap water and
flush down the drain. Use DI H2O for the final rinse.