Free Radical Chlorination of Heptane
Experiment
In today’s experiment, you will be chlorinating n-heptane with chlorine atoms (radicals) to form
monochlorination products of that alkane. You will analyze the product mixture by gas
chromatography to compare the amount of substitution of primary hydrogens versus secondary
hydrogens.
For the most part, alkanes are among the least reactive of organic compounds. Of course they
burn, and this combustion of alkanes and other hydrocarbons provides the energy that powers
nearly all commercial transportation. The other major reaction of alkanes is free radical
halogenation. In this reaction, a halogen atom is formed by one means or another. Historically,
they were first formed by the homolytic cleavage of the halogen-halogen bond by ultraviolet
light.
uv hν
X–X
2 X·
Bromine atoms can be generated this way in liquid solutions or by N-bromosuccinimide, NBS,
which you will learn more about later in the semester. Working with a toxic gas like chlorine,
however, is difficult without using special equipment. However, if the chlorine is generated in a
liquid solution and immediately irradiated with ultraviolet light, chlorine atoms are formed in
solution without the use of any special equipment. In this experiment, this is done by reacting
sodium hypochlorite (NaClO), from liquid bleach, with hydrochloric acid in the presence of light
as shown in the following sequence of equations.
Cl2(g) + NaCl + H2O
NaClO + 2HCl
Cl2(g)
light
2Cl
Once formed, the chlorine radical can react with the heptane as shown below:
R
H
+
Cl
R
+ HCl
In this first propagation step, chlorine radical abstracts a hydrogen
to form an alkyl radical and HCl gas. The alkyl radical then reacts with
another molecule of Cl2 to form the halogenated hydrocarbon product. This
also regenerates Cl radical to continue propagation of the chain reaction.
R
Cl
Cl
RCl
+
Cl
Unlike bromine, which is very selective, chlorine atoms are more reactive and give multiple
monochlorination products. Gas chromatography of the product mixture will be used to analyze
the monochlorination products that are formed.
 Pre-lab Preparation
Before coming to lab, you must do the following in your lab notebook:
1. Write structural formulas for all of the monochlorination products you would expect from
this reaction.
2. Write in tabular form the relevant physical constants for heptane and the
monochlorination products you have listed above
3. Answer the following questions about the procedures given below.
a. What has been done in this experiment to minimize the reaction of the
monochlorination products with additional chlorine radicals to give di- and
trichloro products? (Consider possible ways to prevent multiple chlorinations.)
b. What is the purpose of the sodium bicarbonate washes in the purification of the
monochlorinated heptanes.
c. The gas chromatogram of your product mixture should show separation of the
primary from the secondary products. Referring to your physical property data,
predict which compounds you would expect to elute from the column first – the
primary product or the mixture of secondary products? Explain.
Experimental Procedure
! Safety Considerations
! Hydrochloric acid can cause burns. Use caution when handling HCl and clean any spills
immediately.
! Bleach can irritate your skin and damage clothes. Clean any drips or spills immediately.
! Hydrochloric acid evolves as a gas forming in the heptane during the reaction. Do not use any
unvented caps in this laboratory procedure.
1. First, assemble an apparatus in which the chlorine gas will be generated. See the photograph
below. Obtain a polyethylene bottle, a dropper cap and a piece of tubing with a syringe
needle attached to one end and a vented cork attached to the other end. Connect the tubing to
the dropper cap of the polyethylene bottle using the adapter on the end of the syringe. This
assembled apparatus is your chlorine generator; when you add the bleach and HCl to this
apparatus, chlorine will be generated and will flow out of the tubing. You will also need a
50 ml beaker to hold the polyethylene bottle to prevent it from tipping over.
You must carry out all of the procedures in steps 2, 3, and 4 be1ow in the fume hood.
2. Obtain a small test-tube, add 3 mL of heptane, and then place it in a test-tube rack in the
hood. Fit the vented cork which is attached to the polyethylene tubing into the top of the
test-tube so that the end of the polyethylene tubing is positioned at the bottom of the test
tube, fully immersed in the heptane. Rest this test-tube set-up back in the rack.
vented stopper for
test-tube with
heptane
chlorine generator with
bleach and HCl
3. Obtain a lamp for irradiation of the sample and position it near the test tube and turn it on.
Now you are ready to add the bleach and HCl to the dropper bottle to begin generating
chlorine. First add 5 mL of bleach to the polyethylene bottle followed by two 1.5 mL
portions of 6.0 M HCl. (Have 3 mL of HCl ready in a graduated cylinder and add about half to
the dropper bottle.) Immediately after adding the HCl, replace the dropper cap to force the
chlorine gas into the tubing. You should see bubbles of Cl2 flowing out of the end of the tube
which is immersed in the heptane. Swirl the test-tube as the sample is irradiated by the light
from the lamp. Once the bubbles slow, add the remaining HCl and cap quickly to generate
more chlorine gas. Note: If you do not see bubbling in the heptane for several minutes, you
will not obtain sufficient product. If necessary, empty the dropper bottle and repeat the bleach
and HCl additions until you are certain that you achieved several minutes of bubbling while
irradiating with the lamp.
4. While the reaction proceeds, continue to swirl the tube in front of the light and occasionally
swirl the bleach solution to continue generating chlorine. Note any color changes that you see
come and go from the heptane solution as the reaction progresses. After allowing the reaction
to proceed for approximately four or five minutes, you should see that the bubbling has
slowed. When the bubbling stops, it is very important to immediately remove the tubing from
the hexane to prevent the product from being pulled back into the bleach solution by the
vacuum created as the chlorine generator cools. You should, however, continue to irradiate
the test tube for another minute or so after removing the tubing. Your reaction should be
complete at this point. Empty the contents of the bleach/HCl apparatus into the waste
container.
5. You will need to do a couple of washings to remove residual HCl (a by-product generated in
the heptane by the chlorination reaction). First, pour the heptane/chloroheptanes mixture into
a large vial and then add 1 mL of 5% sodium bicarbonate solution. Cap the vial and shake to
neutralize any acid. Remove the cap frequently to vent. Allow the layers to separate. (Note
that you are basically using the vial as a separatory funnel for this small scale reaction).
Decide which layer is the aqueous layer and remove the aqueous layer by drawing it into a
disposable pipette and transferring it to a small beaker. (To do this, you will need to squeeze
the air out of the bulb, insert the pipet all the way to the bottom of the bottom layer, and
slowly release the pressure on the bulb to draw the bottom layer into the pipet.) Save all
layers in case of error. Now wash the organic layer with 1 mL of deionized water and again
remove the water layer. Pour the finished organic sample into another clean, dry vial (be
certain to leave any traces of water behind when you transfer) and add enough calcium
chloride pellets to dry the sample.
6. Run a gas chromatogram of the sample to analyze the products that were formed. You
should observe that separation is achieved on the GC between the primary and secondary
products, but within the various secondary products, no separation is observed. Therefore,
because their structures and boiling points are so similar, all of the secondary products will
elute together as one peak. You will also see a very large peak for unreacted heptane, which
will elute with a shorter retention time than the chlorinated products. Look for a reference
GC chromatogram of heptane posted in the lab near the GC.
7. Dispose of waste into the waste container and clean all vials, test-tubes and glassware.
 Post-Lab and Report Requirements
The lab report for this experiment will consist of the following:
1. Write a detailed mechanism showing how the chlorine atom (radical) reacts with heptane
to form one of the products you have just written. Your mechanism must show all chain
propagating steps and chain terminating steps. You do not have to show how chlorine
gas was formed by reaction of the bleach with HCl.
2. Based on the number of secondary versus primary hydrogens in a molecule of heptane,
calculate the expected ratio of substitution at the secondary position compared to primary
positions, assuming all hydrogens to be equally reactive.
3. Assuming that your two chloroheptane GC peaks are triangles, calculate the areas of the
two peaks. (Do this my measuring the width of the peak at half-height and multiplying
this by the height of the peak.) How does the ratio of the 2°:1° peak areas compare to the
2°H:1°H ratio you calculated in question 2 above? If these ratios differ, give a detailed
explanation of why they differ.
4. If you could measure a yield for this experiment, you would find that you did not get a
100% yield of your products. Give chemical reasons why you didn’t.