Experiment 3 - Organic Chemistry at CU Boulder

Experiment 3
Distillation: Separation of a Mixture of Methyl Acetate and Propyl Acetate
Reading: Handbook for Organic Chemistry Lab, Sections on Simple Distillation, Fractional Distillation, and Gas
Chromatography.
Distillation is the process of vaporizing a liquid, condensing the vapor, and collecting the condensed
liquid (or condensate) in a different container. It is a general technique that permits liquid compounds to
be purified or solvents to be selectively removed from non-volatile materials. Simple, fractional, steam,
and vacuum distillation are four modifications of the basic distillation technique.
If a perfect separation of two components A and B is achieved during a distillation, a plot of
temperature vs volume of condensate looks like the ideal graph (Figure 3.1). The entire lower boiling
component A distills at its boiling point until it is removed from the mixture; then, the higher boiling
component B distills at its boiling point. Ideal separations are not achieved when separating mixtures of
compounds with boiling points closer together than 10°C. This is because the component B has an
appreciable vapor pressure at the boiling point of component A. In a laboratory situation, one can plot
the volume of distillate vs. temperature of the distilling vapor to determine how closely a distillation
resembles an ideal separation.
temperature ˚C
pure B begins to distil
boiling point of pure B
pure A begins to distil
volume of distillate
“curve” for an ideal distillation
curve for simple distillation
curve for fractional distillation
boiling point of pure A
Figure 3.1 » Ideal, fractional and simple distillation curves.
In this experiment, you will use the techniques of simple and fractional distillation to separate two
liquids, methyl acetate and propyl acetate. The structures of these compounds are shown below (Figure
3.2). Both compounds are esters based on acetic acid; the acidic hydrogen in acetic acid is replaced with
an methyl group or a propyl group respectively.
Figure 3.2 » Structures of methyl acetate and propyl acetate, and their parent compound, acetic acid
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Experiment 3: Distillation
You will compare the efficiencies of the separations achieved in the two distillation techniques, both by
monitoring the temperature of the condensing vapors as a function of the quantity distilled and by
analyzing the composition of two samples of the distillate by gas chromatography.
Gas Chromatography
Gas chromatography (GC or GLC) is an important chromatographic technique used by the organic
chemist. In the gas chromatograph an inert carrier gas constitutes the moving or mobile phase while a
high-boiling liquid layer deposited on an inert solid support makes up the non-moveable component.
Gas chromatography is explained in detail in the Handbook for Organic Chemistry.
Safety Precautions
Methyl acetate and propyl acetate are flammable. Do not distill to dryness, since it can lead to a
potentially hazardous situation.
Procedure
In this lab the TA will assign each student either simple or fractional distillation. During the lab, make arrangements with a student
who is performing the other type of distillation so you can compare data in your lab report.
You are to distill 25 mL of a 1:1 (by volume) methyl acetate:propyl acetate mixture using either the
simple or fractional distillation setup. Your TA will assign you one of the two techniques; however, you
are responsible for coordinating with another student and making copies of each other’s data. This way
you can discuss the difference between the two data sets in your lab write-up. Set up your apparatus as
illustrated in Figure 3.3 (simple distillation) or Figure 3.4 (fractional distillation). The fractionating
column, if you need one, is available from your TA. This is a condenser with glass prongs at the inside
bottom, to hold a stack of glass beads in place. Please do not allow these glass beads to fall out of the
column – they can be a tripping hazard if they are on the floor.
Place a heating mantle under your 100mL round bottom flask. Plug the heating mantle into a Variac—
never plug a heating mantle directly into an electrical outlet—but do not turn the Variac on yet. Ensure
that the thermometer bulb is just below the bend of the Y-adaptor.
If you are performing a fractional distillation, the apparatus will be taller and you may have some
difficulty fitting it into your fume hood. It will still fit if you: 1) Rotate the ring stand base to one side, so
the heating mantle is resting directly on the bench, 2) Before inserting the thermometer, slide it down
through the thermometer adapter, so it’s temporarily shorter, 3) Tilt the entire setup towards you slightly
and insert the thermometer, then rotate the setup back into place. Remember to slide the thermometer
back up, so the bulb is in the right position.
Place 25 mL of the 1:1 methyl acetate:propyl acetate mixture in the round bottom flask; don’t forget to
add boiling chips to this flask.
Set the Variac to 50, and then turn on the Variac power. When the mixture begins boiling, adjust the
Variac setting as necessary so that the distillate collects at a rate of 1–2 drops/sec.
As the distillation proceeds, you should collect the liquid in a 10 mL graduated cylinder. Record the
temperature of the vapors at the distillation head as a function of the volume of condensate (take a
reading about every 1.0 mL). When 3 mL have been collected, remove the graduated cylinder and
substitute it with a sample vial. Collect about 20 drops (0.5 mL); cap and save this sample for GC
analysis (if you fail to cap your vial, your sample will evaporate!) Then, put the 10 mL graduated cylinder
back under the vacuum adaptor and continue collecting.
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Experiment 3: Distillation
Continue recording the temperature of the vapors every 1.0 mL (You may have to turn up the Variac if
the distillation slows down during the process; try setting it to 60). When 15 mL have been collected,
again remove the graduated cylinder, substitute it with a sample vial, and collect 20 drops. You will have
to empty the 10 mL graduated cylinder once.
Discontinue the distillation after 20 mL have been collected or before the distillation pot is dry.
round bottom
flask
heat source
goes here
thermometer
Y-adaptor
Figure 3.3 » Setup for a simple distillation. Neither
the clamps nor the Keck clips are shown.
Gas Chromatography
Figure 3.4 » Setup for a fractional distillation. Neither
the clamps nor the Keck clips are shown.
Run a Gas Chromatography trace for each of the samples you collected. The general instructions for
GC will be covered by your TA and are also posted above each GC instrument. Use the following
settings for each run:
Start temperature 65C
Hold time 1 min
Ramp rate 10C/min
Final temperature 65C
Hold time 2 min
Total length 6.0 min
Pressure 7.0 kPa
Analysis of Data
water out
condenser
water in
receiving flask
goes here
thermometer
adaptor
condensor filled
with glass beads
(available from
your TA)
condenser
vacuum adaptor
1) Input your data into a spreadsheet program like Excel and use it to generate a graph of
temperature vs. volume of distillate for the distillation you performed. Place the simple and
fractional data on the same graph. Add to the graph the ideal curve expected for a mixture of
methyl acetate and propyl acetate. If you need help, you can search for “how to create a scatter
plot in Excel” – there are numerous tutorials available online.
2) Decide which peak belongs to which compound in your GC printouts and report the relative
water out
heat source
goes here
water in
receiving flask
goes here
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Experiment 3: Distillation
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compositions of the mixtures.
Wastes
Place the pot residue, all distillation fractions, and the GC samples in the “Recovered Distillate” bottle
in the main hood. This mixture of methyl acetate and propyl acetate will be recycled for use in future
laboratory sessions. Make sure that you do not contaminate the recovery bottle with water or acetone.
Used boiling chips can be placed in the white solid waste bins around the lab.
Study Questions
1) In Boulder, CO, the atmospheric pressure is always less than standard atmospheric pressure, and
therefore the observed boiling points will are lower than those reported in the literature. The
barometric pressure in Boulder is usually around 625mm Hg (625 Torr). What will be the
observed boiling points of methyl acetate and propyl acetate? (Refer to the first three pages of
the Distillation chapter in the Handbook for Organic Chemistry.)
2) A gas chromatograph is being used to study a mixture of chlorobutanes. The column is 1.5
meters long and contains 5% silicon elastomer on Chromosorb W; the column temperature is
60°C, the injector and detector temperatures are 200°C; the carrier gas flow rate is 40 mL/min.
Under these conditions, the retention times of five chlorobutane isomers are:
Compound Retention time
1-chlorobutane 0.4 min
1,1-dichlorobutane 0.95 min
1,2-dichlorobutane 1.1 min
1,3-dichlorobutane 1.3 min
1,4-dichlorobutane 2.3 min
a. A small sample of an unknown is injected into the GC under the identical
conditions listed above, and the GC trace below is obtained (see next page). What
compounds might be present in the unknown and what percent of each is present?
START
.61
RT AREA TYPE AR/HT AREA%
0.61 XXXX XX XX 0.009
1.11 XXXX XX XX 55.874
2.27 XXXX XX XX 44.117
TOTAL AREA=XX
MUL FACTOR=XX
2.27
1.11

Experiment 3: Distillation
b. What effect would raising the column temperature of the GC to 90°C have on the
retention times of the compounds in (a)? (Assume all other conditions are identical.)
3) Why does a rapid distillation that floods the fractionating column lead to poor separation of
components?
4) Two alcohols have a low retention time and are not separable on a silicone GC column. Explain
why they have a longer retention time and are separable on a Carbowax column. (Hint: the
structures and relative polarities of the two types of columns are given in the gas
chromatography section in the Handbook for Organic Chemistry.)
5) What are the pros and cons of rinsing a microsyringe with the mixture to be analyzed, rather
than with a solvent such as acetone or ethanol?
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Experiment 3: Distillation
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