Experiment 2
DISTILLATION AND
GAS CHROMATOGRAPHY
OF ALKANES
Objectives



To explore the relationship between molecular
structure, intermolecular forces and boiling
points of various types of compounds.
To learn techniques of simple distillation and gas
chromatography (GC).
To use these techniques to separate and
identify compounds in an unknown mixture.
Before coming to lab…

It is *highly recommended* that you
watch the following video prior to
coming to lab.

http://www.youtube.com/watch?v=3JlI
PnyrZMw
Structure, IMF, and Boiling Point



Alkanes = consist of ONLY carbon and hydrogen.
Straight chain alkanes = greater surface area
contact = greater LDF between molecules =
HIGHER BP!
Branched alkanes = less surface area contact =
less LDF b/n molecules = LOWER BP!
Notice more
contact between
molecules here!
Table 2.1
Unknown
Pentane
(GC
SOLVENT)
Use www.chemexper.com
n-hexane
3-methylpentane
n-heptane
isooctane
(2,2,4trimethylpentane)
Molecular
Formula
bp
(oC)
Structure
These can be drawn in Kekulé
form OR skeletal!
H H H H H
H C C C C C H
H H H H H
Distillation

Distillation = separates/purifies liquids.



Heat mixture of two or more liquids in a flask.
More volatile liquid vaporizes 1st, vapor passes into
a water-cooled condensing column, it can reverts
back into a liquid (condenses) on the cool glass,
then trickles into a collection flask.
With further heating the less volatile liquids
vaporize and distill at higher temperatures.
Distillation
Involves vaporization, condensation, and
collection of the condensate (distillate).
 The temperature of the distillate is
measured by a thermometer placed in
the path of the vapor.

water out
water in
Heating Mantle
iron ring
to
voltage
regulator
Collect four 5 mL fractions
in small test tubes!
Simple Distillation


Simple distillation is most effective when the
mixture contains only one volatile component.
More than one….most effective if the boiling points
of the components differ by at least 70 ºC.
Chromatography



Chromatography = collective term for a family
of laboratory techniques for the separation of
mixtures.
Chromatography involves passing a mixture
dissolved in a mobile phase (which moves)
through a stationary phase (doesn’t move).
Different levels of interaction b/n compounds
and mobile vs. stationary phases separates the
analyte to be measured from other molecules
in the mixture and allows it to be isolated.
Gas Chromatography (GC)
He
Injection
port
Oven
Detector
Recordercomputer
Carrier Gas
Column


In GAS chromatography, the MOBILE PHASE = He(g).
Gas chromatography is only used to separate volatile
compounds in lab.
Gas Chromatography



STATIONARY PHASE = a nonpolar packing
material.
When analyzing compounds of similar polarity, the
compounds will elute in order of volatility.
Lower BP = low retention time (Rt)
Liquid Stationary Phase
A ABA
A
A
A A
BA B B
B
He Carrier gas
AA B
A BA
B
He Carrier gas
A
A
A A
B
B
B
B
B
B
B
A
B B
0
Immediately after injection
Higher
boiling
component
Lower
boiling
component
Capillary Tube
After several minutes
Time
Resulting chromatogram
EXAMPLE GC RESULTS
Original
Original
Sample
Sample
Mixture
Pentane
•The individual fractions
collected
areAlkane
analyzed
by GC during the
pentane
#1
( GC Sample
Solvent)
course of the distillation. Based on peak
is evident
that as
hexane size, it Alkane
#2
Unknown Xmainly the
the distillation proceeds, the later fractions contain
higher boiling component of the mixture.
Notice Fraction 1 contains
more of Alkane #1 (lo BP)
than Alkane #2 (hi BP)
Notice Fraction 5 contains
ONLY of Alkane #2 (hi BP),
and NO Alkane #1 (lo BP)
Fraction 2
Fraction 1
Fraction 5
Fraction 5
Product Analysis
(Quantification of Components…Adjusted Area Percent)



Most often you will dissolve your compound or
mixture in a low boiling solvent for GC analysis.
The relative areas of the components of
interest must therefore be adjusted, to exclude
the large % area of the solvent peak.
Refer to page 21 for further explanation.
Adjusted area % =
area % of peak of interest
sum of area % of ALL peaks of interest*
*omit area % of solvent peak
X 100
OVERVIEW

Select and obtain 25 mL of an unknown.

Set up and perform a simple distillation.

Collect (4)-5mL fractions in small test tubes.




Prepare GC samples of Fraction #1 and Fraction #4.
Submit for analysis.
Pick up GC results when ready and record standard
retention times from STANDARD CHROMATOGRAM.
Identify alkanes in unknown by comparing sample
retention times to standard retention times.
Quantify alkanes at beginning and end of distillation by
calculating Adjusted Area Percent.
Experimental Procedure
(Simple Distillation)







Add the unknown mixture and 3
boiling chips to a round bottom
flask.
Set up a simple distillation
apparatus.
water out
water in
Begin water flow through condenser
and apply heat to the flask.
Collect liquid distillate in small test
tubes as it leaves the condenser.
Keep track of the temperature
range for each fraction as it is
collected.
Collect (4) 5mL fractions, leaving
the final 5mL in the boiling flask.
Prepare GC sample for analysis.
Heating Mantle
iron ring
to
voltage
regulator
Product Analysis
(GC Analysis)
• Analyze your FIRST and FOURTH fractions by
GC.
•SAMPLE PREP:
• Transfer 5 drops of your sample into an auto
sampler vial using a plastic pipet.
• Add 1 mL of GC solvent (pentane) from the
solvent dispenser in balance room.
• Be sure to record your GC vial slot # in your
laboratory notebook.
• You will receive an email to your UNCW
account when GC results are ready for pick-up!
Table 2.3
• You cannot complete this table until you obtain
your GC results from the board outside of Dobo 203.
•The standard chromatogram will be provided on the
front of the envelope containing the chromatograms.
• Use only the standard run for your lab section!
Peak
#
1
2
3
Compound
Identity
Standard
Rt
(min)
Fraction #1
Sample
Rt
(min)
Area
%
Fraction #4
Adjusted
Area %
Sample
Rt
(min)
Area
%
Adjusted
Area %
SAFETY CONCERNS

All compounds used in today’s experiment
are extremely flammable. Wear safety
goggles at all times and use extreme
caution!
WASTE MANAGEMENT


Pour all liquid waste into the bottle
labeled “LIQUID ORGANIC
WASTE”.
Do NOT pour any waste down the
drain!
CLEANING


Disassemble distillation apparatus in your lab
hood.
In your lab hood, rinse all glassware with wash
acetone ONLY directly into a beaker from your
drawer (DO NOT REMOVE DISTILLATION
APPARATUS FROM YOUR HOOD AT ANY
TIME.)


Return distillation glassware to case in your hood.
Return all other glassware to your lab drawer.
LABORATORY NOTEBOOK
(Pre-lab)
o OBJECTIVE
(Must clearly state…)
oWhat is the goal of the experiment?
o What technique will you used to accomplish it?
o How will you determine if it worked?
o TABLE OF PHYSICAL DATA
(Complete the following table using
MSDS sheets from a site on WWW Links ONLY. Wikipedia is unacceptable)
Compound
Pentane
n-hexane
3-methylpentane
n-heptane
isooctane
MW (g/mol)
bp (Co)
o REFERENCE TO PROCEDURE
d (g/mL)
(Must include…)
o full title
o Edition
o authors
o page numbers where actual procedure can be found
HAZARDS
LABORATORY NOTEBOOK
(In-lab)
• DATA/CALCULATIONS
•
•
•
•
•
•
Record which unknown was used
Record distillation range of each fraction collected
Record GC vial slot number for fraction #1
Record GC vial slot number for fraction #4
Record GC sample solvent used
Show one EXAMPLE of an adjusted area percent calculation
• EXPERIMENTAL PROCEDURE
•
•
•
•
In paragraph form, briefly describe the procedure that you actually
followed during the lab.
Paragraph must be written in PAST TENSE, PASSIVE VOICE.
Include any volumes of chemicals used during the experiment
Include any mistakes, accidents, or observations if necessary.