Check In, Melting Point, Infrared Spectra and
Gas Chromatography
Preparation: Reading preparation: Mayo, et al., pp. 1-9, 16-21, 65-71, 166-168, and 237. In addition, look up in the Handbook the melting point of anthracene and the boiling points of 1,1,1-trichloroethane and cyclohexane. They are listed alphabetically in the section entitled "Physical Constants of Organic Compounds". Anthracene and cyclohexane are unmodified elementary compounds so they will be listed as headings and not preceded by a hyphen; 1,1,1-trichloroethane, on the other hand, is modified from ethane. Find Ethane then look down the heading list until you find "-1,1,1-trichloro-". (Super- and subscripts to the numbers often refer to conditions of pressure, next significant figure etc.) Record these melting - and boiling points here:
Become familiar with the Handbook, particularly the Organic Compounds section as it will provide important information such as densities, molecular weights and structures for future labs.
Students will rotate in 4 groups to do four activities:
1) check out equipment and familiarization with the lab
2) determine the melting point of the substance anthracene
3) run a gas chromatogram
4) run an IR spectrum
In your notebook, write the entire lab as one
"experiment" with one Purpose (to become familiar with
the with the equipment, prepare a determine a melting point of
anthracene, run a gas chromatogram and an infrared spectrum,
etc.). Procedure, set of Data, Discussion in which the melting
point from your apparatus is compared with the readings of other
students from other apparatuses, etc. and a Conclusion which
responds to the questions below.
Activity 1: Check to see that your drawer is complete according to the inventory list. Use pictures in Mayo, et al. p 17-18 and wall charts to identify pieces. Nothing should be missing, and you are responsible for the entire set of equipment until it is checked out at the end of the term. Obtain any missing items from the stockroom to complete the drawer. Mark your name on the laboratory map under your drawer number.
Familiarize yourself with the locations of the reagents. What is another name for methylene chloride?
What is the temperature of hot running tap water?
(This can be done as a group) Determine the rate of heating by a hot plate by placing about 50 mL of water in a 100 mL beaker. Heat the beaker on a Thermolyne stir plate with a heat setting of 8 and take readings every 5 minutes. Plot the temperature (vertical scale) vs. time (horizontal scale).
Activity 2: Measure the a melting point of anthracene which is on the reagent shelf. Use the Mel-temp apparatuses. Each student loads a melting point capillary tube by stabbing the anthracene crystals with the open end of the capillary tube; the tube is then bounced closed end down on the lab bench until the crystals reach the bottom of the tube. Place the tube closed end down into a slot of the Mel-Temp. Turn it on, set at 70% of maximum and observe the thermometer and the crystals through the eyepiece. The temperature will rise at about 3-6 degrees a minute. Observe and record the temperature at which the solid begins to melt, and also the temperature at which the last solid melts. Report these temperatures as the melting point range. Compare and record the mp's of anthracene with the members of your the group my setting up a table with the melting points and Mel-temp numbers used by each student in the group, including yours.
Activity 3 - Routine use of the Gas Chromatograph. The instrument is a Gow-Mac with a Carbowax 20 M (polar) column and a DC-200 (non - polar) column. The carrier gas is helium, pressure normally at 14 pounds per square inch; the flowmeter should read 20-25 ml/min. Temperature settings should be 110 oC. (dials at 35-40) for the inlet, outlet and column temperature. The instrument will be set up for your work; make sure that the detector power is on only when helium is flowing. The pen should be inserted and adjusted to the right side of the main chart by the time you inject your sample. Make sure the chart is running.
For the present run each student will be analyzing a 5 µL mixture of 1,1,1-trichloroethane and cyclohexane using the DC-200 column.
The following is repeated from a section entitled "Routine use of the Gas Chromatograph found in the Common Practices".. file:
To inject, take the automatic syringe and clean it several times by filling and emptying it several times with your sample; discharge the syringe into the air. Adjust the barrel of the syringe to the microliter (µL) capacity desired. Take up the sample into the syringe, and inject it into the injector port (1000 µL = 1 mL) corresponding to the desired column. Make a mark with a pen on the chart at the point of injection.
While you await peaks, record the data right on the chart: date, name, column type and temperature used, the nature of the sample and amount injected, carrier gas (helium) flow rate and back pressure, chart speed and attenuation. If the peaks are too large or too small, adjust the attenuation (i.e. sensitivity) and re-inject.
If another student is about to inject, permit him/her to inject and wait until your chromatogram has passed the sprocket. Now tear off your chart and staple it to an empty page in your notebook.
Two calculations are essential in every gas chromatogram, the retention times and the integrations. These calculations can be carried out after the lab period but must be shown on the chart or, if there is no space left, on the notebook page containing the chromatogram:
The retention time for each peak is the time the compound remained in the gas chromatograph before it eluted (= exited the column). It is determined by measuring the horizontal distance from the point of injection to the point the peak was at its maximum. This distance is converted to minutes using the chart speed.
To integrate the peaks (i.e. determine the areas represented by certain peaks in square mm): 1) establish a baseline by carefully drawing a curve where the pen would have gone if the peak had not eluted; 2) measure the distance from this baseline to the top of the peak. 3) then halfway up this line, measure the horizontal width of the peak (this is called "width at half height"). Multiply this width by the peak height to determine the area of the peak. (If you are integrating a shoulder peak, i.e. a peak that is not well separated from another, assume the peak would symmetrical if there were no interfering peak; draw in lines and carry out the integration as best you can and show your logic.) Finally calculate the percentages of each peak to the mixture.
You will be given the percent composition that was used to make the 1,1,1-trichloroethane - cyclohexane mixture. Compare this percent composition with the percent composition you measured from the chromatogram. From this information, which compound eluted first and which last?
Analysis of an unknown liquid using an infrared spectrophotometer (IR). McM sections 12.4-12.5. (most of Mayo's treatment, Ch. 6 is overly detailed for an introduction). In order to prepare the sample you must first be assured that the disk shaped plates of crystalline NaCl are not contaminated. With a plastic glove, hold each plate at its edges in the hood and squirt both faces with about 2 mL of pure ethanol. Place the crystal on its edge on a paper towel, and repeat the procedure with the other plate. (Never wash the plates with water - or anything containing water such as soap - as they will dissolve. Salt plates cost $25 each..) The salt plates are mounted on a holder somewhat like Figure 6.43 except that they are not screwed down but instead held in place by a metal retainer.
For this laboratory, your group will run a sample whose spectrum appears in Mayo, et al. Chapter 6.
The instrument is a Perkin Elmer Model 727 infrared spectrometer. It is almost exactly like Figure 6.40. Manually scan the region between 4000 and 2600 cm-1 on your clean plates: 1) with the SCAN control off (neither lit nor blinking), slide the tray holding the spectrum paper so that the scan position indicator reads 4000 cm -1 on the frequency scale; 2) place the sample holder (containing the two clean plates) in the sample beam aperture; 3) use the 100% control to set the pen holder at 40-80% transmission. 4) without engaging the SCAN selector or inserting the pen, slowly scan the spectrum from 4000 to 2600 cm-1 by gently sliding the paper tray and watch the pen holder. The pen will trend upward through this region but if the pen does not deflect downward then back up by more than 15% there is no contamination.
Remove the plates and place 1/2-1 drop (30-50 µL) of your liquid sample on one of the plates, cover it with the other plate, and remount the plates. Repeat steps 1)-3), then: 5) load the pen into the pen holder; 6) press the SCAN switch and quickly lower the pen to the paper (use the "lower pen" lever); (The scan can be interrupted at any time by depressing the PEN switch. The monochrometer and chart will maintain alignment.); 7) when the tray stops at 600 cm cm-1 depress the SCAN selector, and CAP THE PEN - they cost $13 each.
After finishing, re-clean the plates with ethanol (remember - no water or soap) for the next student who may start by running a blank.
Identify the compound by comparing the frequency (i.e. wavenumber) values, not necessarily the intensities of the peaks.
This spectrum chart must be labeled by filling in the first four entries above the chart: name(s), the substance being analyzed (for example "unknown #12"), the date and the class (chem 3012). (In future laboratories you will be required to fill in the fraction of the extract and the method used to mount the sample; for this lab you used the "neat" - you placed the neat sample between salt plates.)
(Also in future labs you will need to calibrate the spectrum to obtain accurate wavenumber readings. In addition you will need to interpret peaks between 4000 and 1500 wavenumbers, that is, assign specific bonds to particular peaks in this range.)
While you are waiting for the next activity: Calibrate your Pasteur pipettes by drawing up 1 and ½ mL quantities of water from a graduate cylinder. Hold the pipette across the palm of your hand and control the volume by squeezing the rubber bulb between your thumb and forefinger. Make a full scale drawing of a Pasteur pipette inside your notebook front cover and indicate approximate marks. On the same page also report how many drops of water to a mL, and how many mg to a drop of water.
Conclusions:
Which Melt-temp instrument will you rely on? Which will require a correction?
In the Gas chromatography experiment, does the compound with the lower retention time also have the lower boiling point?
What is the identity of the compound your group ran on the infra-red spectrophotometer?
References:
"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991
"McM": McMurry, J. Organic Chemistry, 4th ed., Brooks/Cole Publishing Company, Pacific Grove, CA. 1996
The "Handbook": recent editions of: Weast, R.D. Handbook of Chemistry and Physics; The Chemical Rubber Co.: Cleveland, 1960-present.
"Aldrich IR Library": any edition of: Pouchert, C.J. The Aldrich Library of Infrared Spectra; Aldrich Chemical Co. Milwaukee, 1970-present.
Rev. June, 1999