Check In, Melting Point, Infrared Spectra and
Gas Chromatography
Preparation: Reading preparation: Mayo, pp. 1-9, 16-21, 65-71, 166-168, and 237. In addition, look up in the Handbook and record here 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.)
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 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 dichloromethane?
What is the temperature of hot running tap water?
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 - Use of the Gas Chromatograph. Gas Chromatography is a separation technique that takes advantage of the unique speeds that distinct compounds take while traveling through a column (tube). The mixture of two or more compounds is injected into the column and is forced through it by a steady flow of carrier gas. To maximize the differences in speeds - thus optimizing the separation - the column temperature is set near the boiling points of the compounds, and the column is either lined or packed with materials that will retard the passage of one or more of the components in the mixture. At the end of the column is a detector that registers if any compound - besides the carrier gas - is eluting (exiting the column); the detector is connected to a chart recorder that measures the detector response as a function of time.
Further details of the gas chromatography technique of gas chromatography can be found in Mayo, p. 66-77. For the purposes of this laboratory you need to correlate the basic elements in the block diagram in Mayo, Fig 5.1 with the instrument in the laboratory, a Gow-Mac Series 350. Beginning with the carrier gas source:
| a) | The carrier gas supply tank with its pressure regulator is plumbed into the box and is split into two streams A and B, each with a "Flow Adj." metering valve, injection ports, columns and exit ports. Only one stream is used at a time. |
| b) | The stream passes by an injection port having a rubber septum (barrier). A syringe is used to penetrate the septum to allow the sample mixture to be injected at once into the carrier gas stream. |
| c) | The column that separates the compounds is wound to conserve space in an oven to maintain it at a constant temperature. |
| d) | The carrier gas and separated compounds then pass through the detector before they exit the instrument. |
Conditions are normally set in advance. Typically they are:
| a) | carrier gas (helium) pressure normally at 14 psi (pounds per square inch); the flow is monitored periodically with a flow meter as it exits the column; it should be 20-25 ml/min. |
| b) | columns are packed with either Carbowax 20 M (polar) or a DC-200 (non - polar) depending on which stream is used; the column temperature (110 oC for this experiment) is read on the meter in the front of the instrument |
| c) | attenuation (sensitivity of the detector) is adjusted to produce peaks that can be measured on the chart; the chart speed, 1 inch per minute, is better reported as 25.4 mm/min. |
For the present run each student will be analyzing a 5 microliters (1 microliter = 1 µL = 0.001 mL) mixture of 1,1,1-trichloroethane and cyclohexane of predetermined percent composition.
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.
To inject, take the automatic syringe, preset to the desired injection volume, and clean it by filling and discharging it several times with your sample onto the lab bench. Then fill the sample once more and inject it into the injector port 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 and name; carrier gas (helium) flow rate and back pressure; column type and temperature used; chart speed and attenuation; and the nature of the sample and amount injected. If the peaks are too large or too small, adjust the attenuation (i.e. sensitivity) and re-inject.
After pen reaches baseline following the last peak in you run, permit the next student to inject and wait until your chromatogram has passed the sprocket. Now cut your portion of the chart and attach 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:
| a) | 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 in mm from the point of injection to the point the peak was at its maximum. This distance is converted to minutes using the chart speed. Round off the retention time to the nearest 0.1 min. |
| b) | The areas (integrals) represented by peaks are measured in square mm: |
| i) | establish a baseline by carefully drawing a curve where the pen would have gone if the peak had not eluted; | |
| ii) | measure the distance from this baseline to the top of the peak. | |
| iii) | 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 (rounded off to the nearest 1%) 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 12.5-12.6. (most of Mayo's treatment, Ch. 6 is overly detailed for an introduction). The instrument is a Perkin Elmer Model Spectrum One infrared spectrometer. For this laboratory, your group will run a sample whose spectrum appears in Mayo, et al. Chapter 6.
| a) | The disk shaped plates of crystalline NaCl are used to hold the liquid sample in the instrument's IR beam. These plates must be cleaned first. Stand each plate on its edge on a paper towel and squirt both faces with about 2 mL of 100% ethanol. (Never wash the plates with water - or anything containing water such as soap - as they will dissolve. Salt plates cost $25 each..) |
| b) | Using clean NaCl plates only, run a blank "background" spectrum. Place them in the holder provided and secure them with the retainer. Mount the holder in the instrument's beam aperture and activate "background" on the instrument's screen. |
| c) | Following the background scans, remove the sample holder and return to the laboratory to remove the NaCl plates. Place 1/2-1 drop (30-50 µL) of your liquid sample on one of them. Cover it with the other plate, remount the plates, and return the holder to the instrument. |
| d) | Begin "scan" after filling out the information on the screen. The "Name" of the sample is unknown (fill in "Unknown # ...") The "Sampling Method" for this procedure is "neat, between NaCl plates". The "Analyst" is (fill in your names). The "Fraction" entry is left blank this time. |
| e) | Print one copy of the spectrum. |
| f) | Remove the holder then the plates as before and clean the plates with ethanol (remember - no water or soap) for the next student to start the blank for the next analysis. |
Insert and mark a vertical line completely down your spectrum at 3000 cm-1. This line will be helpful in identifying your unknown since any sharp peaks to the left of this line (between 3020-3100 cm-1 - see McM Table 12.1) represent C-H bonds in which the C is sp2 hybridized. Absorptions to the right of this line, 2960-2850 cm-1 represent C-H bonds in which the C is sp3 hybridized.
Identify the compound by comparing the frequency (i.e. wavenumber) values, not necessarily the intensities of the peaks.
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 detailed, 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?
Because gas chromatography involves vaporization of the liquid mixture, the boiling component will usually - but not always - pass down the column faster and have a shorter retention time. In your experiment, did the lower boiling point component actually have the shorter retention time?
What is the identity of the compound your group ran on the infrared spectrophotometer?
References:
"Mayo": 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, 5th ed., Brooks/Cole Publishing Company, Pacific Grove, CA. 1999
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. January, 2001