NMR gives information about the hydrogen atoms in a compound, and the rest of
the structure ... For GC, study this chapter, the lecture notes on the Chemistry.
16. Gas Chromatography and Nuclear Magnetic Resonance Gas chromatography (GC) is yet another separation method. GC is most often used for analysis of very small samples of mixtures in a way analogous to TLC. In gas chromatography the mobile phase is an inert gas called the carrier gas. The stationary phase is usually a high-boiling liquid coated onto an inert support, and the method is called gas-liquid chromatography (GLC) in those cases. The technique is useful for quantitative analysis (identifying how much of a compound is present); the technique is widely used in industrial and analytical laboratories for this purpose. GC is also used for qualitative analysis (identifying unknowns). GC can be also used on a small scale to separate mixtures into their components and recover them in a way analogous to column chromatography. This is called preparative scale GC. Chemists frequently use proton and carbon-13 nuclear magnetic resonance (H-1 and C-13 NMR) in identifying unknown compounds. Proton NMR gives information about the hydrogen atoms in a compound, and the rest of the structure must be inferred from that information and from knowledge about bonding if proton NMR is used alone. C-13 NMR gives direct information about the carbon skeleton. These techniques are even more powerful when used in combination with others. Among the most useful is infrared spectroscopy (IR; see page 46 of this manual), which is used to identify functional groups. You will use an nmr spectrum to identify an unknown in this lab. PRE-EXPERIMENT ASSIGNMENT For GC, study this chapter, the lecture notes on the Chemistry Department web site, and the introduction to chromatography on page 53 of this manual and on pages 184-186 in Williamson. For proton NMR, study this chapter, the lecture notes on the Chemistry Department web site, and pages 216-224 in Williamson. Do the first parts of your notebook writeup.
A student who has prepared for the gas chromatography and proton NMR portions of this lab should be able to: 1. Define and explain: chromatography, gas chromatography (GC), stationary phase, mobile phase, carrier gas, retention time. Explain the differences between GC and the chromatographic methods you used earlier. 2. Draw a diagram of a gas chromatography apparatus, and label the components of the apparatus in a drawing. 3. Identify and explain the reasons chemists use gas chromatography (to ID compounds in a mixture using retention times, to separate components of small samples, and to determine the amount of given compounds present, for mixtures of volatile compounds only). 4. Draw the structure given the name, or give the name from the structure, of the possible unknowns used in the GC portion of the day's experiment. 5. Predict the number of signals that a given compound will give in its proton magnetic resonance (pmr or proton nmr) spectrum and its carbon-13 (13 C or C-13) nmr spectrum. 6. Identify unknown compounds from proton nmr spectra using the number of signals, chemical shifts, and splitting patterns. Students will be given a simplified table of proton nmr chemical shifts to use on the first quiz on this topic; there is a copy on page 70. 7. Identify an unknown compound from its C-13 nmr spectrum and/or its infrared spectrum, either in combination with its proton nmr spectrum or alone. Students may be given a simplified table of C-13 nmr chemical shifts; there is a copy on page 71. Students may also be given a simplified table of IR absorption frequencies (page 49) or may be given information solvable using the table on page 47 (no copies of this will be provided). Quizzes given after the experiment has been performed may also include: 8. Predict the effects in changes in experimental procedure (e.g. flow rate, column temperature, sample polarity) on retention time and resolution of peaks. Give desirable properties of a carrier gas and a stationary phase. Gas Chromatography Your instructor will give a short lecture on gas chromatography. This will include some information about the instrument and some of the factors that affect the efficiency of the separation. You will also be told how to determine retention time for GC peaks. Each compound gives one peak in gas chromatography. (Exception: Compounds that decompose inside the GC instrument can give multiple peaks. That does not apply in this experiment.) You will be given two chromatograms that have been obtained on an instrument in our instrumental analysis lab. One chromatogram will be a standard containing cyclohexane, methylcyclohexane and toluene. The other chromatogram will be your unknown, and will contain only two of the
compounds above. On the GC unknown spectrum, indicate how you measured the retention time for each peak. Determine the identity of each compound in the unknown mixture by comparing the retention times of each peak given by a compound to the retention times of each component in the standard. If two peaks come from the same compound, they will have the same retention time within experimental error on gas chromatograms obtained under the same conditions. On the GC unknown spectrum write the identity of each component across the top of the corresponding peak. Nuclear Magnetic Resonance and Infrared Spectra Your instructor will give a short lecture on nmr, including both proton and C-13. This will include a discussion of the number of signals to be expected and chemical shifts for both proton and C-13 nmr, and of spin-spin splitting for proton nmr only. You will be given the proton nmr spectrum of an unknown compound. You will identify your unknown based on this nmr. POST-EXPERIMENT ASSIGNMENT Both your GC unknown spectrum and proton NMR spectrum should be turned in as part of your laboratory report. Put your name and the date you received the spectrum on each spectrum. Your instructor will provide further instructions regarding what else has to go in your report and when the report is to be submitted.
Simplified Table of Proton NMR Chemical Shifts Chemical Shift,
Proton(s) If the proton(s) of interest are of interest: attached to a carbon atom
that:
δ 10
C
C
alkene proton(s)
H
aromatic ring proton(s)
H—C— || O
aldehyde proton(s)
H—O—C— || O
carboxylic acid proton(s)
OH protons of alcohols and NH protons of amines can be from
1 - 5.
Simplified Table of C-13 NMR Chemical Shifts Type of C
Approx. Chem. Shift (ppm)
Type of C
Approx. Chem. Shift (ppm)
R—CH3
8-35
C-O
50-80
R2 CH2
15-50
C-N
40-60
R3 CH
20-60
C-Cl
35-80
R4 C
30-40
C-Br
25-65
C-I
0-40
O || R—C—NR2 O || R—C—OR
C
65-85
C
100-150
165-175
165-175 C
O || R—C—OH
175-185
O || R—C—H
190-200
O || R—C—R
205-220
110-170
