SPECTROSCOPIC IDENTIFICATION OF AN UNKNOWN COMPOUND: Aldehyde, Ketone, Carboxylic Acid, Alkene, Alcohol, and/or Aromatic Compounds

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1 SPECTROSCOPIC IDENTIFICATION OF AN UNKNOWN COMPOUND: Aldehyde, Ketone, Carboxylic Acid, Alkene, Alcohol, and/or Aromatic s Discussion In this experiment, you will be given an unknown compound that you are to identify. To do this, you will gather information about the compound through two methods: A. Measurement of physical properties B. Interpretation of spectral data (IR and Mass Spec) The unknown compound will be either aliphatic or aromatic, and can be either an alkene, alkyne, alcohol, ether, aromatic hydrocarbon or aryl halide. Tables of possible unknown compounds can be found at the end of this handout. The lists are compiled in the order of increasing boiling point of for each compound class. This lab is a bit different from many of the experiments you have performed. Thus, for this report, we will skip a formal prelab. However, you are responsible for having read all of the instructions prior to coming to class so you have a definite plan of attack in mind and are prepared to obtain information about your unknown from all the sources described herein. Procedure You will need to perform the following methods, though not necessarily in the order listed. It is important to work efficiently, using the equipment and instrumentation such as the infrared spectrophotometer, the boiling point apparatus, and the Abbey refractometer, whenever they become available for your use. A. Measurement of Physical Properties 1. Note any characteristic properties, such as: a. Odor (Don't ignore even the little things. If your compound smells like bananas, and banana oil is on the list, then that should tell you something.) b. Solubility (Is the compound water soluble, for example?) 2. Some properties you will need to measure include: a. Micro boilingpoint determination (see below) b. Index of refraction c. Density (Halogenated compounds are relatively dense.. hint, hint.). Add 1mL of your liquid to a tared test tube and weigh. 1

2 B. Spectroscopic Data (Padías, pp and pp ; also consult your lecture text) Infrared Spectroscopy: You need to prepare a sample, record an infrared spectrum, and obtain a printout of the spectrum. This is not difficult, but requires some care in both the sample preparation and the treatment of the Fourier transform infrared instrument. Please be efficient while working with the instrument, so as not to occupy it for an extended period of time when other students are waiting. Gas Chromatogram / Mass Spectrometry: You need to prepare a sample for GC/MS. Fill an autosampling vial with 1mL of HPLC-grade dichloromethane and add a single drop of your unknown compound. Working in groups, you will use the autosampler to inject your sample onto the GC column and take a Mass Spec of your unknown compound. The method we will use will not take a Mass Spec of the solvent (dichloromethane). The GC/MS is automated you do not need to monitor the system. However, you will need to instruct the GC/MS as to which peaks are your compound and you will need to obtain a printout of the GC/MS of your unknown. Suggested Basic Strategy 1. Focus on methods that will help you to determine the structure of your compoundthe functional group(s) present, the boiling point of your liquid, and the mass of your compound. 2. Some functional group(s) can be established using infrared spectroscopy. Knowing the functional group will substantially limit your list of possible compounds. 3. The boiling point will also be an important piece of evidence for limiting the number of compounds under active consideration. Please note, however, that boiling points are somewhat difficult to determine accurately, so all compounds plus or minus 10 degrees of the experimentally determined value should be considered as possible candidates. (The lists of possible compounds are arranged in order of increasing boiling point to facilitate this process.) 4. Using GC/MS, you can determine the molecular weight of your compound. In addition, the fragmentation pattern could indicate which functional groups are present and/or the geometry of your compound. 5. Other data you will want for structural confirmation certainly includes an index of refraction (corrected for temperature) and perhaps a density, as mentioned above. 2

3 Specific Experimental Procedures Solubility in Water: s that contain four or fewer carbons and also contain an oxygen or nitrogen are often soluble in water. s having five or six carbons and an oxygen or nitrogen are often insoluble in water or have borderline solubility, but bear in mind that branching of the alkyl chain tends to lead to an increase in the water solubility. Place approximately1 ml of water in a small test tube and add one drop of your unknown liquid directly into the water. Gently tap the test tube with your finger to ensure mixing. The disappearance of the separate drop of liquid indicates that the unknown has dissolved. Add several drops more of the liquid to determine the extent of the compound s solubility. Micro Boiling Point Determination: (Padías, p. 52) A micro-bp determination is used when you have too little sample to do a regular distillation (as is the case with your unknown), or as a quick check to see where a compound boils before purifying it by distillation. The micro-bp method uses a small test tube, filled with the sample to a height of about 1 cm. A shortened capillary melting point tube (about 2 cm long and sealed at one end) is placed with the open end down in the test tube of liquid, which is then heated in any one of two or three oil baths set up in the laboratory for this purpose. As the sample is gradually heated in the oil bath, there will be slow escape of air bubbles from the end of the capillary tube due to an expansion of the air trapped in the tube (PV = nrt). When the temperature of the oil in the bath reaches (or somewhat exceeds) the boiling point of the compound, a rapid and continuous stream of bubbles from the end of the tube will be observed (voilá, boiling!). Reading the thermometer at this time yields a crude boiling point, but can be far from accurate. A more accurate result can be obtained by turning down the heat source. The rate at which the bubbles form will lessen and finally at the instant bubbles cease to come out of the capillary, and just before the liquid is sucked up into the tube, the temperature is read. This gives the most accurate boiling point determination. You may wish/need to repeat the determination to be assured of maximum accuracy. You will need to drop in a new capillary tube for each determination. Please note that, if you significantly exceed the temperature of the boiling point of your compound, all of the liquid may boil away, leaving you with an empty tube. Be observant! Also note that the white oil in these baths should not be heated to temperatures above 210 C, as it will very likely catch fire. We have therefore limited the list to unknowns that boil at less than this temperature. Higher temperatures can be obtained, but require the use of a silicon oil bath (suitable to about 280 C). 3

4 Final Report This final report will be written up as a formal report. You must present a thorough argument for the proof of the structure of your unknown compound. Instructions found in the Introduction section of the lab manual still apply. Here are some suggestions for the structure of the report. You should include your name, date, introduction, references to instructions, etc. and incorporate a statement of the goal of the experiment. This statement should include your Unknown Number. Methods: This section will be quite small for this report. However, you should dedicate at least one sentence to each method used; briefly mention what information you hoped to obtain with each method. Data and Observations: This will be a bigger section in your report, and it should include all data collected and any observations regarding your unknown. Use tables so that you can clearly identify significant data. Tables also allow you to easily compare results from your unknown and the proposed identity of your unknown. In addition to the tables of information, you should briefly discuss your results. Results and Conclusions: 1. Summarize the above data. Try to tabulate this information as much as possible. List literature values as well (remember to cite each literature source!). 2. Both spectra should be fully interpreted. You should assign all relevant absorptions in the IR (e.g. ν ~ = 1640 cm -1, C=C stretch in alkenes). For the massspectum, identify the structure of at least 3 peaks (the base peak, the molecular ion peak, and at least one fragment). 3. Provide final concluding remarks discussing how the data is consistent in pointing to a particular compound. If there are inconsistencies between the data you have obtained and that expected for the compound, these should be discussed. If you are unsure of your final assignment, that should also be included in this section. Finally, discuss what other methods you could use to make a conclusive identification. Table 1: Alkenes 4

5 Boiling Point ( o C) 2,4-Hexadiene 82 1,3,5-Cycloheptatriene 116 2,5-Dimethyl-2,4-hexadiene 134 (m.p. 13) 1,3,5,7-Cyclooctatetraene Nonene Phenyl-1-propene (Allylbenzene) Decene Phenylpropene (β-methylstyrene) 175 Limonene 176 Table 2. Ketones and Aldehydes Spectroscopic ID - Handout B.P. ( C) Butanal (Butyraldehyde) 75 2-Butanone (Methyl ethyl ketone) 80 3-Methyl-2-butanone (Isopropyl methyl ketone) 94 Pentanal (Valeraldehyde) 102 3,3-Dimethyl-2-butanone (t-butyl methyl ketone) (Pinacolone) Ethylbutanal (Diethylacetaldehyde) Methyl-2-pentanone (Isobutyl methyl ketone) 117 2,4-Dimethyl-3-pentanone (Diisopropyl ketone) 124 Hexanal 130 Cyclopentanone Methyl-2-hexanone 145 Heptanal 153 Cyclohexanone 156 Furaldehyde (Furfural) 162 2,6-Dimethyl-4-heptanone (Diisobutyl ketone) Octanone 173 Benzaldehyde Nonanone 192 Table 3. Carboxylic Acids 5

6 B.P. ( C) Propanoic acid (Propionic acid) Methylbutanoic acid (Isovaleric acid) Chloropropanoic acid Methylpentanoic acid (2-Methylvaleric acid) 195 Table 4: Aromatic Hydrocarbons B.P. ( C) Toluene 111 Chlorobenzene 132 Phenylethyne (Phenylacetylene) 142 1,2-Dimethylbenzene (o-xylene) 144 Isopropylbenzene (Cumene) 153 Bromobenzene Chlorotoluene 162 (m.p. 9) t-butylbenzene 169 p-isopropyltoluene (p-cymene) 177 1,2-Dichlorobenzene tert-Butyltoluene 190 Table 5: Alcohols B.P. ( C)) Ethanol 78 2-Propanol (Isopropyl alcohol) 82 1-Propanol 97 2-Butanol 99 2-Methyl-2-butanol (t-amyl alcohol) Butanol Methyl-1-butanol 129 Cyclopentanol Hexanol 156 Furfuryl alcohol Octanol 177 6