Alkane or Alkene Unknown

Reading: B & F Ch. 13 Introduction, Sections 13.3-13.7,13.12-13.13,13.14a&b.  Also review the "Table of Chemical Tests" and "Table of Candidates" Sections in the Common Practices and Procedures file.

The purpose of this experiment is to identify a pure liquid you will be given - an alkane or an alkene from its boiling point, infrared (IR) spectrum, two chemical tests and ¹H NMR and ¹³C NMR spectra. Record the unknown number of the compound in your Purpose.

The data obtained will be organized according to two tables: the Table of Chemical Tests and the Table of Candidates.

Set up a Table of Chemical Tests which will organize the IR, NMR and chemical test data. Head columns "Test" then "Result" then "Inference" then "Comments" to span an entire page and expect the table to continue at least one more page. Under the "Test" column, enter "Boiling point", then 7 preliminary tests to determine whether your unknown is an alkene or an alkane: "Br₂/CCl₄", then "KMnO₄", then "IR/3020-3100 cm^-1", then "IR/1630 - 1680 cm^-1", then "¹³C NMR/100-150 ppm" then "¹H NMR/5-6.5 ppm" and "¹H NMR 1.6-2.2 ppm". These tests are redundant. If they give contradictory implications, they should be repeated, so leave further test entries for repeat tests.

Based on these tests and a preliminary analysis of the spectra you should decide if your unknown is an alkane or an alkene. Once you have determined your unknown’s boiling point, skip a few pages for further tests and start a Table of Candidates from either the "Alkane Boiling Points" file or the "Alkene Boiling Points" file. Details on this appear later in the procedures.

A careful reexamination of the spectra is necessary to obtain specific structural features of your unknown which you will use as criteria to reject all but one candidate from the candidates list. These features should be firmly established by cross checking between spectra. This process is organized in further entries of your Test Table but does not need to follow a particular order. These criteria are typically:

Normally one candidate will survive the structural requirements imposed by the spectra. (If no candidate survives this elimination, or if more than one survives, advice is given in the procedure below.) Usually you can check your conclusion by matching at least one of your spectra to one that is available in either a hardbound or a website catalog of published spectra of known compounds.

In your conclusion, you will point out specifically how all the features of the structure you propose fit the spectral data.

Use your laboratory time to take measurements and perform reactions. Looking up data, searching for spectra and planning reactions are best done outside of the laboratory period.

Before leaving the laboratory, you should repeat tests and the boiling point measurement to ensure that your data can be depended on. Often data from various sources will be contradictory; repeating tests and boiling point determinations usually resolves these contradictions and running test with known compounds will help you know what to look for. With reliable information, the identification of your unknown can be done outside the lab.

Procedures: Run the tests in no particular order and obtain independent information from each. Do not force the results of any test to agree with the results of another. Instead, base your decisions on the preponderance of all the tests.

Boiling Point: A small (10x75mm) test tube is clamped to a ring stand and loaded with 10-15 drops of the unknown liquid. A thermometer is also clamped so that its bulb is within the test tube and and 1mm above the bottom of the test tube. A capillary tube is broken 12-15mm from the bottom and is added to the tube, sealed end up. The test tube is suspended in a 30 mL beaker (also clamped to the ring stand) containing mineral oil. Heat the oil with a flame until the air trapped in the capillary bubbles out in a rapid stream (until you are unable to count them individually). Stop the heating and observe the capillary; record as the boiling point the temperature at which liquid just begins to enter the tube. Make a careful drawing of the apparatus for your records.

Liquids with very low boiling points will bubble rapidly even while the mineral oil bath is below 50 ^oC and evaporate before the sample is heated to 70^o. It may not be possible to make an exact measurement for these volatile compounds, but this narrows the list of possibilities to a few viable candidates.

No comments or inferences need be made for this test. However this test until your measurements are consistent.

The IR Spectrum: Use the same procedure as in the first laboratory if the IR spectrum is not provided to you. You should clearly mark two diagnostic regions (B&F Tables 12.4 and 12.5, Mayo, et al, p.187-8 "C=C stretching" and "Olefinic C-H" ) on your spectrum:

Deciding whether your spectrum has these absorptions or not is often easier if you have examples compare to. IRs for hexane and 1-hexene are found in Fig. 12.13 a) and b). Another example of this is in McM problem 12.36 (the upper spectrum is of cyclohexane and the lower one is of cyclohexene.) Enter the results of these and the following observations into the Test Table.

Further examination of the IR spectrum will be helpful in determining whether your compound has a methyl group or not. The diagnostic peak is at 1370-1380 cm^-1, and will be confirmed with the NMR spectrum, below. Comparison of the spectra cited above show this distinction. Hexane and hexene both have peaks at 1375 cm^-1 while cyclohexene and cyclohexene do not. Enter your findings as a test with its result and inference.

One need not interpret every peak on the infrared spectrum.

Simple Chemical Tests.

Spectral evidence for alkenes should be confirmed by the test using bromine in CCl₄ (McM sect. 7.2): add a 2% solution of Br₂/CCl₄ dropwise to a solution of 50 mg of the compound. The red color of the Br₂ will disappear completely if your unknown is an alkene.

A further test using aqueous potassium permanganate is available (McM, top of p. 254): Add a 1% aqueous solution of KMnO₄ dropwise to 50 mg of the compound. Agitate it well and look for signs of MnO₂ (brown). The reaction takes place between two phases, aqueous and organic.

Enter each test in the table; if you repeat the test, write a new entry. If you are unsure of what a positive test should be, run a compound from the reagent shelf known to have the functional group, and enter the data in the Table. Remember that a non-positive result is just as significant as a positive one. Be as detailed as possible and avoid writing results like "it came out negative"; instead, detail what was actually seen.

¹H NMR Spectrum.

Each uniquely positioned hydrogen atom or group of H atoms in the molecule will give a separate signal. The areas under these peaks and the positions of these peaks will also be useful.

A signal at 0 parts per million (ppm) belongs to tetramethylsilane (TMS), which was added as a reference standard. A small signal at 7.24 ppm representing the solvent chloroform also appears. Both signals are to be marked and identified on the chart.

As a separate "test" in your test table, you should enter "¹H NMR – number of signals." In some cases, the number of signals may not be apparent. This situation is most often found in long chains of CH₂ groups that are similar and often overlap in the spectrum, so an estimate may be appropriate. After you record the result and the inference, your comments would refer to which candidates were ruled out and which remain in consideration.

The explanation for the positions of the peaks in the spectrum – the ppm value or chemical shift – will be detailed later. For this experiment, McM Table 13.3 suffices in relating approximate chemical shift zones with specific parts of the compound studied. Since signals for saturated secondary and tertiary don't always fall into these zones, these assignments may need revision following analysis of the edited DEPT spectrum (see below):

Signals are often split into closely spaced lines. This aspect can be disregarded at this time.

The area of each signal is measured and displayed above each signal. The area reflects the relative number of hydrogen atoms represented by that signal or set of similar signals. Often the symmetry of the molecule will be an important factor. For example, the ¹H NMR spectrum of the compound in problem 13.7 part (c) would consist of 2 signals, with an area ratio of 6:4 or equivalently 3:2. For molecules having long chains of CH₂ groups the signals may not be discernable, but the area over these peaks, compared to the area of the terminal CH₃ group can be helpful. Each of these chemical shift zones should be reported in the Table of Chemical Tests, along with the area results. An example is shown below which fits for the compound 1-decene, CH₂=CH-CH₂-(CH₂)₆-CH₃:

¹³C NMR Spectrum.

Again at the introductory level, each uniquely positioned carbon atom in the molecule will give a separate signal. The edited DEPT "spectrum" of your unknown is actually a set of 4 spectra. Signals of the carbon atoms are sorted by type according to the classifications in B&F p. 60: one spectrum of only the primary (CCH₃ - methyl) carbon atoms, another with only the secondary (C₂CH₂ - methylene) carbon atoms, a third with only the tertiary (C₃CH - methine) carbon atoms. The fourth spectrum is for all the carbon atoms. A signal in the "All Carbon" spectrum that is absent or minor in either the methyl, the methylene or methine spectra can be concluded to represent quaternary carbon, C₄C, one to which no hydrogen atom is attached.

The signal at 0 ppm belongs to tetramethylsilane that has been added as a reference standard. A three line signal at 77 ppm representing the solvent chloroform also appears. Both signals are to be marked and identified on the chart.

Relatively small residual signals in one spectrum of an edited DEPT set will often be found directly above or directly below the much larger true signal in another spectrum of the same set. These residual signals should also be noted.

Signals for C=C carbons appear in the 100-150 ppm range. All alkenes must have at least one signal in this range, so the line in the test table entry "¹³C NMR 100-150 ppm" should have a clear result and inference; one or more peaks in this range means an alkene, no peaks means an alkane.

The edited DEPT information is reported as separate tests in the separately on the Test Table. Thus for the "Edited DEPT – methyl signals" test, the number of different methyl signals is entered, the inference – the number of different methyl groups in your unknown is entered, and for comments, the remaining candidate structures are retained or rejected. Similar entries are made as separate tests: the methylene spectrum, methine spectrum and the all carbon spectrum. In the example below that continues for 1-decene, more information is found that corroborates the ¹H NMR data already obtained:

Marking Your Spectra.

Attach your spectra to your notebook. They are not useful unless they are liberally annotated and interpreted right on the spectra. Thus, diagnostic peaks which point to an alkene, or the absence of these peaks must be pointed out on the spectra. Write, for example "no C=C peak here ­ therefore no alkene" where appropriate.

All peaks must be interpreted for all NMR spectra. Where possible, use lines to connect edited DEPT signals with the corresponding ¹H NMR signals. Thus CH₃ , CH₂ etc. signals in the saturated ranges are linked as well as CH₂ and CH signals in the unsaturated ranges.

Table of Candidates.

Attention now focuses on how the structural information obtained above can be used to eliminate all but one of a list of candidates.

Select your candidates from the appropriate list from either the Alkane Boiling Points or Alkene Boiling Points file from the Chem 3012 web page. Begin by listing all the entries with stars (* - the more common entries) having boiling points 5 degrees below to 10 degrees (in the example below, 15 degrees, just in case) above the boiling point you measured. The structure of each candidate is tabulated, along with the boiling point. Under the "Why Doesn’t This Structure Fit My Spectra?" column, at least one reason should be given (several reasons are often given in the example below)

Example problem: the unknown’s boiling point was found to be 134 ^oC. From the ¹H NMR and ¹³C NMR it is an alkene with the following structural requirements:

Using the List of Candidates, the structures of compounds with boiling points from 129 to 149 ^oC are examined against these requirements (in this example, structures of cyclic compounds cannot be provided to save web page space – but you must supply structures for all the candidates):

Unlike most situations, two candidates survived the requirements of the NMR spectra: 2-methyl-4-octene and 7-methyl-3-octene. These are very closely related structures; neither of them is found in published or web based catalogs of known spectra. The choice between them would likely be decided by an entirely different method, mass spectrometry. For our purposes, both these candidates are viable, and although the conclusion below must contain only one candidate, the information in the Table above is sufficient for full credit for the laboratory.

If no possible candidates fit the data you have obtained for your unknown, widen this 15 degree "window" until suitable candidates are found and also check your data carefully to make certain you have selected the correct Boiling Points file. The instructor will provide or verify structures if they cannot be found.

Confirmation of Your Best Candidate.

Often your remaining candidate can be confirmed as the unknown by matching either its IR, ¹H NMR and/or ¹³C spectrum. NMR

NMR Spectra can often be matched using the following website: http://www.aist.go.jp/RIODB/SDBS/menu-e.html

Another catalog is the Aldrich NMR Library which displays the ¹H NMR and only the All Carbon ¹³C NMR spectra.

IR spectra of known compounds are sometimes found at the above website or in the Aldrich IR Library. Your spectrum is linear in wavenumbers (cm^-1) while the Aldrich Library spectra are linear in microns, so each major peak should be measured separately. There is a tendency for novices to compare their spectra with every one in the Aldrich IR Library; this is unnecessary. One should restrict the search to at most a few compounds in the Table of Candidates.

Conclusions

Give your best candidate: "based on the evidence gathered, unknown number ___ is _____ "

Make a large drawing of the structure you arrived at on the page with the spectra. Link each unique C and each unique H atom with its signal in the NMR spectrum. Do this by marking atoms and signals with matched symbols such as triangles, stars, squares, diamonds, etc.

Questions:

1. Write a structure for a compound that has two ¹³C NMR signals but only one ¹H NMR peak.

2. Write structures for compounds A-C with the following ¹³C NMR signals:

3. Why is it impossible to have a methyl peak in the edited DEPT between 100-150 ppm?

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.

"B&F". Brown, W. H.; Foote, C. S. Organic Chemistry, 3rd Ed., Harcourt College Publishers. San Diego, CA. 2002.

The "Handbook" recent editions of: Weast, R.D. Handbook of Chemistry and Physics; The Chemical Rubber Co.: Cleveland, OH, 1960-present.

The "Aldrich IR Library" refers to any edition of: Pouchert, C.J. The Aldrich Library of Infrared Spectra; Aldrich Chemical Co. Milwaukee WI 1970-present. On reserve: QD 96.I5 P67.

The Aldrich NMR Library" refers to Pouchert, C.J and Behnke, J. The Aldrich Library of 13C and 1H NMR Spectra; The Chemical Rubber Co.: Cleveland, OH. On reserve: Ref QC 462.85 .A44 1993.

Grasselli, J.G. and Richey, W.M., Atlas of Spectral Data and Physical Constants; Chemical Rubber Co.: Cleveland, OH. QD 257.7 .G7 1975.

Rev. January, 2001