Unknown Alkyl Halide/Alcohol/Alkene

Reading: M&B sections 5.24, 6.22, 9.27, 17.5, 17.6 and the Common Practices.. file (Table of Chemical Tests, Table of Candidates and Interpreting an IR Spectrum).

The purpose of this experiment is to identify a compound you will be given - an alkyl halide, an alcohol, or an alkene. Make sure you have recorded the unknown number of the compound in your notebook.

To prepare for this lab, set up a Table of Chemical Tests which you will use to organize the infrared and chemical test data and show your reasoning during the identification process (see Common Practices and Procedures for an example). Head columns "Test" then "Result" then "Inference" then "Comments" to span an entire page and expect the table to continue for several pages.

You will measure the unknown's boiling point, obtain an infra-red spectrum and perform chemical tests on it to gather information leading to the identification of the unknown from the list of possible compounds. Chemical tests and the IR spectrum will complement each other to reveal the functional groups on the compound (i.e. the chemical classification). The same chemical tests will also often reveal details of the compound (whether it is 1^o, 2^o, or 3^o etc.). The boiling point can be used to estimate the approximate number of carbons in the compound and further narrow the possibilities. Later a list of probable candidates with their boiling points and IR spectral characteristics should yield one or two best matches for your compound.

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.

Procedures:

Boiling Point Determination Techniques. The instructor will indicate which procedures are appropriate. Your boiling point will be the reference point for the selection of candidates (see below) and it also indicates the approximate number of carbon atoms in your unknown (M&B pages 169, 216 and 285):

Ultramicro boiling point - text, p. 51-55.

Modified ultramicro boiling point - place 3-4 mm of the unknown liquid in a capillary tube and shake the liquid to the bottom. Crush a boiling chip with a pair of pliers and insert a stone small enough to also fall to the bottom. Inserting the tube in a Mel-Temp and observing the liquid undergo initial boiling (in the same manner as a melting point) will give an approximate boiling point.

Semimicro boiling point determination - 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 and 1mm above its 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 the temperature at which liquid just begins to enter the tube. Make a careful drawing of the apparatus for your records.

Before the tube is completely filled with liquid, reapply heat and record the temperature at which the bubbles just reemerge from the capillary. If the measurements are correct, the temperatures recorded will be within 2-3^o of each other and at the point at which the vapor pressure inside the capillary just equals the external atmospheric pressure, in other words the boiling point.

The apparatus can be heated and cooled repeatedly to obtain the boiling point except when the liquid has completely filled the capillary. Note also that prolonged heating often decomposes the liquid; this will cause an error in the determination.

Hickman Still determination - Distillation in a Hickman still will also provide the boiling point of your unknown. Use a 5 mL vial to which you add 3 mL of your unknown and a boiling aid. Because some compounds boil at a high temperature it is best to use a copper plate on the hot plate to insure that you can achieve the temperature at which the unknown boils.

The IR Spectrum. Use the same procedure as in the first laboratory if the IR spectrum is not provided to you. You may have to correct the peak measurements as a result of a calibration before making determinations from the spectrum. On the spectrum mark diagnostic peaks with cm^-1 values. Three critical regions of the IR should be checked at this point:

a) a large, parabolic peak centered between 3200 and 3600 cm^-1 indicates the presence of an O-H bond, in other words, the unknown is an alcohol. Examples of this peak are found in M&B fig. 17.3 and Mayo, et al. fig 6.24.

b) a narrow peak between 3020 and 3080 cm^-1 indicates the presence of a bond between H and a sp² hybridized carbon atom, and for this experiment, the presence of an alkene (be sure this peak has been measured after a calibration since peaks between 3000 and 2850 cm are due to the more common bond between H and sp³ hybridized carbon.) A confirmatory peak for C=C appears between 1680 and 1640 cm^-1. An example of this is in M&B figure 17.4b, a spectrum of 2-methylpropene.

c) the absence of both of these features indicates that the unknown (in this experiment) is an alkyl halide.

Enter the results of these and the following observations into the Test Table. Further examination of the IR spectrum should agree with chemical tests on the functional group (below) and will be helpful in further classifying the unknown.

a) alcohols: refer to M&B section 17.6. The C-O stretching bands are most helpful in determining whether the unknown is a 1^o, 2^o or 3^o alcohol, or at least in eliminating one of these possibilities. ("ArOH" refers to OH groups directly benzene rings - phenols - and will not be used as candidates in this experiment.)

b) Alkenes: refer to M&B section 17.5 for further clues on the unknown. Again discussion on aromatic rings can be disregarded at this point.

c) M&B, section 17.5 also has a discussion carbon-hydrogen bending which can identify features of the alkyl portion of your unknown (for example, the isopropyl "split"), whether it is a alkene, alcohol or alkyl halide.

There are far more peaks (absorptions) in a spectrum than need be interpreted; only a few are necessary to determine the functional groups present. Always check the spectra in M&B for examples of diagnostic peaks.

Simple Chemical Tests. You need not run all the tests for all of the functional groups. Once you have determined the functional group from a preliminary examination of your infra-red spectrum and preliminary tests, you should only run further tests for that group. 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 results like "it came out negative"; instead, detail what was actually seen.

Alkyl halides - the Beilstein test: Heat the end of a copper wire to redness in the oxidizing flame (inner cone) of a burner until the flame is no longer colored; let the wire cool slightly and dip it into a sample of the liquid and then reheat. A green flame indicates halogen present. Caution: Sometimes this test is difficult to interpret.

If the Beilstein test is positive, two tests are helpful - the S_N2 and S_N1 reactions.

For the S_N2 test, react 0.2 mL of the unknown in 2 mL of 15% NaI in acetone. Alkyl bromides will react more rapidly than chlorides, while iodides will not appear to react at all. The other effect, the nature of the substrate is also important (1^o > 2^o > 3^o). Compare the reactivity of your unknown with known alkyl halides (you may have notes on this from a previous experiment).

For the S_N1 test (M&B sect. 5.24), combine 0.2 mL of the unknown with 2 mL of 1% ethanolic AgNO₃ solution. Again compare with known reagents or refer to your notes. The rate of the production of silver halide (AgCl, white; AgBr, light yellow; AgI, yellow) is helpful. (Note the rate of the reaction and if necessary warm the solution on the steam bath until a precipitate is formed.) Carefully remove the liquid with a pipet and wash the precipitate in 5-10 drops of deionized water; again remove the water and add 5-10 drops of 3 M NH₃ (often labeled "NH₄OH"); if the precipitate dissolves, the alkyl halide is a chloride. If the solid does not dissolve, remove the aqueous part and add 5-10 drops of 15 M NH₃. If the solid dissolves, the unknown is a bromide; if it is insoluble, the substance is an iodide.

A combination of the results from the S_N1 and S_N2 tests should help you determine if your alkyl halide is primary, secondary or tertiary.

Alcohols - The ceric nitrate test can be run as confirmation if the alcohol has 10 carbons or less: dilute 0.5 mL of the ceric nitrate reagent with 3 mL of deionized water and add 5 drops of the compound. A color change from yellow to red indicates an alcohol.

The Lucas test (M&B sect. 6.22) distinguishes alcohols of 6 or fewer carbon atoms between primary, secondary and tertiary alcohols (higher alcohols do not dissolve in this reagent): add 3-4 drops of the compound to 2 mL of the Lucas reagent in a 10x75 mm test tube. Agitate well and allow the mixture to stand at room temperature. A reaction is indicated by a clouding of the solution due to the formation of a less soluble alkyl chloride. Tertiary alcohols react immediately; secondary alcohols react in 2-3 minutes and primary alcohols in 7 minutes or longer.

The chromic acid test: to a 10x75 mm test tube dissolve 1 drop of the compound in 1 mL of reagent acetone. Add one drop of the chromic acid reagent and agitate. Primary and secondary alcohols react within 10 seconds and give an opaque blue-green suspension. Tertiary alcohols do not react; other oxidizable compounds, phenols and aldehydes do react.

The iodoform test should be run if the above tests are positive for alcohols but it has limited usefulness since it will distinguish only certain secondary alcohols or ethanol. Dissolve 100 mg of the compound in 1 mL of dioxane (use water for water-soluble compounds); add 3 mL of 10% NaOH solution, and then add dropwise the reagent labeled: "10% I₂ with 20% KI" until a slight excess of brown color (due to I₂) appears. Warm the reaction in a 60 ^oC water bath and continue adding the I₂/KI reagent until the color persists for 2 minutes. Add drops of 10% NaOH until the color just disappears. Remove from the water bath and add 10 mL of water. A yellow precipitate of iodoform (m.p. 120 ^oC, antiseptic odor) indicates the presence of methyl secondary alcohols or ethanol. Ethanol can be used as a model positive test. The iodoform test is also positive for methyl ketones and acetaldehyde.

The solubilities of selected alcohols are listed in M&B p. 253. An approximation of the number of carbons in the compound can be made by weighing 50 mg into a 50 mL flask, then adding deionized water and swirling until the alcohol is dissolved. A measurement of the volume of the solution should provide enough data to relate to the table in M&B.

Primary and secondary alcohols also respond to the KMnO₄ test (see below).

Alkenes - the infrared peaks (above) should be confirmed by the test using bromine in CCl₄ (M&B sect.9.27): add a 2% solution of Br₂/CCl₄ dropwise to a solution of 50 mg of the compound in CCl₄. The red color of the Br₂ will disappear completely if your unknown is an alkene (or an alkyne, primary amine, phenol, or an aldehyde or ketone with à hydrogen).

A further test using aqueous potassium permanganate is available: Add a 1% aqueous solution of KMnO₄ dropwise to 50 mg of the compound. Agitate well and look for signs of MnO₂ (brown). The reaction takes place between two phases, aqueous and organic. As stated above, primary alcohols also react with permanganate to produce carboxylic acids. Secondary alcohols also react to form ketones. Other functional groups react: alkynes, aldehydes and phenols.

Before further analyzing the IR spectrum and the tests you have made, you should repeat tests and the boiling point measurement to ensure that your data can be depended on. It is common that data from various tests will be contradictory; repeating tests 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.

If tests remain contradictory, you will have to base your best decision on the preponderance of the data.

Table of Candidates. A sample is provided on the WWW Server. For your unknown select the entries that approximate the boiling point and functional group of your unknown. The instructor will provide or check structures if they cannot be found.

Infra red spectrum - Refer to the section "Interpreting and IR Spectrum" in the Common Practices…File.

Annotate your spectrum liberally; mark all the peaks that have guided you so far. In addition, mark the absence of peaks that also guided you (write, for example "no O-H stretch here therefore no alcohol" where appropriate). Also go back and check for details in M&B sect. 17.5 and make further notations, for example, mark "isopropyl split" or "no isopropyl split" as appropriate. These details will also agree with the structure of your unknown and will be very helpful in the correct selection among the candidates.

One cannot expect to interpret every peak on the infra red spectrum. On the other hand, if you intend to make a positive identification of a particular compound, your and the Aldrich Library spectrum should match for every peak; the spectrum you obtain are linear in wavenumbers 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 the compounds in the Table of Candidates. the spectrum you obtain are linear in wavenumbers while the Aldrich Library spectra are linear in microns, so each major peak should be measured separately

The Aldrich IR Library is organized by functional group. The first page of each section gives helpful information that, for the most part, repeats what is found in M&B.

In the comparison of your spectrum with the spectra in the Aldrich library, you need to focus on the wavenumber values (positions of the peaks) and less on the amounts of the absorption (depth of the peaks). The spectrum you obtain are linear in wavenumbers while the Aldrich Library spectra are linear in microns, so each major peak should be measured separately

Occasionally two spectra in the Aldrich that closely match the one for the unknown. Again, refer to the appropriate section of the Common Practices.. file.

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

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

"M&B:" Morrison, R.T., and Boyd, R.N. Organic Chemistry, 6th ed., Prentice Hall: Englewood Cliffs, NJ, 1992

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.

Rev. Spring, 1998