Alcohol - Alkyl Halide Unknown

Reading: McM sections 12.5-12.7, 13.5 and 13.11with problems 13.18 and 13.19. Also review 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 or an alcohol - from the unknown's boiling point, infra-red (IR) and ¹H NMR spectra and its response to chemical tests. Record the unknown number of the compound in the Purpose section.

In advance of this lab, set up a Table of Chemical Tests which will be used to organize the spectral 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. In the "Test" column, list the following four preliminary tests that you will carry out no matter what kind of unknown you have: IR – 3200-3600 cm^-1, boiling point; ¹H NMR 2.5-4.5 ppm; Beilstein test. Other tests will be needed depending on the results of the preliminary ones.

Chemical tests and the spectra will complement each other to reveal which functional group, an -OH group (for an alcohol) or –X, a halogen (either -Cl, -Br or -I, for an alkyl halide). Further chemical tests and the spectra will also often reveal details of the compound (whether it is 1^o, 2^o, or 3^o etc.).

From the lists of known alcohols and alkyl halides available in the file "Alcohol/Alkene/Alkyl Halide Unknown Candidates", the boiling point you measure will narrow the possibilities to 10 - 20 candidates. Based on the spectral properties, you should be able to select one compound that fits your data best.

Using a large drawing of your selected compound, you will need to point out how its structural features fit all the spectral data you have.

As mentioned in the "Common Practices…" file, the process of identifying an unknown compound often has inconsistencies and apparent contradictions. Do not allow the results of your first test to prejudice your inferences for later tests. Rather, base your decisions on the results of many initial, confirmatory and repeated tests. Repetition of tests is important since your first trials are not likely to be your best. Use your laboratory time to take measurements and carry out reactions. When you finish this lab, you may not have a final compound selected, but you should have evidence that you have confidence in and can base later decisions on to arrive at an identification. Looking up data, searching for spectra and planning reactions are best done outside of the laboratory period.

Procedures:

Boiling Point Determination. Your boiling point will be the reference point for the selection of candidates (see below): 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 suspended within 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, open end down. 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. If you have not done this measurement before, make a careful drawing of the apparatus for your records.

A Note on the Analysis of All Spectra. Mark these spectra liberally. Take measurements of peaks, where appropriate, with a ruler. Mark the values on the spectrum. Indicate what these values mean. Also remember that the absence of a signal is often as useful as the presence of one. If this is the case, this too should be noted on the spectrum paper, for example "no OH absorption here – therefore not an alcohol."

Each finding, or absence of a peak, should be recorded in your Table of Chemical Tests. The entry you begin with, "IR – 3200-3600 cm^-1" should be reported with a result "large peak" or "no peak" and the inference and comments. Further tests as the presence or absence of other peaks should be reported and interpreted.

The IR Spectrum. On the spectrum mark diagnostic absorptions with cm^-1 values. Enter the findings as "tests" in the test table using the Common Practices…File as a guide.

	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 McM p. 454 and 689 and Mayo, et al. fig 6.24.
	b)	Alkyl halides have a few IR absorptions characteristics that distinguish them from alkanes, but they are not reliable. Therefore the absence of alcohol peaks indicates that the unknown (in this experiment) is an alkyl halide.

A further test for alcohols: the C-O stretching absorptions between 1000 and 1200 cm^-1 are usually helpful in determining whether the unknown is a 1^o, 2^o or 3^o alcohol:

	Absorption		Inference
	1075-1000		Primary alcohol, RCH₂OH
	1150-1075		Secondary alcohol, RR'CHOH, except if R and R' are part of a ring*
	1200-1100		Tertiary alcohol, RR'R"COH

* OH groups attached to rings do not follow this pattern; for example, cyclohexanol’s C-O stretch is at 1060 cm^-1 (see McM p. 689) which suggests that it is a 1^o alcohol; in fact cyclohexanol is a 2^o alcohol.

There are far more IR absorption peaks in a spectrum than need be interpreted; only a few are necessary to determine the functional groups present. Always check the spectra in McM and Mayo for examples of diagnostic peaks.

Once a list of candidates (below) is set up, your spectrum may match peak for peak one found in the Aldrich Library. In the comparison of your spectrum with the spectra in this library:

	a)	focus on the cm^-1 values and less on the amounts of the absorption (depth of the peaks). The spectrum you obtain is linear in cm^-1 values while the Aldrich Library spectra are linear in microns.
	b)	restrict the search to the compounds in the Table of Candidates. There is a tendency for novices to compare their spectra with every one in the Aldrich IR Library; this is unnecessary.
	c)	refer to the appropriate section of the Common Practices.. file if two spectra in the Aldrich closely match the one for your unknown.
	d)	note that the Library is organized by functional group. The first page of each section gives helpful information that, for the most part, repeats what is found here.

The ¹H NMR Spectrum. With this instrument, every signal must be accounted for.

Preliminary. Check the spectrum for the following and mark if found on the spectrum:

	a)	a signal at 0 ppm for the internal standard, tetramethylsilane (TMS), which is used for calibration, and a small peak at 7.24 ppm for chloroform.
	b)	impurities; a common one is acetone [ (CH₃)₂C=O ] which is used to clean the NMR sample tubes is found at 2.15 ppm.

Initial data on your unknown. Report this in the Chemical Tests Table

	c)	the number of signals from your unknown. These represent the uniquely positioned hydrogen atoms within the molecule.
	d)	areas - integrals - of the signals are proportional to the number of H atoms in each group that is represented by a signal. Apply a multiplier to all of them if necessary to recast them to whole number ratios. Round them off. Keep in mind that these numbers are ratios. For example, in a spectrum of propane, the areas of the two signals are in the ratio of 1:3, reflecting the 2 H atoms in one group and the 6 H atoms of the other group.
	e)	peaks overlapped on another usually give broad peaks whose fine structure cannot be interpreted (splitting - see i-5 below). The peak areas for the H atoms or groups represented in the overlapped signals will be integrated together. This is common for consecutive -CH₂- groups.
	f)	For alcohols, the H-OR peak is sometimes overlapped with the signals for other H atoms in the molecule. The extra H is included in the integral.

More detailed data. Other aspects, g) to i) need to be examined and recorded in the Chemical Tests Table:

g) The location of signals in the spectrum – the chemical shift, measured in ppm – can be found in McM Table 13.3. The presence of the relatively electronegative O or halogen (X) causes signals of H near them to be shifted downfield, that is to higher ppm. The amount of this shift depends on the number of bonds between the O or X and the H being observed. In alcohols or alkyl halides, signals for H atoms separated from halogen (X) or oxygen atoms by only one carbon atom ( i.e. H-C-X or H-C-O) are typically in the 2.5-4.0 ppm region (Table 13.3). If two carbon atoms separate the observed H and either halogen or oxygen (i.e. H-C-C-X or H-C-C-O), the signal will range from 1.20 to 1.95 ppm. When O or X is distanced by more than two carbon atoms from H, the signal for H will resemble signals for alkanes: methyl group, 0.7-1.1 ppm; methylene group, 1.2-1.4 ppm; methine group, 1.4-1.7 ppm. The chart, below, summarizes these facts:

								\|--	O-C-C-H or X-C-C-H					--\|

															\|---CH₃*---\|
												\|-CH₂*-\|
		\|--	O-C-H or X-C-H		--\|					\|---CH*---\|

\|--	O-H in alcohols				--\|	\|--	C=C-C-H				----\|

5.0		4.0		ppm value	2.5	2.2		2.0		1.7	1.6	1.4	1.3	1.2	1.1	0.7

H* = H at least 3 carbons removed from either O or X

h) Integration (determination of the area) of the peak, if any, in the 2.5-4.0 range will corroborate findings from the IR and chemical tests. A 1^o unknown R-CH₂-OH or R-CH₂-X, will integrate for 2 H atoms on the carbon that also bears O or X. The integral for a 2^o unknown will be 1 H in this range (R₂CH-OH or R₂CH-X.) A 3^o alcohol or alkyl halide will have no H atoms in this range, unless it is the oxygen bound H of an alcohol (RO-H).

i) ¹H NMR signals are often split into lines. The number of individual lines - the multiplicity - relates to the number of H atoms that are neighbors to the H being observed in the signal. Reasons for this appears in McM section 13.11. For our purposes, "neighbor" H atoms are separated from the observed H by two carbon atoms. H atoms in the same group produce the same signal, are not neighbors, and therefore don’t split each other. For the present identification of the alcohol or alkyl halide, note and interpret splitting features if they are apparent in your spectrum.

	1)	If a signal is not split, the observed H has no neighbor H atoms, or it is bonded directly to O (thus RO-H, alcohols)
	2)	If a signal is split into a doublet, the observed H has one H neighbor. Examples would be BrCH₂CHBr₂ or (CH₃)₂CHCl. Note that when we examine the signal for the neighbor H it will be split by H in return, but not necessarily into a doublet.
	3)	If a signal is split into a triplet, the observed H has two H neighbors. Examples would be CH₃CH₂CH₂CH₂OH and ClCH₂CH₂CH₂CH(CH₃)₂. Note that any H atoms distanced any more than carbon atoms from the observed H are not considered neighbors. Also note that actually two signals in the alcohol example are split into triplets.
	4)	Signals can be split into quartets, pentets, etc. but these are often difficult to determine from the spectra you have without magnification. But, following the sequences started above, it is always true that if an observed H has n H neighbor atoms, it will be split into n+1 lines.
	5)	Complex splitting is present in some unknowns, where neighbor hydrogens are very nearly identical. Examples of complex splitting are found in the 0.9-1.8 regions of McM Fig 13.24 a) and b) for cyclohexyl compounds. Long -CH₂- chains also give complex splitting.
	6)	For alcohols, there is no "neighbor" relationship - and therefore no splitting - between the two H atoms in the following arrangement: H-O-C-H.

Treat each separable signal as a "test". Its position (ppm value) and integral should be reported as "results" in the table with suitable inferences and comments. Typically a "test" on an alcohol would be "¹H NMR 2.5-4.5 ppm" with the result "doublet at 3.4 ppm, area 2 H" and the inference would be "CH₂ attached to the OH also attached to another CH" with the comment "therefore this signal represents CH-CH₂-OH".

Whether your unknown is an alkyl halide or an alcohol will be best determined from the IR spectrum, but this determination will be confirmed by the ¹H NMR spectrum.

Every peak in the ¹H NMR must be accounted for. Reference ¹H NMR (and some IR) spectra can be found on a website: http://www.aist.go.jp/RIODB/SDBS/menu-e.html

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 spectra 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 writing results like "it came out negative". Instead, detail what was actually seen, then in the inference "negative for a …" would be appropriate.

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 (McM sect.11.6), 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 as long as 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 (McM sect. 10.7) 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 ¹³C NMR spectrum. As for the ¹H NMR, every signal must be accounted for.

Preliminary. Mark if found a signal at 0 ppm for the internal standard, tetramethylsilane (TMS), which is used for calibration, and for three closely spaced lines at 77 ppm for CDCl₃.

Your unknown. Report on the spectrum and in the Chemical Tests Table

	a)	the number of signals. Because the signals are not split by hydrogen atoms, each line represents a uniquely positioned carbon atom within the molecule. In some cases these lines may be so closely spaced that they appear as a thick line, and a magnifying glass may be needed. In other cases lines may appear just above the noise, so it may be necessary to report "at least" for the number of unique carbons.
	b)	(as in ¹H NMR the chemical shifts of the carbon atoms will depend on their proximity to to the O or X atoms in the molecule, but as can be seen from McM Fig 13.17, these chemical shift ranges are not well separated, so they do not clearly indicate the kind of carbon atoms observed for alkyl halides or alcohols. Therefore the chemical shift information need not be examined closely. )
	c)	(unlike ¹H NMR, the size of the signal is not proportional to the number of C atoms represented. Therefore integration is not examined for ¹³C NMR. )

Before finishing the lab work, 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 in the Common Practices file. For your unknown, select the entries having boiling points are 5 degrees below to 15 degrees above your measured temperature. Supply structures for all candidates; the instructor will provide or check structures if they cannot be found.

Conclusion. Announce your best candidate: "Based on the evidence gathered, unknown number ___ is _____ ." Make a full page drawing of the compound you have concluded to be your unknown,. Then use symbols (stars, squares, triangles, etc) to link each unique C and H atom to the signal it represents in the spectra.

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

"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, 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. January, 2001