Download Qualitative/spectral analysis outline and hints

Chemistry 7B
IDENTIFICATION OF ORGANIC UNKNOWNS
The following is an outline of procedures that you have available to determine the structure of your
unknowns. You will not need to perform all procedures, in fact, doing too many can be detrimental!
I. Preliminary Examination
A. Physical State
B. Crystalline Form
C. Color
D. Odor
II. Physical Constants and Purity
A. Melting Point (solids)
B. Boiling Point (liquids)
C. Density
III. Solubility Tests
A. Water (check pH if soluble)
B. 5% Hydrochloric Acid
C. 5% Sodium Hydroxide
D. 5% sodium bicarbonate
E. Concentrated Sulfuric Acid
F. Organic Solvents (ether, etc.)
IV. Elemental Analysis
A. Sodium Fusion
1. nitrogen
a. p-nitrobenzaldehyde in DMSO (attached handout)
b. Prussian blue test
2. sulfur
a. sodium nitroprusside test (attached handout)
b. lead acetate test
c. p-nitrobenzaldehyde in DMSO (attached handout)
3. halogens
a. silver halide test (presence of halogen)
b. oxidation of halide ion (differentiation of halogen)
V. Spectroscopy
A. IR
B. H1-NMR
C. C13-NMR
C. UV
VI. Chemical Classification Tests
A. Alkanes
1. none
B. Alkenes and Alkynes
1. bromine/carbon tetrachloride
2. Baeyer test
C. Aromatic Hydrocarbons
1. aluminum chloride/chloroform (attached handout)
D. Halides
1. ethanolic silver nitrate
2. sodium iodide/acetone
E. Alcohols
1. Lucas test
2. chromic acid
3. acetyl chloride
4. iodoform test
F. Phenols
1. ferric chloride solution
2. bromine water
3. acetyl chloride
4. colored phenoxide ion
G. Ethers
1. none
H. Aldehydes and Ketones
1. 2,4-dinitrophenylhydrazine
2. iodoform test
3. ferric chloride solution (for compounds with high enol content)
4. For Aldehydes Only
a. chromic acid
b. Tollens’ test
c. Benedict’s test
I.
Carboxylic Acids
1. sodium bicarbonate solutions
2. ethanolic silver nitrate
J. Amines
1. nitrous acid
2. acetyl chloride
3. Hinsberg test
K. Nitro Compounds
1. ferrous hydroxide test
L. Nitriles
1. ferric hydroximate test (attached handout)
2. ammonia liberation from base hydrolysis
M. Esters
1. ferric hydroximate test
N. Amides
1. ferric hydroximate test (attached handout)
2. ammonia or amines liberated from base hydrolysis
VII. Confirmation of Identity
A. Comparison to an "Authentic" Sample
1. physical properties and constants from literature (Merck Index, etc.)
2. IR and NMR spectra (online spectral databases)
Chemistry 7B
PROCEDURE FOR ELEMENTAL ANALYSIS (N, S, and X)
Sodium Fusion
To test for nitrogen, sulfur or halogens in your organic unknown it is first necessary to completely
destroy the molecule by sodium fusion, releasing these elements as NaCN, Na2S, and NaX. The
resulting aqueous solution can be tested for these ionic species.
There are a number of ways to perform a sodium fusion, but the following method works well. First
obtain a test tube containing a small piece of sodium metal. Using a test tube clamp, hold this test
tube over a Bunsen burner flame just until the sodium melts. Do not overheat because you will
destroy the sodium sample. Immediately after it melts remove from the flame, hold it over a 50 mL
beaker containing 5 mL of distilled water and quickly add a small amount of your sample (solid or
liquid) to the melted sodium. Make sure you add your sample so that it makes contact with the
sodium, and not down the side of the test tube. The reaction with the sodium can result in a flash
and may even break the test tube (which is why you perform over a beaker). If nothing is observed it
you may have to repeat the procedure.
If your test tube breaks, try and scrape all remaining residue from the test tube into the beaker
containing 5 mL of distilled water. If your test tube did not break, add about 10 drops of methanol to
the tube and scrape the inside of the test tube with a spatula to loosen it from the sides of the test
tube. Do this over the beaker because occasionally test tubes will crack during the sodium fusion, but
do not appear broken. Pour all of the methanol and residue from the test tube into the 5 mL of
water. If residue remains in the test tube you may add a small amount of distilled water to the test
tube to rinse it out into the beaker. To prevent unnecessarily dilution the best method is to pipet
water from the beaker back into the test tube to rinse. The aqueous solution in the beaker is the
stock solution you will test for NaCN, Na2S, and NaX. You may store this solution in a sealed,
labelled flask in your drawer for later analysis.
Tests for N, S, and X
The first time you do these tests, you should always prepare (by sodium fusion) stock solutions that
you know contain these elements and solutions that you know do not contain these elements. This is
so you can see how both a positive and negative test appear.
Nitrogen (in the form of CN-)
Perform the ferrous sulfate test as described in the lab text. An additional test known as the pnitrobenzaldehyde in DMSO test (found in handout), can be done as a conformation. This test is also
sensitive to sulfur (see below).
Sulfur (in the form of Na2S)
Perform the sodium nitroprusside test as described in the handout.
Halogens (in the form of NaX)
Perform the silver nitrate test as described in the lab text to determine if a halogen is present. If a
halogen is present, a further test using chlorine water should be done to differentiate which halogen
is present. This is described in the lab text under the heading Differentiation of Chlorine, Bromine
and Iodide (p. 464 in Pavia 2nd edition).
Chemistry 7B
MISC QUALITATIVE ANALYSIS HINTS
General
• In the long run, you will save time, minimize the number of confusing results and minimize the need to repeat
tests by:
(a) doing tests slowly and carefully.
(b) using proper quantities of reagents.
(c) running tests while simultaneously running the same test with a known compound as a control.
• Always use earlier results to direct your next choice of test to do. This is particularly important when you get to
the chemical classification tests. Don’t try to do all these tests and then evaluate the results later. That takes
too much lab time and will cause a great deal more confusion.
• No tests are perfect and absolutely trustworthy although some are much better than others. Look for consistency
of results and the overall weight of evidence from all your results.
• Except as noted in later hints, you should assume that you are given unknowns that are “pure enough” for the
purposes of qual analysis tests.
Preliminary Examination, Physical Constants
• Determine the boiling point of liquid unknowns by simple distillation. If the boiling point is over 200 °C, see the
instructor before attempting the distillation. Set the thermometer bulb low in the apparatus and wrap the
outside so that a good boiling point can be measured.
• If you wish to recheck your boiling point (or not do a distillation), the small scale method of refluxing in a test
tube is easier to set up than distillation.
• Measure the refractive index (read to ± 0.0001) of liquids only after distilling them; this index is very sensitive to
impurities. Your values will rarely be off by more than ± 0.005 and will usually be much closer, sometimes really
close. Correct for temperature before comparing your value to literature values.
• Highly colored unknowns are likely to have extensive conjugation. In addition, highly electronegative atoms may
be present.
• Small amounts of impurities may cause some discoloration of your unknown. This may be most noticeable for
solids which are supposed to be white and liquids which are supposed to be colorless.
• When you initially look up possible structures, allow for a 10 to 15 °C error in your melting or boiling point. Your
values will rarely be off by more than this and will usually be much closer, sometimes really close. Really high
melting or boiling points tend to be less accurate. This is partly due to greater thermometer error at higher
temperatures.
• Sharp melting points are generally more accurate than broad ones. Melting points only a little above room
temperature are often quite inaccurate.
• Student measurements of melting and boiling points are lower than the literature value much more often than
they are higher.
• When you look up possible structures for a solid unknown you may occasionally find a boiling point low enough
to measure in the melting point apparatus. In this case (not otherwise) you can rerun a melting point (use 4 to 5
times as much solid) but, after the solid melts keep heating and watch for the liquid to boil vigorously. Such a
boiling point measurement is likely to have low accuracy, but may be used as a guideline for a “low boiling solid.”
Solubility Tests
• Solids may be slow (maybe very slow) to dissolve. Be careful before you assume a solid is insoluble.
• Do not use more unknown than called for; sometimes a soluble compound may be barely soluble.
• If a compound reacts with a solvent, it should be classified as soluble in that solvent even if a precipitate forms.
• In addition to certain low molecular weight compounds being water soluble, certain higher molecular weight
compounds that are polyfunctional may also be water soluble.
• Use pH paper instead of litmus paper to test water solutions. Also test plain water as a control since it will often
test slightly acidic due to dissolved CO2.
• Depending on the rest of the structure of a phenol, its water solution (if soluble) may test as neutral or acidic.
• Some aromatic compounds will dissolve in (react with) concentrated H 2SO4. This may be caused by certain
substituent groups and is especially likely for polynuclear aromatic compounds.
• Higher molecular weight carboxylic acids and phenols may not dissolve in 5% NaOH (or 5 % NaHCO3). This is
most likely if the compound contains more than one benzene ring and/or a really large alkyl group.
• Higher molecular weight amines may not dissolve in 5% HCl. A positive test for N may be particularly important
to help figure this out. This is most likely if the compound contains more than one benzene ring and/or a really
large alkyl group.
• Concentrated H2SO4 reacts with many organic compounds. If reaction causes the formation of a halide ion, it is
likely that the concentrated H2SO4 will oxidize I – to I2 and Br– to Br2. Look for the colors of I2 or Br2, perhaps
as a vapor due to the reaction being exothermic.
• Nitriles are classified as neutral. They are not listed in the solubility diagram in your lab text.
Elemental Analysis
• Students sometimes have problems with getting a sodium fusion to work well. Be sure the unknown is dropped
directly on to the molten sodium rather than mainly hitting the test tube walls.
• You may wish to combine two sodium fusions into one water solution in case one did not fuse well and to get a
higher concentration of ions. Mistakes are easily made when testing for these ions so be very careful.
• Sodium fusion can potentially distinguish among the various halides by the color of the silver halide salt which is
eventually precipitated. Often this distinction is not possible due to impurities in the precipitate obscuring its
true color. You should always do the chlorine oxidation differentiation test to determine which halogen is
present. Mistakes are common here due to poor interpretation and/or because the concentration of X – is too low.
• If other results indicate the possibility of an error, repeat the sodium fusion.
Chemical Classification Tests
• A negative test does not always mean no change will be observed. Be sure you understand what observations are
required to conclude that a test is positive.
• Sometimes an unknown isn’t soluble in a given test reagent. This may make the test difficult or impossible to do.
• Unknown aldehydes may contain a significant amount of the corresponding carboxylic acid as an impurity caused
by oxidation of the aldehyde. This is especially likely for liquid aldehydes. You must expect this with aldehydes
and not confuse them with carboxylic acids.
• The Tollens reagent is a mild oxidizing agent and will react with any compound which is easily oxidized. This
usually means an aldehyde, however, the reagent may react with some phenols, allylic alcohols, etc. or give
confusing results given enough time.
• Unknown esters may sometimes hydrolyze fast enough under the conditions of strongly acidic or strongly basic
tests to give positive results for the hydrolysis products (ROH + RCOOH). These results might be falsely
assumed to be caused by the unknown.
• In addition to alkenes and alkynes, Br2 / CH2Cl2 may be decolorized by amines and by highly reactive aromatic
compounds. Some compounds may react more slowly so decolorization of Br 2 after waiting a long time cannot be
taken as a positive test.
• Unknown amines may give a variety of confusing results and semi-positive results with many of the tests for
other classes of compounds.
• Unknown alkanes are recognized by the lack of positive tests for any other inert compound.
• Unknown ethers are recognized by the lack of positive tests for any other neutral compound.
• Neutralization equivalents are usually within 5 or 6 g/mol but often much closer.
• A concentrated enough solution of a water soluble phenol may cause slow bubbling with NaHCO 3. Be cautious
about confusing a phenol with a carboxylic acid.
• Before assuming that the iodoform test is negative, be sure to add the iodoform reagent until the color persists;
warm if necessary. If the volume gets large, dump out some of the mixture before continuing to add reagent.
Only do this test after you have come up with possible candidates for your unknown and then only if it could help
you distinguish among the choices. Don’t do it as a matter of routine as it is a difficult test.
• Always run a positive control along with the iodoform test.
• Always run a negative control along with the nitro group test. Oxygen in the air will lead to a positive nitro test
so it is important to bubble gas through the solution or boil it before carrying out the test.
• A positive Lucas test depends on the formation of a carbocation. Remember that carbocation formation (and
stability) is influenced by factors (like resonance) other than whether or not it is 1°, 2° or 3°.
Chemistry 7B
QUALITATIVE ANALYSIS TESTS NOT IN LAB TEXT
p-Nitrobenzaldehyde in DMSO Test for Nitrogen (and Sulfur)
Pour about 0.5 mL of the sodium fusion solution into a small test tube and saturate it with solid sodium
bicarbonate (undissolved solid should remain). Then add 1 or 2 drops of this saturated solution to a test tube
containing 1 mL of a 1% p-nitrobenzaldehyde solution in DMSO.
The presence of nitrogen is indicated if the solution changes from the original yellow color to purple. A color
change to green indicates that sulfur is present. If both nitrogen and sulfur are present only a purple color will be
observed, although the color may rapidly pass through green on the way to purple.
If a positive test for nitrogen is observed, a different test for sulfur may be done — especially if a sulfur containing
compound is being seriously considered as the unknown. The p-nitrobenzaldehyde reagent should be discarded if
the initial yellow color darkens.
Sodium Nitroprusside Test for Sulfur
To a 1.0 mL sample of the sodium fusion solution, add 2 drops of a 1% sodium nitroprusside solution; a deep blueviolet color indicates the presence of sulfur. This color may fade.
Na2S
+
Na2Fe(CN)5NO2
+
H2O
Na4[Fe(CN)5NOS]
sodium nitroprusside
+
2 NaOH
a blue-violet complex
Aluminum Chloride / Chloroform Test for Aromatic Compounds
Heat about 100 mg of anhydrous aluminum chloride in a large Pyrex test tube held almost horizontally until the
material has sublimed 3 or 4 cm above the bottom of the test tube. Allow the tube to cool until it is almost
comfortable to touch, and then add 20 mg of a solid or 1 drop of a liquid organic compound down the side of the
tube. Follow this by 2 or 3 drops of chloroform. The appearance of a bright color ranging from red to blue where
the compound and chloroform come in contact with the aluminum chloride is a positive indication of an aromatic
ring.
The test is based on a series of Friedel-Crafts alkylation reactions. It is important to note that aromatic rings that
will not undergo a Friedel-Crafts reaction will give a negative test.
3 ArH
+
CHCl3
AlCl3
Ar3CH
+
3 HCl
The reaction proceeds by way of intermediate carbocations, and disproportionation between the triarylmethane
and the carbocations produces the relatively stable, highly colored (due to extensive conjugation) triarylmethyl
cation, which remain in solution as AlCl4¯ salts.
Ar3CH
+
Ar+ AlCl4¯
Ar3C+ AlCl4¯
+
ArH
Ferric Hydroximate Test for Nitriles and Amides
Prepare a mixture consisting of 2 mL of 1 M hydroxylamine hydrochloride in propylene glycol, 30 –50 mg of the
compound which has been dissolved in a minimum amount of propylene glycol, and 1 mL of 1 M KOH. Heat the
mixture to boiling for 2 to 3 minutes and cool to room temperature; add 0.5 –1.0 mL of a 5% ferric chloride
solution. A red to violet color is a positive test. Yellow colors are negative, and brown colors or precipitates are
neither positive nor negative.
This test converts nitriles and amides to hydroxamic acids which then complex with ferric ion as described for
esters on p. 513 in Pavia, Lampman, Kriz and Engel.