Qualitative analysis
You will receive three unknown organic compounds, which you are to identify by a
traditional qualitative analysis scheme. The report will consist of three forms that you will fill out
and is worth 100 pts or approximately 10% of your total course grade. Read p 468-516 in your
text.
Qualitative Organic Analysis
The analysis experiment is equivalent to identifying a substance about which you have absolutely
no information: the contents of an unlabeled bottle; a natural product isolated from the leaf of a
tropical plant; a component of a competitor's formulated product; etc. Such an analysis requires a
systematic approach, as is described in in your text.
1.
When you have completed the ion tests, obtain a numbered vial containing your unknown
from the instructor. Record the unknown number in your notebook immediately!
2.
Perform the analysis according to the list on page 469, including the indicator tests
according to the attached handout.
3.
If you wish to carry out any classification or functional group tests with known compounds,
ask the instructor who will cheerfully fulfill your request.
4.
Consult the Handbook of Tables for Organic Compound Identification and the tables in
your text (p. 1000-1013).
Qualitative Analysis -- preliminary classification
Solubility tests:
To carry out the solubility tests, approximately 0.1 g or 0.1 mL of
the substance is added to 3 mL of the solvent. If most of the material appears to dissolve,
the compound is considered soluble. If there is no immediate change, especially with a
solid unknown, the mixture should be thoroughly stirred with a glass rod and at least 2
minutes allowed to elapse before a decision is made.
The solubility tests must be applied in the sequence given below to avoid
misleading observations.
a. Solubility in water. A compound that is soluble in water must be at least somewhat
polar.
b. Solubility in ether. A compound that dissolves in water is tested for solubility in
diethyl ether. Organic compounds that contain more than one polar functional group are
not likely to dissolve in ether. Only compounds that have one polar group and a
relatively small number of carbon atoms are expected to dissolve in both polar and
nonpolar solvents.
c. Solubility in aqueous acid or base. A water-insoluble organic acid should dissolve in
an aqueous base; an organic base that is not soluble in water should dissolve in aqueous
acid. The observed solubility in each case is the result of the formation of an ionic salt
which remains dissolved in the aqueous medium. It should be obvious that these tests are
applied only if the original compound does not dissolve in water. If the substance is
found to be soluble in 5% NaOH, indicating that it is an acid, it is tested further with 5%
sodium hydrogen carbonate. Only acids stronger than carbonic acid will dissolve. A
compound may therefore be classified as a strong or weak acid on the basis of these two
solubility tests. An organic base can be identified by its solubility in 5% hydrochloric
acid. No further classification is possible. If a compound is found to be an acid, it should
also be tested with hydrochloric acid on the chance that it may contain both acidic and
basic functional groups (e.g., an amino acid).
d. Solubility in sulfuric acid. A compound that is insoluble in water, hydrochloric acid,
and sodium hydroxide is considered to be neutral. Those substances that contain nitrogen
or sulfur are not tested further and are classed as nitrogen-sulfur neutrals (class M).
Other compounds are tested for solubility in concentrated sulfuric acid. In this test, a
solution in the sense of an ordinary aqueous solution is not necessarily formed. If heat is
evolved, a color develops, or any other change indicative of a reaction is seen, it is
concluded that the substance is "soluble" in H2SO4.
Solubility classification of some organic compounds:
S1 : soluble in water and soluble in diethyl ether
oxygen and nitrogen compounds having less than five carbon atoms:
monofunctional alcohols, aldehydes, amines, carboxylic acids, ketones.
S2 : soluble in water and insoluble in diethyl ether
polyfunctional oxygen and nitrogen compounds: diols, triols, etc.; polyamines;
dicarboxylic acids.
A1 (weak acids) : soluble in dilute sodium hydroxide
phenols, beta-diketones.
A2 (strong acids) : soluble in dilute sodium bicarbonate
carboxylic acids, polynitrophenols, polyhalophenols, acyl halides.
B (bases) : soluble in dilute hydrochloric acid
amines (except diaryl and triarylamines)
N1 (neutrals) : soluble in conc. sulfuric acid
alkenes, some arenes, ethers, water-insoluble: alcohols, aldehydes, esters, ketones.
N2 (neutrals) : insoluble in conc. sulfuric acid
alkanes, halides, diarylethers.
M (nitrogen-containing neutrals)
amides, nitrocompounds, diaryl- and triarylamines, nitroarylamines.
Indicator Classification Method:
The solubility method suffers from several shortcomings. One is that it is difficult
to estimate solubility in borderline cases. There are also some instances in which a solid
substance dissolves, only to react with the solvent to form an insoluble product. The
indicator method overcomes these difficulties and also provides a more specific
classification. That is, it is possible to classify an acid as weak, intermediate, or strong,
rather than just weak or strong as in the solubility system. Bases can also be classified as
weak, intermediate, or strong.
A set of four indicators, A-I, A-II, B-I, and B-II is required. To carry out the test,
1 mL of the indicator is placed in a small test tube and one drop of a liquid or about 30
mg or a solid (about as much as can be carried on the tip of a small spatula) is added to
the indicator. The effect on each indicator solution is described below:
A-I Indicator (original color: purple)
If the color changes from purple to yellow, the compound is an intermediate acid
(Ai) or a strong acid (As). If the color change is from purple to green, the unknown is a
weak acid (Aw). To distinguish between Ai and As, you must use the A-II indicator.
A-II Indicator (original color: blue-violet)
A change from blue-violet to yellow occurs if the unknown is an intermediate
acid. A strong acid causes a change from blue-violet to a shade of red.
B-I Indicator (original color: Purple)
Any base changes the color from purple to yellow.
B-II Indicator (original color: Yellow)
A weak base (Bw) has no effect (color remains yellow), while an intermediate
base (Bi) produces a change from yellow to blue-violet. (There are relatively few strong
organic bases. Although it is possible to detect strong organic bases by special treatment
of the indicators, they will not be considered here.
Caution: The indicators are made up in nonaqueous solvents. The addition of water to
any of the indicators may cause a color change. It is imperative therefore that a clean,
dry test tube be used for each test, and the sample tested must be free of water.
Indicator Classification of Some Organic Compounds:
Strong acids (As): acyl halides, some carboxylic aicds, nitrophenols.
Intermediate acids (Ai): carboxylic acids, o- and p-hydroxyaromatic aldehydes and
ketones, polyhalophenols.
Weak acids (Aw): phenols, beta-diketones, some aryl esters.
Intermediate bases (Bi): aliphatic amines, heterocyclic amines.
Weak bases (Bw): primary arylamines, arylalkylamines, heterocyclic amines.
Neutrals (do not contain nitrogen): hydrocarbons, halides, alcohols, aldehydes, ketones,
esters, ethers.
Neutrals (contain nitrogen): diarylamines, triarylamines, nitriles, nitrocompounds,
amides, polynitroarylamines, polyhaloarylamines.