Download Qualitative Analysis II Notes

Reminder: These notes are meant to supplement, not replace, the laboratory manual.
Qualitative Analysis II Notes
Application and History
It is only within the last 50 years that a chemist may take and unknown sample
and obtain an IR, 1H NMR, 13C NMR, and within a short amount of time know
the class of compound if not the exact chemical structure of the material.
Analytical instrumentation is very expensive. Many schools, universities and
businesses do not own the necessary instrumentation. Wet analytical tests (using
test tube and reagents) to determine composition of materials have been used
for hundreds of years and are usually relatively inexpensive.
Today a series of 8 different tests will be conducted on a set of known and an unknown
material. The response of each sample to each test will be noted. This will allow
the functional group of the unknown material to be deduced.
Following are the chemical details behind each test.
Jones Test. (Chromium trioxide Oxidation)- Chrome (VI) Oxide is a very powerful
oxidizing agent. When CrO3 oxidizes a material, the chrome is reduced. The
reduced Chrome has characteristic colors. This test shows presence of primary
and secondary alcohols and aldehydes. No reaction with tertiary alcohols,
ketones, carboxylic acids or esters. Phenols also react with Jones reagent but do
not produce a blue/green solution.
1o Alcohol
(orange-red)
(intense sky blue to green)
2o Alcohol
(orange-red)
(intense sky blue to green)
Aldehyde
(orange-red)
(sky blue to green)
Safety- Chrome (VI) is a known carcinogen. The solvent sulfuric acid is highly corrosive. All
waste needs to be in segregated container.
Weaver November 2011
Tollen’s Test
Silver (I) is a very mild oxidizing agent. Diaminosilver selectively reacts with aldehydes
(not ketones) to form acids. The silver is reduced to silver (0) or metallic silver in
the process. This precipitates as either a mirror or gray-black grains. This needs
to sit undisturbed for some time for the mirror to develop.
Silver mirror
Ferric Chloride
Ferric Chloride selectively reacts with phenols. Ferric Chloride forms a purple (dark
gray-maroon) colored complex with most phenols. Enols can show red, violet or
tan.
Purple
2,4 DinitrophenylhydrazineThe terminal nitrogen of the hydrazine is nucleophilic. It will selectively bond to the
carbon of a carbonyl group and form a highly colored conjugated hydrazone
material in a reaction similar to that of oxime formation. This test shows the
presence of aldehydes and ketones. There should be no reaction with alcohols
and acids, however many alcohols over time undergo some autooxidation and
form carbonyls and result in a false positive.
Safety- 2,4-DNP is a suspected mutagen. All waste needs to be in segregated container.
Weaver November 2011
Iodoform test
Methylketones undergo multiple halogenations on the carbon of the methyl group. In the
presence of hydroxide, the hydroxide attacks the carbonyl and the triiodomethyl
group leaves. Triiodomethane is insoluble and precipitates as an opaque yellow
suspension. This test distinguishes between methyl ketones and other ketones.
No solid forms.
Sodium Bicarbonate
The carboxylic acid protonates the sodium bicarbonate to form carbonic acid which
decomposes to carbon dioxide and water. The formation of gas bubbles indicates the
presence of a carboxylic acid. The presence of small oil like bubbles simply confirms
lack of solubility.
Solubility
Organic molecules with 7 or more carbons are rarely soluble in water unless the
molecule contains many (more than 2) highly polar groups such as hydroxides or
carboxylic acids. Most organic molecules with 4 or fewer carbons are soluble in water.
Organic molecules with 5 or 6 carbons are on the edge of being soluble. If they have a
very polar group, they may be soluble. If they do not, they will not be. While this test is
somewhat arbitrary, it can be very useful in narrowing down possible structures of
unknowns. A material which is soluble will completely dissolve in water. A material
which is insoluble will maintain a distinct separate phase. This may look like drops on
the bottom, small drops throughout or a layer on the top. Whether the drops are on the
top or the bottom, depends on the density of the material. A material with slight or
borderline solubility will have intermediate results.
pH
If a material is soluble in water, then test the pH of the liquid. Alcohols, ketones, esters,
ethers, aromatics and hydrocarbons are neutral. Carboxylic Acids and to a lesser extent
phenols are acidic. Amines are basic. If a blue litmus paper turns pink when a drop of
the solution is applied, this indicates the presence of an acidic functional group. Most
often this implies a carboxylic acid, some alcohols also respond. Presence of a
carboxylic acid may be confirmed with the sodium bicarbonate test.
Weaver November 2011