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 an 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 quick and inexpensive.
Observations and Terminology. The quality of chemical qualitative tests results are very
dependent upon careful observations and detailed record keeping. Be sure to
record the starting appearances of all test compounds and reagents. Carefully
observe and document all changes in color, phase and opacity.
Opacity- the degree to which light passes through a material.
Transparent- light readily passes through the material. This can also be
described as clear. An example of this is clear class.
Translucent- some light passes through, but some light is obscured. A
shape seen through the liquid is fuzzy. An example of this is frosted glass.
Opaque- light does not pass through. A shape held behind a vial of
solution cannot be seen. An example of this is blacked out glass.
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, but not the exact composition of the unknown material to be
deduced.
The structures and names of the known compounds are below.
Most, none of the unknown compounds are any of the known materials. Each unknown
will have exactly one functional group present. It will contain either a 1°, 2° or 3°
alcohol, phenol, aldehyde, ketone, methylketone or carboxylic acid.
Following are the chemical details behind each test.
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 hydroxyl groups 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 and will be seen
as a homogenous liquid. A material which is insoluble will maintain a distinct separate
phase. This may look like drops of oil on the bottom, small droplets throughout, or small
droplets 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, 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 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, a few phenols may also respond. Presence of a carboxylic
acid may be confirmed with the sodium bicarbonate test. If blue litmus paper remains
blue, the material may be basic or neutral. If red litmus turns blue with a drop of liquid,
this indicates the liquid is basic. If red litmus paper remains red, this means the liquid is
acidic or neutral. Remember what you learned in General Chemistry lab in regards to
reading litmus paper.
Liquid
Acidic liquid
Neutral liquid
Basic liquid
Blue litmus paper color
when spotted with liquid
Pink or Red
Blue
Blue
Red litmus paper color
when spotted with liquid
Red
Red
Blue
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. A positive test looks a bit like the fizz in 7-up. It will only
bubble for a few seconds, so observe carefully after addition. The presence of small oil
like bubbles simply confirms lack of solubility. Aldehydes may autooxidize in the
presence of air and form carboxylic acids. Aldehydes that have oxidized will react with
sodium bicarbonate release carbon dioxide gas.
Ferric Chloride
Ferric Chloride selectively react with phenols. Ferric Chloride forms a purple (dark graymaroon) colored complex with most phenols. Enols can show red, violet or tan.
Translucent yellow
purple or dark blue
Jones Test. (Chromium trioxide Oxidation)- Chrome (VI) Oxide is a very powerful
oxidizing agent. When CrO3 oxidizes a material, the chrome is reduced to chrome +3 or
+2. Chrome (VI) is normally a shade of orange, chrome (III) is normally a shade of
green. If the color of the chrome changes, this indicates the presence of a functional
group that is able to be oxidized. This test shows the 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. Instead they give a black or dark brown liquid.
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.
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 mechanism very 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 may result in a false positive.
The color of the starting materials and hydrazone material can be quite similar, but the
opacity of the starting liquids and the final product changes markedly when a
reaction occurs. In this test it is important to note the opacity of the solutions.
Safety- 2,4-DNP is a suspected mutagen. All waste needs to be in segregated
container. Keep away from skin.
Tollen’s Test
The Tollen’s test is a way to distinguish between a ketone and an aldehyde. Only
perform Tollen’s test on materials which tested positive for carbonyls using the
2,4-DNP. 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 fine grayblack grains. This needs to sit undisturbed for some time for the mirror to
develop.
Silver mirror
Iodoform test
The iodoform test distinguishes between regular ketones and methyl ketones. Only
perform the iodoform test on materials which tested positive to the 2,4-DNP test
and negative to the Tollen’s test. Hence, only perform iodoform test on ketones.
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. Be careful interpreting this test. Normal ketones can undergo Aldol
reactions with NaOH, doubling the molecular weight of the material, and
precipitating. This precipitate will usually not be yellow. (Remember the aldol
experiment a few weeks ago.)
No solid forms.
Revised:
April 14, 2016, S.L. Weaver