Introduction to Experiment 7: “Organic Functional Groups”
One classic method for identifying the type of functional groups in a
molecule employs solution chemistry. Reagent solutions which react with only
certain types of functional groups to produce a color change or precipitate are
used as indicators for those groups. In the real world, tests such as these are
made commercially available by binding the reagent to a test strip. Such test
strips are used to detect the pH of urine, protein in urine, sugar in the blood, etc.
In organic chemistry, we use solution based reagents to identify the
functional groups in unknown organic compounds. That is your task for today’s
lab. You will react a series of known compounds with functional group reagents
and record the results of those tests. You will also run the same tests on an
unknown which contains at least one but not more than two functional groups.
By the end of class, you should be able to identify the functional group or groups
in your unknown. Before the final, you should be able to select which test you
would use to identify a specific functional group.
As you know, there are quite a few different organic functional groups, and
there is more than one qualitative test for each of them. Therefore, entire books
have been published on the topic of qualitative analysis of organic compounds.
Of course, our lab is only two hours long. Therefore, we will study a small subset
of these qualitative reactions. Our focus will be on identifying alcohols, ketones,
and aldehydes.
Three of the fundamental tests for alcohols are (a) the color change with
ceric ammonium nitrate, (b) the color change with chromic acid, and, for those
that are water soluble, (c) the formation of an insoluble layer with zinc chloride in
HCl. Before considering the chemistry described below, one should quickly
revisit the concept of solution chemistry and solubility. Remember, many organic
compounds are not polar and do not exhibit hydrogen bonding. If the molar
mass is high and/or the molecule is non-polar, a substance will not be soluble in
the very polar (and hydrogen bonding) solvent water. If a reagent is dissolved
in water and the organic compound to be tested is not water soluble, it is not
reasonable to expect a standard response to that reagent. Frequently you may
have to use a solvent other than water or one that is miscible with water. This
lab has been designed so that all organic unknowns are sufficiently water soluble
to give the standard tests. Unless there are multiple highly polar functional
groups, organic compounds of 6 or more carbons are water “insoluble”.
Functional Group Tests:
1. Ceric ammonium nitrate test --
OR
(NH4)2Ce(NO3)6 + ROH
(yellow)
(NH4)2Ce(NO3)5 + HNO3
(red)
Primary, secondary, and tertiary alcohols of up to 10 carbons react to form
a red alkoxy cerium (IV) compound. This color change indicates that the Ce(IV)
has become Ce(III), which is a reduction, and that the organic compound has
been oxidized. For those compounds which are very easily oxidized, the red
color may fade out in a few minutes leaving a colorless solution. The absence of
the orange color or the red one, indicates that further oxidation has occurred to
the organic molecule. This is considered to be a gentle oxidation reagent.
2. Chromic acid or Chromic Anhydride tests –
O
3 RCH 2OH + 4 CrO 3 + 6 H 2SO 4
(orange)
3 RCOH + 9 H 2O + 2 Cr 2(SO 4)3
(blue-green)
O
3 R 2CHOH + 2 CrO 3 + 3 H 2SO 4
(orange)
3 RCR
O
3 RCH
+ 6 H 2O + Cr 2(SO 4)3
(blue-green)
O
+ 2 CrO 3 + 3 H 2SO 4
(orange)
3 RCOH + 3 H 2O + Cr 2(SO 4)3
(blue-green)
Safety Note: Since the reagent solvent is primarily sulfuric acid, you should
avoid getting this solution on you or your clothes or books.
Chromic acid is a “brutal” oxidizing agent. Any organic compound which is
even mildly susceptible to oxidation, will be oxidized by this reagent. Therefore,
aldehydes as well as alcohols react with this reagent. As the organic compound
is oxidized, the chromium ions are reduced from Cr+6 which gives an orange
color to Cr+3 which gives a blue-green color. In addition, organic compounds of
more than 6 carbons are still reactive. Both primary and secondary alcohols give
a positive test, whereas tertiary alcohols do not react. Aldehydes react in 5-15
seconds while ketones usually do not react.
3. Zinc Chloride in HCl (a.k.a Lucas Test) --
R2CHOH + HCl
(water soluble)
R3COH + HCl
(water soluble)
ZnCl 2
ZnCl 2
R2CHCl + H2O
(water insoluble)
R3CCl + H2O
(water insoluble)
The basis for this test is that low molecular mass alcohols are polar
enough to be water soluble because of hydrogen bonding. Alkyl halides of
analogous formula are not capable of hydrogen bonding and are, therefore, not
water soluble. Secondary alcohols which are water soluble will react with
shaking to form a cloudy solution in 3 – 5 minutes. Tertiary alcohols which are
water soluble will turn cloudy immediately and form an insoluble second layer in
the test tube. Primary alcohols, however, do not undergo this substitution
reaction (nor do aldehydes or ketones).
4. Tollen’s test -O
O
RCO - NH 4
RCH + 2 Ag(NH 3)2OH
(Tollen's Reagent)
+
+ H 2O
+ 3 NH 3
(salt of acid)
Safety note: Silver/ammonia complexes can produce explosive
compounds when stored long enough to go to dryness. This can be prevented
by ensuring that the waste from this test is acidified with HCl. In addition,
remember that silver nitrate will stain you skin and clothes as exposed to light.
The Tollen’s reagent is a selective oxidizing agent which reacts with
aldehydes. The basis of the reaction is that the organic compound is oxidized to
the carboxylic acid while the silver ion is reduced from Ag(I) to metallic silver. If
the test tube in which this test is run has a clean glass surface, the silver will
plate out on the glass giving the appearance of a mirror. Of course, the solvent
is water; therefore, compounds which are not water soluble can not be tested
with this reagent.
5. 2,4-Dinitrophenylhydrzine test -NO 2
NO 2
O
RCH
NO 2
+ NH 2NH
RC
H2O
NO 2
H
NO 2
NO 2 +
N-NH
O
RCR
+ NH 2NH
NO 2
RC
N-NH
NO 2 +
H2O
R
Aldehydes and ketones react with 2,4-dinitrophenylhydrazine in acid
alcoholic solution to produce 2,3-dinitrophenylhydrazones. While the hydrazine
form is soluble, the hydrazone forms a precipitate. The original reagent is orange
and the precipitate that forms for aldehydes and ketones can range in color from
canary yellow to scarlet.
Tests such as these no only provide evidence of a specific functional
group, but they can be used to identify the specific compound. The precipitate
formed can be isolated and purified and a melting point can be determined for
comparison to those listed in standard tables of derivatives. However, in this
modern age, chemists frequently use instrumentation such as Nuclear Magnetic
Resonance Spectroscopy, Infrared Spectroscopy, Ultraviolet Spectroscopy, and
Mass Spectroscopy to identify compounds since instrumental methods are more
precise and frequently less time consuming.
6. Bromination tests -CH2=CH2 + Br2
Br-CH2-CH2-Br
CH2Cl2
Br
Br2
OH
Br
OH
+ 3 HBr
H2O
Br
Bromine is used as a color reagent to test for the presence of pi bonds (alkenes
& alkynes) but will also react with phenols (aromatic alcohols). The
disappearance of the red bromine color indicates that a chemical reaction has
occurred. However, the organic compound could be reacting either by addition
(alkene or alkyne) or substitution ( phenols). How can you tell the difference?
When a compound is reacted with bromine in a non-aqueous solvent such as
dichloromethane, a wet piece of blue litmus paper can be held just over the
surface of the solution. If HBr is produced, it is very water soluble and will turn
the litmus red. In addition, bromination can be run in an aqueous solution of
bromine. In water, the phenol is partially ionized (it is a weak acid) and thus even
more susceptible to bromination than it was in the dichloromethane. Not only will
it decolorize the solution, but it will produce a water insoluble product – a
precipitate.
7. Ferric Chloride test -FeCl3
3
OH
(
O- )3
Fe+3
Another characteristic reaction for water-soluble phenols is ferric chloride. Ferric
chloride is an orange-yellow color in water but when reacted with phenols, the
color becomes more burgundy or wine like.