Download Identification of Functional Groups

Identification of Functional Groups
Purpose:
The purpose of this experiment is to observe chemical reactions characteristic of common
organic functional groups. The observations will be used to identify a set of unknown
compounds
Introduction:
Most organic and biochemical compounds are composed of carbon, hydrogen, and a few
other elements (such as oxygen, nitrogen, and sulfur). These compounds participate in a
wide variety of reactions generally related to molecular structure. Distinctive
arrangements of atoms and bonds, known as functional groups, are responsible for the
reactivity of families of organic compounds.
The compounds introduced in this experiment all have oxygen-containing functional
groups and include representatives of the following chemical families:
Family
General Formula
R-OH
Alcohols
O
Ketones
Carboxylic Acids
C
O
O
C
C
OH
CH3
O
(carbonyl)
C
H3C
H
O
C
H3C
C
C
R
R
O
(carbonyl)
R'
O
Aldehydes
Example
CH3CH2OH
O
C
R
Functional Group
-OH (hydroxyl)
H
O
(carboxyl)
OH
C
H3C
OH
Note the use of the abbreviation R- or -R’ to indicate the carbon-hydrogen structures
common to nearly all organic molecules. In most organic reactions, only the functional
groups are changed, the carbon-hydrogen chains remain unreacted. For comparison, a
hydrocarbon (a compound containing only carbon and hydrogen) will be tested along
with representatives of the families indicated above.
By observing the characteristic reactions of known compounds, unknown compounds can
be characterized as belonging to a given family. The characteristic reactions for
qualitative organic analysis used in this lab are as follows:
Sodium Bicarbonate Test for Carboxylic Acids
When sodium bicarbonate reacts with carboxylic acids it produces bubbles of carbon
dioxide. The bubbles are only present with a carboxylic acid because they are the only
family that has an acidic hydrogen.
O
O
C
C
R
R
OH
O-
+
H+
O
O
C
R
OH
+
C
NaHCO3
R
O- Na+
+
H2O + CO2
Oxidation of Alcohols, Aldehydes, and Ketones with Dilute KMnO4
The permanganate ion, MnO4-, is a strong oxidizing agent and will react with many
organic compounds. Alcohols, Aldehydes, and Ketones can all be oxidized to form salts
of carboxylic acids:
O
RCH2OH + KMnO4
C
R
C
H
+ KMnO4
C
R
O
O- K+
+ MnO2(s)
O
C
R
+ MnO2(s)
O
O
R
O- K+
R'
+ KMnO4
C
R
O
O- K+
+
C
R'
O- K+
+ MnO2(s)
Note that the above equations are unbalanced skeleton equations showing only the major
organic and inorganic products of the reactions.
The most important feature of the reactions with KMnO4, is the disappearance of the
purple color characteristic of the MnO4- ions in solution and the appearance of solid
MnO2 as a brown precipitate. In addition to the obvious color change, the speed of the
reaction often provides a valuable clue to the identification of unknown compounds.
Permanganate ions will oxidize different functional groups at different rates. In the case
of especially slow reactions, gentle heating in a hot water bath may be required.
Hydrocarbons and carboxylic acids are the only two of all the types of compounds being
studied that will not react with dilute neutral KMnO4 solution. The hydrocarbon contains
no oxidizable functional group and carboxylic acids can be oxidized further, only in the
presence of concentrated KMnO4 in strongly acidic solution, to carbon dioxide.
2,4-Dinitrophenylhydrazine (2,4-DNP) Test
Compounds containing a carbonyl group, either at the end of the carbon chain (as in
aldehydes) or in the middle of the carbon chain (as in ketones) form easily recognized,
brightly-colored precipitates
O
R
C
R
H
+ H2NNHC6H4(NO2)2
(2,4-DNP)
O
C
R
NNHC6H4(NO2)2 + H2O
C
colored ppt
H
R
+ H2NNHC6H4(NO2)2
(2,4-DNP)
R'
NNHC6H4(NO2)2 + H2O
C
R'
colored ppt
None of the other families of compounds used in this exercise will react with 2,4-DNP.
Tollens’ Test
The 2,4-DNP test is used to identify compounds that are aldehydes and ketones.
However, the 2,4-DNP test does not distinguish between the two. Therefore, a second
test is needed to classify unknown compounds. The Tollens’ test gives positive
identification of aldehydes by the following reaction:
O
O
C
R
H
+ 2Ag(NH3)2OH
Tollens'
Reagent
2Ag(s) +
C
R
O- K+
+ H2O + 3NH3
The Tollens’ reagent is prepared by precipitating silver hydroxide from a silver nitrate
solution then dissolving the precipitate in aqueous ammonium hydroxide:
AgNO3(aq) + NaOH(aq) → AgOH(s) + NaNO3(aq)
AgOH(s) + 2NH4OH(aq) → Ag(NH3)2OH(aq) + 2H2O(l)
Tollens’ Reagent
If the reagent is freshly prepared and the reaction is carried out in a very clean test tube,
the silver metal produced will be deposited on the walls of the tube as a bright silver
mirror. If the test tube is not clean, a black suspension or precipitate of finely-divided
silver particles will be observed. Ketones do not react with the Tollens’ reagent.
A series of known compounds representing the chemical families in the table on page 56
will be used to study the results of each of the tests described above, and then the
observations made of the known compounds will be used to identify the compounds in a
set of unknowns.
Procedure:
A. Bicarbonate Test for Acids
1. In separate, labeled, clean, dry test tubes, obtain 1mL samples (1mL is about
20 drops) of the known hydrocarbon, alcohol, aldehyde, ketone, and carboxylic
acid compounds.
2. To each sample, add 1mL of 5% NaHCO3 solution and stir. On the data sheet
note any signs of reaction, particularly, the evolution of bubbles of CO 2 gas. In
some instances it may be easier to detect the formation of gas bubbles by hearing
rather than by sight.
B. Oxidation with Dilute KMnO4
1. In separate, labeled, clean test tubes, obtain 0.5mL (about 10-drop) samples of
the known hydrocarbon, alcohol, aldehyde, ketone, and carboxylic acid
compounds.
2. Add 0.5mL of 0.05M KMnO4 solution to each sample and stir briefly. On the
data sheet note the appearance of each solution before and after the reaction. If no
reaction is observed after three minutes, try heating the sample by placing the test
tube in a beaker of hot tap water for an additional two minutes.
C. 2,4-Dinitrophenylhydrazine (2,4-DNP) Test
***WARNING: 2,4-DNP reagent stains skin and clothing a bright orange. If contact
with 2,4-DNP occurs wash the affected area with soap and water
immediately.
1. In separate, labeled, clean, dry test tubes obtain 2-drop samples of the known
hydrocarbon, alcohol, aldehyde, ketone, and carboxylic acid compounds.
2. Add 2mL of 2,4-DNP reagent solution a few drops at a time to each of the
samples. Stir occasionally during the addition of 2,4-DNP and note the color and
appearance of any precipitates formed.
D. Tollens’ Test
***WARNING: Immediately after performing the test, dispose of the results of the
Tollens’ test and any excess Tollens’ reagent in the appropriately labeled waste container.
Some of the silver compounds formed may be explosive when dry.
1. In separate, very clean test tubes, obtain 0.5mL samples of those compounds
which give positive tests with the 2,4-DNP reagent solution.
2. In a separate, very clean test tube obtain 4mL of 5% AgNO3 solution and
prepare the Tollens’ reagent as follows:
--To the 4mL of 5% AgNO3 add 15 drops of 10% NaOH solution then stir
thoroughly to mix. A brown precipitate should form.
--Add a few drops at a time of 6M NH4OH solution to the test tube,
stirring after each addition until the precipitate just dissolves.
--Add one drop at a time of 5% AgNO3, stirring after each addition until
the brown precipitate just reappears (this may mean that the precipitate
forms and disappears). The resulting solution is the Tollens’ reagent.
3. Divide the freshly prepared Tollens’ reagent equally among the samples to be
tested and stir briefly. Place the test tubes in a beaker of warm tap water and note
any signs of a reaction on the data sheet.
4. Dispose of the samples and any excess Tollens’ reagent in the appropriately
labeled waste container.
E. Analysis of Unknown Samples
1. Obtain a set of three unknown compounds. Record the number of each
unknown in the spaces provided on the data sheet.
2. Repeat the test procedures given in parts A through D above, using samples of
the unknowns. Record the results of each test on the data sheet.
3. By comparing test results obtained for the unknown compounds with the
results obtained for the known substances, identify the family of organic
compounds to which each of the unknowns belongs.
Supplementary Question:
How could you use each of the following instrumentation methods to distinguish between
your unknowns. Be specific in your answer.
a. IR
b. 1HNMR
c. 13CNMR
d. MS