Name
Lab Day
Chemistry of the Functional Group
Introduction:
Organic molecules comprised only of carbon and hydrogen would
be relatively unreactive and biologically unimportant. Inclusion of atoms
of other elements in organic molecules or changes in the oxidation state
increases the numbers and kinds of reactions of the compound. The
biological activity of molecules is primarily a function of these changes.
The inclusion of heteroatoms in the organic molecule does not
produce an infinite number of related physical and chemical properties.
The chemist has learned that compounds containing heteroatoms can be
classified and organized into classes of compounds whose chemical
properties are, to a large extent, determined by the heteroatom or
oxidation state of the compound itself.
The particular heteroatom or group of heteroatoms and the various
oxidation states comprise the functional groups. Organic molecules are
classified with respect to various functional groups contained in the
molecule.
Unsaturated Hydrocarbons:
Carbon–carbon double (alkenes) and triple (alkynes) are functional
groups which undergo various addition reactions to produce saturated
hydrocarbons or their derivatives. The reactivity of these functional
reactions which characterize unsaturated molecules are the addition of
bromine to unsaturated bonds and the oxidation of these bond by
potassium permanganate. Bromine has a dark red color in non-polar
solvents which disappears upon addition of an alkene or alkyne. An
aqueous solution of potassium permanganate is a deep purple. Addition
of an alkene or alkyne reduces the permanganate resulting in a dark
brown or black precipitate of manganese dioxide.
amber
colorless
OH
C
C
+ KMnO4
purple
C
C
OH
+
MnO2 (s)
brown
2
Halides:
Halogens in organic compounds are detected by the Beilstein flame
test or by the formation of a silver halide precipitate when treated with
alcoholic silver nitrate. Not all halides are susceptible to the silver nitrate
test. The test reactivity is dependent upon: (1) the degree of branching
on the carbon to which the halogen is bonded, (2) whether or not the
carbon is sp or sp2 hybridized, and (3) the type of halogen present. The
rate of formation of the precipitate is a measure of the reactivity of the
halide which generally follows the order, I > Br > Cl >> F.
C
Cl
+
AgNO3
H2O
C
OH
+
AgCl
(s)
Alcohols:
Alcohols possess the functional group, –OH. This group is sensitive
to the Lucas test reagent which is an acidified (HCl) solution of zinc
chloride. Addition of an alcohol to the reagent results in the formation of
an insoluble alkyl halide causing the solution to become turbid. The rate
at which the alkyl halide is formed is dependent on the nature of the
carbon to which the –OH is attached. Branching results in an increased
rate, while hybridization other than sp3 results in slowing the rate.
C
OH
+
ZnCl2
HCl
C
Cl
+
H2O
O
Acids and Bases:
C
OH
Organic molecules having the functional group,
are
classified as acids and those containing a nitrogen atom are classified as
bases. Those acids and bases which are soluble in water yield the
characteristic color change when the solution is tested with litmus paper.
Insoluble acids react with dilute base (10% sodium bicarbonate) to yield
a soluble salt and insoluble bases react with dilute acid to yield a soluble
quartenary ammonium salt.
O
O
C
OH
+
N
..
+
H+
HCO3
C
+
N
H
O
+
H2CO3
3
Aldehydes and Ketones:
Aldehydes and ketones possess an oxygen atom attached to a
carbon with a higher oxidation number than alcohols. These compounds
have similar, though not identical, properties and can be distinguished
chemically. Both will react with 2,4–dinitrophenylhydrazine reagent to
give a solid derivative. However, aldehydes may be readily oxidized to the
acid, while ketones can not.
O
R
C
O
H
R
aldehyde
C
ketone
O
O
R
C
H
+
2 Ag(NH3)2OH
O
R
C
R'
H
+
2
Cu+2
+ 5 OH
2 Ag
_
+
R
C
O
NH4+
+ 3 NH3 + H2O
O
Cu2O
+
R
C
O
+
3 H2O
Experimental:
Unsaturated Hydrocarbons
A.
Bromine in methylene chloride (or dichloromethane). Place 2
mL of CH2Cl2 in a test tube and add 6 drops of the compound to be
tested. To this solution, add 5% Br2 in CH2Cl2 drop-wise and record the
results. A positive test is the disappearance of the red amber color.
B.
Potassium permanganate (Baeyer test). Place 2 mL of 95%
ethyl alcohol (ethanol) in a test tube and add 6 drops of the compound to
be tested. Add 5 drops of 2% KMnO4 solution and record the results. A
positive test is the disappearance of the purple color and the formation of
a brown or black precipitate.
Compound
Observations
Br2 in CH2Cl2
cyclohexane
cyclohexene
toluene
oleic acid
mineral oil
KMnO4
Conclusions
4
Organic Halides
A.
Beilstein flame test. Coil the end of a piece of copper wire
into a loop and heat with a burner until the flame is no longer colored.
Dip the cooled loop into a drop of material to be tested. Slowly bring the
loop with the organic halide into the flame. A positive test is a flash of
green flame as the compound is brought into the flame.
Compound
Structure
Observation
Br
2–bromopropane
CH3
CH
CH3
cyclohexane
1–chlorobutane
CH3
CH2
CH2
CH2
Cl
B.
Silver nitrate. Place 2 mL of 2% alcoholic AgNO3 in a test
tube and add 2 drops of the compound to be tested. Note the time
required for the reaction to occur. If nothing happens in 5 minutes,
carefully heat in boiling water on a hot plate (the alcohol is flammable).
Compound
Structure
Br
2–bromopropane
CH3
CH
CH3
Cl
2–chloropropane
1–chlorobutane
CH3
CH3
CH
CH2
CH3
CH2
CH3
tert-butyl chloride
CH3
C
CH3
Cl
bromobenzene
Time
Br
CH2
Cl
Observations
5
Alcohols
Lucas test. Place 10 drops of the compound in a test tube and add
5 mL of Lucas reagent. Shake the tube and then allow to stand for a few
minutes. Note the time required to observe the appearance of a turbid
(cloudy) solution.
Compound
Structure
Observations
OH
2–butanol
1–butanol
Time
CH3
CH3
CH
CH2
CH2
CH2
CH3
CH2
OH
CH3
tert-butyl alcohol
CH3
C
CH3
OH
Acids and bases
A.
Carboxylic acids. To 2 mL of water in a test tube, add 3
drops (or a small pinch if a solid) of the compound. Shake the tube. If the
material is soluble, test with litmus paper. If the material is insoluble,
add 3 mL of 10% NaHCO3.
Compound
propanoic acid
acetone
Structure
O
CH3
CH2
C
OH
O
CH3
C
CH3
O
benzoic acid
Solubility
C
OH
Observations
6
B.
Amines. To 2 mL of water in a test tube add 6 drops (or a
small pinch) of the compound. Shake the tube and the test the mixture
with red litmus paper if the compound is soluble. If the compound is
insoluble, add dilute HCl.
Compound
Structure
pyridine
Solubility
Observations
N
O
acetanilide
aniline
NH
C
CH3
NH2
Aldehydes and ketones
A.
2,4–dinitrophenylhydrazine (2,4–DNP). To 2 mL of 2,4–DNP
reagent in a test tube, add 2 drops of the compound. A positive test is the
formation of a yellow or orange precipitate. If no precipitate forms
immediately, allow the tube to stand for 3 to 5 minutes.
Compound
Structure
O
propionaldehyde
propionic acid
1–butanol
3–pentanone
glucose
Observation
CH3
CH2
C
H
O
CH3
CH2
CH3
CH2
CH3
CH2
C
OH
CH2
CH2
OH
O
HO
C
CH2
CH3
H
H
H
OH H
H
C
C
C
C
C
H
OH OH H
C
OH
O
7
B.
Benedict’s test. Place 2 mL of Benedict’s reagent in a test tube and
heat just to boiling. Add 6 drops of the compound and heat the mixture in a
boiling water bath for 1 minute. Allow the tube to stand. The formation of a
cloudy greenish solution is a positive test.
C.
Tollen’s test. Use a very clean test tube. Place 2 mL of Tollen’s
reagent in the clean test tube and add 2 drops of the compound. If no reaction
occurs immediately, place the test tube in a beaker of warm water. A positive
test is the formation of a silver “mirror” on the inside of the test tube or the
formation of a black precipitate.
Compound
Structure
H
glucose
HO
Benedict’s
H
H
OH H
H
C
C
C
C
C
H
OH OH H
C
Tollen’s
O
OH
O
3–pentanone
CH3
CH2
C
CH2
CH3
O
propionaldehyde
CH3
CH2
C
H
Combustion
Like most organic substances, hydrocarbons are combustible. The
products of their complete combustion are carbon dioxide and water. Aromatic
compounds do not burn completely and generally give off a “sooty” flame
containing bits of unoxidized carbon. Place about 1 mL of the compounds
below into separate evaporating dishes and (in the hood!) start them burning
by bringing a lighted splint to the edge of the evaporating dishes. Note the
characteristics of the flames.
Compound
Structure
Observation
n–hexane
CH3
CH2
CH2
CH2
CH2
CH3
1–hexene
CH2
CH2
CH2
CH2
CH2
CH3
CH3
toluene