Download Chemical Properties of Organic Compounds

Chemical Properties of Organic Compounds:
Hydrocarbons, Alcohols, Aldehydes, Ketones and Carboxylyc Acids
The chemistry of carbon compounds is a special field because carbon has the unique property of uniting
with other carbon atoms through strong covalent bonds to form long chains and rings. Compounds containing
C, H and O give rise to a large section of organic chemistry. This experiment provides an introductory study of
some of these compounds. These compounds may be classified into different functional groups depending on
the type of bonding between carbon and oxygen atoms. If the letter R is used to represent an arbitrary
hydrocarbon radical (methyl, CH3-; ethyl CH3CH2-; etc.) the formulas of these different groups of compounds
may be written as shown in the following table.
Functional Group
General Formula
Example
Common Name
IUPAC Name
Saturated
R-H
CH4
Methane
Methane
R-OH
CH3OH
Methyl alcohol
Methanol
Acetaldehyde
Ethanal
Acetone
Propanone
Acetic acid
Ethanoic acid
Methyl acetate
Methyl ethanoate
Hydrocarbons
Alcohol
Aldehyde
Ketone
R
R
O
C
O
C
H
R
H3C
H3C
O
Acid
Ester
I.
R
R
C
O
C
OH
OR
H3C
H3C
O
C
O
C
O
C
O
C
H
CH3
OH
OCH3
Hydrocarbons
Organic compounds that contain only carbon and hydrogen are known as hydrocarbons. Saturated
hydrocarbons (alkanes) have the general formula CnH2n+2 and all the carbon atoms have only single bonds to
other carbon atoms or hydrogen atoms. Hydrocarbons that contain one double or triple bond are unsaturated.
If there is more than one double bond it is called polyunsatured. Double bonds give rise to alkenes and triple
bonds to alkynes. From a chemical standpoint, alkanes are relatively unreactive (except for combustion) while
alkenes and alkynes undergo many chemical reactions.
II.
C-H-O Compounds
It is easier to see the relationships among the different functional groups by considering the stepwise
oxidation of a saturated hydrocarbon.
[O]
[O]
[O]
[O]
CH4 → CH3OH →
H-C=O → H-C-OH →
CO2


H
O
Hydrocarbon
Alcohol
Aldehyde
Acid
Carbon dioxide
In each successive step of oxidation, the oxidation number of the carbon bonded to oxygen increases by 2 units
resulting in the formation of a different functional group. We expect the properties of the functional groups to
differ as the oxidation number changes, but there are certain characteristic reactions of each group. For
instance, an alcohol forms when the OH group replaces an H in a hydrocarbon, and the chemical behavior of the
resultant alcohol is similar to water H-OH. Aldehydes contain the CHO fragment in which the H atom is easily
oxidized, making aldehydes good reducing agents. The carboxylic acid group shows typical acid properties in
that the proton is easily ionized in water to give H1+ (H3O1+).
Experimental Procedures
I.
Hydrocarbons and alcohols
A.
Combustion
The products of complete combustion of organic compounds are carbon dioxide and water.
Incomplete combustion can result in intermediate oxidation states of carbon and form any of the
functional groups. One way to observe complete and incomplete combustion is to adjust the collar near
the base of a Bunsen burner. Light your Bunsen burner with the collar closed. When the collar is
closed, little extra oxygen reaches the flame as the fuel (natural gas) is burned. Record the color of the
flame. Slowly open the collar and record the color of the flame as you do so. As the collar is opened,
additional oxygen reaches the flame as the fuel is burned. It is best not to have open containers of any
volatile substance near the flame.
Another way to observe complete and incomplete combustion is to burn some representative fuel
samples. Ignite a small sample (two or three drops) of methyl alcohol (methanol) on a watch glass and
observe the characteristics of the flame. Be careful burning the methanol and other samples! Work
only in a hood! Be sure not to burn a sample larger than 1 mL. Be careful not to spill or splash
sample near the flame. It is best not to have open containers of any volatile substance near the
flame. A blue flame indicates complete combustion; a yellow flame is indicative of incomplete
combustion. Compare the combustion characteristics of several other compounds such as ethanol
(C2H5OH), isopropyl alcohol (C3H7OH) and acetone (CH3COCH3) to the combustion methanol
(CH3OH). Try burning a sample of the aromatic hydrocarbon toluene (C6H5CH3). Compare the
alcohols and relate the completeness of combustion to molar mass. What type of flame would you
expect for the combustion of pentanol? Write a balanced chemical reaction equation for the combustion
of pentanol.
B.
Solubility
There is a general rule for solubility that states "like dissolves like". In this context, like means
similar in terms of polarity. More correctly stated, the rule for solubility could be stated: polar solutes
dissolves in polar solvents, non-polar solutes dissolve in non-polar solvents, while polar solutes do not
dissolve in non-polar solvents and non-polar solutes do not dissolve in polar solvents. We will test this
rule by trying to dissolve a number of alcohols in a polar solvent (water, H2O) and a non-polar solvent
(hexane, C6H14).
Determine the solubility of methanol (CH3OH) and pentanol (C5H11OH) in water by adding 1
mL of water to a 1mL sample of each alcohol in separate test tubes. Shake each test tube to ensure
mixing. Is a homogenous solution formed or do separate liquid layers remain? Repeat the two tests
using hexane instead of water as the solvent. Draw some conclusions about the effect of the length of
the hydrocarbon chain on the polarity of the alcohol group. Predict the solubility of a tri-hydroxyl
alcohol (glycerine, C3H5(OH)3) in these two solvents and test your prediction.
C.
Oxidation
Potassium permanganate (KMnO4) is an oxidizing agent that can convert alcohols to the higher
oxidation state of an aldehyde or a ketone. Prepare three test tubes, each containing 1 mL of methanol
and 4 mL of water. Test the oxidizing power of KMnO4 in acid or base by adding 1 drop of 10% NaOH
to the first test tube and 1 drop of dilute H2SO4 to the second. Now add 1 drop of KMnO4 to each
sample of the alcohol and mix well. Measure the relative times it takes a reaction to occur as determined
by the disappearance of the purple KMnO4 color or the formation of brown MnO2. Repeat this
procedure using isopropyl alcohol and note any difference.
Please balance each of the following equations. In the first equation, methanol, a primary
alcohol, is oxidized to an aldehyde (formaldehyde, HCOH). In the second equation, isopropyl alcohol, a
secondary alcohol, is oxidized to a ketone (acetone, CH3COCH3). Tertiary alcohols are not oxidized by
KMnO4. (See pages 534 - 536 in the text.)
CH3OH + KMnO4 → HCOH + KOH + H2O + MnO2
CH3CHOHCH3 + KMnO4 → CH3COCH3 + KOH + H2O + MnO2
II.
Aldehydes R-CHO
Silver Mirror Test
Aldehydes can be oxidized to acids by Tollen's reagent. At the same time, Tollen's reagent will
be reduced to produce metallic silver which plates out as a silver mirror on a clean glass surface. This is
a common test for the presence of aldehydes. Prepare Tollen's reagent as follows: place 5 mL of 0.1 M
AgNO3 in a clean test tube and add dilute ammonia (NH3 or NH4OH) dropwise until a precipitate of
silver (I) oxide (Ag2O) forms. Keep adding the ammonia with mixing until the precipitate redissolves to
form the soluble silver complex Ag(NH3)21+. Divide this fresh Tollen's reagent into two portions, then
test with an aldehyde as follows. To one portion of Tollen's reagent, add 1 mL of acetaldehyde
(CH3CHO), warm slightly and observe the silver mirror formed on the test tube wall. The second
portion of Tollen's reagent can be used as a blank or reference. Caution: Do not throw the silvered test
tube away! To properly dispose of the silver, first add a few drops of nitric acid (HNO3) to the test tube
to dissolve the mirror. Then pour the contents of the test tube into the waste container labeled "Waste
Silver Residue".
III.
Organic Acids R-COOH
A.
Preparation
Some organic acids are quite volatile and can be prepared from their salts by adding an acid with
a high boiling point and distilling the organic acid from the mixture.
Place about 1 gram of sodium acetate (NaC2H3O2) into a test tube and carefully add 1 mL of
conc. sulfuric acid (H2SO4). Be careful handing the sulfuric acid! Warm the mixture gently until a
color change is detected. Be certain to keep the container pointed away from people. Also be sure that
you and everyone around you is wearing safety goggles. Cautiously test the odor of the vapors by
wafting them from the test tube toward your nose. Test the effect of the vapors on moistened litmus
paper. Be sure to record all your observations and write a balanced chemical equation for the reaction
you observed.
B.
Oxidation in Steps
An alcohol may be oxidized in steps to an aldehyde and then to an acid. Dissolve 1 gram of
potassium dichromate (K2Cr2O7) in 5 mL of 1.5 M sulfuric acid (H2SO4) and add 5 drops of ethanol.
Warm the mixture gently until a color change is observed. What caused the color change? Cautiously
waft the vapors from the reaction container toward your nose. A pear-like odor indicates the presence of
acetaldehyde. Keep warming the mixture and test the vapors with moistened blue litmus paper. A
vinegar-like odor indicates the presence of acetic acid. Be sure to record all your observations and write
a balanced chemical equation for the two reactions you should have observed.