Chem 4563 Organic Qualitative Analysis
Alcohols-Functional Group Tests
Introduction
Alcohols are an interesting and important group of organic compounds. They are used in
beveages, in medicines, as solvents and as synthetic intermediates. The characteristic functional
group is the hydroxyl group (-OH) attached to a saturated carbon.
Alcohols are classed into three categories: promary (1˚), secondary (2˚) and tertiary (3˚); the
classification is based on the type of carbon to which the hydroxyl group is attached. If the carbon is
attached to only one other carbon, it is a primary carbon. If the carbon is attached to two other
carbons, it is a secondary carbon and the alcohol is a secondary alcohol and so on.
H
H
R''
R C OH
R C OH
R C OH
H
R'
R'
1˚
2˚
3˚
Alcohols have a much higher boiling point than ethers or alkanes of similar molecular weight.
This is due to the association of alcohols in the liquid phase through hydrogen bonding. For
example, the boiling point of butyl alcohol is118˚C whereas the boiling point of the isomeric diethyl
ether is 36˚C.
Hydrogen Bonds
R
-∂
O
+∂ H
1.
H +∂
O
-∂
R
Functional Group Tests
Before the advent of modern spectroscopic methods,the determination of chemical properties
was of paramount importance for the identification, characterization and determination of structure of
pure substances. Many reagents or conditions were found to be specific with substances that
contained certain functional groups. Thus these reactions can be used to confirm or deny the presence
of a specific functional group. The following tests are designed to indicte the presence of a hydroxyl
functional group within a molecule and to further illustrate its structural type (ie. 1˚, 2˚ or 3˚). You
will then draw onthe results of these tests to deduce the structure of an unknown alcohol sample.
a.
Solubility of Alcohols
Because of the hydrogen bonding possible between the polar hydroxyl group of an
alcohol and water, many alcohols are soluble in water. However as the carbon content of an
alcohol goes up, the solubility decreases. This is often described as the 5-Carbon Rule, since
the point at which an alcohol is no longer soluble in water is often (but not always) reached
when it contains 5 or more carbons. However, alcohol structure (either 1˚, 2˚ or 3˚) also has
an effect on solubility.
PROCEDURE
Add the alcohols dropwise to 1 mL of water. Count the number of drops needed to
cause a phase separation, but do not use more than 10 drops. If you have added 10 drops of
alcohol, and no phase separation has been observed, the alcohol should be classified as
completely miscible in water. Record your observations as very soluble(6-9 drops), soluble
(2-5 drops), or insoluble (1-drop)
b.
Sodium Test for Active Hydrogens
This is a test for reactive hydrogens, that is hydrogens which are sufficiently
acidic that they will react with sodium metal by the following equation.
2 ROH + 2 Na(s)
2 RONa + H2(g)
The liberated hydrogen, being a gas, rapidly diffuses out of the liquid as a series of bubbles.
The order of reactivity of an alcohol to sodium metal is known to decrease with increasing size
of the alkyl portion of the alcohol. This test is most useful in connection with medium weight
alcohols of 3 to 8 carbons. Smaller alcohols are difficult to obtain in anhydrous conditions
(see below) and larger alcohols react to slowly.
Other compounds with active hydrogens will also give a positive result for this test.
Thus amines, thiols, and acetylenes will also liberate hydrogen when treated with sodium
metal.
2 RSH + 2 Na(s)
2 RSNa + H2(g)
2 R2NH + 2 Na(s)
2 R2NNa + H2(g)
Note that many compounds will give a reaction with sodium metal even if that compound
does not have a reactive hydrogen such as alcohols do. This is often do to small amounts of
water in the sample. It is important that any compound to be tested is first dried by allowing it
to stand over a drying agent such as anhydrous MgSO4 or CaCl2 .
CAUTION SODIUM IS VERY REACTIVE AND MUST BE HANDELED WITH CARE.
USE ONLY SMALL PIECES AND HANDLE IT WITH FORCEPTS. SODIUM REACTS
VIOLENTLY WITH WATER! CARRY OUT ALL REACTIONS IN A HOOD. IT IS
NORMALLY STORED UNDER MINERAL OIL .
PROCEDURE
Cut small pieces of sodium metal about half the size of a pea and add,to 1/2 mL of the alcohol,
in a 6-inch test tube. Devise a method to describe the rate of the reaction (such as the rate of
gas bubbles evolved over time). When finished, slowly add 3-5 mL of methanol to react with
all the remaining sodium metal. DO NOT POUR THE ALCOHOL DOWN THE DRAIN
UNTIL YOU ARE SURE NO SODIUM REMAINS.
c.
Lucas Test
The hydroxyl (OH) group of an alcohol is a poor leaving group and is not redily
displaced by nucleophiles. However, in strongly acidic solutions, protonation of the -OH can
occur to give -OH2+ and water may be displaced. When the alcohol is treated with HCl, the
alkyl halide results. The alcohol may, depending on its structure, give substitution by either
Sn1 or Sn2 mechanisms.
This test depends on the appearence of an alkyl chloride as an insolublesecond layer
when an alcohol is treated with ZnCl2/HCl. Primary alcohols do not react at room
temperature. Secondary alcohols will react slowly,while tertiary, benzylic and allyl alcohols
react instantly. The Lucas Test is limited to alcohols with some solubility in water.
R
R C OH + ZnCl2
R
d.
R
+∂
R C O
R H
-∂
ZnCl2
R
R C
R
+Cl-
R
R C Cl
R
Jones Test
This test is based on the reduction of chromium (+6) which is orange to chromium
(+3) which is green, when an alcohol is oxidized by the reagent. A change in color of the
reagent from orange to green represents a positive test. Primary and secondary alcohols are
oxidized by the reagent to carboxylic acids and ketones respecitevely. Although primary
alcohols are first oxidized to aldehydes, the aldehydes are quickly oxidized to acids. The
ability of chromic acid to oxidized aldehydes but not ketones is further used in a chemical test
to distinguish between aldehydes and ketones.
Tertiary alcohols are not oxidized at all by this reagent. Hence this test can be used to
differentiate between primary and secondary alcohols on the one hand, and tertiary alcohols on
the other. Unlike the Lucas test, this test can be used with all alcohols, regardless of
solubility.
O
H
Cr(+6)
R C OH
R C OH + Cr(+3)
orange
green
H
H
R C OH
R
Cr(+6)
orange
O
R C R
+ Cr(+3)
green
A combination of the Lucas Test and Jones Test can differentiate between 1˚, 2˚, and
3˚ alcohols.
Alcohol Type
Lucas Test
Jones Test
1˚
NO REACTION
FAST REACTION
2˚
SLOW REACTION
FAST REACTION
3˚
FAST REACTION
NO REACTION