Experiment 6: Identification of an Unknown
General Principles
Identification and characterization of the structures of unknowns are important parts of organic chemistry. It is
often of necessity in drug analysis and other organic syntheses. It is possible to establish the structure of a
compound on the basis of IR and NMR spectra alone, but in most cases, these spectra are used with other
information about the unknown such as physical state, solubility, pH, melting point range, and other chemical
tests confirming the functional groups. The general procedure in identifying an unknown is done in four parts.
Part I. Identification Using Physical Properties
a) Melting point and boiling point. Assuming the unknown is pure, the melting point or boiling points are a
good start to identify unknowns.
) Color. Some compounds are recognizable by their specific colors. Nitro, nitroso compounds and α
diketones are yellow; quinones, azo compounds and polyconjugated olefins and ketones are yellow or red;
Phenols and amines are often
own to dark purple because of traces of air oxidation.
c) Odor. Some compounds are recognizable by their specific odor. Some esters have a pleasant, fruity
fragrance; alcohols, ketones, aromatic hydroca
ons, and aliphatic olefins have characteristic odors; thiols,
iso-nitriles, and low molecular weight ca
oxylic acids have an unpleasant odor.
d) The ignition test. This test consists of heating a small amount of the substance on a spatula over a
Bunsen burner flame. If the flame is luminous, this indicates that the compound is aliphatic. If the flame is
sooty or smoky, it indicates that the compound is aromatic.
Part II. Identification Using Spectroscopy
IR, NMR and sometimes Mass spectrometry are used to confirm the structure of unknowns.
IR is an easy and fast method to determine the functional groups. NMR and Mass spectrometry are used to
confirm the exact structure of the compound.
Using IR to determine the functional groups.
a) The presence of an OH group (alcohol vs. ca
oxylic acid).
) The presence of an unsaturated compound (alkene vs. alkyne vs. aromatic)
c) The presence of an amine (primary vs. secondary, vs. tertiary)
d) The presence of a ca
onyl group (aldehyde vs. Ketone)
e) The presence of a ca
onyl group (Esters vs. Ca
oxylic acids)
f) The presence of a amide; Ca
onyl group + amines (1°, 2° or 3°)
Pease use the IR handout and the information in the link below to determine the diagnostic signals for the
functional groups listed above.
https:
chem.li
etexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_
Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vi
ational_Spectroscopy/Infrared_Spectrosco
py/Infrared%3A_Interpretation
Part III. Identification Using Solubility Test and Acid-base Properties
The solubility of compounds is based on “like dissolves like rule” and is used to determine whether the
compound is polar or not.
The best way to test for solubility is to add a spatula tip of solid unknown (about 10 milligrams), or 1-2 drops of
liquid unknown, to about one milliliter of solution in a small test tube and then using a glass sti
ing rod stir the
mixture, or sometimes, the test tube is covered with a clean ru
er stopper and shaken vigorously. In certain
cases, it may take a minute or two for all the unknown to dissolve. If a very small amount of the sample fails to
Experiment 6: Identification of an Unknown
dissolve when added to some of the solvent, it can be considered insoluble; and, conversely, if several portions
dissolve readily in a small amount of solvent, the substance is obviously soluble. It is easy to see whether a
solid substance is soluble or not, but the solubility of liquids is harder to observe. If the solution is cloudy, the
liquid unknown has probably not dissolved, it is just dispersed in the aqueous layer. If an unknown is partially
soluble, the solution is gently heated to see if this causes all the substance to dissolve.
Density. If the liquid unknown does not dissolve in water, it is important to observe whether it appears on the
top of the water layer or the below the water and therefore gives hints about its density. Water has a density of
about 1.0 g/mL Most organic liquids have a density of less than 1.0 and hence float on water. Organic liquids
with density greater than 1.0 often contain halogens (Cl, Br, or I). They will sink to the bottom when added to
water.
The solubility can also be tested in dilute acid or base than in water, the observation can be confirmed by
neutralization of the solution; the original material will precipitate if it is less soluble in a neutral environment. If
oth acidic and basic groups are present, the substance may be amphoteric and therefore soluble in both acid
and a base.
The solubility tests are not infallible and many borderline cases are known.
Below is a flowchart (Scheme 1) that can be used to ca
y solubility tests. These tests are particularly effective
for determining the presence (or absence) of acidic or basic functional groups in the unknown.
Note: The solubility tests need to be followed by pH testing to confirm the presence of acid, base or neutral
functional groups.
a) Solubility in H2O
If the unknown is soluble in water, it suggests that the substance is very polar or has at least one functional
group capable of hydrogen bonding with the water per 4-5 ca
on atoms. For example, simple alcohols
containing 1-3 ca
ons (methanol, ethanol, propanol) are completely soluble in water. Butanol and pentanol
(containing 4 and 5 ca
on atoms respectively) are slightly soluble in water. While hexanol (6-ca
ons), and
larger homologues, are essentially insoluble in water.
Experiment 6: Identification of an Unknown
If the unknown is soluble in water, then it can be determined whether it contains an acidic or basic functional
group by testing with litmus paper. If the red litmus paper turns blue when it is dipped it into the solution of the
unknown in water, then the unknown contains a basic functional group. In simple organic compounds this
normally indicates the presence of an amine or an amide. If the blue litmus paper turns red when it is dipped it
into the solution, then the unknown contains an acidic functional group. In simple organic compounds this
normally indicates the presence of a ca
oxylic acid. Of course it is possible that the unknown has no acidic or
asic functional groups, in which case there is no change in the color of the litmus paper.
If the unknown is soluble in water then there is no reason to test solubility in any of the other aqueous solutions
(5% sodium hydroxide, 5% hydrochloric acid, or saturated sodium bica
onate) since these solutions are
predominantly water and consequently the unknown will dissolve in them as well. However, even if the
unknown is insoluble in water it may still dissolve in one of these aqueous solutions if it reacts to form an ionic
(charged) species. For example, benzoic acid is insoluble in water but when treated with a base, such as
sodium hydroxide or sodium bica
onate, it reacts to form a ca
oxylate salt which is soluble in water.
) Solubility in Aqueous NaOH
If the unknown is insoluble in water, but does dissolve in 5% sodium hydroxide (NaOH) solution, then it
probably contains an acidic functional group (pKa < 15) that is deprotonated by the sodium hydroxide
producing an ionic compound. Two common functional groups with this property are ca
oxylic acids (pKa ~5)
and phenols (pKa ~10) (Scheme 2).
c) Solubility in Aqueous NaHCO3
Sodium bica
onate (NaHCO3) is a weaker base than sodium hydroxide. Thus, if the unknown does not
dissolve in 5% sodium hydroxide there is no reason to test its solubility in a solution of sodium bica
onate.
However, if the unknown does dissolve in the hydroxide solution, it is useful to test the solubility in sodium
ica
onate since sodium bica
onate will deprotonate functional groups with a pKa < 8, but not those with a
pKa > 8 (Scheme 3). Thus, if the unknown dissolves in the aqueous NaOH, but does not dissolve in aqueous
NaHCO3, it probably has a functional group with a pKa of between 8 and 14, possibly a phenol. If, on the other
hand, the unknown is soluble in the bica
onate solution, it is likely to have an acidic group with a pKa < 8,
probably a ca
oxylic acid.
Experiment 6: Identification of an Unknown
As an additional observation, when bica
onate solutions are treated with acids they react to form CO2 (a gas)
which bu
les out of solution.
d) Solubility in Aqueous HCl
If the unknown is not soluble in water, but does dissolve in 5% HCl, then it probably contains a basic functional
group that is protonated by the hydrochloric acid producing an ionic compound.
The most common organic functional group with this property is an amine (Scheme 4).
Solubility table of organic compounds
Reagent Class Group of Compounds
Soluble in cold or hot water Neutral, acidic or basic. Test
with litmus or universal indicator
paper
Neutral such as alcohols
Acidic such as phenols
Basic such as 3° amines
Soluble in diluted HCl Basic compounds Most amines except 3° amines
with only aromatic groups
Soluble in diluted NaOH Acidic compounds Most acids, most phenols
Soluble in NaHCO3 Strongly acidic Most ca
oxylic acids
Insoluble in water, acid and
alkali
Neutral Hydroca
ons,
nitrohydroca
ons, alkyl or aryl
halides, esters and ethers,
higher molecular mass
alcohols, aldehydes and
ketones
Experiment 6: Identification of an Unknown
Part IV. Identification Using Chemical Tests
Please refer to the text below for a detailed description and detailed procedures for some of the chemical tests
listed below:
https:
chem.li
etexts.org/Bookshelves/Organic_Chemistry/Book%3A_Organic_Chemistry_Lab_Techniques_(
Nichols)/06%3A_Miscellaneous_Techniques/6.03%3A_Chemical_Tests
A. Testing for Alkenes, alkynes, and aromatic compounds
a) Bromine water Test. A few drops of
omine water solution is added to the unknown compound. A
positive test is confirmed when the solution turns from orange (the color of free
omine) to clear indicating
that the
omine reacted. A negative test