OF IDENTIFICATION AN UNKNOWN ORGANIC SOLID
Abstract:
The experiment was performed so that student could familiarize with the basic techniques
used in the organic chemistry lab such as the solubility test, thin layer chromatography
(TLC), and melting point test. Different solvents allow different type of reaction to occur;
therefore some worked better than the other and allowed the elimination of many given
known solids. Thin layer chromatography compared the retention factor of the unknown
to a list of known compound and gave a more convincing evidence of the identity of the
unknown. Lastly the melting point of the unknown compared to the suggested identified
solid yielded by TLC test confirmed once again the identity of the unknown has been
found.
Introduction:
It is known that organic compounds have a variety of organic solvents in which they
could dissolve in. By knowing that fact, a chain of observation was done by allowing an
unknown organic solid to dissolve in water, diluted HCl, diluted NaOH, and 5% Sodium. With
the experience of how organic compounds behave, we hypothesized the structure of the unknown
through polarity by how well the solid dissolved in each given solvent. Thin layer
chromatography test was done on a thin silica film on which a very small amount of dissolved
unknown and the known compounds that needs to be compared were introduced on a straight
horizontal line. The retention factor (Rf) compared the distance which the unknown could travel
relatively to a more narrowed down list of known compounds to give a better sense how polar
the unknown is in certain column solvents including diethyl ether, hexane and ethyl acetate.
While TLC narrowed the final following up known compound, melting prove the true and last
physical property of the unknown and confirmed that the unknown solid to be identified.
Experimental procedures:
Solubility test
The unknown (22) was dissolved in water, diluted NaOH, diluted HCl and sodium
bicarbonate (NaHCO3) and followed deducting logic of solubility test. An amount of 15mg of the
unknown was placed in a test tube and dissolved with 1.0mL of each listed solvent and noted
down the observations. A pH indication test (litmus paper) was done in the case of water as
solvent to analyze the components of the unknown.
Dissolving series was done and indicated following:
-Unknown was soluble in water and shown to be neutral therefore narrowed down to the
list of alcohol, aldehydes, ketones and amines.
-Unknown was soluble in diluted NaOH and sodium bicarbonate therefore belong in the
group of carboxylic acids, phenols with EWG (electron withdrawing groups) and amines.
-Unknown was also soluble in diluted HCl therefore contains amine group.
The evidences indicated that the unknown was very polar which was the reason that the
solubility fell into a quite range of solvents. Since the group of side chains were narrowed down,
it was possible that the unknown could contains more than one listed groups.
-The first narrowed down list contained: thymol, 3,4-dimethoxybenzaldehyde, resorcinol, 2,4dinitrophenol, 2-napthol, 4-acetamidophenol, p-dichlorobenzene, phenacetin, phydroxybenzaldehyde
The next step, the list of known solid organic compounds was to be tested using one of the three
solvents to narrow the list down even further. For this test NaOH was chosen.
Name
thymol
3,4-dimethoxybenzaldehyde
resorcinol
2,4-dinitrophenol
2-napthol
4-acetamidophenol
p-dichlorobenzene
phenacetin
p-hydroxybenzaldehyde
Soluable NaOH
Y
N
Y
N
Y
Y
N
N
N
The in second narrowed down list of possible known consisted of: thymol, resorcinol, 2-napthol
and 4-acetamidophenol.
Thin Layer Chromatography test (TLC)
The unknown was dissolved in each of the three possible TLC solvents including diethyl
ether, toluene and ethyl acetate (same amount was used like the previous solubility test). The
result showed that only diethyl ether and toluene were the possible solvent for the unknown. The
TLC test was carried out as following:
-Each of the four possible known as well as the unknown were dissolved in ethyl ether with the
same amount as described above and placed on a thin silica film using a micro tip.
-All five were placed on a horizontal pencil separated by an appropriate spacing.
-The film then placed in a jar containing a minimum amount of toluene as column solvent that
did not go past the indicated pencil line.
-The system was closed and allowed to sit until the entire silica film was soaked with the column
solvent by capillary effect.
-The same procedure was repeated using toluene as solvent for the compound and diethyl ether
as column solvent.
The order of each compound was:
1) Thymol
2) Resorcinol
3) 2-napthol
4) 4-acetamidophenol
5) Unknown
Retention factor in Diethyl ether
Name
thymol
resorcinol
2-napthol
4-acetamidophenol
Unknown
Retention factor
0.81
0.54
0.67
0.14
0.14
Retention factor in Toluene
Name
thymol
resorcinol
2-napthol
4-acetamidophenol
Unknown
Retention factor
0.49
0.32
0.0
0.03
0.03
The possible choice was narrowed down to 4-acetamidophenol.
Melting point test
-The final test was done comparing the melting point of the known in this case was 4acetamidophenol and the unknown solid
Name
Melting point (C)
4-acetamidophenol
Unknown
168.9-170.1
168.0-169.8
-The melting point [2] of 4-acetamidophenol was conclusive enough to determine the identity of
the unknown.
This compound contains amine group and phenol which has an electron withdrawing group
located at the para-position [3] in the ring. These indications were clarified by the solubility test
and proven true once the final possible structure was confirmed by the other two tests.
Conclusion
This experiment showed that the basic techniques such as solubility test, TLC and
melting point test are crucial. With a long list of possible choices the solubility test has done
great by eliminating all the impossible candidates and point out such structural properties that the
unknown must have in order to yield such soluble range . TLC was one of the possible tests but
inefficient due to the number of the plates must be done and the preparation was not as favorable
as the other two. Melting point test was the fastest procedure to consider when time frame is
limited. The three tests concluded that the unknown in this experiment was 4-acetamidophenol.
References
1) Organic chemistry, KPC Vollhardt and NE Schore, Mar 2006.
2) Properties of 4-acetamidophenol, Chemicalbook[Online],
http://www.chemicalbook.com/ChemicalProductProperty_EN_CB1413658.htm, [Accessed
2011]
3) Ring substituents and reactivities, McGraw Hill [Online],
http://www.mhhe.com/physsci/chemistry/carey/student/olc/graphics/carey04oc/ref/ch12substitue
nteffects.html, [Accessed 2011]