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Emily Tesnohlidek
O CHEM 302
April 2014
Unknown Qualitative Analysis
Introduction
Qualitative analysis is the identification and characterization of unknown
compounds. It is a very important part of organic chemistry. We were asked to establish
the identity of an unknown compound. I first found the physical characteristics of my
compound, and then identified the main functional group of the compound through a series
of tests. Using this information, I found a group of possible compounds for my unknown.
This allowed me to choose an appropriate derivative to make and test to narrow down
which compound my unknown was.
Procedure and Results
I first found the physical properties of my unknown compound. To take the boiling
point, I put a capillary tube with one end closed off into a 1 mm glass tube with a little of
the unknown. This was heated until a steady stream of bubbles was coming out of the
bottom of the capillary tube. I then stopped the heat and recorded the temperature when
the stream of bubbles stopped. This gave my unknown a boiling point of 132°C.
I then weighed out a 1 mL volumetric flask (18.179 grams), and added 1 mL of my
unknown into it and weighed this. It came to be 19.112 grams with both the volumetric
flask and 1 mL of unknown, so taking the difference made my unknown’s density to be
0.933 grams/mL.
Using the index of refraction machine, I found my unknown’s index of refraction to
be 1.4542.
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To conduct a flame test, I heated the end of a copper wire in a Bunsen burner. After
allowing it to cool, I put a few drops of my unknown on the end. I burned this in the
Bunsen burner to observe the color changes. I observed to no color change, indicating that
the unknown does not contain a halide. However, some black soot did burn off, indicative
of possible aromaticity.
I then conducted solubility tests to see if my unknown was soluble in water, 5% HCl,
5% NaHCO3, 5% NaOH, and concentrated H2SO4. Knowing the solubility or insolubility of
the unknown in each of these mixtures can help determine the main class of the functional
group present. To prepare each mixture, I added 2 mL of solvent into a test tube, then
added two drops of unknown and observed any reaction – change in color or change in
temperature. The unknown ended up dissolving in all of the solvents. When a compound is
water soluble, you test the compound on litmus paper to see if it is acidic or basic. The
unknown did not change the litmus paper color – meaning it is a neutral compound.
Aldehydes, ketones, esters and alcohols are the neutral compounds in this
experiment. This made my next step a DNP test. I added 2 drops of unknown and 1 mL of
2,4-dinitrophenylhydrazine reagent and shook vigorously. A positive test is when it
immediately turns into a yellow or red precipitate. Aldehydes and ketones give a positive
DNP test, and esters and alcohols usually do not give a positive DNP test. The unknown
was unreactive and no precipitate formed, so the unknown is probably an ester or alcohol.
To determine whether an alcohol group was present, and narrow down what type of
alcohol, I ran the Lucas Test. I placed 2 mL of Lucas reagent in a small test tube with 4
drops of unknown. This was stoppered and shaken vigorously. The solution turns
immediately cloudy for tertiary, benzylic and allylic alcohols. However, the solution took
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longer to turn cloudy and show signs of reaction. This is indicative of a secondary alcohol.
Primary alcohols dissolve in the Lucas reagent and show no cloudiness.
The chromic acid test was also run to verify what type of alcohol the unknown was.
Two drops of unknown were dissolved in 1 mL of acetone. One drop of chromic acid
reagent was added, and results were shown immediately by a change in color form orange
to a blue-green color. Tertiary alcohols do not react, so this test indicates that the unknown
is either primary or secondary.
Combining the results of the Lucas test and chromic acid test, my unknown alcohol
is likely a secondary alcohol. It is now time to pick a derivative. Knowing it’s a secondary
alcohol with a boiling point of 132°C, I narrowed the possible compounds by looking at the
Appendix 1: Tables of Unknowns and Derivatives from our Introduction to Organic
Laboratory Techniques book. I looked under the Alcohols section, and made a list of the
compounds with boiling points within +/-10 of 132°C boiling point, so about 120°C-140°C.
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*from Introduction to Organic Laboratory Techniques: A Microscale Approach Appendix 1,
p. 967-968
I first made the 3,5-Dinitrobenzoate (DNB) derivative with my compound. I
dissolved 0.25 grams of 3,5-dinitrobenzoyl chloride in 0.25 mL of the unknown compound.
I heated this mixture for 5 minutes, and then allowed it to cool. After cooling to room
temperature, I added 1.5 mL of 5% sodium carbonate solution and 1 mL of water. I stirred
the solution vigorously, crushing any solid that formed. I then collected the product with
vacuum filtration, and washed with water. I then put the product into the oven for fifteen
minutes in an attempt to dry the product out enough to be able to take the melting point
that day. However, the product was still too wet to take an accurate melting point after
putting it in the oven, so I let the product sit for a day to dehydrate. The following day I
determined the melting point of the DNB derivative to be 110-112°C. This number is still
not very accurate, because one - I took the melting point many times, as slowly as possible,
and still had many different ranges, and two – with these many ranges, this derivative
really did not narrow down the possible compounds very much to be very useful.
To hopefully obtain more precise data, or back up the results of the DNP derivative, I
also made the second possible derivative with my compound with phenylurethane. I did
this by placing 0.25 grams of the unknown compound in a dry test tube with 0.25 mL of
phenylisocyanate. The reaction was spontaneous, so I did not need to heat the mixture in a
water bath to catalyze the reaction. The product crystallized, so I collected the product and
dissolved it in 3 mL of hot hexane. This was cooled and recrystallized, then collected by
vacuum filtration. I let this sit for a day to dry, then took the melting point of this solid. I
found it to be 132-133°C.
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This greatly narrowed down my possible compounds from the list above to – 4methyl-2-pentanol (phenylurethane derivative melting point: 143°C) and cyclopentanol
(phenylurethane derivative melting point: 132°C). However, when taking into account the
DNB derivative, cyclopentanol seems much more likely because it’s melting point is 115°C,
which is only about 4°C off of the melting point I found for the DNB derivative. On the
other hand, 4-methyl-2-pentanol has a melting point of 65°C for the DNB derivative, which
is less than the experimentally found melting point of the DNB derivative. The unknown
must not be 4-methyl-2-pentanol, because impurities may lower the melting and boiling
points, but you will never find a higher melting or boiling point for a compound than it’s
pure literature value. This rules out 4-methyl-2-pentanol, and leads me to believe the
unknown could be cyclopentanol.
Conclusion/Discussion
I believe my unknown compound is cyclopentanol. The boiling point of the
unknown was 132°C, and cyclopentanol has a boiling point of 140°C. Boiling points for the
unknown may be lower due to impuritites, but may not be higher, than the compounds
literature value, so this is reasonable. The density of my unknown was 0.933 g/mL, and the
density of cyclopentanol is 0.949 g/mL. The index of refraction of my unknown was 1.4542
and the index of refraction of cyclopentanol is 1.453. The flame test conducted did burn
some soot, indicating possible aromaticity – but this test is not always accurate, so it should
not be used to eliminate non-aromatic compounds, so this test should just be disregarded.
The DNP test resulted in no reaction, so the unknown is not a ketone or aldehyde, and
likely an alcohol. Both the Lucas test and chromic acid test indicate that the unknown is a
secondary alcohol, as cyclopentanol is. The 3,5-dinitrobenzoate derivative had a tentative
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melting point of 110-112°C, and the melting point of cyclopentanol with this derivative is
115°C, which is reasonable. The phenylurethane derivative had a melting point of 132133°C, and the melting point of cyclopentanol with this derivative is 132°C, which is right
on! At this point I could take the IR, C13, and NMR of the unknown to verify whether the
unknown compound is cyclopentanol.
IR spectrum of unknown
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DEPT C13
HNMR
Final conclusion is that the unknown compound is cyclopentanol.