A neutral organic compound was isolated from an unknown
mixture containing organic impurities and an inorganic salt. This was done by
solvent extraction to separate compounds using two immiscible solvents such as
DCM and water – this removed the inorganic salt in the aqueous layer. A basic
and acidic wash was performed where all the acidic and basic impurities were
removed from the organic phase. DCM was removed, the neutral product was
recrystallised, and the melting point was determined.
Sample 4 (2.65g), water (10cm3) and dichloromethane
(20cm3) were added to a 100cm3 conical flask and swirled
until the solid dissolved. The solution was transferred into a separating funnel,
rocked and inverted to mix the DCM and aqueous layers. Also, the tap was opened
to release the pressure. The lower organic layer was separated into a clean
conical flask. This was repeated with additional DCM (10cm3) and
added to the flask.
To remove the acidic impurities, excess sodium hydroxide
solution (10cm3) was added to the separating funnel, mixed and inverted.
The pH of the aqueous layer was tested with red litmus paper, which turned
blue. Also, to remove the basic impurities, excess dilute hydrochloric acid
(10cm3) was added to the funnel. The pH of the aqueous was tested
again with blue litmus paper, which turned red. After each test, the organic
layer was separated into a clean conical flask.
Magnesium sulphate was added to the conical flask to remove
traces of water, swirled and filtered into a pre-weighed round-bottomed flask.
DCM was removed using a rotary evaporator until there was no change in the
weight of the flask. The product was a fine, white powder.
Ethanol and water were tested in a test tube to determine
the best solvent system for the product in sample 4. It was soluble in cold
ethanol, but insoluble in cold water. However, small volumes of hot
ethanol/water and 2-3 anti-bumping granules were added to the flask on a
hotplate, until the solution was clear. The flask was left to cool and the
crystallised neutral compound was vacuum filtered and weighed. The crystals
were white, shiny and needle-like. Then, the melting point of the product was
determined to figure out the identity of neutral compound.
Results & Discussion
% crude yield =
% recovery yield =
Melting point of NS-1-23-1 = 69-70OC
The % crude yield was ~50% because
after the first solvent extraction, the inorganic salt was removed. Also,
almost all the acidic and basic impurities were removed once the basic and
acidic wash was performed. This suggests that only 46.47% of the mixture was
made up of neutral organic compound.
The % recovery yield of recrystallisation was 2.44% but the best
theoretical yield would be ~90%, since 100% isn’t achievable due to loss of product
whilst transferring it to different equipment. Some product may also be lost in
the recrystallisation solvent as there is finite solubility of the solid, even
at lower temperatures. If the rinse solvent (cold water) isn’t cold enough, the
solid may redissolve, leading to a lower % recovery.
The melting point of the product was 69-70oC, which
is within a degree of the literature melting point (71oC) of the
organic compound ‘biphenyl’. It melted completely over a narrow temperature
range of 1.0oC, indicating that the product was pure1.
However, additional melting point trials can be carried out to confirm the
After isolating the neutral organic compound, the melting
point was determined as 69-70oC. In conclusion, the identity of the
organic compound was biphenyl as it was closer to the literature melting point
Date accessed: 28/01/2018