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Organic Chemistry Lab 1- Recrystallization

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Recrystallization
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Report Sheet
Solubility tests: (“s” for soluble, “is” for insoluble )
Water
Ethanol
Acetone
Cold
Hot
Cold
Hot
Cold
Hot
Benzoic Acid
is
s
s
-
s
-
Acetanilide
is
s
s
-
s
-
Naphthalene
is
is
is
s
s
-
Best recrystallization pairs:
 Benzoic acid in water
 Acetanilide in water
 Naphthalene in ethanol
The ideal solvent should dissolve the compound at high temperature and be insoluble at low
temperature.
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Questions
1. A compound is quite soluble in toluene, but only slightly soluble in petroleum ether. How
could there solvents be used in combination in order to recrystallize this compound?
If a suitable solvent with the desired solubility properties cannot be found: soluble in hot solvent
and insoluble in cold solvent, a mixed-solvent recrystallization can be used. For instance, the
compound is quite soluble in toluene, but only slightly soluble in petroleum ether. Firstly, the
compound can be dissolved in the minimum amount of the hot solvent, toluene, in which is soluble.
Secondly, small amounts of hot solvents, toluene, in which it is not soluble, petroleum ether, are added
to the hot mixture. The mixture is kept hot, and the insoluble hot solvent, petroleum ether, is added
dropwise until the mixture appears cloudy. Thirdly, more of the solvent in which it is soluble is then
added to clear the mixture, resulting in a hot, saturated solution of the compound in a mixed solvent.
When the solvent is cooled, crystals of the compound will form. [1]
2. How can you separate the mixture of meta-nitrobenzoic acid ana para-benzoic acid?
To separate a mixture of meta-nitrobenzoic acid and para-benzoic acid, use a technique known as
chromatography. By using this technique, two acids are separated according to how polar they are.
After being dissolved in a solvent, the mixture is run through a stationary phase-filled column.
Typically, silica gel with polar groups on its surface serves as the stationary phase. In comparison to
the less polar compound (para-benzoic acid), the more polar compound (meta-nitrobenzoic acid) will
interact more strongly with the stationary phase and move through the column more slowly. The two
compounds are eventually separated as a result of this.
Meta-nitrobenzoic acid and para-benzoic acid can also be separated by converting the mixture into
the corresponding methyl ester mixture through the use of diazomethane or other methods. Simple
chromatography on silica gel should make it simple to separate the two regioisomeric esters. The
separated esters should then hydrolyze with CH3ONa/CH3OH with little difficulty. [2]
3. During recrystallization, a yellow solution of a compound in hot recrystallizayion solvent
was treated with acitivated carbon and then filtered through fluted paper. On cooling,
the filtrate yielded gray crystals, although the compound is colorless. Explain why the
crystals were grey, and how can you prevented this?
The crystals' grey colour is most likely caused by the presence of impurities that activated carbon
cannot remove. It's possible that impurities existed in the original compound or were added during
the recrystallization procedure. During crystallization, impurities may also become trapped in in
the crystal's lattice.
It is crucial to make sure that the compound is free of all impurities prior to recrystallization in
order to prevent this. This can be accomplished by employing an appropriate solvent for
recrystallization and making sure it is impurity-free. Additionally, impurities that are still in
solution prior to crystallization should be eliminated using activated carbon. Larger, more pure
crystals can also be created by slowly cooling the solution. [3]
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References
http://www.orgchemboulder.com/Labs/Experiments/12%20-%20Recrystallization.pdf [1]
http://www.orgsyn.org/demo.aspx?prep=CV1P0391 [2]
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Suppl
emental_Modules_%28Physical_and_Theoretical_Chemistry%29/Physical_Properties_of_Matter/Solu
tions_and_Mixtures/Case_Studies/RECRYSTALLIZATION [3]
Peynircioğlu, N. Bekir (Editor). Organic Chemistry Laboratory Manual. Middle East Technical
University Department of Chemistry, 2022.
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