lab 2 Flashcards
why might we do a mixed solvent recrystaliazation?
ten, a
single solvent with the desired solubility characteristics cannot be found. In these cases, a two -
solvent system is used, and the purification process is referred to as a mixed solvent
recrystallization.
what must be true for the to be purified?
2-1
Experiment 2: Mixed Solvent Recrystallization of an Unknown
Compound & Identification by Mixed Melting Point Determination
Introduction
In experiment 1 you were introduced to the single solvent method for recrystallization. Often, a
single solvent with the desired solubility characteristics cannot be found. In these cases, a two -
solvent system is used, and the purification process is referred to as a mixed solvent
recrystallization.
Selection of Solvent for a Mixed Solvent Recrystallization
For a mixed solvent recrystallization, the compound to be purified must be soluble in one of the
solvents and insoluble in the other solvent at all temperatures. The two solvents being used
as a mixed solvent must also be miscible.
what are some common solvent pairs for recrystalization?
Methanol-Water Ether-Acetone
Ethanol-Water Ether-Petroleum ether
Acetic acid-Water Toluene-Ligroin
Acetone-Water Dioxane-Water
how is a mixed solvent recrystalization performed?
To perform a two solvent (or mixed solvent) recrystallization, the compound is first dissolved in a
minimum volume of boiling solvent in which it is soluble. The second solvent (the one in which the
compound is insoluble) is heated close to its boiling point and then added dropwise to the boiling
solution of the compound in the first solvent until the mixture just begins to become cloudy. This
cloudiness is due to the formation of a precipitate of the compound being recrystallized. In order
to clear the cloudy mixture, a small amount of the first solvent is added dropwise. After
clarification, the solution is saturated and as such it should afford crystals upon cooling. I
what may cause a solution to oil out?
is
important not to add an excess of the second solvent or to cool the solution too rapidly. Either of
these actions may cause the solute to oil out. If this happens, the solution should be reheated and
more of the first solvent (in which the compound is very soluble) should be added.
what can a melting point assess other than purity? how?
Melting point
analysis can also be used as supporting evidence in identifying a compound. When identifying an
unknown, a useful practice is to measure its melting point and at the same time, measure that of a
mixture of the unknown and a known compound with a similar melting point and suspected to be
the same compound as the unknown. If the known and the unknown samples are the same
substance, there will be no depression of the melting point.
what may happen if the two compounds are too different?
However, if the mixed compounds are
different, a depression and broadening of the melting point range will occur. This procedure is
called a mixed-melting point determination. The mixed-melting point determination requires that
an authentic sample (a “standard”) of the compound be available in the laboratory.
how are the samples prepared?
Sample preparation for a mixed-melting point determination begins with the thorough mixing and
crushing to a fine powder of approximately equal amounts of the standard and the unknown. A
melting point tube is charged with the mixture of the unknown and the standard compound. This
process is then repeated using the other standard or standards. The mixtures are placed in the
melting point apparatus and the melting points are recorded simultaneousl
what does a broad range melting point indicate? what is the standard if the melting point is sharp?
A melting point
depression or broadening of the range indicates that the standard has acted as an impurity toward
the unknown and thus they are different compounds. If the melting point of the mixture remains
sharp and is not depressed, the standard is the same compound as the unknow
what happens to impurities that fail to enter the lattice? is the impurity solid?
Impurities that fail to enter the crystal lattice of the compound, such as sand or charcoal, will not
depress the melting point. The impurity does not need to be a solid. It can be a liquid, such as the
one used to recrystallize the compound. Hence it is necessary to dry the compound thoroughly
before determining its melting point
example of a typical melting point test:
Suppose unknown X melts at 133-135 °C and is known to be one of the compounds A, B or C below.
To identify the unknown X, a small amount of X has been mixed with an equal amount of A. Two
similar mixtures have been prepared by combining X with B and C. The melting points of all three
mixtures have been measured. The results are summarized on the next page.
Compound Melting Point Range
A. trans-cinnamic acid 133-134 °C
B. phthalic anhydride 131-134 °C
C. urea 133-135 °C
X 133-135 °C
X + A 110-121°C
X + B 119-126 °C
X + C 132-134 °C
Since the melting point of the mixture of X + C has a narrow range and is not depressed compared
to the melting point of pure X, the conclusion can be made that X and C are the same compound.
Therefore, X is urea.
what are the possible unknowns and their melting point ranges?
group 1 - benzil 94-95 , methyl 4-nitrobenzoate 94-96,
group II - 3-nitroaniline 112-114, 2-methyl, 4-napthoquinone 105-107, ,
group III - 4-methylacetanilide 149-151, 4-nitroaniline 149-151
list the chemicals used:
Benzil • Methyl 4-nitrobenzoate • 3-Nitroaniline • 4-Nitroaniline • 4-Methylacetanilide • 2-Methyl-1,4-naphthoquinone • Methanol
read over procedure
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- How would the melting point range of the products from Experiments 1 and 2 be affected if the
crystals were not completely dried? - A compound which melts over the range 132-134 °C was suspected to be either aspirin (M.P.
132-135 °C) or urea (M.P. 131-133 °C). Explain how you would determine the identity of the
unknown. - List at least three methods that can be used to reduce product loss during the recrystallization
procedure
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