Chapter 6. Recrystallization

Chapter 6. Recrystallization
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Recrystallization
- One of the major tasks in the organic chemistry laboratory is purifying
chemicals
- Solid organic compounds can be purified successfully by
recrystallization.
- Recrystallization involves several steps
 choosing a good recrystallization solvent
 dissolving the sample in the minimum amount of boiling solvent
 filtering the hot solution to get rid of insoluble impurities.
 cooling down the solution to induce crystallization.
 separating the solid from the solution (called the mother liquor)
by vacuum filtration (cold filtration).
 washing the solid on the filter with a small amount of cold
solvent to remove traces of the mother liquor.
 drying the solid to remove traces of solvent.
If the hot solution is clear and transparent without insoluble
impurities the hot filtration can be skipped.
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- the solubility of organic compounds
increases as the temp. of the
solvent rises.
- The upper limit of temperature is
dictated by the boiling point of the
solvent.
- ex. Purification of the 10g of
benzoic acid that is contaminated
with 0.2 g of salicylic acid
Solubility (g/l)
Recrystallization
The solubility of
benzoic acid in water
Temp.(oC)
O
C
OH
O
C
The solubilities of both acids in water
OH
OH
Compound
At 200 C (SC )
At 1000 C (SH )
Benzoic acid
2.9 (g/L)
68.0 (g/L)
Salicylic Acid
2.2 (g/L)
66.7 (g/L)
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Recrystallization
147 mL of
boiling water
10 g of benzoic acid
impurified with 0.2 g
of salicylic acid
Compound
Cool
down
Both acids in hot
solution
At 1000 C (SC )
9.57 g of benzoic
acid precipitates out
salicylic acid in
solution
At 200 C (SH )
Benzoic acid 68.0g/L=10.0g/147mL)
2.9g/L = 0.43g/147mL,10.0-0.43=9.57g
Salicylic Acid 66.7g/L=9.8g/147mL)
2.2g/L = 0.32g/147mlL
- Recrystallization is one of the most powerful techniques to purify solid
compounds even though the nature and amount of impurities are
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unknown.
Recrystallization Steps
1. Selection of the solvent
- Solvent I is a better recrystallization solvent
than solvent II because it yields a larger
percentage recovery of sample upon cooling.
I
II
Solubility
- The compound to purify should be very soluble
in the hot solvent and have very limited
solubility in the cold solvent.
Temperature
- The recovery % depends on the ratio the solubilities in the hot and cold
solvent Sc /SH ; when the ratio is small due to a limited solubility in the
cold solvent or a high solubility in the hot solvent, the % recovery is large.
- Because information on the variation of solubility with temperatures is
usually unavailable, solvent selection is an experimental problem solved
by trial and error.
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Recrystallization Steps
1. Selection of the solvent ; check points
- The compound to purify should be very soluble in the hot solvent
and have very limited solubility in the cold solvent.
- Ideally the impurities should either be soluble in the cold solvent
or very insoluble in the hot solvent.
- The boiling point of the solvent should be in the range of 50-1200
C and lower than the melting point of the product.
- The solvent should not react chemically with the product to be
purified.
- Given several choices, the solvent with the lowest toxicity should
be chosen.
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Recrystallization Steps
2. Dissolving the sample
- the bulk of the sample is dissolved in an Erlenmeyer Flask; its
conical shape minimize the losses of solvent due to
evaporation.
- Small portions of hot solvent are added and the suspension of
solid and solvent is brought to a boil by heating on a hot plate. A
glass rod is used to stir the system and avoid boil over.
- When very volatile solvents are used it is
recommended to use 10 to 20% excess of
solvent after the sample is dissolved, this
extra volume would make up for evaporation
during the filtration.
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Recrystallization Steps
3. Hot filtration
- Insoluble impurities present in the hot solution are removed in this step.
- Hot filtration is usually performed with a short stem funnel and flutted filter
paper; preheat the funnel with boiling solvent to avoid crystallization of
the solid in the filter.
- Filter papers of fine porosity should be avoided during hot filtration
because they result in very slow filtration which leads to premature
crystallization.
- The whole assembly is placed on a hot plate; both
Erlenmeyer flasks the one that receives the filtrate
and the one that contains the sample should be kept
on the hot plate.
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Recrystallization Steps
4. Cooling down
- After the solution has been hot filtered it is left to crystallize
at room temperature first and then placed in a cold bath.
- An intermediate rate of crystallization is recommended;
∙ Too rapid crystallization gives small crystals with a large surface area.
→ These crystal can adsorb large amounts of impurities
∙ very slow crystallization results in the formation of big crystals that
sometimes may occlude impurities.
- An ice water bath should not be used with solvents such as acetic acid,
cyclohexane and dimethyl sulfoxide, which have a freezing points
between 20 and 00 C and solidify upon cooling.
- In case of no crystal growth; “Scratching” or “Seeding” is helpful
(scratching) to gently scratch the walls of the flask with a glass rod with
a vertical motion → loosen small particles of glass that act as a nucleation
points where crystals grow.
(seeding) to add the seed crystal
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Recrystallization Steps
5. Cold filtration
- Once the compound has completely precipitated out of solution the
suspension is vacuum filtered using a Buchner or Hirsch funnel depending
on the total amount of solid.
- the vacuum is turned on, and the filter paper is wetted with a small portion of
the cold solvent to adhere it to the funnel
- The crystals are then collected by pouring the
cooled suspension through the funnel. The filtrate
called the mother liquor, is saturated with the
compound of interest and contains the soluble
impurities.
- The mother liquor is sometimes saved to recover
the solid in solution by the second
recrystallization; the purity of the second crop is
less than that of the first crop.
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Recrystallization Steps
6. Washing and Drying the solid:
- The solid on the filter paper called the cake, contains traces of the mother
liquor with soluble impurities, all traces of mother liquor should be
eliminated from the cake to increase the purity of the final product.
- a small portion of fresh cold solvent is added to the funnel with the vacuum
off. The cake is carefully stirred with a glass rod or spatula and the vacuum
is turned on again.
- The drying of the solid starts at this point; Air
forced to pass through the solid by the effect
of vacuum accelerates the evaporation of the
solvent
- The solid on a piece of filter paper or an a
watch glass can also be dried in an oven at a
temperature below its melting point or in a
desiccators.
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Recrystallization : http://www.youtube.com/watch?v=Q47hTa1KvN0&feature=related
Recrystallization
▶ Recrystallization is a procedure for purifying compounds.
The most typical situation is that a desired "compound A" is
contaminated by a small amount of "impurity B".
Single-solvent recrystallization
Multi-solvent recrystallization
Hot filtration-recrystallization
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Recrystallization
Single-solvent recrystallization
Ideal solvent : In soluble in R.T but soluble in b.p.
→ Solvent added (clear) to compound (yellow)
→ Solvent heated to give saturated compound solution (yellow)
→ Saturated compound solution (yellow) allowed to cool over time
to give crystals (yellow) and a non-saturated solution (pale-yellow).
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Recrystallization
Multi-solvent recrystallization
- This method is the same as the single-solvent recrystallization but
where two (or more) solvents are used.
- This relies on both "compound A" and "impurity B" being soluble in a
first solvent. A second solvent is slowly added. Either "compound A" or
"impurity B" will be insoluble in this solvent and precipitate, while the
other of "compound A"/"impurity B" will remain in solution.
- Thus the proportion of first and second solvents is critical; Typically the
second solvent is added slowly until one of the compounds begins to
crystallize from solution and then the solution is cooled. Heating is not
required for this technique but can be used.
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Recrystallization
Multi-solvent recrystallization
Ideal solvent : Miscible two solvent
Soluble in solvent 1 at RT and insoluble in solvent 2 at b.p.
→ Solvent added (clear) to compound (yellow)
→ Solvent heated to give saturated compound solution (yellow)
→ Second solvent (blue) added to compound solution (yellow) to give
mixed solvent system (green)
→ Mixed solvent system (green) allowed to cool over time to give
crystals (yellow) and a non-saturated mixed solvent system (green-blue).
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Recrystallization
Hot filtration-recrystallization
Hot filtration can be used to separate "compound A" from
both "impurity B" and some "insoluble matter C".
→ Solvent added (clear) to a mixture of compound (yellow) + insoluble
substance (purple)
→ Solvent heated to give saturated compound solution (orange) +
insoluble substance (purple)
→ Saturated compound solution (yellow) filtered to remove insoluble
substance (purple)
→ Saturated compound solution (yellow) allowed to cool over time to give
crystals (yellow) and a non-saturated solution (pale-orange).
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