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Requirements for successful recrystallization?
- Solvent must not react with desired compound.
- Purified compound can be easily removed (filtration).
- Purified compound should be very soluable in hot and almost insoluable in cold solution.
- Impurities must be very soluable in cold or almost insoluable in hot solvent.
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Characteristics desirable for a recrystallization solvent?
- High solubility at its boiling boiling point and low solubility at lower temeratures.
- Impurities remain in solution as it cools.
- Cannot be reactive with compound.
- Volatile (to evaporate)
- Cheap, non-toxic, non-flammable.
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What are mixed melting points?
Melting point of compounds that have been mixed (impure).
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What should the recrystalization range of heating be?
3 – 5 degrees/minute
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In extraction what is the formula for the distribution coefficient?
Korganic/water = [A]organic/[A]water
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In extraction what is the basic idea of a distribution coefficient?
- A measure of the tendency for the solute to reside in
- one phase versus the other and is equal to the ratio of solubility’s for A in the respective solvents.
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What was the role of biphenyl (Ph-Ph) in extraction?
Nonpolar nonreactive compound that stayed in the organic phase.
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What was the role of benzoic acid (Ph-CO2H) in extraction?
Nonpolar acidic compound that in a basic environment forms a polar salt.
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What was the role of p-nitroaniline (C6H6N2O2) in extraction?
Nonpolar basic compound that in an acidic environment forms a polar salt.
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What was the role of methylene chloride (CH2Cl) in extraction?
Nonpolar solvent serving as the organic phase.
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What was the role of water (H2O) in extraction?
Polar solvent serving as the aqueous phase.
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What was the role of hydrochloric acid (HCl) in extraction?
Created an acidic environment.
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What was the role of sodium hydroxide (NaOH) in extraction?
Created a basic environment.
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Understand why the various chemicals or solutions
could be used in an extraction, for example, sodium bicarbonate (NaHCO3), HCl, NaOH, and MgSO4 or Na2SO4.
- NaHCO3 & NaOH: Nucleophilic bases
- HCl: Electrophilic acid
- MgSO4 & Na2SO4: in anhydrous form used as a drying agent
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What was the role of water in recrystallization?
Solvent
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What was the role of decolorizing carbon (charcoal) in recrystallization?
Removes impurities (large polar surface area)
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What was the role of: benzamide, benzoic acid, acetanilide, succinimide, cyanoacetamide, glucose pentaacetate, adipic acid, benzilic acid, salicylic acid, cinnamic acid in recrystallization?
Substance being purified and crystalized.
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What effect does a volatile impurity mimiscible with the liquid phase have on melting temperature?
Lower broader melting point (geometric differences)
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What is done if a single solvent does not give suitable results for recrystalization?
- Use mixed solvents (solvent pair)
- One solvent dissolves readily the other sparingly eg. (water & hexanol, hexane & benzene)
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What are the characteristics of a solvent pair at high and low temperatures?
- Higher temperatures characteristics of the better solvent dominate.
- Lower temperatures characteristics of the weaker solven dominate.
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How do you calculate Rf?
Distance traveled by the compound divided by the distance traveled by the solvent front.
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What was the role of B-carotene in chromatography?
Nonpolar pigment.
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What was the role of dichloromethane in chromatography?
Mobile phase that is very slightly polar.
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What was the role of iodine in chromatography?
Stains organic compounds brown.
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What was the role of lycopene in chromatography?
Slightly polar pigment.
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What was the role of petroleum ether in chromatography?
Mobile phase composed of nonpolar solvent.
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What was the role of sand in chromatography?
Inert packing compound.
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What was the role of silica gel in chromatography?
Stationary phase composed of an inert solid with a polar functional group.
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What was the role of sodium sulfate (anhydrous) in chromatography?
Dehydrates the crude pigments.
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What was the difference between simple and fractional distillation?
- Fractional distillaton uses a longer distilling column containing packing.
- The paking used in fractional distillation gives it more areas for the liquid and gas phase to be in equilibrium (theoretical plates).
- Higher number of theoretical plates in fractional distillation gives better separation.
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