Organic Chemistry Lab
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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.
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.
What are mixed melting points?
Melting point of compounds that have been mixed (impure).
What should the recrystalization range of heating be?
3 – 5 degrees/minute
In extraction what is the formula for the distribution coefficient?
Korganic/water = [A]organic/[A]water
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.
What was the role of biphenyl (Ph-Ph) in extraction?
Nonpolar nonreactive compound that stayed in the organic phase.
What was the role of benzoic acid (Ph-CO2H) in extraction?
Nonpolar acidic compound that in a basic environment forms a polar salt.
What was the role of p-nitroaniline (C6H6N2O2) in extraction?
Nonpolar basic compound that in an acidic environment forms a polar salt.
What was the role of methylene chloride (CH2Cl) in extraction?
Nonpolar solvent serving as the organic phase.
What was the role of water (H2O) in extraction?
Polar solvent serving as the aqueous phase.
What was the role of hydrochloric acid (HCl) in extraction?
Created an acidic environment.
What was the role of sodium hydroxide (NaOH) in extraction?
Created a basic environment.
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
What was the role of water in recrystallization?
What was the role of decolorizing carbon (charcoal) in recrystallization?
Removes impurities (large polar surface area)
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.
What effect does a volatile impurity mimiscible with the liquid phase have on melting temperature?
Lower broader melting point (geometric differences)
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)
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.
How do you calculate Rf?
Distance traveled by the compound divided by the distance traveled by the solvent front.
What was the role of B-carotene in chromatography?
What was the role of dichloromethane in chromatography?
Mobile phase that is very slightly polar.
What was the role of iodine in chromatography?
Stains organic compounds brown.
What was the role of lycopene in chromatography?
Slightly polar pigment.
What was the role of petroleum ether in chromatography?
Mobile phase composed of nonpolar solvent.
What was the role of sand in chromatography?
Inert packing compound.
What was the role of silica gel in chromatography?
Stationary phase composed of an inert solid with a polar functional group.
What was the role of sodium sulfate (anhydrous) in chromatography?
Dehydrates the crude pigments.
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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