Quiz 2 - Crystallization/Recrystallization/Extractions Flashcards
extraction
technique used to separate a mixture into its individual components via differences in polarity and solubility
polarity
Polarity: distribution of partial charges or electron density throughout a molecule
solubility
- the extent to which a solute will dissolve in a solvent
- “Like dissolves like” rule – relates solubility and polarity
- Like polarities solutes will dissolve in like polarity solvents
miscibility
refers to the interaction between 2 liquids
miscible
similar polarities will appear as one phase when mixed
immiscible
dissimilar polarities will generate 2 layers when mixed
Identify layers using density
typical densities
Halogenated organics > water > non-halogenated organics
***typically aqueous solution is on the bottom BUT cannot assume
solid-liquid extraction
- Separate a mixture of solids by adding a solvent that will dissolve desired solids and leave behind insoluble solids
- Filtration is done to finalize separation
Ex/ mixture of sand and salt
Salt will be dissolved by water through filter paper and sand will remain on top of filter paper
liquid-liquid extraction
- Separate two solutes by using 2 immiscible solvents with opposing polarity
- using a separatory funnel (i.e. our week 3 experiment)
- drain out solution to get desired layer/solution
chemically active extraction
- Separate 2 solutes with similar polarity
*Liquid-liquid extraction will not be effective. - Must make compound prefer water for a solvent by making it more polar via acids/bases protonation/deprotonation
- The charged ion will stay in the aqueous layer and the less polar molecule will separate in the organic layer
Kd
- distribution coefficient that provides a numerical representation of the solubility equilibrium of a given solute in a given solvent system
Kd equation
- Kd = solute organic / solute aqueous
- Kd > 1 → solute prefers organic layer
- Kd < 1 → solute prefers aqueous layer
- Kd = 1 → solute is distributed equally between both layers
most effective extractions?
- Multiple smaller extractions are more effective than one large extraction
overall chemical reaction of week 3 extraction
- mixture in methyl orange is filtered and heated
- insoluble cellulose filtered off
- soluble benzoic acid and methyl orange in organic layer
- NaOH makes benzoic acid negative into benzoate ion
- Benzoate ion is separated into aqueous layer and methyl orange stays in organic layer
- HCl added to reform benzoic acid (impure solid with some MO)
Crystallization
- formation of new, insoluble product by chemical reaction
- This product then precipitates out of the reaction solution as a solid containing many trapped impurities
Recrystallization
- does not involve any chemical reaction
- The crude product is dissolved into solution; conditions are changed that allow crystals to reform
**produces a more pure final product
**can be used as a method to remove impurities
where do impurities come from?
- side products of chemical reactions
- Unreacted starting material
- Contaminants
crystal formation
- When crystals are being formed → the same individual molecules will fit in a specific crystal lattice structure
- Equilibrium process that produces very pure material
- Form quickly ⇒ impurities can become trapped within the crystal lattice resulting in an impure solid
- Form slowly/controlled ⇒ impurities are excluded from the crystal lattice resulting in a pure solid
Extraction of Benzoic Acid Activity
- Extract containing benzoate will contain impurity of methyl orange
- Benzoate ion is soluble in water
- Adding HCl will reform benzoic acid from benzoate (insoluble in water)
- Crystals form too rapidly trapping impurities within growing crystal lattice
***Benzoic acid crystals with methyl orange tapped inside (crude mixture)
recrystallization
- purification method of non-volatile solids
- General technique involves dissolving the solid crystals in a hot solvent and then cooling slowly so pure crystals form
- then filter pure crystals off leaving impurities in solution
When can you use recrystallization ?
- Desired compound is more concentrated than the impurity in the solid material
- Impurity is easily dissolved or not dissolved at all
- Will not be incorporated into the crystal so long as crystal formation is slow
- If the impurities are not dissolved, they can be filtered off
3 stages of solubility
- collision
- dissociation
- solvation
collision
when a solvent (S) molecule collides with the solid crystal surface
Dissociation
- when collision results in the breakaway of a molecule from the solids
***Boiling solvents results in collisions with sufficient energy for dissociation.
solvation
when the solute molecules are “in solution”. The solute molecules are solvated by the solvent
Choosing an ideal recrystallization solvent →
- High temperature coefficient: Solid is insoluble in the solvent at low temp but very soluble at a high temp
- The solvents boiling point needs to be less than the melting point of the solid (dissolve the solid, not melt it/oil out)
- The solid and solvent must be unreactive
- The solvent needs to dissolve impurities easily/not at all
- The solvent should be volatile; Solvent will easily evaporate from the surface of the reformed crystals
Charcoal purpose
- be used to remove colored impurities
- Color indicates conjugation
- Charcoal binds to methyl orange (pink color)
- Charcoal is insoluble in H2O; Charcoal is filtered off to remove methyl orange
hot gravity filtration
- Removes insoluble materials from the solution while the desired solid remains dissolved in the hot solution
- Must remain hot so that crystals do not begin to form
nucleation
Methods of inducing crystal formation
- seeding: addition of a small crystal of the material to provide a surface for crystal formation to adhere to
- scratching: scratch bottom of flash with glass rod
melting point
- Used to test the purity of crystals
- Melting point is a range (temp at beginning to temp at end)
melting point range meaning
(Temp @ melting beginning - temp @ melting end)
- More pure solid ⇒ high melting point, narrower range
- More impurities ⇒ low melting point, wider range
— Bc the lack of homogeneity of the sample allows for different melting points of the molecules
