Ochem Lab: Extraction & Recrystallization

purpose of experiment

given a 1:1:1 mixture of acid, base, and neutral compounds to extract, purify by recrytallization, and identify based on the melting point of each compound

Extraction and recrystallization are both...

separation techniques

extraction

technique for separating and isolating individual compounds from a mixture (solvents will be immiscible)

relies on there being a difference in solubility between each component of a mixture

4 criteria of extraction

1. solvent must be immiscible with the initial solvent. If the two solvents are not immiscible, they will not form two distinct layers and so you won't be able to separate the layers.

2. extraction solvent should NOT react with the compound such that it cannot easily be reversed.

3. pick an extraction solvent that is as specific with the compound you are trying to extract as possible. The fewer purification steps needed to isolate your compound, the larger the overall yield in the end.

4. choose a solvent that can easily be removed once the extraction is finished. Normally, this means that you want an extraction solvent with a relatively low boiling point

general theory of extraction

exposing a mixture of compounds to two different solvents with two different polarities at the same time, the compounds will migrate into the solvent that best matches the polarity of the individual compound

acid/base extraction: what/how to modify the compounds of the mixture

change the pH --> mix becomes charged ions (protonation upon addition of acid OR deprotonation upon the addition of base)

short story of extraction procedure

take unknown mix and separate it into the unknown neutral and unknown base with extraction

ions vs neutral organic compounds

ions - souble in aq solns, moving into the aq layer

NO - not soluble in water and thus stay in organic solvent

recrystallization

purification that relies on solubility of a PURE compound and that of an impurity

solvent chosen based on props that it is no very soluble at room temp, but @ higher temps it will dissolve

steps of recrystallization

1. heat soln to boiling to ensure all of sample dissolves

2. let soln cool slowly to room temo

3. desired compound precipitates as a crystalline solid & impurities remain in soln

4. filtration is then used to isolate the solid product

Why must the seperatory funnel be vented?

1. To allow the liquid to drain out.

2. Because we need air in the seperatory funnel to ensure a complete reaction.

3. To ensure efficient mixing of the aqueous and organic phases.

4. To relieve the pressure that builds up upon shaking.

4

What should you do if you spill HCl or NaOH on your skin?

1. Neutralize the spill by applying the opposite type of compound to the spill (acid to a base or base to an acid).

2. Wait to see if it hurts before taking further action.

3. Use a paper towel to wipe off the liquid.

4. Rinse the affected area with running water for at least 15 minutes.

4

short story of recrystallization procedure

dissolve compounds in a small amt of hot solvent

let cool & collect the 2 solids by vacuum filtration

In the recrystallization step, what size crystals are we looking for?

1. As large as possible.

2. As small as possible.

3. Our products will not crystallize.

4. Not too large and not too small.

4

In regards to recrystallization, what is seeding?

1. The addition of base to an acidic solution or the addition of an acid to a basic solution.

2. The slow addition of water to an organic solvent that leads to formation of a precipitate.

3. The addition of a small amount of the pure, crystalline compound to a saturated solution of the compound.

4. Seeding is when one scrapes the sides of a glass vessel.

3

what do I do if the base does NOT precipitate?

try blowing air over the top of the solutions

sometimes some ethyl acetate floats on top of water and dissolves all of the desired product

blowing air evaporates the ethyl acetate and forces the product to precipitate

how to use the mel-temp (MP)

load small amts of crystals to capillary tubes (3 mm of sample per tube) then watch thru the eye piece as the temp increases

solvents/solids in experiment

ethyl acetate, NaOH, HCl, sodium sulfate

bases used in experiment

2-methyl-4-nitroaniline, 3-nitroaniline, 4-nitroaniline

neutrals used in experiment

9-fluorenone, anthracene, fluorene, phenanthrene