CHM 205 Lab Final

Why would you use a filtering pipet?

Used to remove unwanted solid from a liquid
-When you want the liquid, cotton plug stops solid

Why would you use a filter-tip pipet?

-to remove solids from very small amounts of liquid by leaving solid behind (as opposed to trapping it)
-when you want the solid

Why (in exp. 0) do you rinse w/ 1 mL of methanol and 1 mL of hexane?

-methanol dissolves and hhexane has a low bp to evaporate liquid

What m.p. indicates purity?

-a narrow melting point range
-an impurity lowers and broadens m.p.
-to lower freezing point, add impurity as well
-an intensive property

What does mixed melting point do?

-If the unknown and known are the same, then there is no change in melting point range
- if not, then the range will react like an impurity was added

Melting point range

temperature of first liquid- temperature of completely melted

When using the mel-temp, why shouldn't you heat faster than 1-2 ˚C?

-can result in misread temperature due to crystal lagging

What does it mean if the solution in the mel-temp turned from white to brown?

-Decomposition occurred

What happened if the solution in the mel-temp disappeared?

-sublimation occurred, seal capillary

What is the literature m.p. value for ferrocene?

-173-174˚C

What is the literature m.p. value for adipic acid?

-151-154˚C

What is the literature m.p. value for acetyl ferrocene?

-81-86˚C

What is the literature m.p. value for citric acid?

-153-159˚C

How do you calculate the number of drops from a given mL?

-x mL x \# drops/ mL \= x drops

How do you calculate the number of drops from a given weight (g)?

- X g x cm^3/g x \# drops/mL \= x drops

How do you calculate the \# g from a given number of drops?

- \# drops x 1 mL/ 28 drops \= \# mL x g/mL\= \# g

max of heat dial for a hot plate?

4-390 degrees celsius

Why should you always add boiling chips before heating them to their boiling point?

-to prevent bumping and local overheating (decreased evaporation)

Why should you try to dissolve a compound in as little solvent as possible? (during recrystallization)

-to increase compound crystallization from increased concentration

How are insoluble impurities removed?

-from hot gravity filtration

How are soluble colored impurities removed?

-from charcoal and subsequent HGF

why should you allow recrystallization to occur slowly?

to obtain a more pure compound

explain what the colors of drierite mean.

blue: fresh and ready to use
pink: used; already contains water

Why should you never add charcoal (or an solid substance) to a boiling solution?

-it could boil over

why do you scratch the flask? (in macroscale recrystallization)

-to create a surface for nucleation

After macroscale recrystallization, what are two ways you can get more recovery?

-cool further for more recovery
-evaporate for a higher concentration

What is the purpose of CaSO4?

-to dry solution/ absorb moisture

What is the solvent/solute used in the macroscale recrystallization?

-water and organic unknown

When would you use microscale recrystallization?

-for ranges 10-100mg in .5-.15 mL of solvent (in this case it's water)

Why do you rotate the microspatula rod while adding water?

-to improve dissolution

Why, in microscale recrystallization do you want to avoid the presence of residual charcoal?

-Will interfere with % yield and proper ID of substance

If crystals haven't appeared in microscale recrystallization, what should you do?

-reduce the volum of filtrate by ~20%, or until the crystals begin to appear in the heated solution

What do you use instead of vacuum filtration in microscale recrystallization?

-centrifugation

The solubility of an unknown compound X in ethanol is found to be 0.21 grams per 10 mL at 0 °C and 1.4 grams per 10 mL at 78°C. What is the minimum amount of ethanol needed to recrystallize a 2.0 gram sample of compound X? After recrystallization of the 2.0 gram sample, how much of compound X will remain dissolved in the cold solvent (i.e., how much of compound X is not recovered after the recrystallization)?

2.0 grams X * 10 mL ethanol / 1.4 grams X
\= 14.29 mL of ethanol
0.21 grams X / 10 mL ethanol * 14.29 mL ethanol
\= 0.30 grams X remain dissolved in the cold solvent.
.315 grams remains in solvent

Why is it important to use only the minimal amount of hot solvent needed to achieve a recrystallization? What would be the effect of using too much solvent while dissolving a solid for recrystallization?

It is important to use minimal amount of hot solvent because if the amount of hot solvent exceeds 25 mL, then it will be more difficult to recover the crystals. Upon cooling there will be very little actual recrystallization.

What do you think would happen to your recrystallization solution if you did not use a preheated filter during the "hot" gravity filtration step

If a pre-heated filter wasn't used during the "hot" gravity filtration step, then when the hot solution touched the cold (or room temperature) filter paper, there is a risk that the crystals would crystallize on the fluted filter paper.

Why is benzyl alcohol a poor choice of solvent for recrystallization of fluorenol?

-its b.p. is too high
-the boiling point of the solvent must be lower than the melting point of the solute

why is acetone a bad solvent?

-low bp and flammable

how do you remove a very soluble contaminant?

-let it recrystallize very slowly

What is the partition coefficient?

-accords distribution of mixture between two immiscible solvents
-KD \= C2/ C1

What solvent do we use in extraction?

-methyl tert-butyl ether (MtBE) bc less flammable than diethyl ether (effective solvent)

Why would you use the liquid-liquid extraction method?

separate compounds making up a mixture using two immiscible solvents

what happens if you add a base (NaOH) to water containing RCO2H?

-solubility of the acid increases
-Turns to RCO2Na, which is a salt and water soluble

How can aqueous NaHCO3 be used to separate two weak organic acids (a phenol and a carboxylic acid)?

aqueous NaHCO3 converts carboxylic acids to their sodium carboxylates (making them more soluble in aqueous layer) but does not have an effect on phenolic compounds (leaves phenol on organic layer)

How do acids & phenols increase in solubility?

-by addition of a base

Why would separation not be possible if aqueous NaOH was used instead of NaHCO3 in a mixture of carboxylic acid and phenol?

Because NAOH would deprotonate both and both would reside in the aqueous base layer

How are amines extracted?

with the addition of acid

Why do we add anhydrous sodium sulfate?

As a drying agent to remove traces of water from the organic layer we dry for \`5 minutes

What do you use to remove the solvent from liquid-liquid extraction?

-a warm sand bath and a gentle stream of air

What is the solute in the extraction experiment?

-Benzoic acid

What layer is the organic layer?

-Top layer, has benzoic and MtBe

If you were trying to separate a sample mixture with base, carboxylic acid, and neutral compound, what would you use?

-Acid

If you were trying to separate a sample mixture with carboxylic acid, phenol, and neutral compound, what would you use?

-requires weak base

What is back extraction?

-adding MtBe to a separated solvent to back extract any compound that should not have gone into the aqueous layer

in a neutral compound, what kind of solvent would we use?

-h2o to dissolve any kind of aqueous dissolving stuff

Extraction scheme for separating an organic base with an organic neutral compound

Dissolve the base and neutral compound in an organic solvent
Add HCl unitl the solution becomes acidic
Separate the organic layer and aqueous layer carefully
Dry the organic layer to obtain neutral compound
Add NaOH and an aqueous solvent to the aqueous layer
Remove the organic layer and evaporate to obtain the base

Extraction scheme for separating an organic acid and phenol

Dissolve acid and phenol in organic solvent
Add NaOH
Separate the organic and aqueous layer carefully
Add HCl to the aqueous layer
Add organic solvent to the aqueous layer
Evaporate the organic layer to obtain acid
Add HCl to the aqueous layer and remove the aqueous
Add organic solvent and NaOH to the aqeous layer
Remove the organic layer and evaporate to obtain phenol

If you aren't sure which layer is which what can you do

To see what layer is what add a small drop of water and put it in the test tube and see where it goes

why should potassium carbonate not be used with some classes of organic compounds?

RNH2 would be a better choice to use with potassium carbonate because if you use rcooh it forms water

What is used to make the TLC plate?

Silica gel, SiO2
-stationary phase

What is the mobile phase in thin layer chromatogahpy?

the solution

What determines the motility of solution?

1. polarity of the solvent (mobile phase
2. polarity of the compound (solute)
3. polarity of adsorbant

\** more polar the solute \= greater affinity it will have for stationary and less it will travel**

What are the two most common adsorbants in tlc?

silica gel and alumina

a compound w stronger interaction w the stationary phase moves slower than a compound with a weak interaction with the adsorbent. True or False?

True

what force causes the mobile phase to rise up the TLC plate?

capillary action

What is Rf?

-retardation factor
-distance traveled by the compound to that of the solvent front
-C2/ C1

What is the solute used in Part A of tlc analysis?

-orthohydryoxyacetophenone and para-hydroxyacetophenon

Why do o- and p hydroxy have a big dif. in mP?

-O hydroxy has hydrogen bonding and a delocalized e- pair \= higher MP

What does an rf of .91 indicate?

- use less polar solvent

what does an rf of .15 indicate?

-use more polar solvent

List in order of decreasing polarity: naphthalene, fluorenol, o-toluic acid

o-toluic acid, fluorenol, naphthalene \=\>
Rf: naphtalene, fluor, o-toluic

arrange the solvents in order of increasing polarity: ethanol - ethyl acetate - n-propanol - dichloromethane - toluene - heptane

heptane-toluene-dichloromethane- ethyl acetate- n-propanol- ethanol

What will elute from the column last?

-acetylferrocene bc. it's more polar and you switch to the more polar solvent (like dissolves like)

why does ferrocene elute from the column first but is the higher of the two spots in TLC analysis?

-because the solvent used in TLC analysis was more non-polar and therefore ferrocene had a stronger interaction with it than acetylferrocene.

What is the absorbent in dry column chromatography?

-alumina

Why is it imporant the sand is level when packing the column?

-there will be portions of the eluent that are not filtered if the sand is slanted

What solvents do you use in column chromatography?

-petroleum ether (nonpolar) and MtBe (more polar)

what force allows the eluent to percolate in column chromatography?

gravity

choosing a column for column chromotography:
diameter of column related to ?
length of column related to ?

diameter : amount of sample
length : difficulty of separation

What is the order of changing test tubes in column chromatograpy?

- yellow orange begins to elute, switch, once yellow collected & becomes colorless, switch, change to MtBe, once red orange band has hit bttom switch, until collected, switch

Why must the temp. of sand bath be no greater than 60 C?

because mp of acetylferrocene is ~80 so may melt

arrange these solvents in order of increasing densities: water, methyl tert-butyl ether, chloroform

MtBE\>H2O\>chloroform

assume a solution of 100mL of water contains 10g of "P" in it. you add 20 mL of diethyl ether to the solution. the partition coerfficient of of water and diethyl ether is 8.5. write down an equation for the partition coefficient and calcluate amount of P in water layer.

Kd\=C2/C1\=8.5

(10-x)/20
\------------- \= 8.5
(x/100)

x\=3.704g

nonpolar solvents are (miscible/immiscible) in water

immiscible\-- diethyl ether, MtBE, and Benzene

polar solvents are (miscible/immiscible) in water

miscible\-- ethanol

column chromatography: you run a column on a mixture of two unknown halogenated alkenes and they elute at the same time. the solvent you used was dichloromethane. what should you do?

use a less polar solvent like hexane first so that it is specific enough to distinguish between the two compounds and then switch to dichloromethane.

column chromatography: you run a column on a mixture of a thiol and an amine and neither elute. the solvent you used was a mixture of of petroleum ether and dichloromethane. what should you do?

slowly increase the polarity of the solvents used until one compound begins to elude from the column and then follow up with a more polar solvent

condensation polymerization

nylon-10,6; reaction of monomers produces a large unit and a small unit like HCl or H2O is eliminated.

how did we synthesize nylon-10,6?

via interface condensation polymerization

chain addition polymerization

polystyrene; uses an initiator that forms a reactive species (benzoyl peroxide is used because it generates free radicals at high temps) that adds monomer and grows rapidly
\>involves a vinyl-type monomer
\>polymers have simple hydrocarbon backbone
\>empirical formula of monomer same as that of polymer

cross-linked polymerization

slime; borate ion coordinated with to the alcohol groups of four PVA molecules. cross-linked bonds break and form easily which allows the elastic substance to exist

monomer

small identical molecules that bond and form larger polymers

repeating unit

identical monomers that bond to form polymers

what is fractional distillation used for?

to separate two volatile liquids with similar boiling points

what is simple distillation used for?

purifying a compound that is

which of the two distillation products is most pure and why?

the more volatile one because as has gone through a serious of continuous transitions between condensate and vapor, each time enriching the vapor in the more volatile product. when the vapor is finally condensed, it is almost pure.

what is the best method to achieve quality separation during distillation?

even heating and a low rate of distillation to mainain thermal equillibrium and a high reflux ratio