Obtain approximately 3.0 g of analgesic mixture from the instructor. Weigh the v

Question

Obtain approximately 3.0 g of analgesic mixture from the instructor. Weigh the vial and its contents - it will be reweighed later when empty to obtain the weight by difference. Set up for gravity filtration with a tared (weighed) piece of filter paper and a 125 mL Erlenmeyer flask. Use several portions of dichloromethane to dissolve as much of the solid as will go into solution and transfer the contents to the gravity filtration apparatus. Use a total of 50-60 mL dichloromethane; with each portion stir thoroughly and transfer as much of the solid (starch) to the filter as possible. Set the filter paper aside to dry (not in a desiccator) until the next period, at which time it will be weighed to obtain the weight of starch and discarded. If stopping at this point, use a cork (not a rubber stopper) to tightly seal the flask and set it near the front of the locker (the rear of the lockers gets hot due to the steam pipes).

Next the aspirin is extracted as its sodium salt. Transfer the filtrate (which is now a dichloromethane solution of aspirin, organic unknown, and tetraglyme) to a 125 mL separatory funnel and extract sequentially with two 25 mL portions of 1 M NaOH as follows. The dichloromethane layer is on the bottom and must be drained before decanting the upper sodium hydroxide layer. Then return the dichloromethane solution to the separatory funnel and extract with a fresh 25 mL portion of 1 M sodium hydroxide. Drain the dichloromethane layer into a 125 mL Erlenmeyer flask; cork

and set aside. Decant the second sodium hydroxide extract into the flask with the first sodium hydroxide extract - the combined extracts are 50 mL in volume. Because aspirin hydrolyzes in base the aqueous hydroxide solution must be worked up immediately.

With stirring, add 10 mL of 6 M HCl in portions to the combined aqueous hydroxide extracts. Check that the pH is below 2 using pH paper. Chill the mixture for 10-15 minutes in an ice bath while setting up for vacuum filtration. Isolate the aspirin on a Buchner funnel, washing with two small portions of chilled water. Air dry using vacuum for five minutes prior to storage in a tared vial inside a jar desiccator.

Finally, the crude organic unknown is isolated by evaporation of the dichloromethane. Place the organic solution in a 125 mL side arm flask and evaporate under vacuum. Some heat may be applied to offset the cooling due to evaporation, but do not heat much above room temperature. Once the solid has formed, leave under vacuum for an additional five minutes at room temperature. Using a spatula, scrape the solid out into a tared vial and dry in a desiccator between periods, or proceed directly to the recrystallization (Part II).

PART II:

Set aside three melting point samples of the crude unknown. Weigh the remaining solid and calculate how much hot water would be needed to dissolve it if it is acetanilide, and how much would be needed if the unknown is phenacetin. The following solubility data will be useful:

Compound

Sol. in 0 °C Water (g/100 mL)

Sol. in 100 °C Water (g/100 mL)

Acetanilide

0.54

5.0

Phenacetin

0.076

1.22

You should realize the steam cone will not quite heat your solution to 100 °C, so you will observe somewhat lower solubility than these data indicate.

Recrystallize the unknown from hot water using a steam cone for heat as described below. It is helpful to have some boiling water handy for the washes during hot gravity filtration: for this purpose attach a utility clamp to a 100 mL round bottom flask charged with three or four boiling stones and 50 mL DI water (which is more than enough) then heat to a slow boil on the thermowell.

Place the crude unknown in a 125 mL Erlenmeyer flask. Since it is important not to use too much water (why?) assume your unknown is the more soluble compound and add this amount in portions with continual swilrling and heating on the steam cone. The water you add need not be preheated, but allow time for it to warm after each addition. You should see the solids dissolve a little more with each addition. If a small amount of solid does not dissolve further upon addition of water then this represents a solid impurity which will be removed during the hot gravity filtration. Keep the solution heated on the steam bath while setting up for hot gravity filtration using a 125 mL Erlenmeyer flask and a wide stem funnel; the filtration apparatus should be heated on the steam cone also. Hot gravity filter using a couple of minimal-volume washes with the boiling water to disslove crystals which may form on the filter paper. You can estimate the results of using too much water in this process using the solubility data above. Note also that the solubility of your compound is a clue to its identity.

After the hot gravity filtration, set the filtrate on the benchtop to cool slowly. This would be a good stopping point. Prior to

isolation of the solid, cool the mixture in an ice bath for 10-15 minutes to maximize recovery. Collect the crystalline solid by vacuum filtration, washing with a small volume of chilled water. (NOTE: measure the total volume of filtrate before you discard it – recall pre-lab problem.) The material is air-dried on the Buchner funnel under vacuum for several minutes, and transferred to a tared vial for drying in a jar desiccator.

Note: Sometimes when far to much water is used no solid precipitates, even after chilling and scratching of the flask. In this case salting out by addition of sodium chloride in the ratio 20 g per 100 mL is often effective.

Perform five melting point and mixed melting point analyses as follows:

one mp of crude unknown (three samples were prepared so spares are available)

two mp's of purified unknown
one mp of unknown mixed with acetanilide one mp of unknown mixed with phenacetin

Present the data in a table in the observations section of the lab report.

Compound

Sol. in 0 °C Water (g/100 mL)

Sol. in 100 °C Water (g/100 mL)

Explanation / Answer

As far as it is understood,you need to separate the components from the mixture of aspirin and an unknown organic compound.The unknown organic needs to be identified as well using their solubility data and the melting point measurements done by you at the end of your experiment.

It is believed that your unknown could be phenacetin or acetanilide so you measure the solubility of your unknown in DI water at 0 degree C and 100 deg C after separation and compare them to that of acetanilide and phenacetin.

The hot water solution of your unknown orgnic compound is then cooled in ice-bath for recrystallization so that you get the pure form of your unknown.

Now, as far as the procedure for the experiment is concerned ,i will brief you about it.

You will need to keep the data table for various organic compounds handy.It should be -a table of molecular weights and physical properties of compounds including solvents and should include acetanilide, phenacetin, apsirin, starch, dichloromethane, and tetraglyme with reported soluabilities at 0 degrees celcius and at 100 degrees celcius in units of g/100mL.

Now separation needs to be done,using solvents like tetraglyme (a polar aprotic solvent that dissolves aspirin with -polar -COOH group. Add dichloromethane in parts to dissolve the organic compound as well.Dichloromethane is imiscible with aqueous NaOH which is to be used for extracting aspirin as sodium salt.

After aspirin is separated in its aqueous solution as it sodium salt ,HCl is added to get back aspirin by the salt hydrolysis,

The solid aspirin is obtained by filtration any drying

The organic solvent you get contains the unknown compound.The solvents are required to be evaporated to get the crude compound.Then it is dissolved in minimum amount of water and in hot water and its solublity noted.You need to note the volume of the filtrate after filtering the pure compound from the solution.So that you can calculate the solubility of the pure compound.(Solubility of compound =amount of compund/volume of filtrate )

This solubility measurement can be compared to that of the compounds in the table.You can identify the unknown if you get a match for your solubility.

Also measure m.pt of crude, mp of pure,mp of pure compound+acetanilide, mp of pure +phenacetin

If for (mp of pure compound+acetanilide, mp of pure +phenacetin) there is no change in mp ,then it means your pure compund and the one you mixed is the same.

Related Questions

Navigate

• Browse (All)
• Browse O
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.