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13 Dec 2019

Â· Question 2

Following is a list of mistakes commonly made during recrystallizations. Match each of them with their unfortunate results.

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Selected Match

don't premoisten the filter paper in a Buchner funnel

E.

weight of crystals will be wrong (inaccurate, higher yield)

rinse crystals with solvent that isn't ice cold

A.

water can be drawn into the evacuated filter flask

to stop a vacuum filtration, turn off the water

B.

crystals may pass through the funnel with the filtrate

vacuum filter a hot solution

C.

solvent will rapidly boil under reduced pressure

forget to remove the boiling chips

D.

crystals will dissolve (lower yield)

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Calculate the mass of zinc you will use. One equivalent of zinc is the mass needed to react with the amount of stilbene dibromide used. Give your answer in grams and show your working below.

Reaction = C14H14Br2 + Zn ----> C14H12 + ZnBr2

Method= In this procedure, you will recycle the stilbene dibromide that you made last week. The solvent and reagent are non-toxic and the procedure is neither time nor energy intensive. Note that the by-product, zinc bromide, can cause burns.

1. Into a 100 mL round bottom flask place 0.5 g stilbene dibromide, 10 mL ethanol and a stir bar. 2. Add 1.25 equivalents (i.e. 1.25 fold excess) of zinc powder

3. Reflux and stir the reaction using a hot water bath (at 90-1000 C) as described in Padias.

4. When all the solid has dissolved (after 30-60 minutes or so), preheat a fluted filter paper and filter funnel with 3 mL boiling ethanol in a 50 mL Erlenmeyer flask on a hot plate.

5. Remove the reflux condenser. Using the clamp as a handle for the hot round bottom flask, filter the reaction mixture while keeping the solution and filter paper hot.

6. Allow the filtrate to cool to room temperature. Crystals should start to form

7. Cool the flask in ice to bring down more crystals

8. Collect the crystals by vacuum filtration. Rinse with a small volume of ice-cold ethanol. Continue to draw air through the crystals to dry them.

9. Record the yield.

10. Record the melting point of your product (which should be above 1000 C) and a mixed melting point with stilbene

11. Compare your product with stilbene and stilbene dibromide by TLC.

Provide the mechanism for the haloform reaction in part B of this experiment

Also explain how the added acetone in step 4 destroys any unreacted hypochloride.

Need isolated yield in moles, theoretical yield in moles and grams, and percent yield!

synthesis of Dulcin, 4-Ethoxyphenylurea

1. In a 50 mL round-bottomed flask, place 1.4 mL (1.4 g, 0.01 mole) of p-phenetidine, 2.4 g (0.04 mole) of urea, and 5 mL of distilled water.

2. To this mixture, add 1 mL of concentrated HCl and 5-6 drops of glacial acetic acid.

3. Fit the round-bottomed with a reflux condenser, add a boiling stone, swirl the material well and boil the mixture vigorously until the reaction is complete (30 min. maximum). At first, the dark colored solution remains clear, but in 15-20 minutes the product begins to separate rapidly. When the mixture sets to a semi-solid crystalline mass, stop the heating immediately.

4. After cooling the round-bottomed flask to room temperature, add 3-4 mL of cold water, stopper the flask tightly and shake it thoroughly to make a slurry of crystals.

5. Collect the product by vacuum filtration, wash it with cold water and dry the crystals by drawing air through them while they are still in the Hirsch funnel.

6. Recrystallize the crude product from boiling water (30 mL/g of product) using decolorizing charcoal to remove the brown color, as follows:

a. In a small beaker (100 mL) bring to a gentle boil the crude dulcin in the appropriate amount of water (donât forget the boiling chip).

b. Add a small amount of decolorizing charcoal over the boiling liquid and keep boiling for 1 minute.

c. If the liquid (not the dark, floating particulates) is colorless, filter the hot mixture through a pleated paper filter, placed in a stem less glass funnel, into another beaker. If the liquid is still pink, repeat step b.

7. Chill the filtrate in an ice bath before vacuum-filtering the purified crystals. Dry the product, weigh it, determine a melting point.

For step 5 the product was weighed and turned out to be .32 g then after finishing the lab with step 7 the final product weighed in at .07 g

Isolated Yield _____ moles _____ grams

Theoretical Yield _____ moles _____ grams

Percent Yield ______ %

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