Calculate the degrees of unsaturation for each compound

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LAB 4: EXTRACTION – ACID/BASE REACTIONS

PRE-LAB 4:

Complete the pre-lab assignment BEFORE entering lab. Your instructor will give you instructions about how it should be handed in.

1) Complete the safety section for this experiment.

2) Is NaHCO

3

a strong or weak base? What about NaOH?

3) Rank the chemicals in the background information section from strongest acid to weakest acid.

4) Briefly define the words

aqueous

and

anhydrous

. What is the solvent in the

aqueous

solution?

5) Look up the melting points of benzoic acid, ethyl

p

-aminobenzoate, and acetanilide.

PURPOSE:

You will learn how to separate complex mixtures using differential distribution of substances in two immiscible liquids and then you will perform a separation of a strong acid, a weak acid and a neutral substance via acid-base liquidliquid extraction.

BACKGROUND INFORMATION:

Some solid mixtures are hard to purify by crystallization techniques especially when the mixture is complex and no one substance is present in large excess. Liquid mixtures can sometimes be separated by extraction techniques when distillation methods

(see a later experiment) would be more difficult. Extractions are very often done as the first steps in the purification of a product in a synthetic reaction. This is generally called a "workup procedure".

Extractions can separate components of a mixture because the substances distribute themselves differently in the two immiscible liquids depending on their distribution coefficients. The distribution coefficient is the ratio of the concentration of the substance in solvent A divided by the concentration of the substance in solvent B. It can be shown mathematically that multiple smaller extractions are better in separating components of a mixture than one larger extraction. Hence, we will do two "washes"

(another term often used for extraction) instead of one. The term wash is often used in older literature to mean extraction. Another consideration is mechanical loss. This is the loss of material by not completely placing it in the desired container. Any solution that is left in the separatory funnel will have some solute dissolved in it which will be lost in the process. To minimize mechanical loss, one often adds a small amount of pure solvent to rinse the container and then adds this rise to the solution that contains the bulk of that substance.

Since there will be two layers, you must determine which layer is which. The less dense layer is the upper layer and the denser layer is the lower layer. So a mixture of water and diethyl ether will have the ether as the upper layer and the water as the lower layer. However, when substances are added to a solvent, the density is changed (for example, salt water is denser than distilled water). Smelling a layer is not an effective way to determine which layer is which as each solvent will be partially miscible in the other. In the above case, both layers will smell like ether. A good method to determine

which layer is the aqueous layer is to add a few drops of one layer to a few milliliters of water. If the drops dissolve, that layer is the aqueous layer. If they don’t dissolve, that layer is the organic layer.

O

C

OH

OCH

3

OH

H

3

C

C

H

3

C

CH

3

H

3

CO

Benzoic Acid

mp 120 o

C

First Neutralization Reaction:

4

-t butylphenol

mp 101 o

C

1, 4-dimethoxybenzene

mp 57 o

C

O

C

OH

+ NaHCO

3

O

C

O

-

Na

+

+ H

2

CO

3

(aqueous layer)

(organic ether layer)

Second Neutralization Reaction:

OH

H

3

C

C

H

3

C

CH

3

(organic ether layer)

+ NaOH

H

3

C

C

H

3

C

CH

3

(aqueous layer)

O

-

Na

+

+ H

2

O

Very often changing the pH of the aqueous solvent can have dramatic effects on the distribution coefficients of substances as will be noted in the experiment below. For example, benzoic acid is reasonably soluble in ether and not very soluble in water.

However, benzoic acid is converted to sodium benzoate in a 5% sodium bicarbonate solution (see the first neutralization reaction on the previous page). Sodium benzoate is very soluble in this aqueous solution and insoluble in ether. Hence, we can effectively move benzoic acid between ether and water by pH adjustment as will be done in today’s experiment.

An extraction is usually done using a separatory funnel which is a pear shaped glass container with a stopper at the top and a stopcock at the bottom. The mixture to be separated is placed in the funnel along with the two extracting solvents. The closed funnel is then shaken (with periodic venting) to thoroughly mix all the contents and allowed to stand so that the layers can separate. If the density of the two solvents is close or if there are any emulsifying materials present, the layers may not separate which poses an experimental difficulty. Once the layers separate, the stopper is removed and the lower layer is drained through the stopcock. If the upper layer is to be removed it is poured from the top of the funnel. Students often forget to remove the stopper before opening the stopcock. The stopper keeps air from entering the top and hence very little or nothing will drain from the bottom. Students then think the stopcock is clogged but this in not the case. You must always remember to remove the stopper before the stopcock is opened.

EXPERIMENTAL PROCEDURE:

DO NOT DISPOSE OF ANY SAMPLES UNTIL YOU ARE COMPLETELY DONE!

1) Dissolve 300 mg of a 1:1:1 mixture of benzoic acid, 4-t-butylphenol and 1,4dimethoxybenzene in 25 ml diethyl ether.

2) Place the solution in a separatory funnel. Be sure the bottom stopcock is closed before pouring the solution in the funnel. Add about 10 ml of an aqueous 5% sodium bicarbonate solution (immiscible in ether) through the top of the separatory funnel.

Question

: Do you see two layers? What is the top layer and what is the bottom later?

3) Put the glass stopper in place to seal the container. Holding the stopper tightly, invert the funnel and open the stopcock to vent the carbon dioxide and volatile ether that have contributed to a pressure buildup. Close the stopcock and shake to mix the contents.

Open the stopcock to vent the pressure buildup again. Repeat this process of shaking and venting a few times and then place the separatory funnel in a ring stand in the hood to allow the layers to separate.

Question

: Did you observe bubbling? What is the likely cause of the bubbles?

4) Once an interface is seen between the two layers (generally 10 to 50 seconds) remove the stopper and drain off the lower aqueous layer into a labeled small beaker. Leave the ether layer in the separatory funnel and repeat this process with a fresh 10 ml of 5%

sodium bicarbonate solution. When this solution is removed from the separatory funnel combine the two 10 ml portions. This solution will be used later to recover the benzoic acid that was present in the original mixture which has been now separated from the other two components which remain dissolved in the ether. Again, be sure to leave the ether solution in the separatory funnel.

Question

: What is happening in the sep funnel? What compound(s) is/are now in the organic layer? Which is/are in the aqueous layer flask?

5) To separate the 4-t-butylphenol which is still in the ether layer with the 1,4dimethoxybenzene, add 10 ml of aqueous 5% sodium hydroxide solution (see the second neutralization reaction shown earlier) to the ether layer and extract the two layers as done above. Place the 10 ml of the sodium hydroxide solution in a labeled small beaker.

Repeat this process with another 10 ml of fresh sodium hydroxide and then combine the two 10 ml portions as was done above. The 20 ml solution will contain sodium 4-tbutylphenoxide which will be used later to get the pure sample. The ether solution now has only the 1,4-dimethoxybenzene in it (along with some dissolved water which will have to be removed).

Question

: What is happening in the sep funnel? What compound(s) is/are now in the organic layer? Which is/are in the aqueous layer flask?

6) At this point, you have separated the three substances from each other but you must do some work to purify them. (IMPORTANT: Clean the spearatory funnel and leave it on the bench –vmake sure the funnel is NOT stored with the stopper in the funnel.) To obtain the benzoic acid, very carefully and slowly add concentrated hydrochloric acid to the 5% sodium bicarbonate/sodium benzoate solution from above. Gas will be evolved as the white benzoic acid precipitates. Record the amount of HCl needed in your notebook.

TO TEST pH: Obtain a pH testing strip. Rip it into small squares and place them on a clean dry watch glass. Try adding about 3 drops of acid to the solution at a time and stir carefully with a glass stir rod. Use the glass stir rod and place a small drop of the solution on a square of the pH paper and record the color change. Use the color chart to determine the pH of the solution. After you have used all of the pH paper squares on your watch glass make sure it is clean and dry before repeating the process.

NOTE: BE CARFUL when you dispose of any acidic solutions – make sure they get placed in the correct waste container.

7) Test the pH with pH paper to be sure the solution is acidic and then set the flask in an ice bath. After the solution has become acidic suction filter the benzoic acid, dry it, weigh it, and determine its melting point. The reported melting point for benzoic acid is

122-123 degrees C.

Question

: What is the purpose of adding the acid? What reaction took place?

8) To obtain the 4-t-butylphenol, carefully add concentrated hydrochloric acid to the 5% sodium hydroxide/sodium 4-t-butyl phenoxide solution from the extraction. (Try 10 drops at a time for this sample) The product may ‘oil out’ (which means it is separated from the aqueous solution as a liquid instead of as a solid) and crystallize slowly over time. After it solidifies and has been cooled in an ice bath, filter by suction, dry it, weigh it, and determine its melting point which should be about ___________ºC.

Question

: What is the purpose of adding the acid? What reaction took place? Why was more acid needed for this sample?

9) The final compound, 1,4-dimethoxybenzene, is obtained by adding solid anhydrous sodium sulfate (a drying agent) to the ether solution to remove dissolved water. Add the drying agent until it not longer clumps together but do not add too much. Your Instructor will assist you in determining the appropriate amount.

Question

: No acid this time, why?

10) After letting this stand for 5 to 10 minutes, gravity filter the solid sodium sulfate through a small piece of cotton or glass wool into a pre weighed 50 ml beaker. The clear ether solution is then subjected to a slow stream of air to evaporate the ether. The pure

1,4-dimethoxybenzene remains in the glass container.

11) Weigh the container and solid to obtain the weight of the solid. Take the melting point of the solid which should be about __________ºC.

12) Dispose of your waste carefully. Place the 3 products in the appropriate solid waste containers. Put any organic solvents in the organic solvent waste container. Place acidic aqueous waste in the aqueous waste container.

IMPORTANT INFORMATION ABOUT THE REPORT:

A full report is required. From the weight of starting material used and the weights of each of the pure compounds obtained, calculate and report the percent recovery of each component (assume there are 100 mg of each component). The percent recovery of component A would be the weight of component A divided by the weight of the starting component in the mixture times 100%. Be sure to report the correct measurements and the correct number of significant figures. In an ideal world, the sum of your three percent recoveries should add up to 300%. Add your three percents and discuss how your results compare to 300%. Also, do a total percent recovery for all three components.

Explain your results. Also, report the melting points of each recovered substance and compare to the literature values. Show each reaction that took place during this experiment.

Questions to consider in your report:

Why was crystallization a poor choice as a purification technique for this mixture?

How was acid-base chemistry used to isolate the components of a mixture?

How did we exploit the pKa differences of the compounds?

When you performed the isolations of the aqueous layers, what caused a precipitate to form?

For the products you isolated: How well did the melting points of the recovered compounds match the literature values? How pure do you think your components are based on melting point? If your separated components are not pure, what are the likely impurities?

END OF EXPERIMENT.

© 2011 Carberry and Carreon

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