Recrystallization and Melting Point Determination of Benzoic Acid

R

ECRYSTALLIZATION

AND

MELTING

POINT

DETERMINATION

OF

BENZOIC

ACID

J.V. DE GUZMAN

DEPARTMENT OFCHEMICAL ENGINEERING

,

,

,

DATEPERFORMED: JANUARY 28, 2015

INSTRUCTOR’SNAME: ALLANKENNETHREGUNTON

ANSWERS TO QUESTIONS

1. How does a fluted filter paper hasten filtration? Why is it

advisable to place a small piece of wire between the funnel and the

mouth of the flask during hot filtration?

Compared to a conventional filter paper, fluted filter paper hastens the

filtration by maximizing the contact area between the filter paper and the

solution and minimizing the contact area between the funnel and the filter

paper. The rate of liquid flow through the filter paper increases because the

flutings of the filter paper permits air to enter the flask along its sides

allowing for pressure to rapidly equalize. Placing a small piece of wire

between the funnel and the mouth of flask, on the other hand, relieves any

pressure increase brought by the hot filtrate. The wire prevents formation of

a solvent seal between the flask and the funnel since such seal will block the

vent of air displaced by the filtrate thus relieving the pressure.

2. Enumerate 2 techniques that can help prevent premature

recrystallization. Explain each item. What can be the consequence if

premature recrystallization was not avoided to occur?

a.) Keeping the filtration set-up warm- the hot solution cools as it runs

through the filter allowing crystallization to occur. Warming the funnel and

the filter paper thus prevents the hot solution from crystallizing during

filtration. Heating the receiving flask helps prevent premature

recrystallization as the filtrate accumulates since this produces vapors that

keeps the funnel and filter paper warm thus preventing crystallization.

b.) Pouring the hot solution in small amounts- It prevents

recrystallization since it lessens the solution's time in the filter preventing it

to cool and crystallize. If crystallization already occurs, pouring small amount

of the hot solvent dissolves the solids.

If premature recrystallization was not avoided, it gives a lower percent

yield and a negative error since the sample that prematurely recrystallized

cannot be accounted in the total yield (unknown mass) and thus considered

to be lost sample.

3. A mixture containing 2.0 g of A and 8.0 g of B is to be separated

and the components purified by one crystallization from 100 mL of

solvent. Their solubilities are given in the table below:

Solvent

A

B

cold

hot

cold

hot

Ethanol

0.5

10.0

5.0

20.0

Acetone

5.0

20.0

0.1

8.0

water

0.6

15.0

0.5

10.0

a. What solvent would you choose? Support your answer by

means of a schematic diagram.

The appropriate solvent to be used is acetone since it satisfies

the requirement for an ideal solvent: dissolve the solute of interest at

high temperature and sparingly only at lower temperature. The desired

solute should also recrystallize specifically from the solvent when

cooled. Acetone would effectively dissolve both solutes but it will

selectively recrystallize solute A upon cooling.

100 ml of hot acetone solution can dissolve 20 g A and 8 g B

100 ml acetone x

20 g A

100 ml acetone

=20 g A

100 ml acetone x

8.0 g B

100 ml acetone

=8.0 g B

100 ml of cold acetone solution can dissolve 5 g A and 0.1g of B

100 ml acetone x

5 g A

100 ml acetone

=5 g A

100 ml acetone x

0.1 g B

100 ml acetone

=0 .1 g B

Mixture of 2 g A and

8 g B dissolves in

hot acetone solution

Cool solution.

7.9 g of B

crystallizes out

2 g of A and 0.1 g

of B remains in

the mother liquor

b. Theoretically, what is the purity of A and B after one crystallization?

The recrystallized B is 100% pure though its yield is only 98.75%

(based from 8 g on the sample initially). On the other hand, compound A is

only 95.24 % pure.

4. A mixture of three compounds, A, B and C, is to be separated and

purified by crystallization. Their solubilities in g/100 mL ethanol are

given below.

Solvent

A

B

C

Cold

1.05

5.6

4.2

Hot

20.5

2.3

3.5

a. If a mixture containing 4.0 g each of A, B and C is recrystallized from 100

mL

UNDISSOLVED

CRYSTALS: 1.7

g B and 0.5 g C

Heat

solution

MIXTURE: 4.0

g A + 4.0 g B +

4.0 g C

_{MOTHER LIQUOR:}

1.05 g A + 2.3 g B

+3.5 C

MOTHER

LIQUOR: 4.0

g A + 2.3 g B +

3.5 g C

Cool

solution

_{RECRYSTALLIZED:}

2.95 g A

[1]University of Sydney Faculty of Science. Skills. [Online] 2014.

https://scilearn.sydney.edu.au/fychemistry/LabManual/Skills.pdf

(accessed February 9, 2015).

[2]University of Calgary Department of Chemistry.Organic Laboratory

Techniques.

[Online].

2014.

http://www.chem.ucalgary.ca/courses/351/laboratory/filtration.pdf

(accessed February 9, 2015).

[3]Pavia, D., Kriz, G., Lampman, G., Engel, R. A Microscale Approach to

Organic Laboratory Techniques. Chapter 11: Crystallization: Purification

of Solids. Brooks/Cole: California, 2013. pg 684.

[4]Gilbert, J. and Martin, S. Experimental Organic Chemistry 5

_{ed. Chapter}

2: Techniques and Apparatus. Brooks/Cole: California, 2013. pg 66.

[5]University of Wisconsin-Madison Department of Chemistry.

Recrsytallization.

[Online].

2014.

http://chem.wisc.edu/deptfiles/genchem/Chm346/pdf/recrystallization.p

df (accessed February 9, 2015)

[6] University of Toronto Department of Chemistry.Recrystallization.

[Online].2014

http://www.chem.utoronto.ca/coursenotes/CHM249/Recrystallization.pd

f (accessed February 9, 2015).

Portland Community College. Extraction of Caffeine from Tea Leaves.

[Online].

2014.

http://spot.pcc.edu/~chandy/241/CaffeineExtractionCH2CCl2.pdf

(accessed February 9, 2015)

http://www.xula.edu/chemistry/documents/orgleclab/Caffeineprocedure.pdf

http://infohost.nmt.edu/~jaltig/Caffeine.pdf

1. Solvents for extraction experiments should have desirable properties. Here is a list. For both liquid-liquid and liquid-solid extractions, the solvent should have a relatively low boiling point for easy removal by evaporation; and it should not react with any of the substances present (unless you are performing an acid-base extraction). In liquid-liquid extraction, the compound being extracted should have a favorable distribution coefficient in the extracting solvent; in liquid-solid extraction, the solvent must dissolve the compound being extracted. 2. The caffeine extraction procedure has several features that may seem pointless at first.

Here are some explanations.

Caffeine is an alkaloid, an organic base. Sodium carbonate also a base, and it is added in the first extraction to make sure that the caffeine remains in the free base form (that is, to prevent it from reacting with any acids that may be present). An emulsion is a suspension of one liquid as droplets in another (the two liquids must be insoluble in one another). Emulsions are almost always undesirable. To avoid them, you can shake mixtures of insoluble liquids gently and add salt to aqueous layers. You will use a centrifuge in this experiment break up emulsions once they form. The visualization technique used in the TLC portion of this experiment involves the use of ultraviolet (UV) light. The caffeine absorbs the UV light and gives off visible light. This phenomenon is called fluorescence. 3. The equation for calculating the percentage of caffeine in tea is as follows:

amount of caffeine recovered

percentage of caffeine = --- x 100% weight of tea There are some systematic errors in this experiment as we perform it. For example, you do

not weigh the tea, you weigh the tea bag, which has not only tea but also string and

paper in

its weight. When you squeeze the tea (liquid) out of the tea bag, some of the liquid (containing caffeine) remains behind in the wet bag. The distribution coefficient of caffeine between water and dichloromethane is not perfect; some caffeine will remain behind in the water. Dichloromethane evaporates rapidly, cooling the watch glass as it does; water can condense on the cool watch glass, influencing the weight of your final product.

http://www.xula.edu/chemistry/organic/No...

Caffeine can be extracted easily from tea bags. The procedure one would use to make a

cup of

tea, simply "steeping" the tea with very hot water for about 7 min, extracts most of the caffeine.

There is no advantage to boiling the tea leaves with water for 20 min. Since caffeine is a white, slightly bitter, odorless, crystalline solid, it is obvious that water extracts more than

just caffeine.

When the brown aqueous solution is subsequently extracted with dichloromethane, primarily caffeine dissolves in the organic solvent, leaving the other substrates in the aqueous layer. Evaporation of the solvent leaves crude caffeine, which on sublimation yields a relatively pure product. When the concentrated tea solution is extracted with dichloromethane, emulsions can form very easily. There are substances in tea that cause small droplets of the organic layer to remain suspended in the aqueous layer. This emulsion formation results from vigorous shaking. To avoid this problem, it might seem that one could boil the tea leaves with dichloromethane first and then extract the caffeine from the dichloromethane solution with water. In fact, this does not work. Boiling 25 g of tea leaves with 50 mL of dichloromethane gives only 0.05 g of residue after evaporation of the solvent. Subsequent extractions give less material. Hot water causes thetea leaves to swell and is obviously a much more efficient extraction solvent. An attempt to sublime caffeine directly from tea

leaves also was unsuccessful.

Procedure

stick.

Bring the water to a boil on the hotplate, remove the boiling stick, and brew a very concentrated

tea solution by immersing a tea bag (2.4 g tea) in the very hot water for 5 min. While wearing latex gloves, squeeze as much tea from the bag as possible after it cools enough to handle. Be careful not to break the bag. Again bring the liquid to a boil, and add a new tea bag to the hot solution. After 5 min, remove the tea bag and squeeze out as much water as possible. This can be done easily on the Hirsch funnel. Rinse the bag with a few milliliters of very hot water, but be sure the total volume of aqueous extract does not exceed 12mL. Pour the extract into a 15-mL centrifuge tube, and cool the solution in ice to below 40°C (the boiling point of dichloromethane).

Using three 2-mL portions of dichloromethane, extract the caffeine from the tea. Cap the tube

and use a gentle rocking motion to carry out the extraction. Vigorous shaking will produce an intractable emulsion, while extremely gentle mixing will fail to extract the caffeine. If you have ready access to a centrifuge, the shaking can be very vigorous because any emulsions formed can be broken fairly well by centrifugation for about 90 sec. After each extraction, remove the lower organic layer into a reaction tube, leaving any emulsion layer behind. Dry the combined extracts over anhydrous sodium sulfate for 5 or 10 min in an Erlenmeyer flask. Add the drying agent in portions with shaking until it no longer clumps together. Transfer the dry solution to a weighed 25-mL filter flask containing a boiling chip, wash the drying agent twice with 2-mL portions of dichloromethane, and add the washes to the test tube or filter flask. Evaporate the dichloromethane to dryness by carefully placing the tube/flask into a hot water bath (NOT ON A HOTPLATE) and allowing the ether to boil away. You can facilitate the removal of the vapors by holding a Pasteur pipette connected to a water aspirator just above the surface of the boiling liquid. Be careful not to touch the liquid or your product will be drawn into the pipet and lost down the drain. Continue the evaporation until no trace of the smell of dichloromethane can be detected. The residue remaining in the filter flask will be crude caffeine (determine its weight) that is to be purified by sublimation.