Sample Lab Report

5/20/2018 Sample Lab Report

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Patrick Apipanyakul

Ben Furlan

September 7, 2011

Recrystallization of Impure Naphthalene

Introduction

Recrystallization is the preferred method for purification of organic solids with less impuritybecause because some of the impurity would co-crystallize.1The most important aspect of therecrystallization technique is the selection of the solvent. A solvent, which works for onerecrystallization, may be totally unsuitable for purification of a different compound because ofthe variation of solubility of different organic compounds in these solvents.2Some characteristicsdesirable for a recrystallization solvent are that the substance to be purified must have a highsolubility in the solvent at its boiling point, and a significantly diminished solubility at lower

temperatures.3

The solvent should be able to dissolve the impurities at all temperatures.2,3

Andone of the most important is that the solvent must not react with the compound to be purified.3

The seven steps of recrystallization

The process of recrystallization can be broken into seven discrete steps: (1) choosing the solventand solvent pairs; (2) dissolving the solute; (3) decolorizing the solution with pelletized Norit;(4) filtering suspended solids; (5) recrystallizing the solute; (6) collecting and washing thecrystals; and (7) drying the crystals.2These steps can be categorized into three distinguishablepartsheating, cooling, and drying. First of all, it is known that the solubility of mostcompounds in a given solvent will increase at elevated temperatures. Therefore heating is used to

dissolve the sample. It is also known that the solubility near the boiling point greatly increases;thus it is imperative to bring the solution to a boil.4Furthermore, it is important to keep thevolume of the solvent to the minimum since recrystallizations from dilute solutions usually leadto poor yields of recovery of the material.5In addition, this step usually uses a boiling chip orboiling stick in heated reactions since a solution can become superheated or heated above itsboiling point without actually boiling with almost explosive violence, a process called bumping.2To prevent this from happening, a wood applicator stick can be added to the solution. Air trappedin the wood comes out of the stick and forms the nuclei on which even boiling can occur.5

During the cooling step, the molecules of a given compound will be assembling into aggregateswith the molecules that have a similar nature. These aggregates will grow in size and eventually

will be deposited from solution to form a solid with a particular crystal lattice.

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Other types ofmolecules that are not capable of engaging in the same types of interactions with the aboveaggregates (and/or possess a different size, shape) will be excluded from the solid and, ideally,will remain in the solution. To maximize the amount of product that crystallizes out of solution,the solution is cooled from boiling to 0oC. If only cooling this solution to room temperature, forexample, instead of 0oC, the result is lower percent recovery yield.2At room temperature, not asmany crystals would form as they would at 0oC. The colder the solution, the more crystals willcome out of solution.


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For the last part, drying of the crystals will rid of the solvent molecules to produce the desiredcompound in a purer form than the initial solid. The drying step is usually done either on air overa prolonged period of time or under a vacuum. For air- and moisture-sensitive materials, thedrying should be done under vacuum, and the solid should be stored in a desiccator.6

Solvent selection

Obviously, the most crucial step is the choice of an appropriate solvent. There are few empiricalrules that can be used as a guide to select a proper solvent, but for the most parts, the selection ofthe optimal recrystallization solvent is a trial and error method.4The age-old rule is the keyLike dissolves like. Also, the solid should be insoluble or only sparingly soluble in a coldsolvent, and completely soluble in a hot solvent.5The impurities should have a differentsolubility profile in the particular solvent than the compound of interest.3The melting point ofthe compound should be higher than the boiling point of the solvent.4Finally, given the choice ofsolvents, the one with a lower boiling point will be a better option.5

Decolorizing agents

On a number of occasions, the presence of highly colored impurities will significantly contributeto the color of the compound. Colored impurities can be removed by the addition of decolorizingcarbon (charcoal) to the hot, yet not boiling, solution.5The charcoal has a very fine surface,which is filled with air. Once it is placed in a solution, the air is displaced by the solventmolecules; at boiling temperatures, the displacement process is extremely rapid, which usuallyresults in frothing of the solution and usually leads to the loss of the compound and potentialinjury.4After the charcoal is added, the solution is heated at or near the boiling point for severalminutes, while continuously being swirl to avoid bumping. Subsequently the charcoal is removed

by gravity filtration, and the filtrate is allowed to cool to yield the crystalline material. Note thatthe traces of decolorizing charcoal can cause the gray color in the pure sample afterrecrystallization.2

Filtration

After the crystallization is finished, the crystals should be separated from the liquid. In principle,the supernatant solution can be decanted from the solid.4However, a loss of the solid as well asan incomplete removal of the liquid, which might contain the impurities, take place. A bettertechnique to separate the solid from the liquid is by filtration. Fundamentally, filtration is aprocess that involves passing the suspension of the solid in the liquid through a membrane that is

permeable only to a liquid.

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The filtrate, often called mother solution or mother liquor, iscollected in a separate receiving flask, whereas the solid remains on the membrane.4there aretwo types of filtration used for recrystallization: gravity and vacuum filtration.2Gravity filtrationis the best way to remove insoluble material is to filter the hot mixture, while the desired materialis dissolved. Insoluble debris will be trapped on the filter paper. This technique is mostly used toremove impurities, drying or decolorizing agents.1Vacuum filtration, on the other hand, isusually used to collect solids from the solvents after either precipitation of recrystallization.2However, this technique should not be used for filtering hot solutions. During filtration some of


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the solvent is evaporated, which cools down the solution. Since application of vacuum willenhance the rate of solvent evaporation, the solution will cool off much faster than at ambientpressure. This can lead to premature crystallization of the solid on the filter paper. The cloggingof the filter paper will occur, which will significantly slow down the filtration process.4Thus,one should keep in mind that vacuum filtration is faster, sometimes much faster, but carries an

increased risk of losing some of the solids if the person doing the filtering is not careful. Itshould be used only to collect insoluble solids, because applying a vacuum will sharply increaseloss of the solvent from evaporation.1,4

As one can tell, gravity filtration rather than suction filtration is used to remove suspendedimpurities and charcoal from a hot solution. During suction filtration of a hot solution,evaporation of the solvent takes place with attendant evaporative cooling. The combination ofthese two effects causes crystals to form and clog the Bchner or Hirsch funnel. However,vacuum filtration is a much more efficient method for removing solvent from the crystals than isgravity filtration.

Purposes:From the concepts and theories, one should learn that recrystallization is a purification techniqueused for solid compounds. In addition, the keys to recrystallization are: 1) Finding a solvent thatwill dissolve your sample while hot; 2) That same solvent should not dissolve your sample whilecold; and 3) The cold solvent must keep impurities dissolved in it forever.

Other techniques used in this lab

Thin-layer chromatographyor TLC will also be used in this experiment. TLCis a solid-liquidform of chromatography where the stationary phase is normally a polar absorbent (solid) and themobile phase (liquid) can be a single solvent or combination of solvents. TLC is a technique that

can be used to determine the number of components in a mixture and verify a substancesidentity.

After the TLCexperiment is completed, the retention factor () value is calculated. The

retention factor of a compound is a measure of how far it has moved up a plate under certainconditions and it is can be use a quick way of identification. The value is calculated using this

formula:

=Distance spot travels

Distance solvent travels

Once recrystallization is performed the melting pointwill also be determined to verify the purityof the compound and used for identifying the unknown solids. The melting pointor meltingrangeof a solid is defined as the temperature at which the solid and liquid phases of a substanceare in equilibrium.


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Experimental Procedures and Observations

Part 1: Determine solvent/solvent system.

0.010 g of pure naphthalene was weighed out and placed in a reaction tube. 0.5 mL of eachsolvent (water, dichloromethane (DCM), methanol, and hexanes) was added and the tube wasshaken. The solubility of naphthalene was recorded. If all of naphthalene sample dissolved atroom temperature then it was not a good choice of solvent for recrystallization. If the sample wasnot dissolved, the tube was heated in a sand bath. A sand pit burner was used instead of thetraditional boiling water like in the MIT video. If the sample completely dissolved while hot, thesolvent would be chosen for recrystallization. If not completely dissolved, that particular solventshouldn't have been used. Then, the test tube was cooled down until room temperature and thencooled in an ice bath. If no crystals formed, the inside of the tube would be scrapped with aspatula or glass stirring rod. If the precipitation was seen, the solvent for recrystallization wasfound. The best solvent found for this experiment was methanol.

Part 2: Recrystallization

10 mL of methanol was heated it in Erlenmeyer flask on a hotplate. 0.454 g of impure (red)naphthalene was added in another Erlenmeyer flask. Small amounts of heated methanol waspipetted and added to the flask with sample until the crystals were dissolved. The solution turnedred. Thus, a small amount of activated charcoal was added because of the sample was colored.Activated charcoal traps high molecular weight molecules such as colored impurities. While thesolution was hot, gravity filtration was utilized with a stemless funnel. During the gravityfiltering process, a very high frequency of recrystallization could prevent the solution to passthrough the filter paper. Then the solution was allowed to cool to room temperature and placed in

an ice-bath. The recrystallization process was left undisturbed for five more minutes, andvacuum filtration was used. The obtained crystals were dried by squeezing technique with alarger filter paper. The obtained crystals looked grayish. The weight was recrystallizednaphthalene was recorded as 0.39 g.

TLC

The developing chamber was prepared with a beaker with a watch glass on the top. The solvent(1:1 Ethyl acetate/hexanes) was decanted into the beaker to a depth of just less than 0.5 cm andthe beaker was covered with a watch glass. A TLC plate was cut with a size that only fit thebeaker. The size of the TLC plate was 3 cm x 5 cm. A line was drawn across the TLC plate with

a pencil, 1 cm from the bottom of the plate and two hatch marks were drawn evenly spaced alongthe line. The impure and purified naphthalene were spotted with a separated clean TLC spotterson the hatch marks on the TLC plate. The plate was placed in the developing chamber. As themobile solvent traveled up the plate and got close to the top (3.5 cm), the plate was removedfrom the beaker, and a line was drawn across the solvent front (0.5 cm from the top) before thesolvent evaporated. The spots were visualized under the UV lamp and the spots were circled witha pencil. The retention factor, , valueof each spot was calculated. Before recrystallization,

there were two visible spots. The lowest spot traveled 0.4 cm and the highest spot traveled 2.7


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cm. TheRfof the lowest spot or impure substance was 0.114 and of the top spot or naphthalenewas 0.771. However, TLC was performed twice because the plate became contaminated fromtouch.

Melting Point Determination

The open end of a melting point capillary tube was thrust into the purified naphthalene severaltimes. The closed end of the capillary was dropped down several times to fill and pack thesample. This step was repeated until the height of accumulated sample is about 1-2 mm in thebottom of the tube. The start temperature was set at 70C, the ramp rate at 1C and the stoptemperature at 260C. Once the apparatus was ready, the capillary was inserted into the chassishole and the start button was pushed. The melting point range was recorded when the first dropsof liquid were observed and completely melted. While doing the Melting point determination,the fast ramp was performed as well as a slow ramp even though identity of the compound wasknown. However, to be certain that the process of recrystallization was successful, free fromimpurity, the fast ramp was performed. The melting point range was recorded as 80.581.4C.

Cleaning Up

Any organic waste was disposed in the organic waste container. The melting point capillarytubes were disposed in the broken glass container. For faster drying, acetone was used forcleaning the glassware in the lab kit.


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Experimental Data

Table 1: Solubility Test

ROOM TEMPERATURE HEATED

WATER INSOLUBLE INSOLUBLEDCM SOLUBLE SOLUBLEMeOH INSOLUBLE SOLUBLEHEXANES INSOLUBLE Partially-SOLUBLE

Chart 1: TLC (before and after Recrystallization)

Table 2: Recrystallization Results

Impure naphthalene 0.454 g

Amount recovered 0.390 g

% recovery 85.9%

value 0.777

MP fast ramp 79.982.0

MP slow ramp 80.581.4

Before After


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Results and Discussion

In Part 1 of the experiment, the solubility of naphthalene in different solvents was determined toidentify the best solvent used for recrystallization. The solubility was recorded as soluble,partially soluble, or insoluble in water, dichloromethane (DCM), methanol (MeOH), and

hexanes. In water, naphthalene was insoluble at room temperature and insoluble while heated. InDCM, naphthalene was soluble at room temperature and soluble while heated. In hexanes,naphthalene was insoluble at room temperature and partially-soluble while heated. In MeOH,naphthalene was insoluble at room temperature and soluble while heated. Therefore, MeOH wasthe best solvent and could be used for part 2 of the experiment. This result confirmed oneconcept of recrystallizationthe suitable solvent used for recrystallization that the solute shouldbe insoluble at room temperature, but should be soluble at the boiling point of the solvent. Notethat this part of the procedure was crucial because without the right solvent, recrystallizationwould not be accurate.

In Part 2 of the experiment, 0.5 g of impure naphthalene was recrystallized and verified the

purity by performing TLC and melting point analysis. The amount of the solid recovered wascalculated. 0.454 g of impure naphthalene was used to perform the recrystallization. 0.39 g ofpurified naphthalene was recovered. Thus, the recovery yield was 85.9%. Next, the TLCexperiment was performed with 1:1 Ethyl acetate/hexanes. The visual results of the TLC can befound in Chart 1. The impurity spot traveled 0.4 cm and the purified spot traveled 2.7 cm. Thesolvent front traveled 3.5 cm. Thus, theRfvalues of the lowest spot or impure substance was0.114 and of the top spot or naphthalene was 0.771. Next the Melting point determination wasperformed which gave a melting range of 80.581.4C. Consequently, TLC and melting pointanalysis results confirmed that the recrystallized naphthalene was pure. And the technique ofrecrystallization was effective. Moreover, 85.9% of the initial amount of the solid was recovered

meaning that approximately 14.1% was impurities.

As one can tell, recrystallization technique allows for the purification of a crude material. Thesmall loss in percent recovery was made up by the high gain in purity as shown in the TLC plate.Furthermore, the purity of the material obtained was correlated to the sharpness and veracity ofthe melting point data and this was observed. And the result was proven by determining themelting point range of the recrystallized material, which was very narrow and was close to theliterature value

Conclusion

Recrystallization is an important method for purifying solid organic compounds. It is also a verypowerful, convenient, and efficient method of purification, and it is an important industrialtechnique that is still relevant in the chemical world today.


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References

1. Caron, S. Practical synthetic organic chemistry; 1st ed.; Wiley: Hoboken, N.J., 2008.

2. Williamson, K.; Masters, K.Macroscale and microscale organic experiments; 6th ed.;

Brooks/Cole: Belmont, CA, 2011.

3. Bunnelle, W.; Meyer, L.; R. E. Glaser, R. Recrystallization Notes by Tiyupanisshttp://www.studymode.com/course-notes/Recrystallization- Notes-1832858.html(accessed Sep 7, 2011).

4. Tabacco, S.; Siddiqui, A. Recrystallization | MIT Digital Lab Techniques Manualhttps://www.youtube.com/watch?v=7LBGQHjgHEw (accessed Sep 7, 2011).

5. Allen, S. Organic Chemistry Lab: Recrystallizationhttps://www.youtube.com/watch?v=XK0MZk3Q4jk (accessed Sep 7, 2011).

6. Harwood, L.; Moody, C.; Percy, J.Experimental organic chemistry; 1st ed.; BlackwellScience: Malden, MA, 1999.

7. Lehman, J. Operational organic chemistry; 1st ed.; Pearson Prentice Hall: Upper SaddleRiver, N.J., 2009.


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Post Lab Questions

1. A sample of naphthalene, which should be pure white, was found to have a grayish color afterthe usual purification procedure. The melting point was correct, and the melting point range wassmall. Explain the gray color.

The gray color of the naphthalene is due to traces of decolorizing charcoal. It can be removed byfiltration through fine filter paper.

2. How many milliliters of boiling water are required to dissolve 25 g of phthalic acid? If thesolution were cooled to 14C, how many grams of phthalic acid would recrystallize out?

The amount of water required to dissolve 25 g of phthalic acid (using the solubility informationin Experiment 2) is 139 mL. On cooling to 14C, this solution should deposit 24.25 g of phthalicacid.

3. Why should activated carbon be used during a recrystallization?

Activated carbon is added to adsorb high-molecular-weight impurities, which are often coloredand relatively less soluble.

4. If a little activated charcoal does a good job removing impurities in a recrystallization, whynot use a larger quantity?

Activated carbon will adsorb some of the product, decreasing the yield.

5. Under which circumstances is it wise to use a mixture of solvents to carry out arecrystallization?

A mixture of solvents is used when no single solvent has the desired property of rendering the

substance soluble when hot and insoluble when cold.

6. Why is gravity filtration rather than suction filtration used to remove suspended impurities andcharcoal from a hot solution?

During suction filtration of a hot solution, evaporation of the solvent takes place with attendantevaporative cooling. The combination of these two effects causes crystals to form and clog theBchner or Hirsch funnel.

7. Why is a fluted filter paper used in gravity filtration?

A fluted filter paper has a larger surface area through which the solvent can flow than an

unfluted filter paper, which presses against the glass side of the funnel.

8. Why are stemless funnels used instead of long-stem funnels to filter hot solutions throughfluted filter paper?

If a long-stem funnel is used for filtration, the filtrate will cool and deposit crystals as it passesdown the stem, clogging the stem.


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9. Why is the final product from the recrystallization process isolated by vacuum filtration ratherthan gravity filtration?

Vacuum filtration is a much more efficient method for removing solvent from the crystals than isgravity filtration.

10. Why should wood applicator sticks not be used when carrying out a chemical reaction?

Wood applicator stick will often interact with the reagents being used in the reaction,contaminating the product.

11. Why should you never use a beaker for recrystallization?

The large opening in a beaker allows solvent to evaporate too easily when trying to dissolve asolute. A crust of impure material appears around the edge on cooling. The Erlenmeyer flasksolves these problems.

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Sample Lab Report