Recrystallization and Identification of an Unknown

Kaitlyn Greiner

Organic Chemistry 2270 Laboratory, Section 027

Instructor: Maria Swasy

October 9, 2014

“My signature indicates that this document represents my own work. Outside of shared

data, the information, thoughts and ideas are my own except as indicated in the

references. I have submitted an electronic copy through Blackboard to be scanned by

TurnItIn.com. In addition, I have not given aid to another student on this assignment.”

Abstract

In this experiment, an impure compound was recrystallized and its identity was

determined by finding its melting point. The best recrystallization solvent was determined to be water by observing solubility in different conditions. The impure substance was

recrystallized with this determined solvent. The melting point of this product was found by comparing the melting points of just the unknown compound with the melting point of a 50/50 mixture of this compound and the hypothesized compound, cinnamic acid. The

melting point with a ramp rate 5°C was 134.9°C and 134.1°C with a ramp rate of 2°C. The melting point range of the 50/50 mixture was 135.5°C. This melting point was

further confirmed by comparing it to other known melting points and the identity of the unknown was determined to be cinnamic acid. The known melting range of cinnamic acid was 132°C to 135°C. An impure compound, p-Dibromobenzene, was recrystallized

using ethanol-water solvent to demonstrate that a mixed solvent can be used for recrystallization.

Introduction

A solid sample can be purified through the process of recrystallization. In this process, impurities are dissolved out of the sample. Melting point can be used to

determine the purity of this sample.1 The goal of this laboratory project was to purify an impure sample of an unknown compound by recrystallization and identify the compound by its melting point. The melting point method is used because, “An impure solid melts at

a lower temperature and over a wider range. Thus, a solid's melting point is useful not only as an aid in identification but also as an indication of purity.2” The first step of

recrystallization was to identify a suitable solvent. The proper solvent was needed to be used in the recrystallization of the impure

compound. In part 1 of the experiment, solubility observations were made for each

combination of four different solvents and the unknown that was placed in certain conditions in order to determine the most suitable solvent. There are certain solubility

criteria that must apply, “To remove soluble impurities, first, by doing solubility tests, a suitable solvent is chosen (high solubility in hot solvent, low solubility in cold solvent).4”

For part 2 of the experiment, another goal was to conduct a larger scale

recrystallization process of the impure compound using this solvent. This was done in order to purify the sample. The unknown compound was dissolved and filtered, resulting

in a clear solution. This clear solution was then crystallized and dried. In part 3, the goal was to find the melting points of this unknown compound in

order to hypothesize the identity of the unknown compound. A 50/50 mixture was

created, to confirm or negate the hypothesis by revealing the identity of the unknown compound. This is because it is known that, “If the two samples are the same compound,

a 50:50 mixture will still be a pure sample and the melting point will be unchanged. If they are different compounds, the 50:50 mixture will be very impure and the melting point range will be lowered and broadened.5” This was how the identity of the compound

was determined. For part 4, the last goal was to recrystallize p-Dibromobenzene to demonstrate the

technique of using a mixed solvent for recrystallization. It is possible to combine two different solvents when a single appropriate one cannot be found.6

Results

Part 1-

Unknown: C

Table 1: Solvent Solubility at Room Temperature

Solvent Choices at Room

Temperature:

Dissolved? Comments/Additional

Observations:

Acetone yes Everything dissolved except for sawdust

Ethanol yes Everything dissolved except

for sawdust

Toluene no Solids appeared to dissolved at first but there

were two distinct layers of liquid visible

Water no Mixture was very cloudy

*Characteristics and solubility of four solvent choices at room temperature observed

Table 2: Solvent Solubility When Heated

Dissolved? Comments/Additional

Observations:

Toluene in Steam Bath no Boiled, let it cool, no distinct layers observed but

cloudy, never dissolved

Water in Sand Bath yes Did not dissolve at first, kept increasing temperature,

eventually dissolved when boiled

*Characteristics and solubility of solvents not dissolved at room temperature were observed while heating

Table 3: Solvent Crystal Formation

Crystals? Comments/Additional Observations

Toluene in Ice Bath no

Water in Ice Bath yes Some crystals formed

before in ice bath but even more formed when placed in ice bath, also scraped

with glass rod to promote crystal formation

*Characteristics and crystal formation of solvents observed

Recrystallization Solvent: Water

Part 2-

50mg of solute unknown C

3mL of solventx

2mg

x mL

æ

èç

ö

ø÷= 120mL of solvent (use half to start with: 60 mL)

Comments/Additional Observations: When solvent was boiling, undissolved compound

still existed so 10-15mL of water was added until no further solute was detected Part 3-

Table 4: Melting Point of Unknown

Melting Points: (°C)

With a ramp rate of 5°C 134.9

With a ramp rate of 2°C 134.1

Of 50/50 Mixture (ramp rate of 2°C) 135.5

*Melting points of unknown compound observed at ramp rates of 5°C and 2°C and melting point of 50/50 mixture observed at ramp rate of 2°C

Percent Yield:

Percent yield = actual yield

starting material*100

Unknown: .14g

2gx 100% = 7%

p-Dibromobenzene: .18g

1gx 100% = 18%

Known melting range of cinnamic acid: 132-135°C

Part 4- Comments: 5 mL of water total was added to make the solution become cloudy

Discussion

Solubility is the amount of the material that will dissolve in a certain amount of solvent. Polar compounds are more likely to be soluble in polar solvents, such as water,

and nonpolar compounds are more likely to dissolve in nonpolar solvents.3 The most polar bonds have the largest electronegativity difference between the atoms in the bond and the highest boiling points.7 The attractive forces between polar molecules are dipole

forces. These attractive forces of dipoles increase the boiling point. The stronger these forces are, the more the compound must be heated to overcome the attractions, therefore,

the higher the melting and boiling points.8 As a compound is heated to its melting point, its lattice breaks and it dissolves.

The first goal of the experiment was to identify a suitable recrystallization solvent. In part 1, the sample that was insoluble at room temperature (Table 1), but

soluble when boiled was the one with water as the solvent. This was the best solvent. This finding was not surprising, “And, water is called the ‘universal solvent’ because it dissolves more substances than any other liquid.9” When solutes dissolve and a solution

becomes supersaturated, the solvent cannot contain the amount of dissolved solute. A crystal is a solid formation of solute molecules leaving the solution.10 Mainly ionic

compounds form crystals because of their giant lattices.11 These lattices are made up of ions of opposite charges. A substantial amount of crystals formed in the mixture of the solvent and the unknown compound after being placed in an ice bath as seen in Table 3.

The water dissolved the solute or unknown compound when boiled (Table 2) and was effective in forming crystals when placed in an ice bath. One problem that arose was that

at first, it was difficult to tell which sample was going to dissolve when heated. Therefore, both samples containing water and toluene were placed in the ice bath. This confirmed that crystal formation was only apparent in the sample containing water.

The goal of part 2 of the experiment was to recrystallize the impure compound using this solvent. A common mistake in recrystallization is the addition of too much

solvent so the amount of solvent was calculated and half of this amount was used to start with. Samples are purified through recrystallization. The aim is to dissolve the impure solute from the solution. Crystals will form as the sample becomes more pure. If the

compound still had impurities, insoluble materials would be present in the solution and would not dissolve. Filtration will further separate the sample from the impurities.1 In the

experiment, the unknown compound was dissolved and filtered, resulting in a clear solution. This clear solution was then crystallized and dried. Recrystallization was successful in the experiment. It is important to understand the process of recrystallization

because it is commonly used in day-to-day life. It is used in the pharmaceutical industry because it, “is a simple and inexpensive method for scaling up the drug developments to a

commercial level.12” In part 3 of the experiment, the goal was to identify of the unknown compound by

determining its melting point. The compound was confirmed to be cinnamic acid. It is

actually better to use the term melting range rather than melting point because, “a solid usually melts over a range of temperatures rather than at one specific temperature.4” A

common mistake in recrystallization is that the compound is not completely dry when finding the melting point. In this experiment, the compound had completely dried because a week had gone by between part 2 and part 3. Cinnamic acid was the predicted

compound after having measured the melting points of the original unknown compound, “The melting point is a physical property of a solid and can be used to help identify a

substance.4” Cinnamic acid was the known compound that was used in the 50/50 mixture. The melting point of the 50/50 mixture was comparable to the melting points of the original unknown compound (Table 4). This meant that unknown compound was

confirmed to be cinnamic acid. If two samples that are mixed together are the same, the melting point will stay the same. This is what occurred in the experiment. It is known

that, “Since different molecules do not fit into each other's crystal lattices in exactly the same way, the presence of even a small amount of unknown in a sample of A will lower

the mp of A unless of course the unknown is A.13” If the two mixed samples were different, the melting point would have been lower. Compounds have unique melting

ranges due to the varying intermolecular forces between the atoms. As the forces increase in strength, the boiling point also increases.7 Another common mistake in

recrystallization is heating the compound too quickly when taking its melting point so ramp rates of 5°C and 2°C were used.14 Sagging can also occur before melting. It gives a false lower end of the melting range because the crystalline structure sags but there is not

actually any liquid yet. The lower end of the melting range should only be recorded when liquid forms.15

In part 4 of the experiment, the goal was to recrystallize an impure sample of p-dibromobenzene. The contaminant was successfully separated from the product. An orange impurity was left behind and a white product was obtained using an ethanol-water

solvent. Incorrectly pairing the mixed solvents could occur as a common mistake in this process. Crystallization occurred using this solvent. This proved that a mixed solvent

could be successfully used for recrystallization. It is known that, “compounds having similar structural features will be soluble in one another”. Examples of these features are polarity and the interaction of forces.16 This is demonstrated in the images below (Figure

1, Figure 2). Using ethanol by itself as a solvent for p-dibromobenzene would not work because the substance was soluble in cold and hot ethanol. The use of water by itself as a

solvent for p-Dibromobenzene would not work as well because the compound had a low solubility in hot and cold water. Kathleen Armstrong of Foothill College Chemistry stated that, “When a suitable solvent cannot be found, search instead for two miscible

solvents that together provide the appropriate solubility properties.6” Ethanol and water were capable of being mixed together. A certain mixture of these two caused the

compound to dissolve when heated but be insoluble when cold.

Figure 1

*Demonstration of solubility

Figure 2

*Solubility of compounds with similar structural features Conclusion

It was concluded from part 1 that the water solvent dissolved the solute or

unknown compound when boiled and was effective in forming crystals when placed in an ice bath. It was the most suitable recrystallization solvent. For part 2, the impure compound was successfully recrystallized. The impurities were completely left behind.

For part 3, the identity of the unknown compound was cinnamic acid. The melting point of the 50/50 mixture was comparable to the melting points of the original unknown

compound. Cinnamic acid was the other component of the 50/50 mixture. The melting range of the 50/50 mixture was 135.5°C and the known melting range of cinnamic acid is 132 to 135°C. This comparison confirmed the hypothesis that the unknown compound

was cinnamic acid. In part 4, an impure sample of p-Dibromobenzene was successfully recrystallized using a mixed solvent system of ethanol and water. This shows that it was

possible to use a mixed solvent for recrystallization. Experimental Procedure

First, for part 1, 50 mg of the unknown was mixed with 3 mL of each of the four

solvents in four separate small test tubes. Each test tube was labeled according to the solvent that it contained. The four solvent choices were acetone, ethanol, toluene, and water. Each of the four combinations was shaken to determine if the solid would dissolve

at room temperature. Dissolved mixtures, acetone and ethanol, were recorded and no longer used. If the samples did not dissolve at room temperature, such as water and

toluene, they were placed over steam baths or hot plates and swirled with wooden sticks to allow the solvent to boil and to break down the compound. If acetone, ethanol, or toluene solvent samples were still being used, they were heated over a steam bath

because they are organic solvents. If the water solvent sample was still being used, it was heated in a sand bath on a hot plate set over 200°C. It is important to note that not

everything will dissolve in a good solvent. This is because of impurities assorted with the unknown compound. After the samples boiled, they were allowed to cool. The samples that did not dissolve were recorded and no longer used. The sample that did dissolve after

being boiled was used as the recrystallization solvent. Observations were made based on whether or not crystals formed in the sample after cooling. The inside of the test tube was

scratched with a glass rod to prompt crystal formation. The tube was then placed in an ice bath to further promote crystal formation. Our product was saved after this experiment.

In part 2 of the experiment, the amount of solvent needed to recrystallize a 2g sample of the unknown was calculated and half of this amount was used. Sixty mL of

solvent was mixed with 2g of the unknown in an Erlenmeyer flask. This sample, along with a wooden stick was heated directly on the hot plate until it boiled. About 25 mL of the solvent was heated on the hot plate as well if needed to further dissolve the remaining

solute after the sample was boiling. Small amounts of the extra solvent were added with a glass pipet until no further solute could be seen. Gravity filtration was performed on the

dissolved sample by pouring it into a stemless funnel lined with fluted filter paper and into a second Erlenmeyer flask. The fluted filter paper was soaked with more of the extra solvent that was heated alongside the sample before gravity filtration took place. This

clear filtrate solution was cooled set aside to cool then cooled further by being placed in an ice bath. This cooling was performed to promote crystallization. When crystallization

was complete, vacuum filtration was performed to dry the crystals. The sample was vacuum filtered by using suction. This was performed using a side-arm flask and a Buchner funnel with filter paper and a filter adapter. The water was turned off. The

sample was placed on the filter paper and washed with three small portions of pure solvent that was cooled in an ice bath. The crystals were then dried by drawing air

through the vacuum system. Finally, they were placed on a watch glass to dry. In part 3, this new sample of purified compound was crushed up and placed in a

melting point capillary tube. The melting point apparatus was used to determine a melting

point for the sample. The apparatus was started at 100°C and with a ramp rate of 5°C to give an approximate range. Once this melting point was determined, a more accurate

reading was obtained by using a ramp rate of 2°C. This melting point was recorded as well. The temperature was raised much slower when at the melting range for the more accurate reading. By comparing the melting points of the unknown compound with the

melting points of a known compound, the identity of the compound hypothesized. A 50/50 mixture was created using the hypothesized compound and the original compound

and the melting point was measured using a ramp rate of 2°C. If both samples are the same, the melting point will also stay the same. The identity was confirmed with the TA and recorded.

For part 4, one gram of p-dibromobenzene contaminated with orange dye methyl orange was mixed with 10 mL of ethanol and a wooden stick in a 50 mL Erlenmeyer

flask. This mixture was heated on a steam bath and stirred until the solid dissolved. Perform gravity filtration with fluted filter paper, a stemless funnel, and an Erlenmeyer flask if some parts remain insoluble. Once it dissolved, water was added at .5 mL

increments until the solution became cloudy. .5 mL of ethanol was then added to the solution to make the cloudiness vanish. The flask was removed from the steam bath and

cooled to room temperature. The flask was then placed in an ice bath for several minutes. Crystals were obtained through vacuum filtration. The water was turned off and the crystals were rinsed on the Buchner funnel with a few mL of a 50/50 mixture of ice-cold

ethanol-water. This combination was stirred on the Buchner funnel briefly and the water was turned on. This was repeated there was a white product and the orange impurity was

removed. The crystals dried on the funnel with the water on for a bit and were placed on a new piece of filter paper to dry.17

References

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30, 2014). 2. Houston Community College. Dr. Gholam Pahlavan’s Chemistry Homepage.

http://swc2.hccs.cc.tx.us/pahlavan/2423L2.doc (accessed Oct, 2014).

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11. Important Things to Know About Ionic Compounds.

http://misterguch.brinkster.net/ionic.html (accessed Oct 30, 2014). 12. Yousef, J.; Hamedeyazdan, S.; Asnaashari, S. Recrystallization of Drugs:

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13. Penn State Department of Chemistry. Recrystallization and Melting Points. http://courses.chem.psu.edu/chem36/Experiments/recrstal.pdf (accessed Oct 30, 2014).

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http://www.foothill.edu/psme/armstrong/meltingpoints.shtml (accessed Oct 5, 2014). 15. Minnesota State University Moorhead Department of Chemistry. Chem 355 Lab

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