The Process of Recrystallization of Benzoic acid PDF

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Dr. Srinivasan 2123-540 Organic Chemistry Lab 2 Report...

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Recrystallization Brooklin Ellis CHEM 2123-540 September 22, 2021

1 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 Recrystallization ABSTRACT The purpose of the experiment was to purify and identify the Unknown A compound by using physical properties, such as, recrystallization, polarity, and melting points. The melting point portion was found by using a modern digital melting point apparatus. The apparatus showed that Unknown A had a melting point of 122.1 C, making it Benzoic acid.

INTRODUCTION The experiment was done to determine the unknown compound used based off the experimentation of its physical properties. The main objective of the experiment was to learn to conduct basic purification of a solid by recrystallization and to familiarize the student with using specialized lab equipment. There was no specific apparatus used for the recrystallization portion of the experiment. A modern digital melting point apparatus was used to confirm the unknown’s identity for the melting point portion. The same unknown and solvent was used throughout the entire experiment.

MATERIALS & METHODS Equipment and materials used: 



Recrystallization o Beaker

o Solvent- DI water

o Weighing paper

o 0.75g of solid sample

o Ice

o Vacuum tubing

o Erlenmeyer flask

o Buchner flask

o Magnetic stir bar

o Buchner funnel

o Thermometer

o Filter paper

o Hot plate

o Scapula

Melting point

2 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 o A modern digital melting point apparatus o Weighing paper

o Scapula o 3 capillary tubes o A solid sample to test

Methods used: 

Recrystallization I.

An appropriate amount of solvent was heated, on a hot plate, to just below its boiling point in a beaker.

II.

Using weighing paper, the solid was placed into an Erlenmeyer flask. A magnetic stir bar was also placed into the flask.

III.

Small portions of the hot solvent were added to the Erlenmeyer flask. The mixture was then brought to its boiling point on a hot plate.

IV.

Once the solution was completely dissolved, the Erlenmeyer flask was removed from the hot plate to examine. If there are insoluble particles, add additional amounts of hot solvent in small increments and resume heating the solution. If the solution is colored, add activated charcoal to remove the color. After, a gravity filtration will need to be done to remove charcoal particles. If there are no insoluble or highly colored impurities in the hot recrystallization solution, cool the solution.

V.

The solution was cooled to room temperature before being placed into an ice bath to further crystallize. If crystallization doesn’t occur scratch the bottom of the flask vigorously with a glass stirring rod. If crystallization still does not occur, there is probably too much solvent. Boil off some of the solvent in the hood and cool the solution again.

VI.

The crystals were recovered through vacuum filtration using a Buchner funnel, a Buchner flask, vacuum tubing and filter paper. The vacuum source was turned on and the paper was wetted with the recrystallizing solvent. The crystals and solvent were poured into the funnel. All the crystals were removed from the flasks using a spatula.

3 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 VII. The crystals were washed on the Buchner funnel with a small amount of cold recrystallization solvent and allowed too completely dry. Any magnetic stir bars were removed. VIII.

A spatula was used to lift the filter paper and crystals out of the Buchner funnel and the dry crystals were weighed and recorded.

IX. 

The percent recovery was calculated.

Melting Point: I.

The powdered, dry solid sample was introduced into a capillary tube that was sealed on one end. This was done by tapping the open end of the capillary tube into the powdered sample. The tube was inverted, and the sealed end was tapped to move the solid down to the base. The tube was filled with 2-3mm of the solid sample.

II. III.

The capillary tube was placed into the melting point apparatus. The heat rate on the melting apparatus was set to change at a moderate rate of 2 C per minute. For a digital melting apparatus, the start temperature should be set 10 C below the expected melting point. The stop temperature was set to 5 C above the expected melting point.

IV.

If the temperature began rising more than 1 - 2 C per minute at the time of melting, the melting point would be determined with a new sample.

V.

The melting range was recorded as the temperature between the onset of melting and when the solid was completely melted.

VI.

The used capillary tubes were removed from the melting apparatus and discarded in the sharp’s container located in the laboratory.

Chemicals Used:

4 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 Benzoic Acid

Water

MW: 122.12g/mol

MW: 18g/mol

MP: 121-125 C

MP: 0 C BP: 100 C

BP: 249 C 3

Density: 1.32g/cm

Density: 1g/cm3

PROCEDURE 

Recrystallization I.

250mL of Water was heated, on a hot plate, to just below its boiling point in a beaker.

II.

Using weighing paper, 0.779g of Unknown A was placed into an Erlenmeyer flask. A magnetic stir bar was also placed into the flask.

III.

Small portions of the hot water were added to the Erlenmeyer flask. The mixture was then brought to its boiling point on a hot plate.

IV.

Once the solution was completely dissolved, the Erlenmeyer flask was removed from the hot plate to be examined. There were no insoluble or highly colored impurities in the hot recrystallization solution.

V.

The solution was cooled to room temperature for 15 minutes before being placed into an ice bath to further crystallize. If crystallization doesn’t occur scratch the bottom of the flask vigorously with a glass stirring rod. If crystallization still does not occur, there is probably too much solvent. Boil off some of the solvent in the hood and cool the solution again.

VI.

The crystals were recovered through vacuum filtration using a Buchner funnel, a Buchner flask, vacuum tubing and filter paper. The vacuum source was turned on and the paper was wetted with the recrystallizing solvent. The crystals and solvent

5 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 were poured into the funnel. All the crystals were removed from the flasks using a spatula. VII.

The crystals were washed on the Buchner funnel with a small amount of cold recrystallization solvent and allowed too completely dry. Any magnetic stir bars were removed.

VIII.

A spatula was used to lift the filter paper and crystals out of the Buchner funnel and the dry crystals were weighed and recorded.

IX.



The percent recovery was calculated.

Melting Point: I.

First, a small sample of Unknown A was placed on weighing paper and crushed with a scapula until it was the consistency of powder.

II.

The powdered, dry solid sample was introduced into a capillary tube that was sealed on one end. This was done by tapping the open end of the capillary tube into the powdered sample. The tube was inverted, and the sealed end was tapped to move the solid down to the base. The tube was filled with 1mm of the solid sample.

III.

The heat rate on the melting apparatus was set to change at a moderate rate of 2 C per minute. For a digital melting apparatus, the start temperature 10 C below the expected melting point, so it was set at 60 C. The stop temperature was set to 5 C above the expected melting point, so it was set at 230 C.

IV.

If the temperature began rising more than 1 - 2 C per minute at the time of melting, the melting point would be determined with a new sample.

V.

The melting range was recorded as the temperature between the onset of melting and when the solid was completely melted.

6 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 VI. The used capillary tubes were removed from the melting apparatus and discarded in the glass disposal container located in the laboratory. The chemicals were discarded into the trash can.

DIAGRAMS Filtration:

RESULTS & DISCUSSION

Melting Point Apparatus:

7 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 Recrystallization:

Observations

The unknown sample was Unknown A.

Name of Solid

Unknown A

Nature of Crystals

Large flaky, shards

Color of Solid

White

Amount of Solute

0.779g

and dried. The dried crystals were weighed, and

Volume of Solvent

3mL

it was discovered that 0.643g of the sample was

Time of cooling

15 minutes

First 0.779g of the unknown was dissolved into 3mL of the solvent, which was water. The water was heated to 90 C before being used to dissolve the sample. The solution was then cooled to room temperature for 15 minutes, then was placed into an ice bath for additional 5 minutes. The crystals were filtered out by using vacuum filtration, seen in the diagrams section,

recovered. Then the percent recovery was calculated, shown in the calculations section, and found that 82.5% of the compound was recovered. Melting Point: Capillary Tube # #1

Name Unknown A

Amount 1mm

MP Range 121 - 123.4 C

(Before #2

recrystallization) Unknown A

1mm

122.1 - 123.8

#3

(After recrystallization) Benzoic Acid mixture

1mm Unknown A

C 121.3 - 123.8

(1:1 ratio)

1mm Benzoic Acid

C

The start temperature was set at 60 C, with a 2 C ramp rate, and the stop temperature was set at 230 C. When the melting apparatus was finished preheating, capillary tube #1 with approximately 1mm of Unknown A before the purification was added. The unknown began melting at 121 C. Another trial of Unknown A after the recrystallization and purification was tested. Approximately 1mm of Unknown A into the capillary tube #2, this sample began to melt at 122.1 C. Based off the melting, it was

8 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 expected to have Benzoic Acid. To confirm the determination, a 1:1 ratio of Benzoic acid and the Unknown A was tested for purity. This sample began to melt at 121.3 C. The results proved that Unknown A was pure for Benzoic Acid, thus confirming that Unknown A was Benzoic acid.

CALCULATIONS Recrystallization: Percent Recovery of Compound Percent Recovery=

Amount of solid crystallized 0.634 g × 100 , Percent Recovery= ×100 Intial amount of solid 0.779 g Percent Recovery=82.5 %

SAFETY AND DISPOSAL For safety, wear tight long sleeves, googles, gloves, floor length pants with no holes in them, and closed toed shoes. Dispose of capillary tubes in glass disposal box located in the laboratory. Chemicals, filter paper, and weighing paper may be disposed of in the trash can. All glass and plastic materials must be cleaned with soap and water.

CONCLUSION Through performing this experiment, it was determined that Unknown A was Benzoic acid. The recrystallization method to first purify the unknown. After performing the recrystallization, it was found that 82.5% of the compound was recovered. The melting range of the unknown was performed to determine its identity. This was done by comparing the melting range to a chart of known compounds melting ranges. Unknown A had a melting range of 122.1 - 123.8 C and tested pure with Benzoic acid, which has a melting range of 122 -123 C.

REFERENCE Mohrig, J.R., Albert, D.G., Hofmeister, G.E., et al; Laboratory Techniques in Organic Chemistry, 4th ed., W.H. Freeman and Company, New York: 2014.

9 Brooklin Ellis Dr. Srinivasan 2123-540 September 22, 2021 POST-LAB QUESTIONS

1. Describe the characteristics of a good recrystallization solvent. 

A good solvent should allow the solute to have maximum solubility in hot solvent and minimum in cold. A solvent should also have a similar structure and polarity to its solute.

2. The solubility of a compound is 59 g per 100 mL in boiling methanol and 30 g per 100 mL in cold methanol, whereas its solubility in water is 7.2 g per 100 mL at 95°C and 0.22 g per 100 mL at 2°C. Which solvent would be better for recrystallization of the compound? Explain. 

Water would be the best solvent, because the more soluble the solute is in the solvent the more difficult it will be to recrystallize the solute.

3. Explain how the rate of crystal growth can affect the purity of a recrystallized compound. 

If the recrystallization process occurs too quickly impurities will become trapped in the crystals.

4. In what circumstances is it necessary to filter a hot recrystallization solution? 

When the solid will not completely dissolve in the hot solution due to insoluble impurities.

5. Why should a hot recrystallization solution be filtered by gravity rather than by vacuum filtration? 

The vacuum filtration process will cool the hot solution and cause premature recrystallization and the impurities will be trapped within the solid.

6. Low-melting solids often “oil out” of a recrystallization solution rather than crystallizing. If this were to happen, how would you change the recrystallization procedure to ensure good crystals? 

I would add more of the solvent and if that doesn’t work ill switch to another solvent with a lower boiling point....