Recrystallization and Melting Point Measurement- Identifying a Component of Panacetin

Introduction:

As this experiment is being carried out it is important to use the correct techniques of weighing, using heat sources, recrystallization, vacuum filtration and finding the melting point. In order to weigh the correct amount of the starting solid first weigh out a clean watch glass on an electronic balance and the slowly add the solid to the watch glass until the amount needed has been weighed out correctly. In the experiment to perform a recrystallization a heat source is needed. The heating implement used in this experiment was a hot plate. Using a hot plate is an ideal heating source to be used since the part of the recrystallization that requires heating will be in an Erlenmeyer flask. Also by keeping heat-resistant gloves or beaker tongs nearby the flask can be removed quickly afterwards from the hot plate. For recrystallization first heat the solid in the flask and water in a beaker separately and when the water is hot enough add the minimum calculated to dissolve the solid. Keep adding the hot water until the entire solid has completely dissolved and quickly remove the flask from the hot plate. Let the flask cool at room temperature before putting the flask into an ice water bath. After the recrystallization is complete in the ice water bath remove the flask and continue to vacuum filtration. After setting up the apparatus for vacuum filtration, and using only cold distilled water then vacuum filtrate the crystals from the flask that formed during recrystallization. Also let the solid dry after the vacuum filtration is complete by letting the vacuum run while the solid is still in the Buchner funnel. In order to find the melting point of the solid that had formed in recrystallization and had been vacuum filtrated a capillary melting was used. First the thermometer was attached to the capillary melting point. Then the solid gathered was crushed and divided into three parts; the solid, the solid and phenacetin, and the solid and acetanilide. Each of the three parts was then put into separate capillary tubes and the melting point ranges for all three were found. The mixture of the solid and phenacetin or acetanilide would be depressed and large for one of those mixtures and the other would match the melting point range of the solid very closely.

Experimental:

The amount of the unknown substance that was used was 0.235g. The minimum amount of water to add to the recrystallization was 4.7mL and the maximum amount was 19.3mL.

After adding the hot water to the unknown solid, the solid began to dissolve in the water and then some more hot water was added to dissolve the solid completely. After heating and letting the flask cool to room temperature some crystals began to form on the top of the solution. While letting the solution cool in the ice water bath white crystals where formed. After vacuum filtration the final product that was formed are fine white crystals.

Reaction Mechanism:

There was no reaction mechanism in this laboratory experiment because it was a recrystallization and melting point experiment continued from experiment 2 of last weeks lab.

 

 

 

 

 

 

 

 

 

Experimental Data:

Unknown: 0.235g

Minimum water to add to recrystallization: 4.7mL

Maximum water to add to recrystallization: 19.3mL

Weight of watch glass: 25.163g

Weight of watch glass and solid: 25.292g

Weight of unknown: 0.129g

Temperature of pure unknown sample: 126-130 degrees C

Temperature of unknown and phenacetin: 128-133 degrees C

Temperature of unknown and acetanilide: 92-100 degrees C

The unknown sample was determined to be phenacetin from the melting points.

The final product that is formed after vacuum filtration are fine white crystals.

Calculations:

 

 

 

 

 

 

 

Discussion:

The initial amount of the unknown sample that was started out with was 0.235g and using this amount and the solubility of acetanilide and phenacetin in hot water the minimum and maximum amounts of water to be added during recrystallization was determined to be 4.7mL and 19.3mL. After heating the flask with the unknown sample and water separately add enough of the hot water into the flask so that the unknown sample can be dissolved without going over the maximum amount of 19.3mL. After the unknown sample had been dissolved the flask was removed from the hot plate and allowed to cool at room temperature. Some crystals began to form on the top of the solution in the flask as it cooled to room temperature. Then the flask was put into an ice water bath and allowed to cool inside. As the flask was cooled within the ice water bath white crystals formed and after the flask was cooled proceed to vacuum filtration. The solution was then vacuum filtrated and allowed to dry by letting the vacuum filtration apparatus run while the solid was still in the Buchner funnel. The weight of the watch glass was 25.163g and the weight of the watch glass and the unknown solid was 25.292g. The final weight of the unknown product after recrystallization was 0.129g. The unknown solid was then crushed and divided into three equal portions. The first portion was used to find the melting point of the unknown sample so that it could be compared to the melting points of the unknown and phenacetin and the unknown and acetanilide. The melting point range for the pure unknown sample was 126-130 degrees C. The melting point range of the unknown and the phenacetin was 128-133 degrees C and the melting point range for the unknown and acetanilide was 92-100 degrees C. From within the organic chemistry laboratory manual the exact melting point for acetanilide was 114 degree C and the exact melting point for phenacetin was 135 degrees C. After adding the hot water to the unknown solid, the solid began to dissolve in the water and then some more hot water was added to dissolve the solid completely. This was the first part of the recrystallization, and it was important to heat the solid and the water separately and afterwards add the hot water to the flask with the unknown in order for the unknown solid to dissolve. Also it is important not to add too much hot water to the flask with the unknown; it is best to use the minimum amount of hot water that was calculated and slowly add more if needed. After heating and letting the flask cool to room temperature some crystals began to form on the top of the solution. It is important that the solution was cooled to room temperature before putting it in the ice water bath to cool in order to get the highest yield of the purest substance for the final product. While letting the solution cool in the ice water bath white crystals where formed. It is important to let the crystals form completely before continuing to the vacuum filtration in order to get the highest yield of the pure unknown substance to precipitate out of the solution. After vacuum filtration the final product that was formed are fine white crystals. After crushing and dividing the samples of the unknown solid it is important not to get the sample mixed up.

Conclusion:

It can be concluded that the unknown sample in the Panacetin was phenacetin because the melting point range of the pure unknown sample, 126-130 degrees C, closely matches that of the unknown sample and phenacetin, 128-133 degrees C, without any melting point range depression.