Objective: The purpose of this lab is to help us practice the process of purifying compounds using chromatography. We will practice this technique by synthesizing benzoin from benzin and purifying this compound using both column chromatography and recyrstalization. We can then compare and contrast these techniques and our performance of these techniques through analysis of the purification process using both TLC and melting point. We will also gain an understanding of good technique for the proper care of a laboratory notebook.

Experimental:

Conducted on a 2g scale instead of 4g scale

• Measured out 2g of Benzoin and 7ml DI water, to be heated in a steam bath for 11 minutes. 

• Measured 37.5 ml of DI water to add after the synthesis process, some solids could be seen.

• Placed the solution into ice to accelerate the crystallization of the product. 

• 2.21 grams of a pale yellow, solid, precipitate was obtained. This precipitate was referred to as the crude product in later experiments.

• Split the crude product into two halves, half will be used in recrystallization, and the other half will be used for unreacted starting material and column chromatography (0.90g of crude product for recrystallization and 1.31 g of crude product for column chromatography).

Experimental: 

• Added NaOH to a small amount of remaining crude product to test for unreacted starting material. 

  • No color change occurred -  this implies that no uncreated starting materials were left in the crude Benzil. 

Experimental:

• Pipetted drops of reagent alcohol 0.90 grams of the crude product until a slurry was formed with the help of our lab professor.

• Approximately 10 ml of reagent alcohol was heated in an Erlynmyer flask on a hot plate set to its highest heat setting. 

• Allowed reagent alcohol to come to a boil.

• Placed the slurry of crude product and reagent alcohol onto the same hot plate that the regent alcohol was boiling on. 

• Pipetted drops of the hot reagent alcohol onto the slurry until the crude product was totally dissolved.

• Removed both flasks from heat and turned off the hot plate. 

• Pippetted drops of cold DI water into the solution containing the crude product until the "cloud point" was reached.

• Allowed solution to sit undisturbed while crystals formed.

• Once crystals had formed, a chem wipe was placed over the top of the beaker and the beaker was placed in a drawer to dry so crystals could be analyzed in the next lab period. 

• Weighed the crystals after being left dry. The weight of the crystals was equal to 0.86 grams.

• Analyzed purity of crystals using TLC and melting point. 

Experimental: 

 1.31 grams of crude product was used for column chromatography.

The solvent used to run the column chromatography was approximately 50 ml of 4:1 Hexane ethyl acetate. 

• Started by preparing the column for liquid chromatography, measured 15.00 cm of silica gel, and created a slurry with hexane solvent. 

• Silica gel and slurry were loaded into the column. The silica was well packed, compact, and flat with no obvious channels seen.

• "Packed" the column by adding  15.00 ml of excess hexane and allowing it to run through. 

• Hexane was drained into a 150.00 ml Erlynmyer flask.

• Loaded the crude product into the column.

• An additional layer of sand was placed on top of the crude product. 

• Samples were collected after the yellow band containing the benzil began to move through the column. The band was thin at first but became thicker as the solvent ran through the column.

•  6 test tubes of product were collected but only the last tube will be placed onto a plate for TLC analysis. 

• Excess was evaporated down on low heat (hot plate set to 3) to form a solid product which will be analyzed using MP before comparing with the purified product from recrystallization.

Discussion:  The synthesis of benzil yielded a large amount of crude product, which allowed me and my lab partner to have plenty of product to work with as we purified our benzil. Based on the test for an unreacted product, due to the lack of color change in our precipitate, no unreacted product was found in our crude product before we proceeded to conduct our recrystallization or column chromatography experiments.

During the recrystallization process, it is likely that the benzil we recrystallized was pure based on the obtained melting point value (93–95 °C) falling within the expected literature value 

The synthesis of benzil yielded a large amount of crude product, which allowed me and my lab partner to have plenty of product to work with as we purified our benzil. Based on the test for an unreacted product, due to the lack of color change in our precipitate, no unreacted product was found in our crude product before we proceeded to conduct our recrystallization or column chromatography experiments. range, the TLC had the same Rf values for both the control and the lab-collected sample (0.6), and the crystals appeared a pale-yellow color, which is consistent with what we would expect the appearance of pure benzil to be. The column chromatography process also indicated a likely pure sample based on a similar logic of melting points falling within the expected literature values (92–94 °C), the Rf values of the standard, and the lab obtained benzin matching on the TLC plate (0.38), and the appearance of the crystals being similar not only to the expected outcome but also to the appearance of the crystals from the previous experiment.

Based on the percent yield for these experiments, it is clear that recrystallization is a more effective purification technique for benzin, as it has a much higher yield of 95% when compared to the yield for column chromatography of 59%.

Post-Lab:

1.  What experimental detail was missing from the lab report you were given?

  Our lab report did not recommend that we put our solution on ice to help accelerate the recrystallization of the product. This process was, however, recommended by our lab instructor. This recommendation was beneficial because it allowed us to work more efficiently within the given time constraints of our lab. 

2.  Why is it important to record all that you do in the laboratory?

Recording the steps taken in the lab allows for the procedure to be repeated by others. This is particularly important for other chemists who may be conducting experiments using the experimental procedure you have created or who may be designing their own experiments based on your results. 

3.  After today’s lab, what changes will you make in your own recordings keeping in mind that someone next year might be asked to follow your instructions?

In my own recordings, I will make sure that I have the exact measurements used for each of my chemicals rather than approximates of the amounts used. I will also strive to provide accurate recordings for the length of time that each step took, for example, how long the steam bath was run. I feel that it would be helpful to record any notable changes that occur in my chemicals, such as obvious color changes or noticeable smells, which will not only help me to have a better understanding of my results but will also give a future student an idea of things to look for as they conduct their experiment.