Max Muller - Lab 9: Synthesis Chroma

Objective Statement: In this lab, I will learn about the importance of writing through my lab notes in my laboratory notebook. It is also to focus on continuing to practice very important purification methods of chromatography. By qualitatively analyzing percent yield by testing my reaction mixture for unreacted starting materials.

Benzoin

Polarity: very slightly-polar

Mass: 212.24 g/mol

MF: C14H12O2

Density: 1.310 g/cm3 (20 °C)

BP: 330 to 356 °C

MP: 135 to 139°C

Solubility:

Slightly soluble: Water, Ether

Soluble: Chlorine

Very soluble: Ethanol, Chloroform

Benzil

Polarity: non-polar

Mass: 210.23 g/mol

MF: C14H10O2

Density: 1.23 g/cm3, solid

BP: 346.0 to 348.0 °C

MP: 94.0 to 96.0 °C

Solubility: not soluble in water

soluble in, ethanol, diethyl ether, and benzene

Pre-Lab Questions:

For the most efficient time management, you should outline the lab in advance. First, you need to gather all materials for the synthesis and then start the synthesis. What do you do after the synthesis is complete and then what comes next?

From general chemistry you know that in an oxidation reaction, electrons are lost and in a reduction reaction, electrons are gained. Please find and write a description of an organic oxidation reaction (remember that your Wade text is a good resource). This needs to be a written response. Drawing the chemical equation of an oxidation reaction will not suffice as an answer.

An organic oxidation reaction causes an increase in the number of carbon-halo atoms (N, O, and S) bonds and a decrease in the amount of carbon-to-hydrogen bonds in a molecule.

Your sample needs to be absorbed into silica gel. How should this be done? Your answer needs to include information about what rotatory evaporation is. You can refer back to your techniques book for information.

Rotary evaporation is how to absorb our sample into the silica gel. Rotary evaporation works by reducing the volume of a solvent by distributing it as a film thin across the interior of the vessel at elevated temperatures and reduced pressure. This promotes the rapid removal of excess solvent from less volatile samples.

Procedure:

Activity 1: Synthesis of Benzil from Benzoin

Prepared a steam bath with water boiling vigorously. Wrapped the bath in foil and covered the top with a watch glass to prevent heat from escaping.
Added a 200 ml round bottom flask onto the steam bath with 2.15g of benzoin and 7 mL of concentrated nitric acid in the flask. Erhlenhmeyer flask would work better so you do not have to spend 15 minutes scrapping the round bottom for your crude benzil later.
Let the flask boil for 11 minutes, nothing longer. Ensures that the benzoin molecules do not fracture from exposure to the heat for too long. Mixture changed from a light yellow-orange to a dark red-orange, and back to yellow after sitting. Also, colored steam or vapor coming out of the top of the flask.
Removed the round bottom flask from the steam bath and add 37.5 mL of water to the solution in the flask. Let it sit to reach room temperature. Swirl the solution every few minutes to ensure that the compound coagulates. Also, put the solution in ice to form solids.
Vacuum-filtered solids out of solution.
Weighed crude benzil on a watch glass for the efficiency of the reaction. See the results below.
Split the crude solution in half by weight for the two types of analysis: recrystallization column chromatography.

Activity 2: Test for Unreacted Benzoin

~5mg crude benzil was dissolved in a test tube of hot ethanol, and 10 drops of NaOH were added to check for color change. This would indicate the presence of unreacted benzoin.

Activity 3: Purification of benzil by Recrystallization

.98 g of the crude into an Erhlenmeyer flask with 10 mL of hot ethanol.
Gravity filtered while hot to remove impurities; benzil remained dissolved in solution.
Drops of water added until solution reached cloud point, then cooled to room temp.
Vacuum filtered to isolate purified crystals. Rinsed with cold water and left to air dry.

Activity 4: Purification by Column Chromatography

Set up a column chromatography chamber
Crushed 1.1g of crude benzil in a motor and placed in the chamber.
Used 40 mL of 1:1 hexane/ethyl acetate for chromatography. Note this system is too polar, would recommend 4:1 hexane/ethyl acetate
Run the chromatography chamber until the yellow band is ready to be collected in fractions. One yellow band, the benzil being completely non-polar.
Collected a total of 15 1-2 ml benzil fractions, and tested fractions 1, 5, and 10 in the results below.

Results:

Activity 1 photos:

Crude benzil: 2.08 grams
- 2.08 grams of yellow solid recovered/2.15 grams starting) x 100% = 97% mass recovered
Benzil used in column Chromatography: 1.1 g
Benzil used for recrystallization: .98 g

Column product

The first test is the benzil standard, the second is fraction 1, third point is fraction number 5.
- Highest points shared Rf values from standard in a 1-.5 range difference. Too many spots so conducted more accurate Rf values with TLC to the right.
For melting point half melted at 70-75 degrees Celsius, other half melted at 89-91 degrees Celsius

4:1 hexane: ethyl acetate solvent system

TLC Analysis

S is the benzil standard
- Rf = 3.3 cm / 4.0 cm = .83
C is column chromatography fraction number 10
- Rf spot 1 = 3.2 cm / 4.0 cm = .8
- Rf spot 2 = 2.7 cm / 4.0 cm = .675
- Rf spot 3 = 1.9 cm / 4.0 cm = .475
- Rf spot 4 = 1.0 cm / 4.0 cm = .25
R is the recrystallization product
- Rf = 3.3 cm / 4.0 cm = .83
4:1 hexane: ethyl acetate solvent system

Recrystallization product

.45 grams of fluffy yellow powder
- - (.45 g of recrystallized product / .98 g of starting material) x 100% = 45.92 % yield
first melting point test 94-96 degrees Celsius
second melting point test 93-95 degrees Celsius

Discussion/Conclusion:

The 97% recovery indicates that the synthesis of benzil from benzoin in nitric acid went to nearly full chemical completion. Along with the test for unreacted starting material, there was no unreacted benzoin in the solution. A nearly 46% recovery from recrystallization was lower than expected. We did have some issues recrystallizing the product with water and it could have also been possible that we experienced heat loss in vacuum filtration causing some of the benzil to not be properly extracted. The melting point of the recrystallized product indicates pure benzil when compared to its literature value. But TLC analysis showed the presence of some small vague spots below the highest spot. The highest spot, however, the most present and dark spot, had the same Rf value as the benzil standard. Our column product was all over the place. We experienced to different melting points, one a lot lower and the other slightly lower than the literature value. Where TLC analysis showed multiple dark spots with Rf values much lower than the standard. We still did have one spot at the Rf value of the standard. The multiple spots of impurities are most likely due to the improper solvent system when conducting column chromatography. We used a too-polar system and that caused all of the products in our crude to be pulled out together and that would explain the multiple spotting shown on the TLC paper.

Reflection:

In this lab, we took techniques learned from previous labs to conduct column chromatography and recrystallization for the comparisons of these technique's ability to purify. using an Erhleynmeyer flask instead of a round bottom flask would have made the extraction of the crude product easier. A less polar solution mixture would have created more pure results from column chromatography. However, recrystallization yielded a very pure product, despite it being lower than expected. If I were to repeat this lab I would make sure to run a more effective column chromatography technique by using a different hexane: ethyl acetate mixture and I would let my recrystallized product cool for longer before filtration to ensure all the benzil recrystallizes before extraction.

Post Lab Questions. What experimental detail was missing from the lab report you were given?

The lab procedures did not mention anything about chromatography and TLC analysis so those were up to use to determine and create. The lab's procedures also did not mention anything about the fumes/color changes of the mixture during the steam bath.

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

So the experiment can repeated properly and exactly as attended. Also to explain results/things that happen during the procedure for analysis and understanding of the effectiveness of each technique.

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

I will make sure to make notes of observations that happen during my experiments and all the small details about setting up certain apparatuses and why I made those decisions.