Lab 9: Multistep Reaction

Objective: The purpose of this experiment is to practice performing complex synthesis by performing a chemical reaction over multiple steps. This lab will allow us to practice both precision and accuracy measures to create a high yield of a desired product. We will test products for purity using a variety of methods, including TLC, IR, UV, NMR,  or mass spec, and, if desired, we will also use this lab as an opportunity to learn about rotary evaporation under the direction of our lab instructor. 

Methods:

Activity 1: Benzaldehyde to Benzoin

• Placed 1.3g of thiamine hydrochloride in an Erlenmeyer flask and added 4 mL of water.

• Swirled flask until all contents fully dissolvent. 

• Added 15 mL of ethanol to the flask.

• The flask was placed into an ice bath until the temperature of the solution was brought to below room temperature.

• Slowly added 0.5 g of solid sodium hydroxide to the flask while carefully monitoring the temperature, preventing the solution from rising above  20 degrees Celsius. A yellow precipitate was formed. 

• Added 7.5 mL of distilled benzaldehyde and heated the solution at 60 degrees Celsius for just over 1.5 hours on a hot plate. A watch glass was placed over the top of the flask to minimize the loss of product. With heat, the solution became a brilliant red color. A sweet smelling aroma was noted as solution heated.

• Several TLC plates were run to monitor reaction progress using a lab-provided standard against the student-prepared sample.

• At the 1.5 hour mark, an additional 0.5 g of sodium hydroxide was added to the solution causing the solution to turn a deep, muddy brown color. 

•  The reaction was allowed to run for another 30 minutes.

• The solution was then pulled off of heat and allowed to cool on the benchtop. 

• Clumpy, "sand like" crystals formed, which were separated from the solution via vacuum filtration.

• The crystals were washed with a 1:1 solution of 95% ethanol and water. This resulted in a golden colored filtrate and pale yellow crystals.

• The crude crystals were then purified with 95% ethanol.  The addition of ethanol created a pale purple appearing solution. 

• The solution was brought to a boil before being placed on ice to facilitate crystal reformation. White, powdery crystals appeared in the flask when put in the ice bath.

• The crystals were again separated via vacuum filtration, and a pale yellow filtrate was obtained.  

• Both the golden colored filtrate from earlier "first crop" and the pale yellow filtrate were combined in an Erlenmeyer flask and placed on a hot plate set to 6. As the solution heated, it became a dark brown color. 

• The solution was allowed to heat until 50% of the solution had evaporated.

• The solution was then placed on ice to allow for crystals to form.

• Vacuum filtered crystals from the solution. Analyzed the filtrate by TLC to determine if most of the product had been removed from the solution during recrystallization.

• Combined crystals from both the "first" and "second crop." Covered the flask with a Kim-Wipe and placed all crystals in the lab drawer for further drying and analysis next week.

Activity 1: Analysis:

• Retrieved the flask containing product from the previous week. A cream colored, powdery solid was noted in the flask.

• The flask was weighed, and the % yield was calculated.

• Analyzed via melting point.

• Analyzed via TLC.

• Analyzed via NMR.
  
  
  

Activity 2: Benzoin to Benzil

• 2 g of benzoin and 7 mL of concentrated nitric acid were combined in an Erlenmeyer flask.

• The reaction flask was placed over a boiling steam bath for 11 minutes, ensuring to keep the flask inside the fume hood for the entirety of the boiling process. A bright yellow precipitate formed in the glass, and orange fumes were noted as the reaction mixture heated.

• Removed flask from heat and added 40 mL of water to the flask.

• Allowed flask to cool to room temperature.

• Extracted precipitate from solution via vacuum filtration.

• Recrystallized the solvent on low heat using a solvent of 1:1 water to ethanol.

• Vacuumed filtered bright yellow crystals from solution, weighed the wet product, and placed the product to dry in a lab drawer until it could be analyzed next week.

Activity 2 Analysis:

Activity 3: Benzil to 2, 3-Diphenylquinoxaline: 

Sublimation was not performed at the direction of the lab instructor.

• Ground 1 g of benzil in a mortar and pestle until a fine powder was formed

• Added 1 g of ground benzil and 0.5 g of 1,2-phenylenediamine to a test tube. Mixed contents until thoroughly combined.

• Placed on a steam bath and allowed the reaction to run for 10 minutes. A light tan solvent was formed.

•  Allowed product to come to room temperature before placing on a hot plate set to a low setting.

• Recrystallized with a methanol and water solvent pair. 

• Placed product in the lab drawer for analysis next week.

Activity 3 Analysis:

• The product was retrieved from the laboratory drawer and the sample was prepared for analysis via melting point, TLC, and IR.

Activity 5: Benzil to Hydrobenzoin/Stilbene Diol (racemic mixture):

• Combined 0.5 g of benzil and 5 mL of 95% ethanol in an Erlenmeyer flask.

• Placed flask on ice until a fine, near film-like suspension was noted in the flask. 

• Added 0.1 g of sodium borohydride to the flask and removed from the ice bath.

• Allowed reaction to run for 10 minutes, letting solution naturally heat to room temp. No yellow color was observed within the solution.

• Ran a TLC using lab-provided benzil standard to determine rxn process. Solvent of 3:1 hexane and ethyl acetate was used.

• Placed the flask on low heat to dissolve all solid products. Recrystalized with about 10 mL of DI water. 

• Filtered white, fine powder crystals under vacuum filtration and placed in a lab drawer to dry.  

• Reheatedthe  solution and attempted to recrystallize with sodium chloride. No success.

• Attempted to recrystallize after heating and placing in an ice bath. No success.

• Attempted to scratch the glass to encourage crystallization. No success.

Unable to successfully get second crystals before end of lab period.

• Placed the solution in the drawer to see if crystals would form after being allowed to sit. Will repeat the experiment next week if necessary.

Activity #5 Analysis:

• Removed product from lab drawer - no crystals had appeared within the solution.

• Consulted with lab instructor, decided to proceed with analysis despite lack of crystal formation under the assumption that the final product was a racemic mixture, which inhibited crystal formation. 

• Conducted melting point analysis of meso product. 

• Conducted RI analysis of racemic mixture.

• Conducted TLC analysis of both meso product and racemic mixture. 

Discussion:

Melting Point: The melting point value for the benzoin product synthesized for activity 1 was 165.4 °C. This is much greater than the accepted literature value of 137 °C. Thiamine hydrochloride has a melting point of 250 °C, while sodium hydroxide has a melting point of 323 °C, which were both used as reactants within the experimental procedure. These reactants with much higher melting points may be present within the final product rather than having a pure benzoin product.

The melting point of the 2,3-diphenylquinoxaline product was 121.0 °C -123 °C. This range is slightly below the expected range of 125-128°C. The decrease in the expected range may be indicative of a contaminant within the final product, but as the range is only slightly under the accepted literature value, a second melting point analysis could be run to ensure a systematic error is not present in our data recording process. 

The melting point for the meso hydrobenzoin product synthesized in Activity 5 was 135 °C. This fits well within the literature value for the melting point of meso-hydrobenzoin of 134-139°C, lying close to the middle of the expected range. This indicates a high probability that meso-hydrobenzoin was synthesized in this experiment.

 

RI: The RI for stilbene diol ((1R,2R) and (1S,2S)) racemic mix  was1.3432. While not matching the expected literature value of  1.6230, this value is much more consistent with the literature value of RI for NaOH of 1.3576. Due to the difficulty of the recrystallization for the racemic mixture and the inability to form crystals for the final product, a larger-than-expected volume of NaOH was used during the recrystallization process. The large NaOH volume present within the final sample likely explains an RI reading that is more closely aligned with the predicted literature value for NaOH rather than the literature value for stilbene diol. 

% yield: The percent yield for activities 1, 3, and 5 was close to the expected yield as indicated by the lab report handout, with each experiment having less than a 5% error, with % errors of 4.47%, 4.34%, and 4.35%, respectively. Activity 2 had a slightly greater % error with a % error calculation of 6.82%. The successful yield of nearly all of these experiments with low margins of error indicates that all four products were likely synthesized with good success.

TLC: The benzoin TLC plate indicated that both samples contain benzoin, as the lab-prepared sample and the standard had an RF value of 0.85, confirming that the expected product is present. The lab-prepared sample has fewer spots, suggesting greater purity compared to the standard, although streaking still suggests impurities or incomplete reaction, which is atypical of what we would expect from a pure standard. The lab-prepared product also exhibits multiple spots and streaking, which indicates that the lab-prepared sample is reasonably pure, but some impurities exist within the final product.

The TLC plate for the benzil reaction showed a laboratory standard benzil with a high RF value (0.82), consistent with non-polar character, as expected for pure benzil. The synthesized sample has a significantly lower RF (0.43), suggesting that the two compounds are not the same chemical. The main compound is more polar, indicating the presence of unreacted starting material (e.g., benzoin or hydrobenzoin) or incomplete oxidation. While the synthesized product had only one spot indicating product purity, it likely doesn't contain much, if any, pure benzil based on their differing RF values (0.43 and 0.82).

The TLC plate for the benzil to hydrobenzoin/stilbene diol (racemic mixture) product shows a benzene standard with Multiple spots and streaking, suggesting impurity. This is unexpected as this standard was taken from the lab-prepared standard, and this abnormality should be taken under careful consideration when comparing these results with the two laboratory-synthesized products.  For the meso hydrobenzoin product, the spot at Rf 0.81 is likely pure benzil, which is non-polar and travels far with the solvent. The spot at Rf 0.81 again corresponds to the calculated RF of the benzil standard, suggesting the presence of residual benzil within the final product. The other spot (Rf ~0.47) likely corresponds to meso-hydrobenzoin, which is more polar due to the two hydroxyl groups. The two spots for the stilbene diol racemic mixture racemic mixture of enantiomers or diastereomers, separated the TLC plate. The lower Rf values indicate greater polarity, which is expected for diols.

IR: The IR for Stilbene Diol (Racemic Mixture) had a broad peak from 1700–2000 cm⁻¹. The broad character of this peak is outside of the standard carbonyl region (~1700 cm⁻¹) and, as such, may not be a significant functional group and may be due to instrumental noise or background. A very broad peak from 2300–3000 cm⁻¹ was noted. This region typically includes: C–H stretches (sp³ and sp² hybridized carbons): ~2850–3100 cm⁻¹ and O–H stretch (alcohols or phenols), especially if it extends broadly over this range. In Stilbene Diol, this broad peak is most likely due to hydrogen-bonded O–H stretches from the diol groups. It is possible there are masked C–H stretches (sp² and sp³): ~2850–3100 cm⁻¹ caused by overlapping with the broad O–H band.

Based on the very broad O–H stretch (2300–3000 cm⁻¹), the IR spectrum is consistent with stilbene diol.

Mass Spec: The mass spec for benzil indicated an [M+] peak of 213 amu. The Westminster Mass Spec machine added a +1 value to all M+ readings, giving us a true amu value of 212 amu. The true molecular ion of 212 amu is close to the proposed literature value for the mass of benzil (C₁₄H₁₀O₂), 210.23 g/mol. The Base Peak of 195 amu (machine), or 194 amu true (machine adds 1 amu to all masses, so always subtract 1 for correct interpretation) is a very common fragment ion for benzil, as the loss of a C=O group (28 amu) or phenyl ring can result in this fragment. Overall, the mass spec data provides good supporting evidence for benzil being the final product of activity #2.

NMR: The NMR data for benzoin synthesis is partially consistent with benzoin, but some inconsistencies suggest either experimental anomalies or impurities within the sample. The 4 ppm (Multiplet) fits the CHOH proton, which is adjacent to both an OH and a phenyl ring, making it deshielded and complexly split. The 6 ppm (Singlet) likely corresponds to the OH proton, which often appears as a singlet and is exchangeable. This value is reasonable, especially if hydrogen bonding is involved. The 7.2 ppm (Doublet) is consistent with expected results for benzoin, as aromatic protons typically appear in this range. Benzoin has two phenyl rings, and under certain conditions or symmetry, a doublet might be observed for some protons.

The 1.5 ppm (Triplet) is unexpected in benzoin, which has no alkyl chains. A triplet here suggests a CH₃–CH₂– type fragment, which benzoin lacks. This might be caused by a contaminant from the solvent. The 8 ppm (Quartet) is also not expected, but could be aromatic coupling misinterpreted as a quartet.

UV/Vis: The UV/vis data for Stilbene Diol (Racemic Mixture) had two prominent λmax peaks and interference noted in lower frequencies. The λmax peak at 320 nm is indicative of a strong π → π* transition, a classic signature of a conjugated stilbene system. The λmax peak at 370 nm likely shows a weaker n → π* transition, which may be influenced by the presence of hydroxyls or extended electronic delocalization. The 200–270 nm interference is expected from intense aromatic absorptions and should be excluded from primary analysis.

Post-Lab: None.