Benzoin condensation - lab report PDF

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Benzoin Condensation

Abstract

This lab was focused on carrying out a catalytic dimerization reaction of benzaldehyde to give Benzoin. The use of a catalyst was also observed. Traditionally, there are two types of catalysts, cyanide and thiamine pyrophosphate (or vitamin B1), but because cyanide is an extremely poisonous substance, vitamin B1 is used as the greener catalyst. The experiment was a nonmicrowave method of observing crystallization and created a vacuum filtration setup. The resulting product had a purity that used several avenues to reach that conclusion, from percent yield and IR spectra analyzation. The following conclusions were observed: the mass of the product was 4.64 grams, a percent yield of 22% was achieved, and there was a melting point of 130.9-133.2. These results demonstrate the success of the reaction. There are some notable observations however that may have hindered success in the lab, such as all the product not being measured out. Additional hindered success could have been possible in the setting up of the hot bath and vacuum filtration, and not a complete crystallization.

Introduction

This lab was focused on carrying out a catalytic dimerization reaction of benzaldehyde to give Benzoin, a reaction that yields in complete atom economy since all the atoms that are in the benzaldehyde are fully retained in the final product of benzoin. The green nature of this experiment came in the form of opting to use Vitamin B1 as the catalyst instead of cyanide, because cyanide is extremely poisonous. The structural features that cyanide and thiamine have in common is that they are both easily deprotonated which makes them both capable of catalyzing the benzoin condensation. Vitamin B1 (or thiamine) is “edible, nontoxic, and significantly greener than cyanide” (Kenneth et al., 2016).

Vitamin B1 (or thiamine hydrochloride) acted as the catalyst in this experiment. A catalyst increases the rates of reactions without being fully consumed, Enzymes are an example of this. The catalyst attacks the carbonyl atom, which results in an intermediate that essentially rearranges itself to give a benzylic anion. The anion then takes on the form of a nucleophile, attacking the carbonyl atom that is on the second molecule.

The process of recrystallization is heavily observed in this experiment. This process involves dissolving a crude product in a hot solvent and then allowing the solution to cool to room temperature, allowing product to be collected (Chem LibreTexts).

Table of Physical Constants Compound Benzaldehyde Thiamine hydrochloride Benzoin Ethanol Sodium hydroxide

-26

Boiling Point (°C) 179

337.3

260

250

1.4

212.24 46.07

137 -114.1

651.2 77.1

1.31 0.789

39.9

318

315

2.13

Molar Weight (g/mol) 106.12

Important Compounds

Chemical Formula

Benzaldehyde

C7H6O

Melting Point (°C)

Structure

Density (g/ml) 1.044

Thiamine hydrochloride

C12H18Cl2N4OS

Benzoin

C14H12O2

Ethanol

C2H6O

Sodium hydroxide

NaOH

Methods Around 0.35g of thiamine hydrochloride was dissolved in of water. To this, 3 mL of 95% ethanol was added, and the solution was cooled by through the use of an ice water bath. Then, 1mL of 2M NaOH was put in a small test tube, and this tube was also cooled. Five minutes of cooling commenced. Then, the cold sodium hydroxide was added to the thiamine hydrochloride solution in a dropwise method. This occurred until the solution turned a yellow color. To this solution, 2mL of benzaldehyde was added, as well as boiling stones. The resulting mixture was heated for about 90 minutes. The solution was cooled to room temperature and then the tube was placed in an ice bath to jumpstart the process of crystallization. The solution spent around twenty minutes in the ice bath to ensure crystallization, after which the crude product was collected through vacuum filtration. The solid product was washed with 5mL of cold water and then recrystallized from 95% ethanol. The product was then dried, weighed, and the perfect yield, melting points and IR spectra were all analyzed and calculated.

Reaction Mechanism

Data & Results Percent Yield of Benzoin Calculation 20 mL of benzaldehyde 4.64 g of benzoin m = Dv m (benzaldehyde) = (1.044g/mL) (20mL) m (benzaldehyde) = 20.88 g

Actual Value: 4.64 g Theoretical Value: 20.88

1 mol benzoin 1 mol benzaldehyde x x 20.88 g benzal . x 106.12 g /mol benzaldehyde 2 mol benzaldehyde Percent Yield=

g benzoin mol =20.88 g benzoin 1mol benzoin

212.24

4.64 g x 100=22 % 20.88 g

Melting Point Dried Product – 130.9 oC – 133.2 oC Literature value – 137 oC

Figure 1. This figure shows the IR spectra analyzed of benzoin. There is an OH stretch at around 3500, an aldehydic CH stretch at 2700, and a ketone carbonyl stretch around 1715.

Figure 2. This figure shows the IR spectra analyzed of benzaldehyde. There is C-H methyl stretching at around 2800, C-H aromatic, C-H methyl bonding, and C-N and C-C bonds.

Conclusion This lab was successful in using the catalyst to create benzoin. The lab adopted the nonmicrowave method. Vitamin B1 was used as a catalyst in the experiment because it is the greener alternative than cyanide. The process of recrystallization was observed, the product was weighed to 4.64 g, with a percent yield of 22%. The purity of the product can be observed through the value of the melting point. The melting point of benzoin was observed to be 130.90-133.2, and the literature value is 137. This means that the lab was very successful. This can also be observed in the theoretical value of benzoin, which should have been 20.88 g. Possible errors that may have hindered successful implementation of the experiment may have been that the solution had not been heated enough or boiled enough to ensure proper recrystallization. Additionally, it is

possible that not all the product was weighed out. Future experimenters should ensure that the full 90 minutes of heating is completed and all the product is allowed to recrystallize and measured out adequately.

Citations Kenneth F. Cerny, Marietta H. Schwartz and Christopher E. Katz, 2016, Laboratory Manual for Organic Chemistry (4th edition) Libretexts. (2020, September 23). Recrystallization. Retrieved February 18, 2021, from https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_ Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properti es_of_Matter/Solutions_and_Mixtures/Case_Studies/RECRYSTALLIZATION...