Synthesis of Benzopinacol and Benzopinacolone

Sabrina Liao

Purpose

1. Lab 1 (Synthesis of Benzopinacol): the purpose of this experiment is to synthesize benzopinacol from benzophenone. This is an interesting example of photochemistry, in which sunlight initiates the reaction. We will use the product as the starting material for a pinacol rearrangement later in the semester

2. Lab 5 (Synthesis of Benzopinacolone): In this experiment, an example of the pinacol rearrangement will be performed, as β-benzopinacolone is synthesize d from β benzopinacol

Reaction

Mechanism

Table of reactants and reagents

Reactant/reagent  (LAB 1)	Amount (g or ml)	Moles (mol)	Molar mass (g/mol)	Density (g/ml)	mp/bp (C)	comments
benzophenone	2.1 g	0.012 mol	182.217 g/mol	1.111 g/cm^3	305.4 celcius/ 48.5 celsius (alphaform) & 26 celsius (beta form)	reactant
2-propanol	¾ of the 250ml beaker	n/a	60.1 g/mol	0.786 g/cm^3	-89 celcius/ 82.6 celcius
Glacial acetic acid	Approx 0.05 mL	0.00021 mol	60.052 g/mol	1.05 g/cm^3	16.6 celsius/ 117.9 celcius

Reactant/reagent  (LAB 5)	Amount (g or ml)	Moles (mol)	Molar mass (g/mol)	Density (g/ml)	mp/bp (C)	comments
iodine	0.21 g	0.00083 mol	253 g/mol	4.93 g/cm^3	113.7 celcius/ 184.4 celcius	reagent
Glacial acetic acid (for Lab 5)	10.0 ml	0.168 mol	60.052 g/mol	1.05 g/cm^3	16.6 celsius/ 117.9 celcius	Solvent, catalyst
benzopinacol	1.9 grams	0.013 mol	366.46 g/mol	1.24 g/cm^3	174-180 celcius/ 508 celcius	reactant
ethanol	A few	n/a	46.07 g/mol	0.789 g/cm^3	-117.3 celcius/ 78.37 celcius	solvent

Procedure

2.1 grams of benzophenone and being placed in a culture tube. Then the tube was filled with 2-propanol and the mixture was mixed to be dissolved. Then, 1 drop of glacial acid and was obtained into the tube followed by the container being filled to the top with 2-propanol. Then the screw cap was placed on the top. The tube with benzopinacol was then labelled and placed below the sun for 3 weeks. Since the yield of benzopinacol or lab 1 was only 1.1 grams while 2 grams were needed, 0.8 more grams of benzopincacol was obtained, making the reactant of Lab 5 weigh 1.9 grams. The benzopinacol was purified by vacuum filtration, followed by being combined with 10.0 ml glacial acid and 0.21 grams of iodine. The mixture was then heated on heating mantle and stirred. Dark red color was observed in the mixture after a few minutes. 40ml of H2O was then obtained inside the mixture. After that, the product was extracted through vacuum filtration.

Table of products

LAB1 product	Yield (g)	yield(mol)	Theoretical yield(g)	Percent yield	Molar mass	Actual mp (C)	literature mp (C)
benzopinacol	1.1 grams	0.003 mol	4.40 g	50%	366.46 g/mol	174.5 celcius	174-180 celcius/

Lab 5 product	Yield (g)	yield(mol)	Theoretical yield(g)	Percent yield	Molar mass	Actual mp (C)	literature mp (C)
benzopinacolone	1.447 g	0.041 mol	1.447 g	79.9%	348.45 g/mol	182.5 celcius	181-182 celcius

Discussion

Based on the comparison of the IR spectra, the presence of the carbonyl peak in benzopinacolone at around 1700-1750 cm⁻¹and the absence of it in benzopinacol indicates the progress of the reaction from benzophenone to benzopinacol and then to benzopinacolone. Besides,the OH stretch at 3300 cm⁻¹ and C-O stretch shows the formation of alcohol functional group. The reppearance of carbonyl group of benzopinacolone at 1700-1750 cm-1  shows that there’s ketone formed again.  The IR spectra provide strong evidence of the formation of benzopinacol and its further reaction to benzopinacolone.  The products being successfully made can also be proven by the measured melting points.  Our experiment yielded a melting point for benzopinacol at 174.5 degrees Celsius which is very closed to the theoretical value. Similarly, the measured melting point for benzopinacolone was  182.5 degrees Celsius. These melting point and IR spectrums illustrates the synthesis of the desired products.

References

PubChem. “PubChem.” Nih.gov, National Library of Medicine, 2022, pubchem.ncbi.nlm.nih.gov/.