Synthesis of Benzpinacolone: Photoreduction and Dehydration Processes with Spectroscopic Analysis

In this experimental procedure, Benzpinacolone was synthesized through a two-step process. Initially, benzophenone underwent photoreduction in 2-propanol, utilizing sunlight to absorb UV rays and facilitate the reaction. The subsequent step involved dehydrating benzpinacol to benzpinacolone, achieved by subjecting benzpinacol to acid-catalyzed rearrangement through the addition of iodine and acetic acid. The reaction involved refluxing, cooling, filtration, and washing with ethanol. The resulting percent yield was 71%, although challenges such as prolonged photoreduction and product loss during transfer contributed to a lower than expected yield.

The observed melting point was slightly lower than the theoretical value, attributed to potential impurities. Various spectroscopic analyses, including IR spectra for benzophenone and 1H NMR, 13C NMR, and IR spectra for benzpinacol and benzpinacolone, were conducted to identify functional groups and compare the compounds.

This experiment focused on utilizing ultraviolet light for the photoreduction of benzophenone and employing carbocation reaction conditions to induce the benzpinacol rearrangement. The absorption of UV light elevated the organic compounds to electronically excited states, enabling reactions that typically do not occur in their ground state.

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In Part 1, the reduction of benzophenone was carried out, driven by the UV radiation it absorbs, in the presence of 2-propanol. The resulting product, benzpinacol, underwent a subsequent reaction with iodine in acetic acid to produce benzpinacolone through rearrangement.

Procedure:

Part 1: Photoreduction of Benzophenone This experiment necessitates an irradiation period of at least 24 hours.

1. Dissolve 6.0 g of benzophenone in 30 mL of 2-propanol in a 125-mL Erlenmeyer flask, possibly requiring warming on a steam bath.
2. Add a dropper of glacial acetic acid to ensure the absence of alkali.
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3. Label the flask, tightly stopper it with a cork, and submit it to the instructor for irradiation for one to two days.
4. After irradiation, cool the test tube in an ice bath for 10 minutes and collect the solid through vacuum filtration.
5. Allow the solid to air dry, determine its weight and melting point, and compare the melting point with the literature value.

Part 2: Dehydration of Benzpinacol to Benzpinacolone.

1. In a 50-mL round-bottom flask (RBF) equipped with a reflux condenser, add 0.05g of iodine (2-3 small crystals) and 15 mL of glacial acetic acid. Introduce 3.0g of benzpinacol and heat the solution slowly until it gently boils.
2. Reflux the reaction solution for 5-7 minutes. Cool the reaction flask in an ice-water bath, leading to the precipitation of benzpinacolone as fine needles.
3. Thin the pasty product with a few mL of cold ethanol before collecting the crystals through vacuum filtration. Wash the crystals until free of iodine color.
4. Dry the benzpinacolone, calculate the percent yield, and determine its melting point.
5. Obtain the NMR and IR spectra and analyze the peaks.

Observations: Part 1:

• Step 5: Obtained 2.11g of benzpinacol; Melting point of 180°C.

Part 2:

• Step 2: Observed benzpinacolone precipitate in the form of fine needles.

Flow Chart: A flow chart details the stepwise progression of both Part 1 and Part 2 of the experiment.

Table of Reagents: A comprehensive table lists the molecular weight, density, melting point, boiling point, and concentration for the various compounds involved in the experiment.

Results (Data/Calculations included): Part 1:

• Mass of Benzpinacol: 2.11g
• Melting Point of Benzpinacol: 179-180°C
• Theoretical Melting Point of Benzpinacol: 188°C

Part 2:

• Mass of Benzpinacolone produced: 1.42g
• Theoretical Yield of Benzpinacolone: 2.01g
• Percent Yield of Benzpinacolone: 71%
• Experimental Melting Point of Benzpinacolone: 164-165°C
• Theoretical Melting Point of Benzpinacolone: 179°C

Calculations: The calculations involve finding the limiting reagent, theoretical yield, and percent yield for Benzpinacolone.

Results (Continued): Benzpinacolone was obtained with a 71% yield, and its observed melting point was close to the theoretical value, indicating a successful reaction. The physical properties of the product, such as color, crystal size, and smell, were noted.

• • IR spectrum results were provided by Prof. Chanmugathas; the major peaks will be discussed in the subsequent discussion.
  • Physical properties of the product were described, including color, crystal size, and smell.
  • NMR spectra for benzpinacol and benzpinacolone are included in the following pages.