Photochemistry: Photoreduction of benzophenone and rearrangement to benzpinacolone
In this experiment Benzpinacolone was synthesized in a process that contained two steps. First the photoreduction of benzophenone in 2-propanol, which was done by placing the flask under sunlightfor the absorption of the UV rays to carry out the reaction. Then the second part was the dehydration of benzpinacol to benzpinacolone, where the benzpinacol product was converted to a ketone by the acid catalyzed rearrangement of the benzpinacol to the benzpinacolone; this was done by adding iodine and acetic acid to benzpinacol. The reaction was then refluxed, cooled in an ice bath, filtered and washed with ethanol. After allowing the benzpinacolone to dry, we found that our percent yield was 71% .Even though the yield was good, considering that the photoreduction period took a couple extra days due to lack of sunlight, loss of product can be attributed to this since we only could get 2.11g/3.0g of benzpinacol from the start of the lab. And since benzpinacol was the limiting reagent, before starting part 2 of the lab this already set us up for some loss in % yield. Also when we transferred our product from the filtration funnel to the watch glass there was product loss because many of the crystals were stuck in the flask and could not be removed to get an accurate weight. The observed melting point of the product was 164-165°C which is a little lower than the theoretical mp value; this can be attributed to impurities in our product. An IR spectrum was given and analyzed for benzophenone. 1H NMR, 13C NMR, and IR spectrums were also given for benzpinacol and benzpinacolone and the peaks were analyzed for the purpose of recognizing functional groups and noting any similarities and differences between the compounds that were made. 1.Introduction: Use ultraviolet light to carry out the photoreduction of benzophenone and use carbocation reaction conditions to effect the benzpinacol rearrangement. The absorption of the UV light creates electronically excited states in organic compounds, making the compounds to be in a higher energy state that will allow the compound to many reactions that normally do not happen when it is in the ground “more stable” state. In this experiment, the reduction of a ketone, benzophenone will be done in Part 1. Benzophenone absorbs ultraviolet radiation and uses the energy obtained as the driving force for its reduction by 2-propanol (isopropyl alcohol). The product, benzpinacol subsequently reacts with iodine in acetic acid to give the rearrangement product, benzpinacolone.
Mechanism: Part 1-Photoreduction of Benzophenone
Benzophenone to benzopinacol:
Step 1: hydrogen abstraction from 2-propanol
Step 2: hydrogen abstraction from 2-propanol radical
Step 3: dimerization (Combine the 2 benzhydrol radicals)
Mechanism: Part 2-Rearrangement to Benzpinacolone
Benzopinacol to benzopinacolone:
Figure 3.1: Conversion of Benzophenone to the Diradical Species Figure 3.2: Reflux Set-up
Part 1:Photoreduction of Benzophenone
This experiment requires all irradiation period of at least 24 hr. 1. In a 125-mL E. flask, dissolve 6.0 g of benzophenone in 30 mL of 2-propanol (isopropyl alcohol). Warming on a steam bath may be necessary....