Electron Paramagnetic Resonance (EPR) Spectroscopy
Source: David C. Powers, Tamara M. Powers, Texas A&M
In this video, we will learn the basic principles behind Electron Paramagnetic Resonance (EPR). We will use EPR spectroscopy to study how dibutylhydroxy toluene (BHT) behaves as an antioxidant in the autoxidation of aliphatic aldehydes.
1. Autoxidation of Butyraldehyde
- Prepare a solution of butyraldehyde (100 mg) and CoCl2·6H2O (1 mg) in 1,2-dichloroethane (DCE) (4 mL) in a 20 mL scintillation vial. Add a magnetic stir bar and fit the vial with a rubber septum.
- Attach the barrel of a 1 mL plastic syringe to a short piece of rubber tubing. Insert the rubber tubing into a latex balloon and secure the balloon to the tube with a rubber band and electrical tape. Inflate a latex balloon with O2.
The autoxidation of butyraldehyde affords butyric acid. The 1H NMR spectrum obtained from the reaction carried out in Step 1 shows the lack of an aldehydic C-H resonance and the presence of the resonances expected of butyric acid. In contrast, the NMR obtained from the reaction mixture from step 2 (with added BHT) displays signals consistent with butyraldehyde, with no butyric acid present. From these data, we observe the butyraldehyde has served as an antioxidant in aldehyde a
In this experiment, we explored the role of antioxidants in inhibiting autoxidation chemistry. We probed the mechanism of inhibition using EPR spectroscopy, which revealed that BHT serves as an antioxidant by quenching reactive radical intermediates via H-atom transfer.
Molecules with unpaired electrons can be challenging to characterize by NMR and thus EPR spectroscopy frequently provides useful and complementary information regarding these species. EPR spectroscopy is an experimental techni
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