30 Oxidation States and Mechanisms

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Freshman Organic Chemistry II (CHEM 125B) • • A difficult exam question shows how visible and NMR spectroscopy related to long-term misassignment of the structure for the triphenylmethyl dimer. Evidence from 1970 shows that Friedel-Crafts propylation involves an SN2 mechanism, not a protonated cyclopropane. Assigning oxidation states from -4 to +4 to the carbon atoms of proposed starting material and product allows choosing whether a reagent that is oxidizing or reducing or neither is appropriate. Beyond belonging to the appropriate redox class, the reagent must have an appropriate mechanism. Alcohol oxidations by elemental bromine and by Cr+6 reagents are shown to involve familiar substitution, elimination, and addition mechanisms. Mechanistic understanding allows adjusting conditions to make oxidation selective. • • 00:00 - Chapter 1. Exam Question on NMR, Color, and Triphenylmethyl Dimerization • 11:15 - Chapter 2. Evidence Against Protonated Cyclopropane in Friedel-Crafts Propylation • 16:30 - Chapter 3. Carbon Oxidation States from -4 to +4 • 32:22 - Chapter 4. Mechanism of Alcohol Oxidation by Bromine • 38:30 - Chapter 5. Mechanism of Alcohol Oxidation by Cr+6 • • Complete course materials are available at the Open Yale Courses website: http://oyc.yale.edu • • This course was recorded in Spring 2011.

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