Friedel-Crafts Revisited Oxidizing/Reducing Scheme Chemistry 125: Lecture 67 April 11, 2011 Triphenylmethyl Spectra Friedel-Crafts Revisited Oxidizing/Reducing Scheme Alcohol Oxidation Mechanism This For copyright notice see final page of this file

Triphenylmethyl Dimer CPh3 Triphenylmethyl Dimer 60 MHz PMR Spectrum in CS2 a,a-coupling a,para-coupling J = 3 Hz J 10 Hz 25 H 2 H 1 H

Triphenylmethyl Dimer 60 MHz PMR Spectrum in CS2 a,a-coupling a,para-coupling 300 400 500 l (nm) Absorbance -60°C 318 nm non-bonding SOMO +21°C 513 nm ultraviolet

Correct dimer dropped for 46 years, because they thought Journal of the American Chemical Society – Next Year Correct dimer dropped for 46 years, because they thought it should be colored!

Third Thoughts on Friedel-Crafts This

Rearrangement in Friedel-Crafts Alkylation Lee and Woodcock (1970) Where? <2% D2 elsewhere 3/7 2/7 < 2% partial or full methide shift en route to n-propyl benzene Deno (1968) D+ D 90% + with one D forms via benzene nucleophile SN2 AlCl3 + Cl 5°C, 45 min D H + gives i-PrPh product (SN1) H What if CH3 gets stuck halfway? CH3 Which of these gives the n-PrPh product in Friedel- Crafts? gives n-PrPh product (SN2) Still + + Ipatieff 1940 J Org Chem conversion of benzene calc. assuming ethyl second subst is 12x faster than first + gives n-PrPh product! Still gives n-PrPh product (SN2?) CH3 Cl AlCl3 Cl AlCl3 + Nu Protonated Cyclopropane (stability between 1° and 2° cations) PROBLEM: How to decide? why not Methide Shift? Hydride Shift

Oxidation/Reduction as Bookkeeping (for purposes of reagent selection) Need a “reducing agent” (CH3)3C C CH3 O (CH3)3C C CH3 OH H ? C C “reduction” +2 +1 -2 H2O Oxidizing: Reducing: Neither: H-Cl KCl Br-Br RS-SR CrO3 CH4 LiAlH4 NaH K RSH Not all reducing agents would work satisfactorily, but it is typically futile to try a reagent from the wrong redox class. H OH -2 -3 -1 But photosynthesis changes H2O into O2 and “H-” (NAD+  NADH) H2C CH2 H2O

Carbon “Oxidation States” (Prof. Siegel, Lecture 47, 2010) This scheme from 2010 is better organized that this year’s which follows. 0:49-11:35

Carbon “Oxidation States” 2011

Alcohol Oxidation (e.g. J&F Sec. 16.14) 2010 Br2 Mechanism

Alcohol Oxidation (e.g. J&F Sec. 16.14) 2011 Br2 Mechanism

Alcohol Oxidation (e.g. J&F Sec. 16.14) 2010 Cr+6 Mechanism

Alcohol Oxidation (e.g. J&F Sec. 16.14) 2011 Cr+6 Mechanism

Pyridinium Chlorochromate (PCC) Stopping at Aldehyde (e.g. J&F pp. 805-6) -O-Cr-Cl O + + CrO3 + HCl Pyridinium Chlorochromate (PCC) n-C9H19-CH2OH n-C9H19-CH=O (92% yield) PCC CH2Cl2 no water; no gem-diol; no overoxidation (Loudon) “In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate matters.” (Wikipedia)

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