Chemistry 125: Lecture 67April 11, 2011
Triphenylmethyl SpectraFriedel-Crafts Revisited
Oxidizing/Reducing SchemeAlcohol Oxidation Mechanism This
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Triphenylmethyl Dimer60 MHz PMR Spectrum
in CS2
25 H 2 H 2 H 1 H
J10 Hz
J = 3 Hz
CPh3,a a-coupling
,a para-coupling
Triphenylmethyl Dimer60 MHz PMR Spectrum
in CS2
,a a-coupling
,a para-coupling
300 400 500 l (nm)
Abs
orba
nce
-60°C+21°C
ultraviolet
318nm
513nm
non-bonding SOMO
Journal of the American Chemical Society – Next Year
Correct dimer dropped for 46 years, because they thought
it should be colored!
What if CH3 gets stuck halfway?
Rearrangement in Friedel-Crafts Alkylation
AlCl3+ Cl +
Cl
AlCl3
+
Hgives
n-PrPh product(SN2)
gives i-PrPh product (SN1)
Cl
AlCl3
+
CH3
givesn-PrPh product(SN2?)
Hydride Shiftwhy not
Methide Shift?
Protonated Cyclopropane(stability between1° and 2° cations)
Still
H+
3/7 2/72/7Deno (1968)
D+
Which of these givesthe n-PrPh product in Friedel- Crafts?
CH3+
+
Nu
with one D
Where?
PROBLEM:How to decide?
givesn-PrPh product!
Still
DD
Lee and Woodcock (1970)
DD
90%
< 2% partial or full methide shift en route to n-propyl benzene
<2% D2 elsewhere
forms via benzene nucleophile SN2
5°C, 45 min
Oxidation/Reduction as Bookkeeping(for purposes of reagent selection)
(CH3)3C C CH3
O
“reduction”(CH3)3C C CH3
OH
H+2C C
0
Need a “reducing agent”
?
Not all reducing agents would work satisfactorily,
but it is typically futile to try a reagent from
the wrong redox class.
H2O
H2O
But photosynthesischanges H2O into O2
and “H-” (NAD+ NADH)
Oxidizing: Reducing:
Neither: H-Cl KCl
Br-Br
RS-SR
CrO3
CH4
LiAlH4
NaHK
RSH
H2C CH2
H
OH-2
-2
-3
-1
+1 -2
0:49-11:35
Carbon “Oxidation States” (Prof. Siegel, Lecture 47, 2010)
This scheme from 2010 is better organized that this year’s which follows.
(Loudon)
Stopping at Aldehyde(e.g. J&F pp. 805-6)
Pyridinium Chlorochromate (PCC)
“In practice the chromium byproduct deposits with pyridine as a sticky black tar, which can complicate matters.” (Wikipedia)
no water; no gem-diol;no overoxidation
-O-Cr-Cl
O
O
++ CrO3 + HCl
n-C9H19-CH2OH n-C9H19-CH=O
(92% yield)
PCC
CH2Cl2
End of Lecture 67April 11, 2011
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