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!

Third Thoughtson

Friedel-Crafts

This

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.

Carbon “Oxidation States”

2011

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

Br2 Mechanism2010

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

Br2 Mechanism2011

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

Cr+6 Mechanism

2010

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

Cr+6 Mechanism

2011

(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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