Sciencemadness Discussion Board

Need help! who knows the mechanism of Kharasch-Brown Reaction

ZHANGNIUBI - 11-4-2012 at 07:50

http://edu.emuch.net/attachment/de/b6/1574191_1333982219_395...

here is the reaction.. please tell me why the group -COCl sit on those specific positions on Cube..
and how it works(mechanism)

thank you!

Nicodem - 11-4-2012 at 08:13

It was not easy to find, but this is apparently the seminal paper for this reaction:

Carboxylation. I. The Photochemical and Peroxide-catalyzed Reactions of Oxalyl Chloride with Paraffin Hydrocarbons.
M. S. Kharasch, Herbert C. Brown
J. Am. Chem. Soc., 1942, 64, 329–333. DOI: 10.1021/ja01254a032

See also their preliminary publication on this reaction (DOI: 10.1021/ja01859a515). (For that depicted reaction on cubane, see DOI: 10.1002/anie.199306121)

ZHANGNIUBI - 11-4-2012 at 08:48

Quote: Originally posted by Nicodem  
It was not easy to find, but this is apparently the seminal paper for this reaction:

Carboxylation. I. The Photochemical and Peroxide-catalyzed Reactions of Oxalyl Chloride with Paraffin Hydrocarbons.
M. S. Kharasch, Herbert C. Brown
J. Am. Chem. Soc., 1942, 64, 329–333. DOI: 10.1021/ja01254a032

See also their preliminary publication on this reaction (DOI: 10.1021/ja01859a515). (For that depicted reaction on cubane, see DOI: 10.1002/anie.199306121)



yes, I know it's hard. But based on my current situation, I can not download these papers by my school's library.


Could you please help download them and put them on here? I ll be very appreciated!!

francis - 21-4-2012 at 00:04

This is a radical chain mechanism. The paper says it can be photochemical but works better with a peroxide initiator.

The cubane is the substrate and the rxn proceeds by benzoyl peroxide decomposition:
(I will call the benzoyl group Bz and include the oxygen-oxygen sigma bond).

Bz-O-O-Bz ---> 2 BzO*

Then the BzO* reacts with oxalyl chloride to form the 'carrier' (the terminal C-Cl bond of the oxalyl chloride is broken):

BzO* Cl-COCO-Cl-> BzO-Cl + *COCO-Cl

Then *COCl decomposes to carbon monoxide (CO) and Cl* radical

*COCOCl -> 2CO + Cl*

The chloride reacts with the cubane to form an alkyl radical by, apparently, breaking of the R-H bond.

R-H + Cl* -> R* + HCl

Apparently the R* reacts with the original oxalyl chloride:
R* + (COCl)2 -> R-COCl + *COCl

The rxn continues when *COCl radical decomposes to CO and Cl* and the Cl* radical reacts with the cubane again as follows:

Cl* + RH -> HCl + R*

Until termination.

Unfortunately the original picture in your post is not up anymore. Maybe you can repost it?

I have attached the paper, see page 3 for the mechanisms (there are no curly arrows but you can imagine they would be single-headed arrows).

Attachment: Kharasch Brown Rxn.pdf (595kB)
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