7 replies to this topic

[TSAD]

Welcome.

Perkin reaction.

The Perkin reaction is an organic reaction developed by William Henry Perkin that can be used to make cinnamic acids by the aldol condensation of aromatic aldehydes and acid anhydrides in the presence of an alkali salt of the acid.

Reaction mechanism:




The above mechanism is not universally accepted, as several versions exist, including decarboxylation without acetic group transfer.

Source: http://en.wikipedia....Perkin_reaction

Thanks.

Edited by TSAD, 26 July 2010 - 10:55 PM.

[mississippichem]

I think I could see this same transformation occur through direct aldol condensation with benzaldehyde followed by an elimination. But I fell the decarboxylation type mechanism is thermodynamically far fetched. I'd like to see the BDE value to break that C-C bond. Decarboxylations are finicky reactions. Seemingly similar substrates sometimes yield very different results.

-interesting post, not enough posts of this type in chemistry forums.

[TSAD]

Thanks mississippichem.

What are the errors in the reaction?

Please explain.

Edited by TSAD, 26 July 2010 - 11:14 PM.

[mississippichem]

There's nothing wrong with the mechanism you posted. I was just saying that I would endorse the mechanism you posted over the direct decarboxylation. There is alot of resonance present in the various intermediates, this tends to hinder the decarboxylations by reducing the electron density on the carboxyl group. Those pesky decarboxylations can stump even the wisest of chemists Another complicating factor is that the decarboxylations are highly favored by entropy assuming the temperature is high enough for the resulting CO2 to exist as a gas (which it always is). Alot of variables to consider. Its one of those borderline cases where entropy could beat out enthalpy. I feel that a calorimetric analysis is needed. Got any grant money?

[TSAD]

Thanks mississippichem.

Yes this is true.

Decarboxylation is a chemical reaction which releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain.



Source : Decarboxylation

Edited by TSAD, 28 July 2010 - 03:36 AM.

[Horza2002]

Lol I've had the same discussion with my professor about this reaction since I did one a few months ago. From what we manage to find out, there are several routes than can be thought of. From our talks and searching the literature, it depends on the exact substrate. I had several benzylic groups that helped by using the delocalised electrons to help.

I personally don't like the cyclisation step forming the 6-membered ring. Theres to many oxygens for my liking and im not sure the bond lengths will be correct for them to work but I might be wrong. Decarboxylation steps are a pain yes...here it depends on the temperature of the reaction to whether it'l occur or not. Normally though, you have to heat decarboxylations reactions to around 200C before they work at all!

[TSAD]

Thanks Horza2002

[mississippichem]

I personally don't like the cyclisation step forming the 6-membered ring. Theres to many oxygens for my liking and im not sure the bond lengths will be correct for them to work but I might be wrong.

interesting observation. Agreed

Perhaps the aromatic ring helps deal with some of the electron density, just a thought; but there is still that bond length problem. I would really love to see some molecular modeling calculations for that intermediate. Anyone with a copy of Spartan willing to waste a couple of hours?