Sciencemadness Discussion Board

Aldol product degradation

pkoushik4 - 14-12-2016 at 20:08

is it possible to get styrene from degradation of aldol product formed by condensation of acetone and benzaldehyde?

Metacelsus - 14-12-2016 at 20:36

What do you mean by "degradation"?

Also, which of these compounds did you have in mind?

Screen Shot 2016-12-14 at 10.35.19 PM.png - 12kB

JnPS - 14-12-2016 at 20:53

Couldn't you oxidize the first condensation product after the dehydration, (benzylideneacetone) to cinnamic acid and then decarboxylate it to styrene? I wouldn't call that a degradation, or how reasonable a synth that is, but it seems possible. Isolating the single condensation product would be a real pain though


[Edited on 15-12-2016 by JnPS]

Metacelsus - 14-12-2016 at 21:08

If you used a strong base such as LDA to quantitatively form the enolate, and then slowly added a stoichiometric amount of benzaldehyde, you could isolate the single condensation product.

However, for the amateur chemist, depolymerization of polystyrene is probably a better option (see: https://www.sciencemadness.org/whisper/viewthread.php?tid=18...).

[Edited on 12-15-2016 by Metacelsus]

pkoushik4 - 15-12-2016 at 12:28

Quote: Originally posted by Metacelsus  
What do you mean by "degradation"?

Also, which of these compounds did you have in mind?


Hi Metacelsus, thanks for the response.

The 1st and 2nd were the structures that I had in mind. I was doing this reaction at hight temperature using a solid acid catalyst. I find that I always get styrene as my side product in varying amounts depending on my reaction conditions

I know aldol addition product could undergo retro-aldol to give the starting compounds but I have no clue as to what kind of chemistry can lead to this type of transformation.

I called it degradation because styrene appears to be the result of C-C bond cleavage( the other product of cleavage being acetaldehyde) between the carbonyl carbon and the one at alpha position to it.

pkoushik4 - 15-12-2016 at 13:43

Quote: Originally posted by JnPS  
Couldn't you oxidize the first condensation product after the dehydration, (benzylideneacetone) to cinnamic acid and then decarboxylate it to styrene? I wouldn't call that a degradation, or how reasonable a synth that is, but it seems possible. Isolating the single condensation product would be a real pain though


[Edited on 15-12-2016 by JnPS]


Hi JnPS, Thanks for the suggested the sequence of steps. I get Styrene as one of the side products of the aldol condensation reaction carried out using an acid catalyst at high temperature (140 C).

I am trying to understand how I could have possibly got styrene and then possibly try to figure out a way to prevent the formation of this undesired product.

pkoushik4 - 20-12-2016 at 13:40

I am just wondering if there could be a net hydration (adding an H,OH) which could be initiated by acid catalyzed attack of water on the ketone carbonyl group? The other product being the acetic acid.

JnPS - 20-12-2016 at 20:50

So you're thinking benzylideneacetone + Water [H+]-> Styrene + Acetic Acid? I would think if the Aldol product got rehydrated it would turn back into the first structure Metacelsus posted, plus if the ketone'a carbonyl was protonated for the acid catalyzed addition of water, the double bond would allow resonance structures making both the beta carbon and the carbonyl carbon electrophilic. Isn't the dehydrated aldol product a conjugated system, why would it spontaneously cleave like that?

What were your exact reaction conditions that yielded styrene as a by-product?

[Edited on 21-12-2016 by JnPS]

[Edited on 21-12-2016 by JnPS]

pkoushik4 - 23-12-2016 at 00:33

I don't know why, but I do observe this product. I got a similar product (1-Phenyl-1-propene) when I did the reaction with butanone. Also, I observed that the yield towards this product increase as I increase the temperature.

The reaction temperature is 140 C. 1:3 molar ratio of benzaldehyde to Acetone and Toluene is the solvent.


[Edited on 21-12-2016 by JnPS]

[Edited on 21-12-2016 by JnPS][/rquote]

[Edited on 23-12-2016 by pkoushik4]

[Edited on 23-12-2016 by pkoushik4]