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Topic: Mixed Aldol Reaction  (Read 4664 times)

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Offline Super Newb

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Mixed Aldol Reaction
« on: July 17, 2012, 09:51:51 PM »
I'm doing a simple aldol reaction and wonder what could be the possible side pdts and how to remove.

The figure below shows what I have predicted but i'm not certain abt it.

Offline PhDoc

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Re: Mixed Aldol Reaction
« Reply #1 on: July 18, 2012, 12:11:12 PM »
First, I have a question for you. What program did you use to make the drawings? If it's something other than ChemDraw I'd like to know about it so I can suggest it to my students.

OK, when presenting a scheme like this, it's best to number all structures to facilitate communication. You appear to have an error in you self-condensation of acetone, combining two 3-carbon species to make a 5-carbon species.

The pent-3-ene-2-one that you are self-condensing (a 5 carbon species) appears to be going to a 7-carbon species. Not possible.

The Robinson Annulation you've proposed will not work from the material you've indicated.

Please let me suggest reading the following four articles, and then re-designing your scheme.

http://lennoxtutoring.com/2011/10/07/the-aldol-condensation-part-one/

http://lennoxtutoring.com/2011/10/09/the-aldol-condensation-part-two/

http://lennoxtutoring.com/2011/10/12/the-aldol-condensation-part-three-stereochemistry/

http://lennoxtutoring.com/2012/05/12/95-of-organic-chemistry-in-5-minutes/

It should take no more than 30 min to read all four articles, and I believe they will be quite helpful for you.

Dr. Joseph Lennox
O-Chem Prof

Offline Super Newb

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Re: Mixed Aldol Reaction
« Reply #2 on: July 18, 2012, 12:46:26 PM »
Hi Sir! Its is the ACS stylesheet in chemdraw which i'm using.

Btw, i'm working on a simple claisen-schmidt reaction condensation of benzaldehyde (2.2equiv) and acetone(1 equiv) in (EtOH with NaOH) as my pratical assignment.

I am trying to predict the side products that could be present in the reaction mixture. I have uploaded a new schematic diagram for easier reference.

My prediction are such:
Compound 1- should be fully consumed since we have excess benzaldehyde , and it will not have other side reaction as the carbonyl should not be reactive(as in acting as electrophile)  since it is ketone (while we still have aldehyde in mixture) and it has alkenyl substituent group (which further decreases its reactivity as electrophile).

Compound 2- The desired product and major product. The main isomer is the E,E isomer since E1cB would prefer a less hindered and stable isomer (usually E).

Compound 3- Should most likely undergo the dehydration in the basic condition.

As for compound 4-6 I do not know whether they will be existent in the reaction mixture.

Thank you for your time, and any help would be greatly appreciated :DD






Offline orgopete

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Re: Mixed Aldol Reaction
« Reply #3 on: July 19, 2012, 02:32:20 AM »
...
My prediction are such:
Compound 1- should be fully consumed since we have excess benzaldehyde , and it will not have other side reaction as the carbonyl should not be reactive(as in acting as electrophile)  since it is ketone (while we still have aldehyde in mixture) and it has alkenyl substituent group (which further decreases its reactivity as electrophile).

Compound 2- The desired product and major product. The main isomer is the E,E isomer since E1cB would prefer a less hindered and stable isomer (usually E).

Compound 3- Should most likely undergo the dehydration in the basic condition.

As for compound 4-6 I do not know whether they will be existent in the reaction mixture.
...

Although (6) has the correct number of carbons, (3) and (5) do not. I suggest numbering (1) and finding those atom numbers in (3).

I don't know what is expected for this problem, however I suggest you read your analysis. It contains good advice on predicting the products. It suggests compounds 4-6 would be unlikely to form in any great amount. Agreed.

Which do you think would be more likely, self condensation of acetone or an E,E, E,Z, or a Z,Z mixture?
Author of a multi-tiered example based workbook for learning organic chemistry mechanisms.

Offline Wastrel

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Re: Mixed Aldol Reaction
« Reply #4 on: July 19, 2012, 06:37:29 PM »
You could add the Cannizzaro reaction.

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