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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: jones106 on December 04, 2005, 03:36:26 PM

Title: double aldol condensation of cinnamaldehyde and acetone
Post by: jones106 on December 04, 2005, 03:36:26 PM

Could someone please help me figure out which molecule forms the enolate ion and attacks the other in the double aldol condensation of cinnamaldehyde and acetone?  I would think that the acetone forms the enolate and attacks the cinnamaldehyde.  I think, however, that my TA told me that the cinnamaldehyde will form the enolate and attack the acetone.  When I write out this reaction, the product just looks a bit outrageous to me, so I think I misunderstood him.  Could someone help me figure out what product is formed in this reaction?

Thanks,
jones106

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: Mitch on December 04, 2005, 03:38:34 PM

is this done under basic conditions?

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: jones106 on December 04, 2005, 05:33:59 PM

yes, it is done under basic condition

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: Mitch on December 04, 2005, 05:43:16 PM

Well, which one is the most acidic proton?

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: jones106 on December 04, 2005, 06:26:17 PM

I think the most acidic proton would be the alpha hydrogens on the acetone.  Am I right?

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: Mitch on December 04, 2005, 06:28:30 PM

you tell me. http://www.chemicalforums.com/index.php?page=pkavalues

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: jones106 on December 04, 2005, 06:46:30 PM

I found the pKa of acetone to be 26.5, but could not find the pKa of an aldehyde.

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: jones106 on December 04, 2005, 07:52:14 PM

I found the pKa of aldehydes to be around 17, which would make them more acidic than ketones.  This would mean that the aldehyde would form its enolate ion and attack the ketone.  Is this right?

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: Mitch on December 04, 2005, 08:03:21 PM

sounds better at least.

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: movies on December 04, 2005, 11:53:12 PM

Woah, hold up.  The alpha proton on cinnamaldehyde isn't like a regular aldehyde!  It's an enal, so the alpha proton isn't very acidic at all!

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: Mitch on December 05, 2005, 12:14:52 AM

good catch.   :P

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: Yggdrasil on December 05, 2005, 01:42:30 AM

Would a Michael Addition be likely in a reaction like this?  Enolates are soft nucleophiles, so I would think that a Michael addition would be more likely than an aldol reaction.

Title: Re:double aldol condensation of cinnamaldehyde and acetone
Post by: movies on December 05, 2005, 04:36:54 PM

Yeah, it's possible, but the Michael addition will be reversable, so will the initial aldol reaction.  The elimination of water from the aldol product (completing the aldol condensation) will be irreversable, so the product would likely all funnel to that under thermodynamic conditions.