[zsinger]

So,
Mixture of phenyl acetone with Br2 and OH-.  Must there be an XS of either Br2 or OH?  It would seem to me that an excess of OH- could perhaps disrupt our final conversion to the carboxylic acid with a milder acidic workup.  Seems as if XS Br2 would do no harm?  Thanks in advance.
     -Zack

[Archer]

In my experience phenylacetone predominantly brominates at the benzylic carbon when treated with Br₂.

That being said I don't use HO^- as the base in this reaction.

[zsinger]

Yea, I know the Rxn. Mechanism.  I was just wondering about XS reagents.

[Archer]

If you brominate the benzylic carbon then the bromoform reaction won't proceed as you need to brominate the terminal methyl group to from CBr₃.

You would more likely form Ar-CBr₂-CO-CH₃. Whether this would react further to brominate to Ar-CBr₂-CO-CBr₃, I am not sure. If 1,1-dibromo-1-phenylacetic acid is your TM then maybe this would work, I have never tried

[zsinger]

ALL,
I have done this RXN with my students, and know exactly how it works…..I was just wondering if somebody who has actually scaled it up noticed any better yields due to XS reagents, or where some $$ can be saved perhaps on reagents.  Thanks.
   

[spirochete]

ALL,
I have done this RXN with my students, and know exactly how it works…..I was just wondering if somebody who has actually scaled it up noticed any better yields due to XS reagents, or where some $$ can be saved perhaps on reagents.  Thanks.
   

Are you a PhD student working with undergrads?

[zsinger]

Masters…..…..hopefully headed towards the PhD but I need to get more practical knowledge.  Books can only take one so far…..

[Archer]

ALL,
I have done this RXN with my students, and know exactly how it works…..I was just wondering if somebody who has actually scaled it up noticed any better yields due to XS reagents, or where some $$ can be saved perhaps on reagents.  Thanks.
   

Please accept my apologies, I was merely giving you my experiences with this compound. Clearly you have more experience with this than I do.

I am slightly surprised that you would use phenylacetone in an experiment like this rather than, say, acetophenone.

Phenylacetone is not recommended for use with undergraduate students. It is actually prohibited by law in Europe.

Clearly the laws where you work are much more relaxed on such things.

[orgopete]

Re: excess reagents

Interesting question. If you add one equivalent of a Grignard reagent to an ester, you recover half of the ester, unreacted. In the halo form reaction, I am assuming the increased acidity and susceptibility to attack will consume Br2 and hydroxide as the reaction proceeds. Therefore we may assume even in the early stages of the reaction, it proceeds with an excess of reagents. Once the reagents are consumed, the reaction should stop.

My reasoning should indicate an excess of reagents should not have a deleterious effect upon the reaction, provided the reaction rates are in line with my premise. However, if other reactions compete, then it would seem that excess reagents could negate the benefit.

Although an excess of base is required as it is being consumed during the course of the reaction, it could also lead to problems. If a large excess (or high concentration) were present, then deprotonation of the haloform could also occur. Similarly, too little hydroxide might lead to the hypobromide of the carboxylate. This might also affect the reaction.

[Archer]

or where some $$ can be saved perhaps on reagents.  Thanks.
 

1Lt acetophenone costs the same as 100ml of phenylacetone from Aldrich, you could save some money by replacing this in your lab class.

[zsinger]

Dumb me…Yes, I meant Acetophenone!!!!!!   

[spirochete]

Haha wait this whole time you were talking about acetophenone? Archer's whole explanation was about the reactivity of a different compound?

[Archer]

Dumb me…Yes, I meant Acetophenone!!!!!!   

  I once used diphenylacetylene instead of diphenylacetone and wondered why my conensation reaction didn't work GC-FID didn't help identify my error.

I'm intrigued now to see what happens in the bromoform reaction with phenylacetone, will publish my results here.

[zsinger]

Yes, I MOST CERTAINLY meant acetophenone……