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Topic: Aldol Condensation-synthesis of tetraphenylcyclopentadienone  (Read 21053 times)

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Offline gobuckskb9

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Aldol Condensation-synthesis of tetraphenylcyclopentadienone
« on: April 11, 2009, 01:14:42 PM »
I have included the reaction as a picture attachment.
But we did this:
Benzil + Dibenzyl ketone (KOH cat., ethanol)  :rarrow: Tetraphenylcyclopentadienone + H2O
I have to tell how I could increase the purity and yield of the reaction. The only thing I can think of is to distill the dibenzyl ketone to insure that it is as pure as possible.

-I know that there are two possible products for this reaction, and that the tetraphenylcyclopentadienone is much for favored due to benzil having no alpha protons and being very reactive. It seems like the lab is layed out so that I get a very high yield under the suggested conditions.
Is there anything that I can change to make it better?

Offline nj_bartel

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Re: Aldol Condensation-synthesis of tetraphenylcyclopentadienone
« Reply #1 on: April 11, 2009, 06:13:15 PM »
The alpha protons there are less acidic than water, and you're using hydroxide as your base.  This means you could get some addition to what you're trying to get the enolate of.  So I'd consider preforming the enolate you want formed with one equiv LDA, or some other such strong base.

Offline Squirmy

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Re: Aldol Condensation-synthesis of tetraphenylcyclopentadienone
« Reply #2 on: April 11, 2009, 07:50:09 PM »
What's the other possible product?

Based on its structure, you might consider the order of addition, how much of the benzil vs. dibenzylketone you use, and whether the rate of addition is important.

As you mention distilling the dibenzyl ketone, certainly purifying a starting material can make a big difference...just because it comes out of a bottle doesn't mean it doesn't require purification. Sometimes it's worth it, sometimes it's not...very much empirical or based on experience with similar compounds.

Overall, I think you're right that the lab is designed to give good yield and high purity by the choice of reactants. The question may just be asking, based on your observations and experience, "was there anything you could've done better?". Maybe you ran the procedure perfectly and got an excellent yield of a very pure sample. Probably not true for everyone in your lab.

Offline Dolphinsiu

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Re: Aldol Condensation-synthesis of tetraphenylcyclopentadienone
« Reply #3 on: April 12, 2009, 05:00:27 AM »
The structure given in the product may be further estimated by mass spectroscopy, IR, NMR etc.

Actually every procedure you use (e.g. heating temperature or recrystallization condition), stiochiometry, reactive site of reactant or the amount of catalyst can affect the reaction kinetics (e.g. kinetic controlled or thermodynamical controlled product) as well as yield.

You may know that low temperature favor beta hydroxyl ketone whereas high temperature favor alpha,beta unsaturatured ketone. Also there are two types of alpha proton in dibenzylketone (two reaction sites), hence, duration of reaction also determine its purity. (One product only have one reaction site reacted, another one may have both sites reacted)

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