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Author Topic: why sometimes we need to add reactant dropwise  (Read 15408 times)

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jiangds05

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why sometimes we need to add reactant dropwise
« on: December 07, 2010, 11:55:13 AM »

for what kind of control?
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MissPhosgene

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Re: why sometimes we need to add reactant dropwise
« Reply #1 on: December 07, 2010, 01:23:04 PM »

You may need to add a reagent dropwise to be able to control a reaction which would be vigorous is the reagent was added all at once.

If you are trying to do a reaction, such as a grignard, with a chemical that has two reactive sites, one more reactive than the other, such as ethyl oxalyl chloride, you may want to add the grignard reagent dropwise to avoid an exotherm which can potentially result in the loss of selectivity and two keep the concentration of the reactive site higher than the concentration of active nucleophile. Control of the temperature is more important in this case, and it can be done by cooling and adding the nucleophile dropwise.
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jiangds05

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Re: why sometimes we need to add reactant dropwise
« Reply #2 on: December 07, 2010, 03:56:53 PM »

thanks so much.
so it is for controlling selectivity.
totally understand it.

You may need to add a reagent dropwise to be able to control a reaction which would be vigorous is the reagent was added all at once.

If you are trying to do a reaction, such as a grignard, with a chemical that has two reactive sites, one more reactive than the other, such as ethyl oxalyl chloride, you may want to add the grignard reagent dropwise to avoid an exotherm which can potentially result in the loss of selectivity and two keep the concentration of the reactive site higher than the concentration of active nucleophile. Control of the temperature is more important in this case, and it can be done by cooling and adding the nucleophile dropwise.
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MissPhosgene

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Re: why sometimes we need to add reactant dropwise
« Reply #3 on: December 07, 2010, 04:30:35 PM »

I meant to instead of two. I can do math but am essentially illiterate in my native language. :/.
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ardbeg

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Re: why sometimes we need to add reactant dropwise
« Reply #4 on: December 07, 2010, 05:15:18 PM »

there is also the prospect of trying to prevent oligomerization
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otivaeey

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Re: why sometimes we need to add reactant dropwise
« Reply #5 on: December 08, 2010, 03:48:47 AM »

Titration of phosphate against KOH: since phosphate has 3 pKa's, there are 3 ranges of pH that the curve would become steep. A single drop of base would make the curve rise so much and at some ranges they are very insensitive. They do even have titration with 0.1ml aliquots which do create vigorous fluctuations, and that would be less than 1 drop.
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jiangds05

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Re: why sometimes we need to add reactant dropwise
« Reply #6 on: December 08, 2010, 04:46:23 AM »

control temperature also is to control selectivity, that is the reason why I said so.
what kind of math you want to do?
I meant to instead of two. I can do math but am essentially illiterate in my native language. :/.
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jiangds05

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Re: why sometimes we need to add reactant dropwise
« Reply #7 on: December 08, 2010, 04:46:59 AM »

could you give me one example?
there is also the prospect of trying to prevent oligomerization
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MissPhosgene

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Re: why sometimes we need to add reactant dropwise
« Reply #8 on: December 08, 2010, 03:09:15 PM »

control temperature also is to control selectivity, that is the reason why I said so.
what kind of math you want to do?
I meant to instead of two. I can do math but am essentially illiterate in my native language. :/.

Any math without numbers.

The reason to add dropwise is pretty much entirely dependent on the reaction you may want to do. For example, to brominate something like 2,5-dihydroxybenzaldehyde, you add bromine slowly to a solution of the benzene in chloroform dropwise, preventing bromination from occurring more than once.

The general statement might be something like "to prevent vigorous reaction, unwanted side-products, and to increase selectivity".
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jiangds05

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Re: why sometimes we need to add reactant dropwise
« Reply #9 on: December 08, 2010, 04:15:31 PM »

you really gave several good examples. thanks a lot!

control temperature also is to control selectivity, that is the reason why I said so.
what kind of math you want to do?
I meant to instead of two. I can do math but am essentially illiterate in my native language. :/.

Any math without numbers.

The reason to add dropwise is pretty much entirely dependent on the reaction you may want to do. For example, to brominate something like 2,5-dihydroxybenzaldehyde, you add bromine slowly to a solution of the benzene in chloroform dropwise, preventing bromination from occurring more than once.

The general statement might be something like "to prevent vigorous reaction, unwanted side-products, and to increase selectivity".
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