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Topic: Isolation of low boiling alkyl bromide  (Read 4024 times)

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AFFA

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Isolation of low boiling alkyl bromide
« on: September 14, 2005, 03:12:12 PM »
I am now getting into trouble in preparing allyl bromide from the corresponding allyl alcohol using NBS/PPh3 condition in CH2Cl2. The estimated b.p. of my bromide should be around 100 oC.

i.e. CH3CH2C(H)=C(H)CH2Br

How could I get rid of the excess PPh3 and O=PPh3, and purify my bromide without significant loss of it??? Or, what's the appropiate workup procedure?
 ???
Thx a lot!!!   :)

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Re:Isolation of low boiling alkyl bromide
« Reply #1 on: September 14, 2005, 06:05:35 PM »
How about distillation?

You have to make sure that all the allyl alcohol is gone though since it boils around 100 degrees too.

Allyl bromide can be a nasty lachrymator and is pretty toxic too.  Be careful.

AFFA

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Re:Isolation of low boiling alkyl bromide
« Reply #2 on: September 15, 2005, 03:34:08 AM »
thx

Distillation is definitively my choice. My problem is that I couldn't get rid of the excess triphenylphosphine and its oxide completely. They dissolve well in n-pentane even I tried to filter via a pad of silica gel. Or, is it not a matter to distill my bromide even in the presence of many solid by-product (phosphine)???  ::)


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Re:Isolation of low boiling alkyl bromide
« Reply #3 on: September 15, 2005, 12:32:25 PM »
The phosphines shouldn't be near as volatile as allyl bromide.  The BP of PPh3 is 377 degrees.

laotree

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Re:Isolation of low boiling alkyl bromide
« Reply #4 on: September 16, 2005, 03:02:14 PM »
Charge your crude residue to a silica gel pad. wash with hexane. The product of yours is such a not polar staff. PPh3 or triphenylphosiphine oxide will stay on column for sure. I have done this reaction (Appel type of reaction) many times.

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