April 03, 2020, 02:27:29 AM
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Topic: Quenching N-bromosuccinimide (NBS) with NaOH / Use of LiBr in organic synthesis  (Read 9541 times)

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Offline Sander1989

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Hi everyone, I have two questions about my master thesis:

1) Recently, I conducted a bromination reaction on a 3-alkylsubstituted thiophene with N-bromosuccinimide (NBS) in a lab synthetic chemistry (the solvent was DMF). The reaction work-up consisted of pouring 2.5 M NaOH solution into the reaction flask at 0°C, followed by washing the organic layer with 2.5 M NaOH, water and brine (saturated NaCl) respectively. I think that NaOH is used to quench the remaining NBS but I'm not sure how this happens:

- is it an SN-reaction between OH- and a C=O bond of NBS (resulting in opening of the NBS ring)?
- will OH- split the bond between N and Br? This seems not a logic option to me, however.

And I have absolutely no clue about this NaCl thing...

2) When performing a nickel catalyzed reaction to couple an organozinc compound (R-ZnBr) to 3-bromothiophene, why would one use LiBr as an additive? I've searched all over the internet --Wikipedia just says that 'it is useful as a reagent in organic synthesis'-- but I can't find a satisfying answer to my question.

Can someone help me with these topics? Your help is greatly appreciated!

Offline Honclbrif

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NBS can act as a source of HBr, the NaOH is probably there to clean that up. As for the sat'd NaCl, they taught you what that's for in sophomore organic lab.

Li salts (LiCl, LiBr, etc) are usually added to break up clusters that organometallic reagents tend to form.
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Offline Sander1989

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As for the sat'd NaCl, they taught you what that's for in sophomore organic lab.

I'm sorry, they didn't. Is it used to make the water very polar to extract the last polar compounds from the organic phase or is there another reason?

Offline fledarmus

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As for the sat'd NaCl, they taught you what that's for in sophomore organic lab.

I'm sorry, they didn't. Is it used to make the water very polar to extract the last polar compounds from the organic phase or is there another reason?

Adding salts usually decreases the solubility of anything else in water - except water. A final wash with a saturated salt solution will pull most of the trace water out of your organic phase, then you can do the final drying with MgSO4 or Na2SO4.

Water is a real PITA in an organic lab - there are a lot of techniques devoted to getting rid of it.

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