December 12, 2019, 05:35:27 AM
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Topic: Debromination using Zinc  (Read 288 times)

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Offline che911

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Debromination using Zinc
« on: November 29, 2019, 12:55:13 PM »
Hello fellow chemists!
I have few questions about my reaction please help me!

So I have been doing a very simple debromination reaction of 5-bromo-2,3-diamino pyridine using Zinc, NH4Cl and methanol. I am facing problems during work up of this reaction (500grams scale).

After the reaction is complete I have filtered zinc etc on celite and the MLR is being basified using 50% NaOH to free the HBr salts if pyridine. Solid separates out ( my assumption is I am getting NaBr). This solid is again filtered out and the MLR is being distilled out. After the distillation is completed I am ending up with lots of inorganic material along with my compound.

1) What am I doing wrong?
2) Is basifying of the MLR necessary?
3) I have a doubt about the compound forming pyridinium salt. Won't the Bromine liberated form Zinc Bromide (I am using excess Zinc)
4) Can any other base be used for basifying? (if basification is required)

Please help me!

Thank you.

Offline Babcock_Hall

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Re: Debromination using Zinc
« Reply #1 on: November 29, 2019, 01:01:22 PM »
I only have two general comments at this time.  One, are you following a literature protocol?  If so, can you provide a reference?  Two, your scale seems large.  Is it possible to work out the kinks on a small scale before proceeding to a large scale?

Offline wildfyr

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Re: Debromination using Zinc
« Reply #2 on: November 29, 2019, 07:40:02 PM »
No liquid liquid extraction? Chances are the inorganic will dissolve in water, or at least be insoluble in ethyl acetate or similar solvents.

Offline che911

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Re: Debromination using Zinc
« Reply #3 on: December 01, 2019, 12:29:13 AM »
I only have two general comments at this time.  One, are you following a literature protocol?  If so, can you provide a reference?  Two, your scale seems large.  Is it possible to work out the kinks on a small scale before proceeding to a large scale?

1) Yes. Following a literature protocol. I am on mobile right now I'll link it when I get access to my laptop.
2) Small scale was fine. I could isolate the product easily. Large scale is giving the aforementioned problems.

Will the bromine liberated during the reaction form salts with my pyridine moiety?

Offline che911

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Re: Debromination using Zinc
« Reply #4 on: December 01, 2019, 12:32:10 AM »
No liquid liquid extraction? Chances are the inorganic will dissolve in water, or at least be insoluble in ethyl acetate or similar solvents.

No. Extraction is next to impossible as the end product is highly water soluble and polar. We have to do multiple extractions to isolate the product which is infeasible for me.

Offline kamiyu2550

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Re: Debromination using Zinc
« Reply #5 on: December 01, 2019, 12:23:10 PM »
I only have two general comments at this time.  One, are you following a literature protocol?  If so, can you provide a reference?  Two, your scale seems large.  Is it possible to work out the kinks on a small scale before proceeding to a large scale?

1) Yes. Following a literature protocol. I am on mobile right now I'll link it when I get access to my laptop.
2) Small scale was fine. I could isolate the product easily. Large scale is giving the aforementioned problems.

Will the bromine liberated during the reaction form salts with my pyridine moiety?

Are you sure there is Br2 liberated? Bromide anion undergoes oxidation (such as air) to give Br2, but look at your reaction condition..

Offline kamiyu2550

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Re: Debromination using Zinc
« Reply #6 on: December 01, 2019, 12:30:10 PM »
Hello fellow chemists!
I have few questions about my reaction please help me!

So I have been doing a very simple debromination reaction of 5-bromo-2,3-diamino pyridine using Zinc, NH4Cl and methanol. I am facing problems during work up of this reaction (500grams scale).

After the reaction is complete I have filtered zinc etc on celite and the MLR is being basified using 50% NaOH to free the HBr salts if pyridine. Solid separates out ( my assumption is I am getting NaBr). This solid is again filtered out and the MLR is being distilled out. After the distillation is completed I am ending up with lots of inorganic material along with my compound.

1) What am I doing wrong?
2) Is basifying of the MLR necessary?
3) I have a doubt about the compound forming pyridinium salt. Won't the Bromine liberated form Zinc Bromide (I am using excess Zinc)
4) Can any other base be used for basifying? (if basification is required)

Please help me!

Thank you.

1. I am wondering if making the reaction mixture basic by NaOH is really necessary. When you add the NaOH solution, solid came out only because of addition of water to force our organic stuff, but this is my guess only, as you did not state how much stuff came out. But I expect there won't be too much precipitate, given your diamino compound should be pretty hydrophilic.

2. My understanding is that the debromination gives ZnBr2, and I do not think there should be any acid generated (and hence no need to add base). The only acid is what you added: NH4Cl, but it is very weak.

3. Of course you will have a lot of inorganic stuff after distilling your MLR (mother liqour? just to make sure, never heard of this abbreviation). Your MLR is basically methanol and lots of water.

Offline che911

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Re: Debromination using Zinc
« Reply #7 on: December 02, 2019, 01:40:47 AM »
Thank you all for taking your time to help solve my problem.

@kamiyu2550

1) Sorry I meant bromide anion when I mentioned about liberated bromine.

2) Yes I am also of the opinion that debromination gives ZnBr2 (my colleagues are of the opinion that pyridinium salt is forming during reaction).

3) When we add caustic solution inorganic [Zn(OH)2 and NaBr] should  form.

And yeah MLR = mother liquor :)

Can we avoid addition of caustic solution?
« Last Edit: December 02, 2019, 02:01:48 AM by che911 »

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