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Topic: Problematic Menshutkin reaction  (Read 3063 times)

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Offline domus121

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Problematic Menshutkin reaction
« on: July 19, 2023, 05:35:00 PM »
Hi,

I'm quite new to organic chemistry and want to try menshutkin reaction. Like the reaction in an attachment.

I use that recipe:

I do everything in a 60ml beaker. I dissolve 18 mmol of pyridine in 35 ml of acetone. I add 115 mmol of 1,6-dibromohexane and stir for 30 minutes. Then I add 76.6 mmol KOH (in the form of crushed pre-flakes). Stir for the next 24 hours.

Then I put the whole thing into a solution of 50 ml of dichloromethane and 100 ml of water. I mix for 1 hour, then pour into a funnel (I have a small 25ml one, so I do it several times).

I evaporate the whole thing using a 60-80 degree water bath until all the dichloromethane evaporates. Then, after cooling down, I wash it a few times with pentane (it cleans the material from the residual dibromohexane, unfortunately I don't have a vacuum pump).

Unfortunately, I only got 5% yield and the product still look little reddish, what am I doing wrong? Could you help me on what to do better?

I can buy equipment but not so sure what to buy
« Last Edit: July 19, 2023, 05:49:16 PM by domus121 »

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #1 on: July 21, 2023, 08:07:13 AM »
Why KOH? You dont need that?

Offline Babcock_Hall

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Re: Problematic Menshutkin reaction
« Reply #2 on: July 22, 2023, 11:28:58 AM »
Did you obtain your procedure using SciFinder or some equivalent searching tool?

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #3 on: July 26, 2023, 02:33:54 PM »
Just dissolve the dibromide in large excess of pyridine, heat to 100C for 1 hour, the evaporate, you will get the product.

Offline Babcock_Hall

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Re: Problematic Menshutkin reaction
« Reply #4 on: July 28, 2023, 09:41:10 AM »
Just dissolve the dibromide in large excess of pyridine, heat to 100C for 1 hour, the evaporate, you will get the product.
What about a second nucleophilic attack?

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #5 on: July 28, 2023, 06:28:54 PM »
Sorry, I mean large excess of the dibromide. Thanx for correcting me.

Offline Babcock_Hall

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Re: Problematic Menshutkin reaction
« Reply #6 on: July 29, 2023, 04:34:57 PM »
@OP, Purifying compounds with pyridinium groups has sometimes been challenging if my memory is accurate.  Do you have a method from the literature or one that you have tried before?

Offline wildfyr

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Re: Problematic Menshutkin reaction
« Reply #7 on: July 29, 2023, 08:51:52 PM »
You can purify quaternary ammonium salts on columns using highly polar eluents, just use a gradient. Mostly pure hexane will flash off the dibromohexane, then a static system should easily separate the singly charged product from its double charged side product.

You can evaporate off pyridine under vacuum before the column (puke), it boils at 115C. Make sure you double trap because if any gets in your pump it will stink forever.

Pyridine is bad for you, use caution, read the SDS. So is dibromohexane for that matter.

I don't think this product is solid, but I am wrong, try recrystallization from something like ethanol.

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #8 on: July 30, 2023, 11:07:10 AM »
You can purify quaternary ammonium salts on columns using highly polar eluents, just use a gradient. Mostly pure hexane will flash off the dibromohexane, then a static system should easily separate the singly charged product from its double charged side product.

You can evaporate off pyridine under vacuum before the column (puke), it boils at 115C. Make sure you double trap because if any gets in your pump it will stink forever.

Pyridine is bad for you, use caution, read the SDS. So is dibromohexane for that matter.

I don't think this product is solid, but I am wrong, try recrystallization from something like ethanol.

If you use large excess of the dibromide it will be pure enough, you can recrystalize probably. If it is 90% pure, thats probably enough. I have made Q-salts, pyridinium, and that was not tricky. We used them as heat-stably Q-salts for SnAr, displacing chlorine for fluorine with KF. Alkyl Q-salts easily undergo Hoffman elimination.

Offline Babcock_Hall

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Re: Problematic Menshutkin reaction
« Reply #9 on: July 30, 2023, 01:55:04 PM »
Going on memory, we precipitated the crude product out of ether, leaving behind less polar impurities.  Recently, we had some luck with low-pressure reverse phase chromatography purifying something entirely different, despite our not having prior experience with this technique.

Offline Babcock_Hall

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Re: Problematic Menshutkin reaction
« Reply #10 on: August 01, 2023, 11:05:03 AM »
Going on memory, we precipitated the crude product out of ether, leaving behind less polar impurities.  Recently, we had some luck with low-pressure reverse phase chromatography purifying something entirely different, despite our not having prior experience with this technique.
Precipitation is a poor man's version of recrystallization.  We collected the precipitate by centrifugation in Corex centrifuge tubes.

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #11 on: August 01, 2023, 03:14:51 PM »
He can use EtOAc maybe, its completely insoluble in ether.

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #12 on: August 01, 2023, 03:15:50 PM »
Going on memory, we precipitated the crude product out of ether, leaving behind less polar impurities.  Recently, we had some luck with low-pressure reverse phase chromatography purifying something entirely different, despite our not having prior experience with this technique.
Precipitation is a poor man's version of recrystallization.  We collected the precipitate by centrifugation in Corex centrifuge tubes.

Great, its a little different with RP-gel, you can have more back-pressure etc.

Offline rolnor

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Re: Problematic Menshutkin reaction
« Reply #13 on: August 02, 2023, 03:03:20 PM »
If he runs the reaction in refluxing toluene, the product will probably precipitate out and it will then be protected from further reaction.

Offline Babcock_Hall

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Re: Problematic Menshutkin reaction
« Reply #14 on: August 02, 2023, 09:32:29 PM »
He can use EtOAc maybe, its completely insoluble in ether.
I was thinking that the reaction would be run in one solvent, which presumably gives a crude liquid product.  Then one next adds the product dropwise into (cold?) ether in a Corex centrifuge tube.  I think that is what we did a long time ago.
« Last Edit: August 02, 2023, 09:50:07 PM by Babcock_Hall »

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