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Topic: gringard reaction with H20 and Cyclohexanone  (Read 9139 times)

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Offline industrial_engineer

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gringard reaction with H20 and Cyclohexanone
« on: April 07, 2011, 12:49:16 PM »
Hi - I am reacting CH3CH2-MgBr using water as the solvent (not my choice) and need to figure out what is going on.
 
I have pieced it together so that the CH3CH2 will steal a hydrogen from the H2O to form CH3-CH3 and MgBr2 with an OH- 

I am just curious if anything else happens between that OH- and the cyclohexanone, because if not... why is the cyclohexanone their you know?

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Offline voidSetup

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #1 on: April 07, 2011, 01:25:08 PM »
I guess the OH- could attack the carbonyl carbon of the cyclohexanone but using a grignard reagent to form OH- is pointless. You could just dissolve NaOH in water to get OH-. Is the CH3CH2-MgBr bought from a manufacturer?  Because you can not form a grignard reagent in water.

Offline industrial_engineer

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #2 on: April 07, 2011, 01:35:23 PM »
It is for an assignment in O chem I and the formed gringard is given in ether.  I have a feeling that it may have been a typo where the prof meant to put in H30+ instead of the H20

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Offline Honclbrif

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #3 on: April 07, 2011, 01:39:38 PM »
Water is kryptonite to grignards. Are you sure you're not supposed to just quench the reaction with H2O once it is complete?
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Offline industrial_engineer

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #4 on: April 07, 2011, 01:40:53 PM »
I know water kills the reactions... but it says add them at the same time - hey may just be throwing us a curveball to screw with us on the last assignment haha

Offline voidSetup

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #5 on: April 07, 2011, 01:41:24 PM »
It might be that the cyclohexanone is supposed to also be in ether, which is writted as ET2O sometimes.  The grignard reagent then should attack the carbonyl carbon of cyclohexanone.  After this is done you do an H3O+ workup to isolate the product.  I actually did this same reaction last week to get my Grignard formation skills down better.

Offline industrial_engineer

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #6 on: April 07, 2011, 01:43:48 PM »
The question states that you don't use any acid workup though haha... That is what I originally thought was going to happen

Offline voidSetup

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #7 on: April 07, 2011, 01:45:08 PM »
That's weird because you'll just have some precipitate because it forms R-O- MgBr+

Offline industrial_engineer

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #8 on: April 07, 2011, 01:51:44 PM »
Yah... the whole question seems fishy

Offline voidSetup

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #9 on: April 07, 2011, 01:54:33 PM »
Maybe he's trying to show you what happens if water gets in?  Do you already know what the desired product is?

Offline jake.n

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Re: gringard reaction with H20 and Cyclohexanone
« Reply #10 on: April 07, 2011, 02:03:33 PM »
Treating the grignard reagent strictly as a base, you still have an acidic hydrogen on your ketone which would be in equilibrium with -OH in your water phase.

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