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Author Topic: Grignard reactions help  (Read 1387 times)
prncess23
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« on: April 23, 2009, 02:58:24 AM »

hello.

Please help me with this question.

Give the products of the reaction of methylmagnesium chloride (Grignard reagent) with:
a.   acetaldehyde
b.   acetone
c.   ethyl acetate (1 equivalent per Grignard)
d.   ethanol
e.   propylene oxide
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chrisf
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« Reply #1 on: April 23, 2009, 06:31:19 AM »

A) propan-2-olate
B) 2-methylpropan-2-olate
C) 2-methylpropan-2-olate + ethanolate
D) I'm not sure this would work...
E) butan-2-olate

Keep in mind that these reactions are forming ions - if you follow the Grignard up with an acid/water, you'll get the corresponding alcohol (really, the whole point of these reactions - in my opinion).
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chrisf
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« Reply #2 on: April 23, 2009, 07:10:54 AM »

After protonation:

A) propan-2-ol
B) 2-methylpropan-2-ol
C) 2-methylpropan-2-ol + ethanol
D) ?
E) butan-2-ol
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prncess23
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« Reply #3 on: April 23, 2009, 06:56:02 PM »

thanks so much. how did u get to those results though and the end product is the protonated one right?
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chrisf
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« Reply #4 on: April 24, 2009, 11:17:41 AM »

Without doing the whole mechanism... the carbanion of the Grignard attacks the carbon of the carbonyl. The electrons forming the double-bond move to the oxygen giving it a negative charge. The resulting ion is a great base to deprotonate acid or water.. making the alcohol.

Unless your professor indicates a source of protonation, the answers are the ions.
« Last Edit: April 24, 2009, 02:29:44 PM by chrisf » Logged
chrisf
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« Reply #5 on: April 24, 2009, 02:39:34 PM »

Here's a visual representation of the acetone reaction.

The product in blue is what you get by only adding the Grignard. The product in red is what you'd get after acidification.
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prncess23
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« Reply #6 on: April 24, 2009, 06:45:50 PM »

thank you.
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chrisf
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« Reply #7 on: April 24, 2009, 07:11:03 PM »

thank you.

Does it make sense to you?
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prncess23
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« Reply #8 on: April 25, 2009, 12:13:11 AM »

thank you.

Does it make sense to you?

yes it does. thanks.

i have a quick question though...i am having a hard time determining alpha carbons... in the following example:

benzaldehyde + acetophenone

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prncess23
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« Reply #9 on: April 25, 2009, 12:30:10 AM »

wait for d...isnt the answer ethanol + methane?

why wouldn't that work?
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chrisf
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« Reply #10 on: April 25, 2009, 05:44:53 AM »

Well, the carbanion is a strong base.. so it may abstract a hydrogen from the alcohol - but even that, I'm not sure. I just don't think there would be a significant reaction occuring there.
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