Grignard

[ChrisM07]

When does a Grignard add an alkyl group and when does it serve only to de-protonate. I feel like I've seen both happen and I can't really predict when.

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[athomas]

It will always deprotonate any remotely acidic H, such as on an alcohol or a terminal alkyne (which makes an acetylide)

 

[Illfavor]

It will always deprotonate any remotely acidic H, such as on an alcohol or a terminal alkyne (which makes an acetylide)

Yep...generally pka of around 25 and lower gets deprotonated. What's I think is important is that you understand that the Grignard is an excellent nucleophile/super strong base. If you understand that you can pretty much extrapolate how it reacts in any scenario.

 

[ChrisM07]

Thanks guys! So, one more relating to Grignards. How do you know when sulfuric acid will be removing the O- if the Grignard is added to a carbonyl, forming a new C=C bond, or if it will protonate the O forming an alcohol.



Here, the ethyl grignard is being reacted with sulfuric acid.

Thanks again!

 

[GsuDent]

Thanks guys! So, one more relating to Grignards. How do you know when sulfuric acid will be removing the O- if the Grignard is added to a carbonyl, forming a new C=C bond, or if it will protonate the O forming an alcohol.



Here, the ethyl grignard is being reacted with sulfuric acid.

Thanks again!

This is not a grinard reaction. This is a wittig reaction with an ylide acting as a nucleophile. If I am not mistaken, forming a c-c double bond.

 

[cmF]

This is not a grinard reaction. This is a wittig reaction with an ylide acting as a nucleophile. If I am not mistaken, forming a c-c double bond.

I was going to say the same thing. I don't think there's any case in which a grinard can eliminate the oxygen all together. In all cases, there will always be an alcohol present where there used to be a ketone.

 

[ChrisM07]

From DATQVault. I was honestly looking for a Wittig reaction in the answer choices.

 

[real3369]

This is not a wittig reaction. From the answer choices, grignard addition (two carbons) happens first, forming an alcohol. Then, elimination reaction of alcohol by strong acid like sulfuric acid.

 

[Illfavor]

And based on what folks say, that is maybe harder than the actual test.

 

[athomas]

People also say on here all the time not to use qvault for Ochem, apparently there are also lots of errors you might run into.

 

[cmF]

From DATQVault. I was honestly looking for a Wittig reaction in the answer choices.

Ah this makes more sense now that I see the answer choices. Your question was, "How do I know if a grinard forms 1) alcohol/alkyl substituent or 2) double bond."

A grinard by itself will always form 1).
A grinard followed by dehydration (H2SO4) will form 2).

I think the confusing part is that a grinard by itself will still use H2SO4 but remember that it's diluted so you really have H3O+. It's just not written as part of the grinard in answer choice E. Thus, the confusion when you see the second step.