quick ochem question

[xjoohn]

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Can someone explain to me why D is the answer? Thanks!

 

[Dotoday]

Because Grignard is a strong base. Since there is acidic proton on the alcohol (OH), it will react with Grignard and leave the methyl group. The water or acid can remove the Grignard. Hope this helps.

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[Dentalbum]

My reaction would be to choose B as the answer. First, the carbonyl would pick up the H+. The pi bond would then form a lone pair on the oxygen resulting in an alcohol while the grignard (acting as a nucleophile) would attack the carbonyl carbon. Since the problem only says 1 equivalence is reacting, this would be the final product. But you're book is saying that it's D. If the answer is D this is because the grignard is acting as a strong base instead of a nucleophile. If its acting as a base, as what the answer implies, then it is picking up the aldehyde hydrogen forming CH4. The H+ will then protonate the carbonly carbon leaving you with what you started with.

 

[Msmouth]

CH3 is the nucleophile......that attacks the carbonyl causing the oxygen to become negative.....follow with an acid work-up where the negative oxygen picks up the proton....

Choice B

 

[xjoohn]

My reaction would be to choose B as the answer. First, the carbonyl would pick up the H+. The pi bond would then form a lone pair on the oxygen resulting in an alcohol while the grignard (acting as a nucleophile) would attack the carbonyl carbon. Since the problem only says 1 equivalence is reacting, this would be the final product. But you're book is saying that it's D. If the answer is D this is because the grignard is acting as a strong base instead of a nucleophile. If its acting as a base, as what the answer implies, then it is picking up the aldehyde hydrogen forming CH4. The H+ will then protonate the carbonly carbon leaving you with what you started with.

My answer choice was B as well... How can I tell when to use grignards as the nucleophile or strong base?

 

[wcombs]

I think the answer is right but I would have definitely made the mistake of choosing B if it appeared on a test. There's even a DAT Destroyer question similar to this asking which reagent would most easily form the Gringard. The answer solution says that Gringards react with acidic protons and it messes everything up, to the point that you have to flame-dry test tubes so it doesn't react with extra water instead of your intended electrophile. The way I see it, the acidic proton on the alcohol would look a lot more appetizing to me, as a highly reactive base, than the partially positive carbon in the carbonyl. Hopefully that helps some!

 

[Msmouth]

Sorry, I didn't see that you wrote the answer is D....

I looked in my book and as previously stated, grignard will deprotonate OH as well as NH, SH, and CO2H

It will add to carbonyls but only in the absence of the aforementioned groups.

 

[TXpredent]

I think the answer is right but I would have definitely made the mistake of choosing B if it appeared on a test. There's even a DAT Destroyer question similar to this asking which reagent would most easily form the Gringard. The answer solution says that Gringards react with acidic protons and it messes everything up, to the point that you have to flame-dry test tubes so it doesn't react with extra water instead of your intended electrophile. The way I see it, the acidic proton on the alcohol would look a lot more appetizing to me, as a highly reactive base, than the partially positive carbon in the carbonyl. Hopefully that helps some!

but realistically an OH hydrogen isn't THAT acidic, unless its bonded to a carbonyl carbon. I think you're right though, however I wouldn't have been smart enough to think of it that way. I guess check if there's an acidic hydrogen around first before choosing a nucleophilic attack.

 

[Junebug1665]

I have a note from Chad's videos that says a Gringard reagent shouldn't be used in any kind of protic solvent. The gringard would deprotonate the alcohol and the H3O+ reprotenates it, so you get the original product.