Grignard and alkyne

[joonkimdds]

Hello~~ I have questions in OCHEM.

I feel comfortable with R-Mg-X (grignard reagent) attacking carbonyl carbon.

But I still have trouble with the grignard reagent when it reacts with alkyne.

Both my professor and destroyer used alkyne as an example that grignard reagent will switch its Mg-X with terminal H of alkyne.
The explanation says terminal H is acidic.
I am sure there are many other acidic structures with acidic proton so is it a coincident that both my professor and Dr. Romano used the same example or is it a dominant reaction over all?

Also, destroyer has NaNH2, NH3 + alkyne does the exact same thing as the above example.
NaNH2, NH3 is supposed to turn alkane into alkyne so I don't know how it further reacts with alkyne. And I don't know why Dr. Romano used alkyne to show the example of acidic structure.

My professor, grignard example, and NaNH2 examples were all alkyne with terminal H. I want to know if it's just a coincident or if it's the only acidic structure that can react this way.

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[tizom]

i had the same problem earlier!

i am still a little cloudy on some of it, but one thing i did conclude was that NaNH3 is a very strong base and the terminal proton on an alkyne is acidic and very reactive toward NH2-, which is why it reacts so well... I know NaNH2,NH3 is supposed to turn an alkane into an alkyne, but i havent seen any examples of those in my practice q's yet 🙁

hope that helps a lil

 

[jbrenn4]

A grignard is an extremely strong base...therefore it will reamove thre terminal H from an alkyne as well as attacking a carbonyl group....the same applies with NaNH2/NH3 they are very strong bases therefore they would react with the terminal acidic proton of an alkyne

 

[joonkimdds]

Ok then I have one more question.
Both Kaplan and destroyer had a question asking
"which can be converted to grignard reagent most easily".

Grignard reagent can swap its MgCl with acidic proton from another structure.

But the answer says the one without acidic proton can react quickest.
shouldn't it be the opposite?
I thought grignard reagents 'want' acidic proton just like the above example that it can take acidic proton from alkyne.

 

[jbrenn4]

That's exactly it...a grignard can form most readily when there is no acidic proton because if there is an acidic proton the grignard will act as a base by removing the acidic proton

 

[PooyaH]

Ok then I have one more question.
Both Kaplan and destroyer had a question asking
"which can be converted to grignard reagent most easily".

Grignard reagent can swap its MgCl with acidic proton from another structure.

But the answer says the one without acidic proton can react quickest.
shouldn't it be the opposite?
I thought grignard reagents 'want' acidic proton just like the above example that it can take acidic proton from alkyne.

hmmmm are you sure? there will be no reaction without an acidic proton! Like ether won't react since it lacks an acidic proton! I'm sure there's something missing lol

 

[jbrenn4]

i think your problem is that there is a Cl or Br attached to an ether, in that case a ether will form the grignard most readily because there is no acidic proton

 

[PooyaH]

i think your problem is that there is a Cl or Br attached to an ether, in that case a ether will form the grignard most readily because there is no acidic proton

Well yeah, if there's a halogen attached to the ether, then it will react, but just an ether won't react!!!

 

[jbrenn4]

Ok so the question that i think joonkimdds had was if you have an ether with a halogen vs an alcohol or acid etc with a hologen attached...a grignard would most readily form with an ether because there is no acidic proton for it to otherwise react with!

 

[PooyaH]

Ok so the question that i think joonkimdds had was if you have an ether with a halogen vs an alcohol or acid etc with a hologen attached...a grignard would most readily form with an ether because there is no acidic proton for it to otherwise react with!

K in that case, it makes sense!

 

[joonkimdds]

Ok so the question that i think joonkimdds had was if you have an ether with a halogen vs an alcohol or acid etc with a hologen attached...a grignard would most readily form with an ether because there is no acidic proton for it to otherwise react with!

basically a question like this~~~
what do you think is right? and why?

 

[tizom]

i would say D, because thats the only one that doesnt have an acidic proton that would react with the MgX. any others that tried to be converted into a grignard would react with themselves and then be unable to act as a reagent

i think ha

 

[iwannabadentist]

so is the answer for that question D or E? Since every other choice, except maybe B has an acidic proton?

 

[joonkimdds]

i would say D, because thats the only one that doesnt have an acidic proton that would react with the MgX. any others that tried to be converted into a grignard would react with themselves and then be unable to act as a reagent

i think ha

D, cuz it doesn't have acidic proton which I think doesn't make sense.

I thought that acidic proton is required to react with grignard reagent.

For example, H-C(triple bond)C-H react with R-Mg-Cl to become H-C(triple bond)C-MgCl so this proves that acidic proton is required.

 

[iwannabadentist]

D, cuz it doesn't have acidic proton which I think doesn't make sense.

I thought that acidic proton is required to react with grignard reagent.

For example, H-C(triple bond)C-H react with R-Mg-Cl to become H-C(triple bond)C-MgCl so this proves that acidic proton is required.

wel i think you are thinkin abt smthn diff and the question is askin smthn else. the question is askin which of the compunds (particularly the halogen in each compund) will react to form grinard most quickly. since they are all R-Cl, you have to look at all the different Rs to see which R would support a gridnard the best. D works becuase with all the other Rs, as soon as a grignard is formed, it will react with another molecule of itself and take of its acidic proton - this will not give you an efficient synthesis of a grinard. Thats why D would work best because you can get the highest yield of a grignard of yor choice as compared to the other 4 options. 👍

 

[tizom]

yeah, if it was asking which reacts the best WITH a grignard reagent to make the grignard product it would prob be the acetelyne cuz that proton is very acidic ( i think, not sure) but it is asking with will act to MAKE a grignard reagent which is R-MgX, and that is the one without the acidic proton so it wont react with itself.

 

[joonkimdds]

gotcha~!

 

[iwannabadentist]

yeah, if it was asking which reacts the best WITH a grignard reagent to make the grignard product it would prob be the acetelyne cuz that proton is very acidic ( i think, not sure) but it is asking with will act to MAKE a grignard reagent which is R-MgX, and that is the one without the acidic proton so it wont react with itself.

even that wouldn't work for that particular problem because the carboxylic acid is WAY more acidic than a terminal alkyne, thanks to resonance with the COO- which wouldnt happen with the alkyne.

 

[joonkimdds]

Since I started a grignard thread, let me post another one.

 

[tizom]

yeah, you;re right, i forgot there was a COOH there, but that proton is the most acidic followed by the alkyne H . righT?

 

[PooyaH]

Wouldn't it be C, the formaldehyde?

 

[joonkimdds]

Wouldn't it be C?

yep!

 

[iwannabadentist]

tizom - i'm not too sure abt that, but if i had to guess, id say alcohol is more acidic than alkyne just because a negative charge on an O is more favorable than a negative charge on the C. Also, Alkynes are very highly basic, so more reason to pick OH to be more acidic.

2nd grignard question - pretty sure its C. A would not make an alcohol. B, D, and E would each make 2' alcohols.

 

[PooyaH]

yep!

I believe that's the case with esters and not carboxylic acids. Even if it happened with carboxylic acids you'd still make a secodary alcohol and not a primary alcohol!

p.s. what happened to your question? LOL