Befuddled by Bromide / Organic Chem Ques.

[GauloisePF]

Greetings fellow SDNers!

I feel that I must preface my question with a quick thank you, as well as good luck to everyone out there studying for the April 2005 test. The board and its participants have been incredibly helpful and willing to mash out any specifics/nuances with test content, registration info, etc. May you be rewarded for your kindness, compassion, and diligent work in whatever form that may take for each of you respectively.

ANYWAY...

Would someone help me out with this question? It's taken up way too much space in my head and while I'm sure it's pretty simple and I'm just missing a step to my logic, I may go mad attempting it any further without professional help of some sort =)

Q. One of the products of a reaction between propyl-magnesium bromide and ethyne is: dum dum dum....

A. 1-pentyne
B. propane
C. 1-pentene
D. propene

Any help would be very much appreciated!

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[Apparition]

Haven't even started with orgo but

1-pentene?

 

[MattD]

Greetings fellow SDNers!

I feel that I must preface my question with a quick thank you, as well as good luck to everyone out there studying for the April 2005 test. The board and its participants have been incredibly helpful and willing to mash out any specifics/nuances with test content, registration info, etc. May you be rewarded for your kindness, compassion, and diligent work in whatever form that may take for each of you respectively.

ANYWAY...

Would someone help me out with this question? It's taken up way too much space in my head and while I'm sure it's pretty simple and I'm just missing a step to my logic, I may go mad attempting it any further without professional help of some sort =)

Q. One of the products of a reaction between propyl-magnesium bromide and ethyne is: dum dum dum....

A. 1-pentyne
B. propane
C. 1-pentene
D. propene

Any help would be very much appreciated!

Interesting question.... is ethyne electrophilic enough to undergo a grignard reaction? If so I'd say the answer is C. otherwise I have no idea, unless the question had some background like the silly chemist tried the reaction in water, might give you answer B. if the grignard decomposes for some reason.... When I get home I'll have to look this one up!


***EDIT: I just looked this one up, and IMHO this is a pretty obscure question. Apparently the acidity of the terminal alkyne is enough to cause a metal exchange. You'd wind up with the magnesium bromide ion jumping to the terminal carbon of the alkyne, and the hydrogen displaced jumps to the former grignard reagent. So the answer is B., but for an actual reason!!

Link: http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm

Interesting question, thanks!

 

[QofQuimica]

The correct answer is B (propane). You are trying to mix two electron-rich species here (a Grignard with an alkyne) so you cannot do any kind of nucleophilic attack on the alkyne using the Grignard. We spend a lot of time in organic chemistry focusing on how Grignard reagents are such good nucleophiles, which they are. However, they are also very powerful bases; the pKa of propyl magnesium bromide is estimated to be about 50. (In comparison, the pKa of NaOH, a common strong inorganic base, is about 16.)The protons of acetylene (the common name for ethyne; no one ever calls it ethyne!) are fairly acidic for an organic compound; they have a pKa of about 25. Remember that each time you go up or down a pKa unit, that is a decrease or increase of 10 TIMES in acidity. So, since the difference in pKa between this Grignard and acetylene is ~25 pKa units, we are talking about an acidity difference of 10 ^ 25, and you are gonna get one heck of an acid-base reaction when you mix the two. The Grignard gets protonated, forming propane, and the acetylene gets deprotonated, forming the anion.

 

[MattD]

The correct answer is B (propane). You are trying to mix two electron-rich species here (a Grignard with an alkyne) so you cannot do any kind of nucleophilic attack on the alkyne using the Grignard. We spend a lot of time in organic chemistry focusing on how Grignard reagents are such good nucleophiles, which they are. However, they are also very powerful bases; the pKa of propyl magnesium bromide is estimated to be about 50. (In comparison, the pKa of NaOH, a common strong inorganic base, is about 16.)The protons of acetylene (the common name for ethyne; no one ever calls it ethyne!) are fairly acidic for an organic compound; they have a pKa of about 25. Remember that each time you go up or down a pKa unit, that is a decrease or increase of 10 TIMES in acidity. So, since the difference in pKa between this Grignard and acetylene is ~25 pKa units, we are talking about an acidity difference of 10 ^ 25, and you are gonna get one heck of an acid-base reaction when you mix the two. The Grignard gets protonated, forming propane, and the acetylene gets deprotonated, forming the anion.

I call it ethyne! So does IUPAC....

 

[QofQuimica]

I call it ethyne! So does IUPAC....

Ok, I stand corrected; there is ONE PERSON who does. But, no practicing organic chemists call acetylene ethyne.

 

[MattD]

Ok, I stand corrected; there is ONE PERSON who does. But, no practicing organic chemists call acetylene ethyne.

I'm trying to start the fad... even if I'm not a practicing organic chemist, maybe it can be my way into the history books! Anyone care to join me, or does the common-name nazi have too firm of a grip on everyone???

Yeah, I know, I'm a dork, so what, wanna fight about it?

 

[Reimat]

I'm trying to start the fad... even if I'm not a practicing organic chemist, maybe it can be my way into the history books! Anyone care to join me, or does the common-name nazi have too firm of a grip on everyone???

Yeah, I know, I'm a dork, so what, wanna fight about it?

Haha, I've taken too many very difficult organic exams that use acetylene as a synthesis starting material to start calling it ethyne. But, then again, after the MCAT I really hope never to see the molecule or either word again.