Conversation started Aug 12, 2020 at 15:51.
Guru Vishnu
Guru Vishnu
Aug 12, 2020 15:51
Hi everyone!
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Here, why did the author refuse to do a carbocation rearrangement for the intermediate shown separately in the bottom?
For me, it seems the tertiary carbocation must be more stable than the vinylic carbocation due to electronegativity and resonance reasons.
It would be helpful if someone can confirm the book is wrong. Thanks!
user434058
Aug 12, 2020 16:16
@GuruVishnu Such rearrangements do happen (it has been experimentally verified), however, in all the questions that I have encountered in practice books for JEE, this rearrangement has always been ignored. So, there's a general consensus among JEE aspirants to not do such a rearrangement.
Guru Vishnu
Guru Vishnu
@FakeMod Ok. Thank you for the clarification :-)
I think, as you said, this might be JEE specific as I was unable to find this example in the international version of Solomons.
ronak jain
ronak jain
Aug 12, 2020 16:29
@Safdar hi
 
10 hours later…
sai-kartik
sai-kartik
Aug 13, 2020 02:53
@GuruVishnu can you show your intermediate please?
i dont see how a tertiary carbocation can be formed... besides, we always get gem di halides on addition of HX to an alkyne. This is a rather common question in jee too...
i dont think vinylic cations can even rearrange until there is scope for resonance
@Safdar What's your take on this?
 
1 hour later…
Guru Vishnu
Guru Vishnu
Aug 13, 2020 04:23
@sai-kartik Hi! I hope you would be able to take from here:
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After this, an iodide ion attacks the tertiary carbocation. Then with the second equivalent of hydrogen iodide, a Markovnikov addition takes place giving the product I got.
I think, now I found a method to arrive at the book's answer even after rearrangements:
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So, the product I arrived at first might be incorrect. Even after two rearrangements (as given above), we get the same product without any rearrangement (as done in the book).
 
1 hour later…
Yuvraj
Yuvraj
Aug 13, 2020 05:42
@GuruVishnu it would only be in excess
sai-kartik
sai-kartik
@GuruVishnu haven't seen this rearrangement anywhere 😬
 
1 hour later…
Safdar
Safdar
Aug 13, 2020 06:55
@sai-kartik I wouldn't be online 90% if the time.. I just have it open on another tab.. so pinging me isn't the best idea
 
7 hours later…
Safdar
Safdar
Aug 13, 2020 14:05
$$\mathrm{ Propanoyl Chloride}\ce{ ->[NH2OH][\text{ethanoyl chloride}/H2O,\Delta] X}$$
@YusufHasan
satan 29
satan 29
Aug 13, 2020 14:17
@GuruVishnu yes! this is my doubt also
And unfortunately solomons-frhyle-snyder has no mention of the mechanism
Although My JEE notes say :"gem di-halides are formed"
Guru Vishnu
Guru Vishnu
@satan29 Are you having the Indian version (adapted by MS Chouhan) or International version?
satan 29
satan 29
adapted by MS chouhan
Guru Vishnu
Guru Vishnu
Then it must be there somewhere after Prins reaction.
satan 29
satan 29
lemme see
Guru Vishnu
Guru Vishnu
Did you (@satan29) and @sai-kartik see this:
10 hours ago, by Guru Vishnu
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This seems to be inline with your notes and also the final product given in the book.
That second rearrangement takes place for a reason the carbocation is stabilized by the +M effect of iodine. But if I recall correctly that must be applicable only for chlorine.
 
Conversation ended Aug 13, 2020 at 14:27.