July 10, 2020, 07:40:32 AM
Forum Rules: Read This Before Posting


Topic: is this mechanism corect??  (Read 5607 times)

0 Members and 1 Guest are viewing this topic.

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
is this mechanism corect??
« on: May 16, 2013, 08:42:16 PM »
https://www.dropbox.com/s/qedxm25xp6lmp65/2013-05-16%2019.41.32.jpg

the second Br reaction is what my question is about. which method is correct? im kind of lost as to why this reaction is occuring in methanol methanol. one methanol in reacting and one methanol is just the solvent??

please help.

Offline TwistedConf

  • Regular Member
  • ***
  • Posts: 67
  • Mole Snacks: +10/-2
Re: is this mechanism corect??
« Reply #1 on: May 16, 2013, 09:44:08 PM »
Neither of your mechanisms / products for the second reaction are really correct- but the second one is closer.

You need to go to your book / notes / etc. and review the "solvolysis" of halides.


Offline TheOrganic

  • Regular Member
  • ***
  • Posts: 39
  • Mole Snacks: +5/-3
  • Gender: Male
  • The Organic Chemist
Re: is this mechanism corect??
« Reply #2 on: May 16, 2013, 10:45:12 PM »
You are thinking that one molecule of methanol is reacting with one molecule of the substrate. Is that it?

Methanol is an important organic solvent because it dissolves many organic compounds. In particular, for this reaction methanol is a good solvent because it'll dissolve the allylic halide very well.

Now, methanol is a solvent for this reaction. What this means is, the alkyl halide ( some millions of alkyl halide molecules ) is dissolved in a huge excess of methanol. Now, a solvent could become a reactant too, if a reaction is possible. Why shouldn't it? If a reaction is possible and if the reactants come in contact, the reaction should happen. A carbocation will be formed after loss of bromine. The carbocation would normally be solvated by suitable solvent molecules. But in this case, a reaction between the two is possible- the carbocation is an electrophile and methanol is a nucleophile - so a HOMO-LUMO electron transfer sounds reasonable.

Thus a solvent can both dissolve an organic compound- and if a reaction is possible it could react with it as well. This is technically called " Solvolysis "

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
Re: is this mechanism corect??
« Reply #3 on: May 17, 2013, 08:47:10 PM »
i looked up on solvolysis....then would my mechanism be like this??


https://www.dropbox.com/s/qex978byvlbs2b8/2013-05-17%2020.45.34.jpg

can i use the Br to deprotonate?

Offline TheOrganic

  • Regular Member
  • ***
  • Posts: 39
  • Mole Snacks: +5/-3
  • Gender: Male
  • The Organic Chemist
Re: is this mechanism corect??
« Reply #4 on: May 18, 2013, 02:49:48 AM »
i looked up on solvolysis....then would my mechanism be like this??


https://www.dropbox.com/s/qex978byvlbs2b8/2013-05-17%2020.45.34.jpg

can i use the Br to deprotonate?

What is the pka of H-Br? Is it a strong acid? Why do you think I'm asking this question?
Are you sure about the concertedness of the first step of the reaction?
« Last Edit: May 18, 2013, 03:00:28 AM by TheOrganic »

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
Re: is this mechanism corect??
« Reply #5 on: May 18, 2013, 05:43:15 AM »
hbr is a strong acid and br is a weak conjugate base...so its no good in this case, right?

as for the reaction...im not quite understanding whats wrong with it.

Offline discodermolide

  • Chemist
  • Sr. Member
  • *
  • Posts: 5038
  • Mole Snacks: +404/-70
  • Gender: Male
    • My research history
Re: is this mechanism corect??
« Reply #6 on: May 18, 2013, 06:05:38 AM »
hbr is a strong acid and br is a weak conjugate base...so its no good in this case, right?

as for the reaction...im not quite understanding whats wrong with it.



Why did you change the stereochemistry in the second link? You now show the OMe as being axial, whereas the bromine was equatorial.
Development Chemists do it on Scale, Research Chemists just do it!
My Research History

Offline TheOrganic

  • Regular Member
  • ***
  • Posts: 39
  • Mole Snacks: +5/-3
  • Gender: Male
  • The Organic Chemist
Re: is this mechanism corect??
« Reply #7 on: May 18, 2013, 08:37:39 AM »
hbr is a strong acid and br is a weak conjugate base...so its no good in this case, right?

Right.

as for the reaction...im not quite understanding whats wrong with it.

You have shown that a carbocation will be formed. What is the shape of the carbocation? How can that affect the stereochemistry of the products?

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
Re: is this mechanism corect??
« Reply #8 on: May 18, 2013, 09:52:07 AM »
it proceeds via sn1....inversion of chem and prod is racemic??

shape of the carbocation....its secondary...i cant make it tertiary by the way of a methyl/hydride shift....im not sure im getting what youre saying...

or does it proceed via sn2??? but ch3oh is a weak nuc..

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
Re: is this mechanism corect??
« Reply #9 on: May 18, 2013, 03:15:26 PM »
hbr is a strong acid and br is a weak conjugate base...so its no good in this case, right?

as for the reaction...im not quite understanding whats wrong with it.

Why did you change the stereochemistry in the second link? You now show the OMe as being axial, whereas the bromine was equatorial.

inversion of stereochemistry??



please help. im not really understanding the problem.

Offline discodermolide

  • Chemist
  • Sr. Member
  • *
  • Posts: 5038
  • Mole Snacks: +404/-70
  • Gender: Male
    • My research history
Re: is this mechanism corect??
« Reply #10 on: May 18, 2013, 03:38:00 PM »
Carbocations are usually planar, therefore if this proceeds via a carbocation I would expect a mixture of "up" and "down" products.
Stereochemical inversion is seen in Sn2 reactions not Sn1.
Development Chemists do it on Scale, Research Chemists just do it!
My Research History

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
Re: is this mechanism corect??
« Reply #11 on: May 18, 2013, 04:57:08 PM »
Carbocations are usually planar, therefore if this proceeds via a carbocation I would expect a mixture of "up" and "down" products.
Stereochemical inversion is seen in Sn2 reactions not Sn1.

how about this??

https://www.dropbox.com/s/d4824c3ysi0j433/2013-05-18%2016.55.07.jpg

it proceeds via an sn1 mechanism.

Offline discodermolide

  • Chemist
  • Sr. Member
  • *
  • Posts: 5038
  • Mole Snacks: +404/-70
  • Gender: Male
    • My research history
Re: is this mechanism corect??
« Reply #12 on: May 18, 2013, 11:37:08 PM »
Better. Note the cation is stabilised by resonance with the double bond, it is an allylic cation.
Development Chemists do it on Scale, Research Chemists just do it!
My Research History

Offline noork85

  • Regular Member
  • ***
  • Posts: 31
  • Mole Snacks: +0/-2
Re: is this mechanism corect??
« Reply #13 on: May 19, 2013, 02:50:24 AM »
Better. Note the cation is stabilised by resonance with the double bond, it is an allylic cation.

gotcha. thanks alot!!

Offline TheOrganic

  • Regular Member
  • ***
  • Posts: 39
  • Mole Snacks: +5/-3
  • Gender: Male
  • The Organic Chemist
Re: is this mechanism corect??
« Reply #14 on: May 19, 2013, 05:05:05 PM »
Better. Note the cation is stabilised by resonance with the double bond, it is an allylic cation.

gotcha. thanks alot!!

By "shape" of carbocation I meant to guide you towards planarity, then the stereochemistry.

As an exercise, try to predict the products, both the stereochemistry and the regiochemistry when the following is subject to the same reaction condition ( kept in methanol ) :

C1C[14c](Br)C=CC1 

Consider that I've already given you a hint.

Sponsored Links