Post by: noork85 on May 16, 2013, 08:42:16 PM
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.
Post by: TwistedConf on May 16, 2013, 09:44:08 PM
You need to go to your book / notes / etc. and review the "solvolysis" of halides.
Post by: TheOrganic on May 16, 2013, 10:45:12 PM
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 "
Post by: noork85 on May 17, 2013, 08:47:10 PM
https://www.dropbox.com/s/qex978byvlbs2b8/2013-05-17%2020.45.34.jpg
can i use the Br to deprotonate?
Post by: TheOrganic 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?
Post by: noork85 on May 18, 2013, 05:43:15 AM
as for the reaction...im not quite understanding whats wrong with it.
Post by: discodermolide 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.
Post by: TheOrganic 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?
Post by: noork85 on May 18, 2013, 09:52:07 AM
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..
Post by: noork85 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.
Post by: discodermolide on May 18, 2013, 03:38:00 PM
Stereochemical inversion is seen in Sn2 reactions not Sn1.
Post by: noork85 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.
Post by: discodermolide on May 18, 2013, 11:37:08 PM
Post by: noork85 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!!
Post by: TheOrganic 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 ) :
C=CC1](https://www.chemicalforums.com/SMILES/a4cd4a64ce9a1345a177.png)
Consider that I've already given you a hint.
Post by: noork85 on May 19, 2013, 07:14:17 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 ) :
Consider that I've already given you a hint.
the carbon 14 is throwing me off....
but the methanol should be able to attack from the front or the back. product would be racemic??
Post by: TheOrganic on May 20, 2013, 07:21:38 AM
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 ) :
Consider that I've already given you a hint.
but the methanol should be able to attack from the front or the back
Good. Go ahead.
Post by: noork85 on May 22, 2013, 11:51:01 AM
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 ) :
Consider that I've already given you a hint.
but the methanol should be able to attack from the front or the back
Good. Go ahead.
how's this? am i on the right track??
https://www.dropbox.com/s/nivqlp2pt36tisr/2013-05-22%2011.49.08.jpg
Post by: TheOrganic on May 23, 2013, 04:24:20 PM
Post by: noork85 on May 23, 2013, 06:27:47 PM
You are ignoring the resonating structures of the carbocation. When you label any one of the carbon, the resonating structures won't be equivalent per se.
:o ofcourse! why didnt i think of that. im going to redo this now. i had something like this on my final...i had to come up with a mechnaism. i finished the exam in 30 mins!! and spent the remaining 1 and a half hour on that problem.