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Topic: Some questions on A level Organic  (Read 14089 times)

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Offline Astrokel

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Re: Some questions on A level Organic
« Reply #15 on: August 04, 2008, 11:11:57 AM »
Thanks for the correction, azmanam. :) In my school, or rather the syllabus, we were taught two mechanism of bromine addition, the simplified one and the more accurate version which is the cylic bromonium ion(but not required). Where is the discussion on the solvent effects? I have a book on organic talking about polar protic and aprotic solvents in nucleophilic and elimination reactions. Is it on that?  :)
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Offline azmanam

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Re: Some questions on A level Organic
« Reply #16 on: August 04, 2008, 11:28:03 AM »
At the link above:

Because they proceed by way of polar ion-pair intermediates, chlorine and bromine addition reactions are faster in polar solvents than in non-polar solvents, such as hexane or carbon tetrachloride. However, in order to prevent solvent nucleophiles from competing with the halide anion, these non-polar solvents are often selected for these reactions. In water or alcohol solution the nucleophilic solvent may open the bromonium ion intermediate to give an α-halo-alcohol or ether, together with the expected vic-dihalide. Such reactions are sensitive to pH and other factors, so when these products are desired it is necessary to modify the addition reagent.
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Offline Astrokel

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Re: Some questions on A level Organic
« Reply #17 on: August 06, 2008, 01:00:06 PM »
Thank you so much sir  ;D
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Offline spirochete

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Re: Some questions on A level Organic
« Reply #18 on: August 06, 2008, 02:41:02 PM »
Thought I'd add to the already great information that's been added so far with a couple of comments on the incorrect mechanism you were taught in class.  With mechanisms I think it's a good idea to always be thinking about why a mechanism happens the way it does.  In this case I see at least 3 reasons why the "simpler" mechanism is far less likely than the actual mechanism which goes via the bromonium ion:

1)  As you noted, a collision of 3 molecules which all have the correct energy to result in a reaction is much less likely than a collison of 2 molecules.

2)  If you keep track of electrons in the incorrect mechanism, one intermediate is a positively charged bromine atom with an incomplete octet!  Bromine is far too electronegative to ever have an incomplete octet in an organic reaction.  On the other hand the positive charge on bromine is resonance stablized in the bromonium ion, and the only atoms with incomplete octets are carbons.

3)  Electrophilic addition to pi bonds is much more common than nucleophilic addition because pi bond electrons are weakly nucleophilic. 

(There are exceptions to this rule when the pi bonded carbons have electron withdrawing substituents, but none are present in this case.)       

Hopefully this added some insight.

Offline Astrokel

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Re: Some questions on A level Organic
« Reply #19 on: August 07, 2008, 07:37:01 AM »
Definitely, thank you so much for the explanations sir spirochete! I understood your points well.  :)
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Offline Astrokel

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Re: Some questions on A level Organic
« Reply #20 on: August 19, 2008, 02:28:40 AM »
hey all, i thought it would be better to post my questions here than starting a new thread since its still under A level.

i have attached a document it was one of my exam question. (It's already over) I have a few questions to ask!

aii) How is the NO2+ electrophile generated through HNO3(aq) only?

aiv) I'm thinking ortho nitrophenol will be the major product because since the probability of nitrating 2 or 6 is higher than para 4. But wouldn't 2/3 of products be ortho while 1/3 is para due to probability? But the question states 15:1?

Or is it because ortho nitrophenol is capable of forming intramolecular hydrogen bonding while para is intermolecular, therefore more para than ortho?

Ci) I put the test as oxidation by acidified KMnO4, reflux because although both will form white ppt of benzoic but ethylbenzene gives rise to carbon dioxide gas, hence bubbles will be observed. my question is will CO2 gas be observed in the actual experiment or the white ppt prevent it from observing? (I have never done this experiment before)

thanks for any helps !

ps: ci) is actually methylbenzene vs ethylbenzene, i just draw the -COOH there.
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Offline azmanam

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Re: Some questions on A level Organic
« Reply #21 on: August 19, 2008, 06:05:18 AM »
O2N-OH  +  H3O+  ⇄  O2N-OH2+ + H2O
O2N-OH2+  ⇄  O2N+  +  H2O

Same as with sufuric acid, just a different acid this time (thus the equilibrium probably does not lie as far to the right, but since phenol is a better nucleophile, it will react with NO2+ faster.   This will remove NO2+ from the equilibrium and drive the equilibrium to the right.
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Offline Astrokel

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Re: Some questions on A level Organic
« Reply #22 on: August 19, 2008, 08:24:01 AM »
thank you azmanam !  :) do you happen to know the other 2 questions?
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Offline Astrokel

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Re: Some questions on A level Organic
« Reply #23 on: August 21, 2008, 06:48:30 AM »
hey all,

i've attached a reaction and after acylating the carboyxylic group and undergo internal reaction between amine and acyl chloride to form a lactam? I'm interested in the mechanism and would be good if you could guide me for the acylation of carboxylic acid. :) I have no idea how to start off with the acylation of the acid with PCl5, i have checked out this http://www.cem.msu.edu/~reusch/VirtualText/crbacid3.htm but i still have no idea. Wat is the nucleophile in this case? This is not school work just doing out of interest.  ;D

Thanks heaps!  :)
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Offline azmanam

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Re: Some questions on A level Organic
« Reply #24 on: August 21, 2008, 09:18:57 AM »
Probably something like this...
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Offline Astrokel

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Re: Some questions on A level Organic
« Reply #25 on: August 21, 2008, 10:48:55 AM »
thanks heaps azmanam, i got it!!
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

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