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Author Topic: Help with carbocations  (Read 548 times)

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rbf17642

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Help with carbocations
« on: November 29, 2012, 08:05:56 PM »

What is the difference in likelihood/plausibility of these occurrences?


1)
catalyzed formation of acyl cation

2)
catalyzed formation of ...hmm 2-oxopropan-1...? idk what to call it.
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orgopete

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Re: Help with carbocations
« Reply #1 on: November 30, 2012, 01:36:24 AM »

ZnCl(-)? Doesn't zinc have electron to donate?
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Babcock_Hall

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Re: Help with carbocations
« Reply #2 on: November 30, 2012, 03:55:06 AM »

Can you draw other resonance forms for either cation?  You have to show an attempt to work the problem.
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rbf17642

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Re: Help with carbocations
« Reply #3 on: December 01, 2012, 07:40:19 PM »

ZnCl(-)? Doesn't zinc have electron to donate?

True, but I know the first case is possible under microwave irradiation.  I found it out researching alternative catalysts for the Friedel-Crafts acylation. 
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rbf17642

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Re: Help with carbocations
« Reply #4 on: December 01, 2012, 08:03:49 PM »

Can you draw other resonance forms for either cation?  You have to show an attempt to work the problem.

Yes I am familiar with the resonance form for the acyl cation, but I am unsure about possible forms of the second cation.
I haven't taken any o. chem classes so there is a considerable amount of knowledge I may not share.  How do I go about figuring out whether that second cation is the most stable form, or if it will even form at all?  What is the difference between the carbonyl carbon and its adjacent carbon in this case?
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orgopete

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Re: Help with carbocations
« Reply #5 on: December 02, 2012, 04:49:48 AM »

ZnCl(-)? Doesn't zinc have electron to donate?

True, but I know the first case is possible under microwave irradiation.  I found it out researching alternative catalysts for the Friedel-Crafts acylation.

Okay, so not having had ochem grants you some latitude, however it also misses describing chemical motivation. Organic chemists help to identify that motivation by showing electron movement with curved arrows.

In the first reaction, zinc is an electron acceptor of Cl(-), so this would have been Zn(++) as ZnCl2. In the second instance, zinc is an electron donor, so this would be zero valence zinc and the net product would be ZnCl2 and an enolate of the ketone. If this reaction were carried out in acetic acid, the result would be donation of a proton and form a ketone without the chlorine.

The curved arrows should clearly show these electron movements and thus rationalize how (and why) the reaction takes place. I might suggest you take a look at my book, A Guide to Organic Chemistry Mechanisms (yellow cover). Although it is somewhat avant garde in some usages, if you are learning chemistry on your own, this will be to your advantage. Curved arrows are inconsistent and ambiguous in certain instances. So for that, you would find showing resonance structures and product formation logical and consistent. Because curved arrows are a language of electron movements, you will easily understand the conventional curved arrows and if you understand the chemistry, you can correctly choose the correct alternate when an ambiguous possibility exists. An example is a Markovnikov addition to an alkene. There is no indication from the curved arrows themselves which of two carbocations should form. This has the insidious effect of students ignoring curved arrows because they had to know which of the carbocations form a priori. You will find you can make all of those predictions from the curved arrow usage in the book, and thus learn which carbocations are favored from the examples. (The first chapter sets up the rules governing carbocations stability.)
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Babcock_Hall

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Re: Help with carbocations
« Reply #6 on: December 02, 2012, 09:49:39 AM »

rbf,  In the second carbocation, are there partial positive or negative charges on any other atoms besides the carbon with two hydrogens?
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orgopete

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Re: Help with carbocations
« Reply #7 on: December 02, 2012, 02:14:55 PM »

rbf,  In the second carbocation, are there partial positive or negative charges on any other atoms besides the carbon with two hydrogens?

That may be, but this reaction is not correct. Zinc inserts to form the anion. In the first reaction, zinc would not form an acylium ion unless it were actually ZnCl2.
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