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Topic: Unimolecular SN2 and bimolecular SN1???  (Read 9117 times)

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Offline Shadow

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Unimolecular SN2 and bimolecular SN1???
« on: September 03, 2014, 11:33:47 AM »
Unimolecular SN2 is present in halohydrins when they form oxirane. Is bimolecular SN1 possible? Also, are bimolecular E1 and unimolecular E2 possible?

Offline clarkstill

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #1 on: September 03, 2014, 11:52:57 AM »
I don't really understand the question... do you mean an Sn1 reaction that is bimolecular in the rate-determining step? If so, then no, by definition. The specific example you give of a "unimolecular Sn2" is a misnomer: although it is mechanistically similar to a conventional Sn2 reaction, kinetically it behaves as an Sn1. 

The analogue in the elimination reaction is the "Ei" elimination (e.g. the Cope elimination) which mechanistically resembles the E2, but kinetically behaves as an E1. I think overall it would be more accurate to describe the example you gave as an "SNi" reaction, but unfortunately this is already an abbreviation for something else.

Offline rwiew

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #2 on: September 03, 2014, 12:48:03 PM »
Yeah, I don't think this is what Shadow was asking about clarkstill, but I'm very confused too. Shadow for what you mean the use of bimolecular and unimolecular is not correct, you should say intermolecular and intramolecular respectively - this is what I understand you mean for the Sn's. For eliminations I have no idea what you mean - of course E1 = unimolecular elimination, E2 = bimolecular elimination and I guess you're asking if you can eliminate the halide and form and enol --> ketone/aldehyde?

Intramolecular Sn1/Sn2/spectrum between the two mechanisms depending on the exact structure and hence stabilization of potential carbocation-like intermediate will be by far the dominating pathway, intramolecular reactions are simply much faster than intermolecular for obvious reasons. However, you might get a dimerization/polimerization products from a different pathway - when you form an oxirane and still have the halohydrin species present, you can envisage attack of the OH of halohydrin on the oxirane, leading to its opening, and so on. Will depend on exact conditions of how you do this.

 Eliminations I would see as slow in this case, if you do them in basic pH you would be forming a quite high energy enolate, this will probably make the TS energetically high and reaction slow. But maybe, especially at higher temperatures.

Offline Shadow

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #3 on: September 03, 2014, 01:39:16 PM »
So basically, SN1 must be unimolecular by definition, eventhough its mechanism can resemble the SN2 mechanism like in the halohydrin. Therefore SN1 can be stereospecific, too.
The same applies for E1 which can resemble E2 (that's Ei).

But can all this go the other way round? Can SN2/E2 resemble SN1/E1 mechanism?

Offline rwiew

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #4 on: September 03, 2014, 11:54:44 PM »
What do you mean mechanism of SN1 can resemble SN2 in halohydrin? Remember there is a spectrum between the two, the two separate mechanisms are defined for limiting cases if you like. I.e. SN1 is a limiting case of SN2, where the C-leaving group bond is stretched to infinity, while the C-nucleophile bond is coming from infinity in the intermediate. Yes, you could say SN1 could be stereospecific, it all depends on the substrate and here on the neighboring groups of the reactive centre. But again, if you have an OH in the molecule you will never get a "pure" SN1 with an isolated carbocation.

I don't agree with E1 resembling E2 in Ei - there is no presence of a carbocationic-like intermediate in Ei, it's just an E2 with the base being internal.

To you last question, I think my first paragraph is the answer, there is a spectrum, you go from an SN2 TS with the breaking and forming bonds of the same length to them stretching to "infinity". Does this help?

Offline clarkstill

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #5 on: September 04, 2014, 02:42:53 AM »
I don't agree with E1 resembling E2 in Ei - there is no presence of a carbocationic-like intermediate in Ei, it's just an E2 with the base being internal.

I did not say that the mechanism resembled E1, merely the kinetics.  It would be impossible, from kinetics experiments alone, to distinguish between the Ei and E1 reaction, whereas E2 would be distinct.

I also don't agree with your assertion that SN1 and SN2 are "limiting cases".  While it is true that many reactions may go via a mixture of SN1 and SN2 mechanisms, each of these individual reactions is either unimolecular or bimolecular, with rate laws that are correspondingly first and second order respectively, and the overall rate law is the sum of the two.  I don't think it can reasonably be argued that any individual reaction goes through an intermediate (1.5)-molecular transition state: what would be the physical meaning of this?

Offline Shadow

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #6 on: September 04, 2014, 06:00:00 AM »
I am not sure if I got this right. Any nucleophilic substitution in which only one molecule participates is SN1 and it may not include carbocations. Therefore in halohydrins, the substitution reaction forming oxirane is SN1, but it's mechanism resembles SN2 because no carbocation is formed during the course of the reaction. Am I correct?

Offline rwiew

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #7 on: September 04, 2014, 12:14:01 PM »
We're both right clarkstill, in a different way. You were talking about kinetics and I was talking about the appearance of mechanisms themselves.

Shadow, now I finally understand what your question is. I see the logic in your understanding of SN1 always being unimolecular and 2 always being bimolecular, but these come strictly from intermolecular (substitution by one molecule on another) I believe. Hence the convention is (and I've never seen it the other way) that the classification is not done on kinetics but on the mechanism - carbocation = Sn1, penta intermediate and inversion = Sn2. Hence the halohydrin mechanism is just an SN2.

Offline Shadow

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #8 on: September 04, 2014, 01:37:12 PM »
It doesn't seem logical to say that the halohydrin reacts SN2 when the 2 means two molecules.
Also, in acetal formation, one step is E2 elimination but it includes only one molecule (the hemiacetal). These signs for substitution and elimination are obviously inaccurate.

Can it go the other way around? Two molecules participating in E1 or SN1?

Offline rwiew

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #9 on: September 04, 2014, 02:24:55 PM »
Ok, you're just confusing yourself very badly. I think they way it went was these reactions were discovered on two molecules and given the names. At some point their mechanisms were discovered. The mechanisms stayed together with the original names, which were never changed. Just accept that the name goes with a mechanism, it is the more logical approach. Otherwise the whole system would be a mess and no one would understand each other. Keep it simple.

Where is there E2 in acetal formation? Are you talking about the lone pair on OH kicking in and kicking another (protonated) OH one? Note that is an E1 elimination, as you are going through a carbocation-like TS (where the lone pair on another oxygen is attacking that developing positive charged centre and taking the charge itself). E2 requires deprotonation on a neighbouring carbon.

It kind of could go the other way, for example imagine using a Lewis acid catalyst to help take off a leaving group and form a carbocation. That would have two molecules involved in the rate determining step.

Offline Shadow

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #10 on: September 04, 2014, 04:06:26 PM »
Yes, that's the step I mean. Now I think it should be something between E1 and E2 ,as the opening of oxiranes in acid is between SN1 and SN2 because the same stabilization effect applies in both cases, right? In the former, it is 1,2 donation of electron pair from oxygen, the latter it is 1,3 donation.

Offline rwiew

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #11 on: September 04, 2014, 10:25:50 PM »
I don't agree, that step in acetal formation is strictly SN1. SN2 requires formation of a new sigma bond in the transition state (as compared to a pi-like stabilization by the lone pair). Also, look at the product of that lone pair attack, you can draw a canonical form where there is a naked carbocation and no new bond exists. Remember that the substitution there is not finished until a nucleophile attacks onto the C=OH, otherwise you haven't substituted anything (lone pair can't substitute). 

Offline Shadow

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #12 on: September 05, 2014, 05:01:49 AM »
OK. I didn't mean strickly SN1, but something in between SN2 and SN1 like epoxide opening in acid.

Offline Shadow

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #13 on: September 05, 2014, 08:16:56 AM »
One more question. Is the formation of benzyne E1 or E2?

Offline rwiew

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Re: Unimolecular SN2 and bimolecular SN1???
« Reply #14 on: September 05, 2014, 01:40:26 PM »
OK. I didn't mean strickly SN1, but something in between SN2 and SN1 like epoxide opening in acid.

Ok, again, seriously it is not even anything between. The lack of any nucleophile incoming is the difference - in acetal you have a lone pair attacking, that is not a nucleophile, just a means of stabilization of the carbocation. You would have to have the second molecule of alcohol attacking in a least "partially" to call that a bit like SN2. Do you see that? In the epoxide opening, yes, sure, but you have a bit of opening of the epoxide and a bit of attack of a nucleophile on it at the same time.

Good question about the benzyne. It's definitely not E1 in carbocation sense if that's what you mean, you won't get a carbocation formed (that's way too unstable, hence need strong base). You can have E2- deprotonation together with the group leaving or E1cB (elimination - conjugate base)- deprotonation first, then that kicks out the leaving group. I've seen both drawn in texts, with the E1cB I'd say more common - but what it is in reality I actually don't know. Maybe both are operating, maybe you get a mixture of the two - I'll try to have a look at some papers later.

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