Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: SVXX on November 21, 2010, 10:35:16 AM
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Greetings forumers.
I read about the benzyne mechanism in Solomons' Organic Chemistry and was also explained on similar lines by a teacher in my competitive exam coaching class two years ago. I have a doubt regarding how we decide which product will be major and which one will be minor.
According to both Solomons and my notes, the inductive effect of the substituent works out the most stable carbanion after the nucleophilic attack of -NH2, and then a proton is added in an acid base reaction to get the final product. Fair enough...
Now we come to the specific case of m-bromo anisole. The methoxy group has a -I effect and a +R effect(which is cancelled due to the triple bond on benzyne). Out of the possible ortho, bromo and para products, some textbooks my friend had referred to depicted the para product(p-methoxyaniline) to be major!
Ideally it should be the meta product, isn't it? If the amino group attacks the meta position(provided the triple bond is positioned accordingly), the carbanion can form on the carbon ortho to methoxy. Being closest to the methoxy group, the inductive effect would stabilize the anion best. Now obviously benzene can resonate into the formation in the first image before the reaction takes place. Then how is the para product major out of all the possibilities?
The following images will explain what I'm confused about -:
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Either the texts are wrong or possibly my understanding of the inductive effect in substituted benzenes is flawed. Could somebody please explain?
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Oh cmon....anybody? :(
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In that first image dont you think that you labelled least & more stable opposite??
A mesomeric effect dominate over inductive
so second one (that u said more stable) will have more mesomeric effect ... +M effect ... and thats why will be less stable!!! ::)
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Pardon me, but the mesomeric or resonance effect is canceled in the benzyne mechanism...and how exactly is that structure supposed to resonate? I'm a bit confused :)
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Benzyne is aromatic . You may think that as it has 8 pi electrons it should be anti aromatic (4n elec. where n=2).
but in that triple bond one pi bond is not in sale plane as of the ring, it is perpendicular. So no of pi elec. participatin in aromatization is only 6 = 4n +2, n=1. So it is aromatic and the groups will show M effect.
Hope it helps.
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Erm....this is what Solomons(and my teacher) says :
"Resonance effects are not important in the carbanions formed as the sp2 orbital that contains the electron pair does not overlap with the pi-orbitals of the aromatic system".
So mesomeric/resonance effects are cancelled...
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i guess that thing is valid till the elec. are bonded to both atoms but when any1 atom accepts the bond elec. ,, the condition changes as now they are much closer to the aromatic system (molecular orbitals are bigger than atomic orbitals obviously) and they do get effected by the M effects.
So after any1 accepts the bond pair ... for ex the first case ... the +M effect is from NH2 only (not OCH3) and 2 -I effects ... 1 from alpha carbon and one from beta .
in second case ... 2 +M effects (both ortho) and 2 -I from both alpha carbon.
As in any case ... M effect dominate over I effect ... first is more stable
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Nice theory of yours.
So how do you explain this?
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If resonance is applicable, shouldn't the meta-carbanion be more stable, as per your theory? After all, trifluoromethyl is meta-directing, isn't it?
Its inductive effect is so strong that only one product is obtained...the one from the more stable carbanion(the ortho side carbanion).
If you're so sure that those structures resonate, show me their canonical structures.
Now do you see my dilemma?
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I don't think Methyl or substituted methyl group has any M effect. for M effect you need at least one free elec.
Your book is right in its example as CCl3 has no M effect so we get meta product
And i dont know why you are mixing two cases ... getting products of 1st image and second image are differently cases.
If you are dealing first image ... para is never formed
and if you are dealing second image ... ortho is never formed
And of course while bulk reaction both rxn will occur as to what in the position of bonds of benzene at time of attack.
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Excuse me, but as per my understanding, any atom directly attached to the ring which has a partial or full positive charge, can withdraw electrons from the ring either through mesomerism or the inductive effect. The mesomerism gives directive influence, and trifluoromethyl is a meta directing group. The inductive effect does not direct groups; it merely withdraws electrons from the closest position it gets.
Now in a normal substituted benzene molecule (substituted with electron withdrawers), the mesomeric effect dominates over the inductive effect, providing m-directivity and deactivation. In halogens, the -I effect dominates largely over the +R effect and even though through mesomerism, electrons are given to the ring providing o,p-directivity, the -I effect overrides it by deactivating it.
In a benzyne molecule which has just lost its triple bond to a nucleophilic addition, the carbanion cannot delocalise over the ring. Resonance simply cannot occur..and hence the inductive effect is taken as the benchmark to decide products. I don't know what you're trying to prove by saying that triflouromethyl does not have a mesomeric effect, it does, in trifluoromethylbenzene the meta directivity IS due to the -R effect. Resonance can withdraw from the ring or give to the ring; in tri-fmbenzene it withdraws.
So I'm not mixing up products....:)
I reiterate my stand. If you say those carbanions can resonate, prove it to me by drawing the canonical structures.
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I don't get it ... C has no empty d orbital so where will the elec. go that she'll accept from the ring?
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Let me give a different perspective on this question. The examples given with the methoxy and trifluoromethyl benzynes are data. The effects are designed to explain why those results were obtained. The degree to which the effects can predict other reactions gives credence to the explanations.
The trifluoromethyl group has a single effect, as an electron withdrawing group. The fact that a meta aniline results from the benzyne suggests an inductive stabilization by the trifluoromethyl group.
With a methoxy group, we have competing possible effects, a Coulombic repulsion from the lone pair of oxygen and the carbanion in an ortho position. Alternately, we can have a similar inductive stabilization by an oxygen atom for the similar ortho carbanion. However, those may not be the only effects. The non-bonded electrons can also coordinate with a metal to direct the nucleophile. I don't know that I will know a priori, which effect should be greater. However, if you give me the data, then I can rationalize which effect might have been greater. I want to use a subjective sense for that rationalization as it too may be incomplete or wrong.
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@cupid, you're right about the trifluoromethyl group not having a mesomeric effect...but mesomerism is not considered in the benzyne mechanism, of this fact I am sure.
@orgopete, I'll get back to you with the data asap. Coulombic repulsions seem to make sense!
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@orgopete : All that is given in the book is that out of the four products, the para product is major in this case. No explanation has been given..please do elaborate on the Coulombic repulsions!
Also, someone clear my doubt as to which effect (resonance or inductive) directs incoming groups! I thought it was resonance, but if trifluoromethyl cannot afford resonance, how does it direct groups to the meta position?
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I searched Internet and found this:
"CCl3 group cannot show M effect But it shows -H effect (H= Hyper conjugation). Here Cl behaves as H and accepts its bond pair forming Cl- and C(+)Cl2 + charge on Carbon. So it draws elec. from ring and therefore is meta directing."
Has it been only on I effect ... it should have been para directing ... Right???
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Nice info cupid! Hyperconjugation makes sense too. Yes, it would've been para directing in that case, I think the same.
As resonance is cancelled, hyperconjugation would be of no use in benzyne, then!
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But are you completely sure resonance wont effect benzyne?
I didn't find any good proof of that on internet and my books are not with.
I am going by your words.
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Yes I am dead sure. Think of a simple vinyl carbanion....would you be able to resonate it? Nope. The case is similar here. It's a proven fact too, so you needn't worry on that part!
Now all that is left is orgopete's opinion on the major product.
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If you perform the search embedded here (http://books.google.com/books?id=gY-Sxijk_tMC&printsec=frontcover&dq=benzyne+Anslyn&hl=en&ei=LLXuTOGyE4GClAeW-dX-DA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CC8Q6AEwAQ#v=onepage&q=benzyne&f=false) and click on page 614, this should give you Anslyn and Dougherty's explanation.
First of all, I did not know what the products were of different substitution patterns. The point I was trying to make is that is that explanations are rationalizations to explain why the products formed. It would not be unusual for someone to suggest another rational. For example, carbonyl addition models, such as, Cram, Felkin, and others, see http://evans.harvard.edu/pdf/smnr_2000-2001_Siska_Sarah.pdf.
One also must exercise caution in analyzing the results. A meta-bromotrifluomethylbenzene probably only produces a single benzyne while meta-bromotoluene gives a mixture. A different mixture of products can result, but it may not indicate a weakness for a para carbanion. That result from the para bromo however does give the same product and thus proves the para carbanion is less favored.
As I recall, a meta-bromoanisole reacts faster than an ortho. I have seen it argued that this is a coordination effect or an inductive effect. A coordination effect could be supported by simple directed metalation experiments.
I would not discount drawing resonance structures for the benzynes. It won't break any rules and won't hurt anything. You may find it to be useful for understanding. Let me give an example of why you might consider it. If you draw the resonance structure for dimethylaminopyridine, the resonance structure increases the electron density of the nitrogen, yet you should recognize that the hybridization of the nitrogen probably does not change and that it is indeed the non-bonded electrons of the nitrogen that are used in acylation reactions. I might argue that a benzyne is not a triple bond in a benzene ring. Triple bonds are linear. A benzyne might also be thought of a benzene ring with a pair or electron orthogonal to the pi-electron of the benzene ring, and thus similar to pyridine. If you can draw a resonance structure for dimethylaminopyridine, then why not a resonance structure for the benzyne of anisole. If you did that, you may well argue that it controls the addition of a nucleophile and the absence of this effect is why a methyl group results in a mixture.
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I don't think the carbanion formed after nucleophilic addition to benzyne can resonate. We could draw resonance structures for benzyne; not that carbanion. In any case benzyne is highly strained and tries to form biphenylene, so it doesn't last long unless trapped.
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Consider the following:
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I have drawn a series of resonance structures. I have drawn the pyridine examples to suggest the pyridine nitrogen might have an increased reactivity. I have drawn a similar example with the anisole benzyne. The pi-bond is presumed to be orthogonal to the benzene electrons. This resonance structure would suggest that an increase in electron density may be found at the ortho and para positions. I have drawn a similar set of resonance structures for the carbanion intermediate. The resonance structures of the carbanion do not suggest an increased stability on that basis.
I don't KNOW what the answer is. After seeing the ratios of products that form, I became skeptical of an inductive effect of an N,N-dimethylamino group and an O-methyl ether to favor meta products from the benzyne. I accept a chlorine having that effect. However, a methyl led to approximately equal o/p products for a weak inductive donor. That resulted in my searching for another possible explanation. Since resonance effects are generally stronger than inductive effects, I thought the resonance effect was possible. Also, I don't see any significance to the resonance structures of the adducts as I don't think there is convincing evidence these reactions are reversible. The resonance structures of the benzyne simply indicate that a higher electron density my repel attack at an ortho position. It is just an hypothesis.
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Well....that was quite a bit of brainstorming on your part, orgopete. My sincere thanks to you for putting in time and effort, and I finally see what you meant by the resonance structures. Neat diagrams!
So the final answer is still inconclusive....can anyone solve this conundrum?
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Before we can solve the mechanism(s) and directing effects, we need more data (and perhaps your text is wrong).
p-Bromoanisole gives equal amounts of m/p products, http://pubs.acs.org/doi/abs/10.1021/ja01584a025
Some reviews: http://euch6f.chem.emory.edu/arynes.html
http://stoltz.caltech.edu/seminars/2006_Tadross.pdf (see p. 12, 13, and conclusions p. 43)
I think one must be careful in deriving mechanisms for reactions solely by looking at the substituents. One cannot look at the electronegativity value of oxygen to figure out why t-butanol has a different acidity than trifluoromethanesulfonic acid. The caltech review had an oxazolylbenzyne adding with different regiochemistry depending on the reaction. Clearly coordination was having one regiochemistry and the lack of it had the opposite.
I foresee a problem in that the generalizations to explain regiochemistry are being contradicted by data. (This is also an objective of people in synthesis, reverse an unfavorable selectivity and success complicates generalizations.) I'm done.
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@orgopete
I agree with your concept of triple bond being linear and hecne under great strain in this case ... but in your resonating structures, you drew C and N with 10 and even 12 electrons ... this is merely impossible for them under any drastic conditions ... though i havnt completely read your links as they are too long :P
@SVXX
I was saying that R effects can effect benzyne but after looking at them ( I didnt drew them myself .. just drew them in my mind ;D) i have to say that i now agree with you now that benzyne is not effected by R effects ... I knew that while triple bond is formed the elec. of the bond orthogonal to the ring wont participate in resonance but now i guess that there wont be any resonance as IMPOSSIBLE STRUCTURES are made!!!!
maybe we have to stick to your earlier statement that R effect donot effect benzyne as now i think there is NO resonance.
In my view, resonance may be present till bond is not shifted to any1 atom, but as bond is shifted, it would block resonance and structure will be totally dependent on I effects. So bond will be shifted so that structure(not resonating) will be max stable ...
SO i can say Benzyne mechanizm donot depend on R effects"
though i will confirm it again and get back on this!!!
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@ orgopete
In your PdF, it just tells how EWG and EDG effects the mechanism. but it didnt tells EW or ED in what sense? I or R? as in case of OCH3 it is EW by I-effect and ED by R effect. which will dominate is the main quest of the question.
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I realize the purpose of the question. However, as SVXX pointed out, either his textbook was wrong or his understanding was wrong, therefore I became more cautious and skeptical. Maybe his textbook was wrong or maybe it really isn't as clear as it might be. So, you read an interpretation in which someone suggests product distribution followed electron withdrawing or donating. I looked up some data and for the 3-bromoanisole example that SVXX related, the product is meta. If 4-bromoanisole were used to generate the benzyne SVXX illustrated, the actual result is an equal mixture of products. Well, what does that do for all of the donating or withdrawing? It looks as though a 3-substituted benzyne will give a meta substituted product whether you interpret it as electron donating or withdrawing, except methyl has little effect and an equal mixture. A 4-substituted benzyne will give mixture of meta and para products. Interestingly, para-fluoro did give a higher ratio of para.
If you asked me to summarize the effect, I would not. I am not sure there is one that I can identify. If there were more reactions that I could find, then maybe I could find an effect that I would be more confident about.
10 or 12 electrons? I guess you have not seen this interpretation,
http://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_118B/Chem_118B_Topics/Activating_and_Deactivating_Benzene_Rings
Google doc (http://docs.google.com/viewer?a=v&q=cache:j-amWzHFGWMJ:www.chm.wright.edu/feld/chm212/eas.pdf+resonance+structure+substituted+benzene&hl=en&gl=us&pid=bl&srcid=ADGEESjINlPlxk-I9AdyBBv1dn5UeTyEiStFIYpsM8rypJL9MtTQTDCAR6h6yL_eI5WnJcgLMXADD7JqbnB4G9uOMHjKhipUpXSRGTpjZmpAxNhHimd1Fow4kyTUS-rrpzAoGLReTHNX&sig=AHIEtbTuTnPMlV9HEWxq5hKSlh6kD7wQoA)