[SVXX]

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 -:





Either the texts are wrong or possibly my understanding of the inductive effect in substituted benzenes is flawed. Could somebody please explain?

[SVXX]

Oh cmon....anybody?

[cupid.callin]

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!!! 

[SVXX]

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

[cupid.callin]

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.

[SVXX]

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...

[cupid.callin]

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

[SVXX]

Nice theory of yours.
So how do you explain this?


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?

[cupid.callin]

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.

[SVXX]

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.

[cupid.callin]

I don't get it ... C has no empty d orbital so where will the elec. go that she'll accept from the ring?

[orgopete]

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. 

[SVXX]

@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!

[SVXX]

@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?

[cupid.callin]

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???