Talk:Electrophilic aromatic directing groups

Merge proposal

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

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Should this be merged with Deactivating groups, into a larger benzene substituents. I only ask this because this is a very incomplete article and are nearly identical related to each other in mechanism. — Preceding unsigned comment added by 130.156.160.65 (talk • contribs) 17:26, 27 March 2007 (UTC)

Yeah, I think it should, most organic chem books I've seen have them together. Maybe a new page called Activating and deactivating groups or something? Davejack (talk) 00:07, 30 November 2007 (UTC)

I agree they should be merged though I'd entitle it Electrophilic Aromatic Directing Groups. How does one merge articles? —Preceding unsigned comment added by 174.253.19.16 (talk) 18:15, 12 January 2011 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

 Done Page moved, merge will be performed shortly. Tomásdearg92 (talk) 22:24, 12 August 2014 (UTC)

Merge completed. Tomásdearg92 (talk) 23:10, 12 August 2014 (UTC)

The signs of inductive effects are wrong, certain EDG groups have positive, and not negative, inductive effect, and certain EWG groups have negative, and not positive, inductive effect. Also, not all EDG and EWG groups must necessarily be donating or withdrawing through inductive effect, there is also mesomeric (resonance) effect, some groups are donating through one effect, and withdrawing through the other. 89.230.205.49 (talk) 21:20, 6 June 2016 (UTC)

Hyperconjugation

Something more should be added like what are the conditions for hyper conjugation , for what type of substances it is valid,etc. Siddharth Jain (talk) 05:02, 23 August 2016 (UTC)

Mechanism section is subtly incorrect

The reasons given for the directing effects is a ground state charge distribution argument. I don't think that's correct. Really, we should be comparing the stabilities of Wheland intermediates, and using Hammond postulate to argue that these stabilities are reflected in the transition states leading to them. Someone who has the time should fix this. Alsosaid1987 (talk) 22:59, 10 February 2019 (UTC)

We should find a ref and write whatever explanation it uses:) McMurry's (undergrad-level) organic text appears to use both reactant (ground-state) electron density (charge distribution) to explain activation/deactivation (actually uses one in the intro but then instead the other in the more detailed analysis!). And then he uses only arene-intermediate resonance to explain ortho/para vs meta effects. Karty's text uses only arene-intermediate stability to explain o/p-vs-m, and further, actually uses terms like "Hammond postulate" and comparison of activation energy leading to their intermediates. And he continues to use intermediate-stability and activation energy to discuss activators and deactivators. He then also uses electron/charge distribution to compare the overall nucleophilicity of the rings of activators vs deactivators. DMacks (talk) 02:29, 11 February 2019 (UTC)

See doi:10.1016/j.theochem.2008.10.014 for use of ground-state analysis of directing effects. DMacks (talk) 03:49, 11 February 2019 (UTC)

Thanks for pointing this out. I should've been more nuanced. I don't mean to suggest that you can't argue with ground states. The energy of the TS is determined by orbital interactions and Coulombic effects as the two reactant approach, so certainly the electronics of the starting material are important. As much as I am a fan of resonance structures, the use of resonance structures is too imprecise. We should not give the impression that these formal charges actually have much to do with real partial charges. What really matters is the pi electron density, or more precisely, the electron density associated with the HOMO. It is harder to make an argument like this precise without being overly verbose or employing computational results. Clayden uses both ground state and TS arguments (Wheland intemediate + Hammond). However, Streitwieser, Carey, and Norman all give only the TS argument. Thus the TS argument is certainly more prevalent in textbooks

I guess I would be happy with a statement like "Resonance structures indicate where there is higher pi electron density available for reaction with an electrophile. The same conclusions can be drawn by comparing the relative stabilities of the Wheland intermediate formed upon electrophilic attack, as differences in the stability of these possible intermediates will be reflected by the energy of the transition states leading thereto." Let me know what you think! Alsosaid1987 (talk) 06:10, 11 February 2019 (UTC)

Halide competing effects

No question that I and M effects are independent ideas (and with independent electronic origins that each have reasonable discussion in our article) that can compete. And no question that because independent effects are involved, the net effect might not follow a uniform periodic-table trend. However, the oveall conclusion in our article:

"Thus the overall order of reactivity is U-shaped, with a minimum at chlorobenzene: PhF > PhCl < PhBr < PhI"

is uncited. And it's contradicted by doi:10.1021/ed080p679, which seems to be a well-cited secondary ref that instead supports PhF > PhCl > PhBr > PhI (and I can confirm that recent McMurry editions agree). That ref also has a substantial discussion of the various halide effects (both relative-rate and position-selectivity). DMacks (talk) 04:12, 11 February 2019 (UTC)

Sorry -- I was sloppy with not giving a citation. My initial numbers were from https://socratic.org/questions/what-is-the-order-of-reactivity-towards-electrophilic-aromatic-substitution-of-f

For a more academic source, I give below the partial rate factors for nitration from Streitwieser's textbook:

ortho: F: 0.06, Cl: 0.03, Br: 0.03, I: 0.2, meta: F: ~0, Cl: 0.0009, Br: 0.0011, I: 0.01, para: F: 0.8, Cl: 0.14, Br: 0.11, I: 0.16. Clayden gives the overall rates (rel. to benzene) of F: 0.18, Cl: 0.064, Br: 0.060, I: 0.12. Thus the order is PhF > PhCl ~ PhBr < PhI. Alsosaid1987 (talk) 04:44, 11 February 2019 (UTC)

phenyl trifluoromethyl sulfone citation request

I think it's reasonable that SO2CF3 is more deactivated than NMe3+ given sigma_m, sigma_p, F, and R parameters of these substituents. Nevertheless, I would like to see numbers (partial rate factors) or k_rel values that confirm this. Alsosaid1987 (talk) 16:09, 11 February 2019 (UTC)

nitroso system

This is one case where the Wheland analysis and starting material orbital coefficient analysis should diverge. In the starting material, nitroso is clearly -M, as the N=O is coplanar with the pi system. The N lp is in plane and cannot donate. However, in the Wheland intermediate, the geometry almost certainly changes, with the N rehybridizing to allow for delocalization of the N lp to stabilize the positive charge. Thus it should (according to my chemical intuition) become +M. I will run some simple HF calculations tonight to verify. In the absence of a reliable citation, this section is highly problematic. I think this section needs to be removed. Alsosaid1987 (talk) 19:50, 13 February 2019 (UTC)

Nitroso is very deactivating experimentally: so –I that it doesn't react enough to see what the directing nature would be. See doi:10.1021/acs.jpca.5b00443 and doi:10.1021/jp409623j. Notable details include use of ¹³C to analyze electronic effects in the ground state and a confounding experimental detail that R–N=O can dimerize (which appears to lead to significant para product). doi:10.1007/s10870-013-0489-8 agrees that the monomeric form would be planar (but that the dimer is not). DMacks (talk) 21:33, 13 February 2019 (UTC)

My calculations confirm this and refute my original intuition. Even though you can draw an octet resonance form of the Wheland intermediate by using the N lone pair, I can't get any geometry with the NO bond "bisecting" the 1,3-hexadienyl ring to converge. It looks like there's a strong preference for the N=O to stay in conjugation as a -M group, even at the Wheland intermediate, which is in line with the experimental data you just gave. I don't think there is any significant +M effect, regardless of whether we are considering the starting material or the Wheland intermediate. Alsosaid1987 (talk) 23:11, 13 February 2019 (UTC)

Given the structure of the dimer, Ph(O-)N+=N+(O-)Ph, this makes sense, since the O- is essentially at a vinylogous (or should I say azologous?) position and can certainly act as a +M group. I think the nitroso system is much to complex to include in a survey of EAS group effects. Also I don't see any experimental evidence to suggest that CR2- and NR- can actually do EAS. These are also probably better left off of the table of activating groups. Alsosaid1987 (talk) 23:18, 13 February 2019 (UTC)

Here I found a small website which may be a little bit helpful: https://chemistry.stackexchange.com/questions/57044/why-is-the-nitroso-group-a-deactivating-group-for-electrophilic-aromatic-substit Fung06831 (talk) 10:15, 14 February 2019 (UTC)

Fung06831, this is exactly the point I was trying to get at. In the starting material, there is absolutely no way N=O can donate, given the placement of the lone pair. I originally thought that the nitroso could donate in the Wheland intermediate, given that all atoms would then obey the octet rule if N takes on sp hybridization. However, some quick and dirty computations showed that not to be the case. Even in the Wheland intermediate, the NO is in-plane, as is the lone pair, precluding donation. I am no computational chemist, and this should not be taken as definitive, but it makes me doubt any +M capability of the nitroso group.

As Dmacks pointed out, the nitroso (a beautiful green) is usually in equilibrium with its dimer, an azodioxide (pale yellow). The oxido groups on the azo can make the azodioxide function a +M group, so I would now attribute the o, p selectivity to reaction of the dimer, rather than the nitroso itself. Since the dimer should be more activated than the monomer, it could account for much of the reactivity, even if the monomer is favored in the solution phase. (In the solid phase the dimer is more stable). Unless you have a source (with experimental evidence) specifically concluding that the nitroso reacts in EAS through the monomer, and that +M from the nitroso N l.p. accounts for the o,p selectivity, I would suggest omitting mention of the +M effect of the NO group. Alsosaid1987 (talk) 00:22, 15 February 2019 (UTC)

But this website doesn't talked much about sp nitrogen Fung06831 (talk) 10:19, 14 February 2019 (UTC)