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Using Lewis structures to describe chemical bonds inside a molecule is still fairly common start in undergraduate education. Lewis structure however can be misleading, or just simply wrong in many cases. Mostly I see this with aromatic systems, Diels-Alder reactions etc.

My question is if there is a rule of thumb when one can safely use Lewis structure and when we should just forget them? Or should I just check it every single time and compare them with eg. MOs?

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    $\begingroup$ I am not sure what you mean when you say Lewis structures are misleading for aromatic systems and Diels-Alder reactions. Can you elaborate? $\endgroup$ – Jan Jensen Jun 5 '15 at 8:09
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    $\begingroup$ My understanding is that the bond-lengths is poly-cyclic aromatic systems are often unlike what would have predicted from resonant structures. Also, that the explanation of Diels-Alder reaction mechanism needs MO theory. Please correct me if I wrong. $\endgroup$ – Greg Jun 5 '15 at 10:21
  • $\begingroup$ The usual explanation for the Diels-Alder reaction mechanism is standard electron pushing (e.g. en.wikipedia.org/wiki/Diels%E2%80%93Alder_reaction) based on Lewis structures. Wrt bond-lengths in poly-cyclic aromatic systems - can you provide an example? $\endgroup$ – Jan Jensen Jun 5 '15 at 11:27
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    $\begingroup$ This is an interesting question for sure, but it seems quite hard. Given the immense complexity and richness of chemical space, I can only imagine that Lewis structures will almost always be inadequate. However, an important factor to consider is whether almost all of these inadequacies will be restricted to exotic compounds we'll never care about. $\endgroup$ – Nicolau Saker Neto Jun 5 '15 at 13:24
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    $\begingroup$ Dioxygen $\ce{O2}$ is well known as a paramagnetic material. While this is predicted by the MO of dioxygen, Lewis structure of $\ce{O2}$ fails to predict this property. So, as Lewis structure fails for a simple molecule like $\ce{O2}$ , I think we should consider it with vigilance. $\endgroup$ – Yomen Atassi Jun 5 '15 at 13:56
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OK, here goes. Chemists use a variety of representations of molecules to try to predict their behavior and reactivity, but none of them come close to preparing you for what happens in the laboratory. In that sense, none of them are "safe" to use. Lewis structures, and the curly arrow formalisms, are good for organic chemists to quickly explore possible avenues of reactivity and synthetic pathways. But they are not very good at dealing with chemistry south of the second row of the periodic table, nor at phenomena such as aromaticity, electrocyclic reactions, bulk properties, solvation, steric effects, polarity, polarizability, photochemistry, free radicals, etc. So we add in to the mix some "rules", try to think of molecules as being best described as "multinuclear atoms", and try to improve and promulgate our best tools for computational chemistry. For all that, we still need to approach the laboratory with a degree of humility--things that look good on paper usually do not look so good when you try them out in the real world.

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