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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?
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.
