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10 Chapter 1 STRUCTURE AND BONDING IN ORGANIC MOLECULES Could we move two electron pairs in toward the N, one from each O? Nope: That would violate the octet rule on N and give us an illegal Lewis structure: H H H-C-N H-C-N H H ILLEGAL! Octet rule 10 electrons on N So the best two structures are the two we derived above, with octets on all nonhydrogen atoms and a pair of charges. The arrows below show how electron pairs shift to go from one to the other: H H 0: 0: + + H-C-N H-C-N H H Since these two forms are identical, they contribute equally to the resonance hybrid. The NO bonds are polar, with a full positive charge on N and a negative charge split, half on each You may ask, what would have happened back at the beginning of this exercise if we started by putting one of our extra pairs of electrons on the N, instead of putting all of them on the oxygens? Good question. Our starting Lewis structure (below, left) would then have an octet on the N and one oxygen, but a sextet on the other O. The remedy, shifting the lone pair from the N toward the electron-deficient O, gets us to one of the same final structures that we obtained above: H H + H-C-N H-C-N H H As a rule, as long as all your σ electrons are in place, and you do not violate the octet rule with the rest, any starting structure will eventually get you to the best answer(s). Now we turn to methyl nitrite. Following the same procedure, we begin with only single bonds, and then arbitrarily add in the remaining electrons as lone pairs, taking care only to avoid violating the octet rule. The structure below left is one result, and it contains a seriously electron-deficient N, just as we found starting out with nitromethane. And, we "fix" it the same way, moving an electron pair "in" from the negatively charged at the end: H H This is pretty good: All nonhydrogen atoms have octets, and none are charged. Can we find any other reasonable resonance forms? There is a common pattern described in the text for systems containing an atom with at least one lone pair attached to one of two atoms connected by a multiple bond. You move the lone pair "in" and move a π bond "out":