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Author Topic: Question about resonant structures with formal charge  (Read 258 times)

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mm2739

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Question about resonant structures with formal charge
« on: January 29, 2019, 04:05:52 AM »

My textbook is explaining resonance and I don't know why this is a valid formation.

The formula is HCONH2. It is one carbon with a hydrogen and oxygen and nitrogen attached, and attached to the nitrogen are two hydrogens. My textbook says the double bond can either be between the oxygen and carbon or between the carbon and nitrogen. However, double bonding the carbon to the nitrogen results in formal charge on the nitrogen, and a double bond on oxygen would not. Why is there resonance between these two structures? Shouldn't the double bonded nitrogen be invalid?
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chenbeier

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Re: Question about resonant structures with formal charge
« Reply #1 on: January 30, 2019, 01:59:58 AM »

One Hydrogen has to move to Oxygen then. Carbon cannot have 5 bonds.  HC=NHOH <=> HC=ONH2
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Babcock_Hall

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Re: Question about resonant structures with formal charge
« Reply #2 on: January 30, 2019, 08:23:15 AM »

I am guessing that the molecule in question is actually formamide.  One way that one can notate the carbonyl group is to put the "O" in parentheses.  The resonance structure of formamide, HC(O)NH2, with a formal negative charge on oxygen and a formal positive charge on nitrogen is a valid resonance structure, although its contribution to the overall structure is probably less than the resonance structure in which both O and N have a formal charge of zero.  It does not violate the Lewis octet rule.  This resonance structure explains the approximate planarity of most amide bonds, among other properties.  One cannot move a hydrogen when one is drawing resonance structures.
« Last Edit: January 30, 2019, 09:07:09 AM by Babcock_Hall »
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