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How do I recognise a molecule which displays resonance?

I need to identify one of the following as a molecule which displays resonance: -Ethane -Ethanol -Ethene I think that it might be Ethanol, but I'm not sure how it displays resonance, or how to represent this in a ball-and-stick molecular representation.

Public Comments

1. Resonance is generally noticeable when you have a region of high electron density - it is the movement of these electrons around the molecule that essentially is "resonance." You can see that ethane has no regions of high e- density; it's a uniform molecule. Ethanol has a partial negative charge on the oxygen, but nothing to speak of for resonance. Oxygen is important for resonance when you have carbonyl groups (an oxygen double-bonded to carbon). The high electron density of the double-bond can be moved onto the oxygen, creating a resonance form. The alcohol would be more useful re. resonance as an enol - when you have the alcohol bonded to a double-bonded carbon. You can then lose the hydrogen and have electrons shifting from the double bond to the oxygen, allowing for tautomerism. You can see that I'm building up to ethene here. The alkene has a double-bond - a region of high electron density. Now, granted, if your professor has you drawing resonance for ethene, he's something of a moron. You *can* shift the double-bond electrons to one of the carbons, creating a resonance form, but a negatively charged carbon bonded to a positively charged carbon... well, to call it a "low ranking" resonance form is putting it mildly. It's just flat-out negligible. Of your three choices, however, "negligible" is better than "zero."