Resonance Review

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Resonance Review

Postby YeseniaGomez_1L » Wed May 30, 2018 9:34 pm

I know this topic is from a while ago, but after the test we just got back, I am confused as to what resonance is now. I thought it was just like a variation to the structure given. I am not sure what are the rules or how to properly give a resonance structure. Anything would help, or going over the question from test 3. Thank you!

Tiffany Chen 1A
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Re: Resonance Review

Postby Tiffany Chen 1A » Wed May 30, 2018 9:50 pm

(In the context of the test question) A resonance structure is like a variation to the given structure, but the resonance structure has to be equivalent to the given structure. So, if there's 1 double bond in a structure and the rest of the bonds are single bonds, you can't just move the double bond wherever you want and then call it a resonance structure. You can only move the double bond to different place if it doesn't change the structure.

A part of the molecule on my test had a carbon that was single bonded to another carbon, double bonded to an oxygen, and single bonded to a another oxygen. In this case, you could the move the double bond on an oxygen to the other oxygen, but you can't turn the C-C single bond into a double bond because that would break octet rules/change the structure. (not sure if this makes sense, sorry)

An easy way to check if you have a resonance structure that works is to visualize flipping/turning the structure you drew and see if it's identical to the given structure.

Rebekah Kaufman 1L
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Re: Resonance Review

Postby Rebekah Kaufman 1L » Wed May 30, 2018 9:57 pm

Hi, I'm in Dis 1L too so we should've had the same test.

Resonance is when the position of an element in a molecule is different but it doesn't change the molecule.

So on the test we were given pyruvate and switching the positions of the H won't be any different so we leave that alone. For the Red C, we see that it is bonded with two other C and one O which is shares a double bond with. If we look at the Blue C, we see that is bonded to one C and two O. It is important to notice the number of bonds. Now, the two green O have the same number of bonds and lone pairs so switching them won't change anything. That leaves the white O. The difference between the white and green O are the number of bonds and the number of lone pairs. If we're looking for resonance structures, then we want to change the element's position without changing anything else (without changing the formal charge). If we traded the white O with the red C's O, then the red C would only be sharing three bonds leaving it with a +1 charge and the blue C would be sharing five bonds and therefore have a -1 charge. This change tells us that this can't be a resonance structure. So next we try switching the positions of both O connected to the blue C. This switch does not cause a change in formal charge and it therefore a resonance structure.

Hope that makes sense :)

Bijal Luhar
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Re: Resonance Review

Postby Bijal Luhar » Thu May 31, 2018 1:56 pm

Resonance is basically when lets say you have 3 oxygen atoms and two of them have 2 pairs of dots and the other has three pair of the dots. In an resonance structure you can basically rotate the one with the three pairs of dots so each oxygen has the opportunity to get the three pairs of dots. You can see the rotation in the diagram I posted.
Screen Shot 2018-05-31 at 1.55.15 PM.png

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