[Fzang]

These are different compounds, right? There's no way electrons can jump between sp3 carbons, so they cannot ever look alike. Just making sure I got it right. Resonance is messing with my head.

[MissPhosgene]

You are right. They are not different resonsnce structures of the same compound.

[Fzang]

Okay, so while this topic is still hot I want to ask about this one as well:


There is <b>no way</b> atoms can shift position (hydrogen) in two different contributors. That would be two different compounds then, right?

[nj_bartel]

Atoms can change position rapidly between two different forms, particularly when the atoms moving are protons.  This isn't resonance though - it's called tautomerization.  Only happens in solution, not solid state.

http://en.wikipedia.org/wiki/Keto-enol_tautomerism

[Fzang]

I see so that would count as two different compounds and not resonance contributors.

[azmanam]

no, in neither case are they two different compounds.  They have the same molecular formula, but different connectivity.  They are constitutional (structural) isomers.  Different compounds would be a wrong answer on my exam.

[Fzang]

I only have the choice between resonance contributors or different compounds :p They seem to be more of the second.

[azmanam]

If it's multiple choice, and those are your only two choices, then yes, choose different compounds.  Then write your instructor a note telling him he's wrong and you deserve extra credit for pointing out an error on the questions

[Fzang]

Nah, it's actually just an assignment where I'm doing whatever the book is telling me

So, instead of writing "different compounds" or "resonance contributors", I've used before said as well as tautomers and constitutional isomers now. Too bad I don't get any extra grades for topping the question... I just have to turn it in in order to gain access to the exam but the assignments aren't rated themselves.

[azmanam]

Rule of thumb:

If they have different molecular formulas, they're different compounds
If they have the same MF, but different connectivity, they're constitutional isomers
If they have the same Mf, but differ in position of electrons and charges (not sigma bonds), they're resonance contributors
If they have the same connectivity, but have different arrangement of atoms in space, they're stereoisomers
If they have the same arrangement of atoms in space, but differ in rotation only about single bonds, they're conformational (rotational) isomers

See if this helps.  I made it for my students

[Fzang]

Thanks

[orgopete]

If it's multiple choice, and those are your only two choices, then yes, choose different compounds.  Then write your instructor a note telling him he's wrong and you deserve extra credit for pointing out an error on the questions

I love it! I knew I made mistakes and gave credit to the first person to point out such an error so I could make an immediate correction.