When does resonance take place?

[kay]

As in what are the conditions for resonance (i.e. in oganic chemistry) to occur? Sometimes it occurs, sometimes it doesn't; for reasons unknown. A little help please? :/

 

[Quantum Defect]

As in what are the conditions for resonance (i.e. in oganic chemistry) to occur? Sometimes it occurs, sometimes it doesn't; for reasons unknown. A little help please? :/

Generally, if you can draw different Lewis structures with different arrangements of double bonds, then the different structures are resonance structures. Some resonance structures may be more or less important. All resonance structures are not create equal -- some represent higher energy structures.

 

[kay]

Generally, if you can draw different Lewis structures with different arrangements of double bonds, then the different structures are resonance structures. Some resonance structures may be more or less important. All resonance structures are not create equal -- some represent higher energy structures.

What i meant to say was that when does resonance occur? what are the conditions necessary for resonance to take place? I'm not asking how to draw resonating structures. My mind was searching for the fact on the basis of which i would be able to decide whether i COULD make resonating structures or not

 

[Quantum Defect]

What i meant to say was that when does resonance occur? what are the conditions necessary for resonance to take place? I'm not asking how to draw resonating structures. My mind was searching for the fact on the basis of which i would be able to decide whether i COULD make resonating structures or not

I don't know of any ready way (without drawing resonance structures) to tell, a priori, that resonance structures are there. Generally, you need some multiple bonds, possibly some lone pairs. But the presence of these does not guarantee that resonance structures are important to describe the molecule.

You can see experimental evidence of resonance if you know things like bond lengths for your structure that you can compare to typical bond lengths. I.e. if the experimental bond length is intermediate between a typical single bond and a typical double bond, you are likely to have resonance structures where the bond in question is either single, or double, depending upon the resonance stucture.

The presence of resonance structures may also be revealed through other experimental measurements (dipole moments, thermochemical data, etc.)

 

[Ygggdrasil]

Resonance structures come about whenever you have a conjugated system that can delocalize electrons over more than two atoms. The requirements for a conjugated system are having three or more adjacent atoms with parallel p-orbitals. Here's a nice page describing ways to look for whether your molecule will exhibit conjugation: http://www.masterorganicchemistry.com/2011/03/08/are-these-alkenes-conjugated/

 

[DrDu]

That's kind of a difficult question. In general, you need much more resonance structures than you are usually considering in introductory classes. E.g. to describe a simple molecule like $\mathrm{H_2}$ starting from its atomic orbitals you need to include H-H, H$^+$H$^-$ and H$^-$H$^+$.

 

[kay]

Resonance structures come about whenever you have a conjugated system that can delocalize electrons over more than two atoms. The requirements for a conjugated system are having three or more adjacent atoms with parallel p-orbitals. Here's a nice page describing ways to look for whether your molecule will exhibit conjugation: http://www.masterorganicchemistry.com/2011/03/08/are-these-alkenes-conjugated/

Thank you so much. :D