Formal Charges in Resonance Hyrbids

Moderators: Chem_Mod, Chem_Admin

Syed Hydari 3E
Posts: 6
Joined: Fri Sep 26, 2014 2:02 pm

Formal Charges in Resonance Hyrbids

Postby Syed Hydari 3E » Sun Nov 23, 2014 5:16 am

When examining resonance structures, does a double or triple bond have to exist in the compound for there to be resonance? I understand that resonance spreads the characters of multiple bonds around the entire molecule, so the double/triple nature of one bond sort of “pervades” into the rest of the single bonds. And also, how can you determine the formal charge of a resonance hybrids (the amalgam of the resonance structures I believe) when you have, essentially, “fraction” like bonds?

GinaYoung1L
Posts: 16
Joined: Fri Sep 26, 2014 2:02 pm

Re: Formal Charges in Resonance Hyrbids

Postby GinaYoung1L » Sun Nov 23, 2014 6:17 pm

Yes, as far as I know, the possibility of having different resonance structures requires that the molecule have double or triple bonds somewhere. However, in the definition of resonance structures, any two structurally identical molecules (in terms of atomic arrangement) with different electron arrangements are considered as resonance structures to each other. For instance, HCN can have one single bond and one triple bond or one single bond and one double bond when the C atom is bonded to the terminal atoms H and N. See page 79 of SSM for the picture as well as for more examples.

I think people in the field of chemistry usually don't bother to calculate the formal charges of the atoms in the resonance hybrid (you can ask Dr. Lavelle to make sure.) Rather, they calculate the formal charges of the atoms in different resonance structures. The structure with the lowest formal charges (lowest charge separation) is considered as the "greatest contribution" to the resonance hybrid. In other words, there can be many different resonance structures for the hybrid, but some are more plausible than others (due to lower formal charges) and are therefore more representative of the actual structure (the resonance hybrid.)


Return to “Resonance Structures”

Who is online

Users browsing this forum: No registered users and 1 guest