I have trouble understanding the concept of resonance

[nemzy]

lets say a structure has 3 resonance structures, i konw that the atoms don't move, but i read somewhere that the electrons do...hmmm and that the resonance hybrid is most stable ofthe resonance forms?? I am confused, what exactly is resonance? in actuallity, isn't there no resonance , just diff orientation of atoms in space?? or is resonance just the transferring of pi bonds or nonbonding electrons to the other?

 

[movies]

Resonance is a way to rearrange electrons on a molecule or ion. Resonance structures really represent the extremes possible electronic arrangements. The true structure is a combination of all the resonance structures, but the "best" resonance structures contribute more to the true structure.

In reality, if you have three equivalent resonance (take NO3-, for example) structures then the electrons are smeared out evenly across each N-O bond, even though each resonance structure suggests that one of the N-O bonds is different from the other two.

 

[chem_tr]

Your second thought is more correct. If there are multiple electronegative atoms with different strengths, then some electron challenges may occur.

In organic chemistry, we explain many instances of reactivity of double-bonded compounds in this way; for example, acetone may be written in its resonance contributor, Me₂C⁺-O^-. Here we assume that oxygen is more electronegative and pulls the pi electrons to itself, forming a reactive single bond and two charges. Then some electrophile or nucleophile can react.

The same approach can be used to determine which carbon atom in an olefin is reacted with hydride ion, i.e., anti-Markownikow rule, etc.

HCN has three resonance structures, the most stable being the one with triple bond. Reactivity increases with decreasing bond order between C and N.

Hope this helps.

 

[jatin9_99]

resonance is just shifting of your electrons from one carbon to another or u can say shifting of bonds

 

[Mertas]

Resonance, briefly, is the delocalization of electrons within the empty orbitals. When the delocalization occurs the energy for the system is lowered, therefore the more the resonance structures, the high probability of energy distribution, and so the stability.

 

[maverick280857]

The name is deceptive at first sight because you tend to believe that there is inherent oscillation of electrons in a resonating system. In fact (as has already been mentioned by movies) the true structure is a resonance hybrid, which for convenience sake is drawn alongside the canonical forms (or resonance contributors as they are sometimes called). There are a few things you must keep in mind while considering molecules (or species) which involve resonance:

1. The greater the number of covalent bonds, the greater is the stability.
2. Identical resonance contributors are usually desirable.
3. Resonance contributors with isolated charges are undesirable and it is often said that their contribution to the resonance hybrid is small in comparison to those structures which have no isolated charges.
4. A negative charge is more desirable on an electronegative atom.

(There are of course some other important ideas but these are the basic ones. You must pardon me for excessive usage of the word "desirable". I really mean greater stability, lower energy.)

It can be shown through advanced quantum mechanical methods that resonance in molecular systems (such as that in organic species like acetone, the acylium ion, the allylic cation, the benzylic free radical and so on) lends stability to the systems and this stability is usually higher than in those species where resonance cannot occur. (An exception, I am told is the tricyclopropyl methyl cation which due to the baeyer strain of three cyclopropyl groups is inherently unstable in terms of free existence since the rings tend to open up and reduce the positive charge on the central carbon atom...stabilizing the carbocation. But again, these are advanced cases which you--as a starting organic chemistry student--need not worry about unless your book/teacher brings them up at an appropriate stage).

If you have trouble coming to terms with resonance, I would suggest that you give yourself more practice than you have now. Try drawing resonance contributors for aromatic and non-aromatic species such as benzene, phenol, aniline, tribenzylic free radical/cation/anion, carboxylate anion, sulfate anion, formate anion, etc. Standard textbooks like Morrison and Boyd have sufficient material to help you out. (You might be lucky to find some resonance diagrams on PF--check the older posts).

Cheers
Vivek

 

[Gokul43201]

resonance is just shifting of your electrons from one carbon to another or u can say shifting of bonds

Actually, there is no shifting of electrons. The charge density is time independent.