When is resonance possible

At this point, the carbon atom has only 6 valence electrons, so we must take one lone pair from an oxygen and use it to form a carbon—oxygen double bond. In this case, however, there are three possible choices:. As with ozone, none of these structures describes the bonding exactly. Each predicts one carbon—oxygen double bond and two carbon—oxygen single bonds, but experimentally all C—O bond lengths are identical. We can write resonance structures in this case, three of them for the carbonate ion:.

Benzene is a common organic solvent that was previously used in gasoline; it is no longer used for this purpose, however, because it is now known to be a carcinogen. Use resonance structures to describe the bonding in benzene. Given: molecular formula and molecular geometry. If we place a single bonding electron pair between each pair of carbon atoms and between each carbon and a hydrogen atom, we obtain the following:. B If the 6 remaining electrons are uniformly distributed pairwise on alternate carbon atoms, we obtain the following:.

C There are, however, two ways to do this:. Each structure has alternating double and single bonds, but experimentation shows that each carbon—carbon bond in benzene is identical, with bond lengths We can describe the bonding in benzene using the two resonance structures, but the actual electronic structure is an average of the two. The existence of multiple resonance structures for aromatic hydrocarbons like benzene is often indicated by drawing either a circle or dashed lines inside the hexagon:.

The sodium salt of nitrite is used to relieve muscle spasms. Resonance structures are particularly common in oxoanions of the p -block elements, such as sulfate and phosphate, and in aromatic hydrocarbons, such as benzene and naphthalene. If several reasonable resonance forms for a molecule exists, the "actual electronic structure" of the molecule will probably be intermediate between all the forms that you can draw.

One would expect the double bonds to be shorter than the single bonds, but if once overlays the two structures, you see that one structure has a single bond where the other structure has a double bond. The best measurements that we can make of benzene do not show two bond lengths - instead, they show that the bond length is intermediate between the two resonance structures. Resonance structures is a mechanism that allows us to use all of the possible resonance structures to try to predict what the actual form of the molecule would be.

Some molecules have two or more chemically equivalent Lewis electron structures, called resonance structures. Resonance is a mental exercise and method within the Valence Bond Theory of bonding that describes the delocalization of electrons within molecules. The very first diagram above shows some curved arrow formalism with a reference to Wikipedia.

Might I suggest that a slightly more consistent approach is to start arrows not so much at charges, but at the associated lone pair also shown in your diagram. One can locate the centroids of electron pairs , and therefore in principle one can define coordinates for the start of an arrow and also by the same principle, coordinates for the destination of the arrow.

I have touched upon this theme here where you might want to contemplate the arrows for the reverse of the three reactions shown. Also here. Thanks for the input Henry. It is indeed more consistent to show arrows starting at lone pairs instead of charges. There are costs and benefits to each approach. The benefit of drawing all the lone pairs in is that there is no ambiguity about the location of the electrons and it is more proper to do so.

There is, however, a cost both in the extra clutter of the drawings and also in the extra time involved in drawing in these lone pairs.

I choose to do this as well. So long as this caveat is understood i. However it is not without regret that some information content is lost. Fixed the diagram. Thanks for the correction. Thanks James. But I remind of the fascinating story of how benzene has been represented over the years. Faraday who discovered it, attributed no structure.

Loschmidt controversially represented it as a circle, and Kekule gets the credit for the final form. But after that chemists got lazy. For perhaps 70 years, it became normal to draw a benzene ring as a pure hexagon, and to leave the unsaturation as implicit. Text books up to around appear to be describing cyclohexane, whereas in fact we might conclude that they are actually referring to benzene itself.. Benzene is also associated with another form of implicit meaning.

Robinson around coined the term aromatic sextet, and mesmerised by this, anointed naphthalene with two rings after all, the two rings are related by a plane of symmetry. Of course we know that two such sextet rings actually implies 10 electrons! See what happens when you take short cuts! Thanks for the historical perspective.

I was unaware about the cyclohexane example. And the doubly circled rings for naphthalene is definitely something to avoid. I have problems when i try to find out the hybridisation in the structures which show the phenomenon of resonance.

Also I had problems regarding how will the electron density be represented in the p orbital or pi orbital. Resonance structures do not "resonate" or flip back and forth as the name implies. Because resonance structures are the same molecules, they must have:. Although, they can differ in whether the connections are single, double or triple bonds.

These are not resonance structures because they have different atomic connectivity rule 3. Whereas in the structure on the right, all the H's are connected to the nitrogen. These are resonance structures because they fulfill all three rules.

Even though the C and O are connected by a single bond in one structure and a double bond in the other, they are still connected to each other in both structures.