If we were to draw the structure of an aromatic molecule such as 1,2-dimethylbenzene, there are two ways that we could draw the double bonds:. Which way is correct? There are two simple answers to this question: 'both' and 'neither one'. Both ways of drawing the molecule are equally acceptable approximations of the bonding picture for the molecule, but neither one, by itself, is an accurate picture of the delocalized pi bonds.
The two alternative drawings, however, when considered together, give a much more accurate picture than either one on its own. This is because they imply, together, that the carbon-carbon bonds are not double bonds, not single bonds, but about halfway in between. When it is possible to draw more than one valid structure for a compound or ion, we have identified resonance contributors : two or more different Lewis structures depicting the same molecule or ion that, when considered together, do a better job of approximating delocalized pi-bonding than any single structure.
By convention, resonance contributors are linked by a double-headed arrow, and are sometimes enclosed by brackets:. In order to make it easier to visualize the difference between two resonance contributors, small, curved arrows are often used.
Nevertheless, use of the curved arrow notation is an essential skill that you will need to develop in drawing resonance contributors. The depiction of benzene using the two resonance contributors A and B in the figure above does not imply that the molecule at one moment looks like structure A, then at the next moment shifts to look like structure B. Rather, at all moments, the molecule is a combination, or resonance hybrid of both A and B. It is very important to be clear that in drawing two or more resonance contributors, we are not drawing two different molecules: they are simply different depictions of the exact same molecule.
Furthermore, the double-headed resonance arrow does NOT mean that a chemical reaction has taken place. Benzene is often drawn as only one of the two possible resonance contributors it is assumed that the reader understands that resonance hybridization is implied. However, sometimes benzene will be drawn with a circle inside the hexagon, either solid or dashed, as a way of drawing a resonance hybrid.
Examples of Resonance. The above resonance structures show that the electrons are delocalized within the molecule and through this process the molecule gains extra stability.
Ozone with both of its opposite formal charges creates a neutral molecule and through resonance it is a stable molecule. The extra electron that created the negative charge one terminal oxygen can be delocalized by resonance through the other terminal oxygen.
Benzene is an extremely stable molecule due to its geometry and molecular orbital interactions, but most importantly, due to its resonance structures.
The delocalized electrons in the benzene ring make the molecule very stable and with its characteristics of a nucleophile, it will react with a strong electrophile only and after the first reactivity, the substituted benzene will depend on its resonance to direct the next position for the reaction to add a second substituent.
Some structural resonance conformations are the major contributor or the dominant forms that the molecule exists. For example, if we look at the above rules for estimating the stability of a molecule, we see that for the third molecule the first and second forms are the major contributors for the overall stability of the molecule.
The nitrogen is more electronegative than carbon so, it can handle the negative charge more than carbon. A carbon with a negative charge is the least favorable conformation for the molecule to exist, so the last resonance form contributes very little for the stability of the Ion.
The different resonance forms of the molecule help predict the reactivity of the molecule at specific sites. The Hybrid Resonance forms show the different Lewis structures with the electron been delocalized. This is very important for the reactivity of chloro-benzene because in the presence of an electrophile it will react and the formation of another bond will be directed and determine by resonance. The lone pair of electrons delocalized in the aromatic substituted ring is where it can potentially form a new bond with an electrophile, as it is shown there are three possible places that reactivity can take place, the first to react will take place at the para position with respect to the chloro- substituent and then to either ortho- position.
Objectives After completing this section, you should be able to use the concept of resonance to explain structural features of molecules and ions. Rules for Drawing and Working with Resonance Contributors Recognizing, drawing, and evaluating the relative stability of resonance contributors is essential to understanding organic reaction mechanisms. Major resonance contributors of the formate ion Representations of the formate resonance hybrid 2 The resonance hybrid is more stable than any individual resonance structures.
Major and Minor Resonance Contributors As previously state the true structure of a resonance hybrid is the combination of all the possible resonance structures. Rules for Estimating Stability of Resonance Structures 1. Examples of major and minor contributors Example 1: Example 2: Example Carboxylate example In the case of carboxylates, contributors A and B below are equivalent in terms of their relative contribution to the hybrid structure.
Apply the rules below The carbon in contributor C does not have an octet. In general, resonance contributors in which a carbon does not fulfill the octet rule are relatively less important. In general, a resonance structure with a lower number of total bonds is relatively less important.
The resonance contributor in which a negative formal charge is located on a more electronegative atom, usually oxygen or nitrogen, is more stable than one in which the negative charge is located on a less electronegative atom such as carbon. An example is in the upper left expression in the next figure. Solution In the structure above, the carbon with the positive formal charge does not have a complete octet of valence electrons.
Exercises 1 For the following resonance structures please rank them in order of stability. Skip to main content. Search for:. The Predicted Stabilities of Resonance Contributors Resonance is a mental exercise and method within the Valence Bond Theory of bonding that describes the delocalization of electrons within molecules.
It compares and contrasts two or more possible Lewis structures that can represent a particular molecule. Resonance structures are used when one Lewis structure for a single molecule cannot fully describe the bonding that takes place between neighboring atoms relative to the empirical data for the actual bond lengths between those atoms. The net sum of valid resonance structures is defined as a resonance hybrid, which represents the overall delocalization of electrons within the molecule.
A molecule that has several resonance structures is more stable than one with fewer. Some resonance structures are more favorable than others. Introduction Electrons have no fixed position in atoms, compounds and molecules see image below but have probabilities of being found in certain spaces orbitals. Delocalization and Resonance Structures Rules In resonance structures, the electrons are able to move to help stabilize the molecule.
Resonance structures should have the same number of electrons, do not add or subtract any electrons. All resonance structures must follow the rules of writing Lewis Structures. The hybridization of the structure must stay the same.
The skeleton of the structure can not be changed only the electrons move. Resonance structures must also have the same amount of lone pairs. Formal Charge Even though the structures look the same, the formal charg e FC may not be. Find the Lewis Structure of the molecule. Remember the Lewis Structure rules. Resonance Hybrids Resonance Structures are a representation of a Resonance Hybrid , which is the combination of all resonance structures.
Where there can be a double or triple bond, draw a dotted line —— for a bond. Draw only the lone pairs found in all resonance structures, do not include the lone pairs that are not on all of the resonance structures. Rules for estimating stability of resonance structures The greater the number of covalent bonds , the greater the stability since more atoms will have complete octets The structure with the least number of formal charges is more stable The structure with the least separation of formal charge is more stable A structure with a negative charge on the more electronegative atom will be more stable Positive charges on the least electronegative atom most electropositive is more stable Resonance forms that are equivalent have no difference in stability and contribute equally eg.
Example 5: Multiple Resonance of other Molecules Molecules with multiple resonance forms. According to resonance effect , the greater the number of resonance contributors, the greater the resonance stabilization effect, and the more stable the species is.
There are some very important rules we need to follow for such purposes. The two resonance structures here are equivalent. It can be moved onto the oxygen atom and become another lone pair on the oxygen atom. The two resonance structures in this example are non-equivalent , so one is more stable than the other. The more stable structure can also be called as the major resonance contributor.
Common errors for drawing resonance structures:. Previous: 1.
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