Rules of resonance pdf
There are rules to follow drawing resonance structures step by step. For molecules and ions, we can draw several resonance structures and their stability is different from one structure to another structure and you should have the ability to identify stability of each structure. In resonance structures, it does not require to show transformation of electrons by arrows. But, to identify each resonance structures, it is good to show arrows. In following examples, arrows are used to show electrons transformation.
Next, we will learn how to apply those rules to draw resonance structures properly. Lets draw resonance structures of nitrate ion. Nitrogen atom has the greatest possibility to be the middle atom than oxygen atom according to theory of an atom which has a greater valence should be the middle atom.
Therefore, three oxygen atoms are located around the nitrogen atom. Then it is as the figure 1. Remember that, octal of oxygen and nitrogen atoms cannot be exceeded because both oxygen and nitrogen atoms are located in the second period and have only s and p orbitals. In the nitrite ion, there is a -1 charge. According to the rule number 4an oxygen atoms should keep negative charges because electronegativity of oxygen is higher than nitrogen.
Now we have one structure. Now we try to draw more structures by changing the bonds and lone pairs keeping locations of atoms without changing rule number 1.
2.5: Rules for Resonance Forms
A lone pair is converted to a bond. With that, a bond should be converted to a lone pair by keeping the octal of nitrogen and oxygen atoms. So it follows rule number 2 which says number of total electron pairs should be same in every structure.
With conversion of a bond to a lone pair and lone pair to a bond, double bond becomes a single bond and single bond become double bond respectively. Study the resonance structures of NO 2 - ion Figure 1. Three resonance structures can be drawn. Therefore all three resonance structures have equal stability. In next examples, you may see, different isomers' stabilities for some other molecules are not same as NO 3.
We can draw three resonance structures for SO 2 molecule. You can see first two structures have charges on atoms. But in third structure, there are no charges on atoms.
So we can understand, structure three is more stable than other two structure. You see in both resonance structures, we have marked the negative charge on oxygen atom and positive charge on nitrogen atom.
Nitrite ion is a -1 charge. So that negative charge should be kept on oxygen atom. Again we see, in most stable structures negative charges should be put on oxygen atoms.AdiChemistry Home. Sometimes, it is not possible to represent the molecule or ion with only one structure. More than one structure have to be proposed. But none of them explains all the observed properties of the molecule. The solution is to write a weighted average of all the valid structures, which explains all the properties.
This condition is usually referred to as resonance or mesomerism or delocalization. The representation of structure of a molecule as a weighted average of two or more hypothetical structures, which only differ by the arrangement of electrons but with same positions for atoms is referred to as resonance. Resonance hybrid: The weighted average of contributing structures is known as resonance hybrid. It is considered as the actual structure.
One should keep in mind that the individual resonance structures do not exist and the molecule do not resonate switch back and forth between these structures. The actual molecule is simply the hybrid of all these imaginary resonance structures.
Hence the delocalization of electrons is also imaginary process which helps in understanding the resonance. Note: The contributing structures are always shown to be linked by using double headed arrows. The following are not the resonance structures, since the position of one hydrogen atom is not same. Indeed, they are different molecules, which are in dynamic equilibrium with each other.
These are called tautomers. Also note that these molecules are linked by two half headed arrows and not by a single double headed arrow.
The reason is to get maximum overlap between the orbitals. Rules to decide the major contributor to the hybrid in the decreasing order of preference are given below:. Among the following, the structure II is more stable since all the atoms have octet configuration and there are more covalent bonds. Among the following resonance forms of phenol, the structure I is more stable since it has no charge.After completing this section, you should be able to use the concept of resonance to explain structural features of certain species; for example, why all of the carbon-oxygen bonds in the carbonate ion are the same length.
This particular compound is discussed in further detail in Section 2. 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 charges creates a neutral molecule and through resonance it is a stable molecule.
The extra electron that created the negative charge on either terminal oxygen can be delocalized by resonance through the terminal oxygens. Benzene is an extremely stable molecule and it is accounted for its geometry and molecular orbital interaction, but most importantly it's 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. The next molecule, the Amide, is a very stable molecule that is present in most biological systems, mainly in proteins.
How to Draw Resonance Structures, Rules, Examples, Problems
By studies of NMR spectroscopy and X-Ray crystallography it is confirmed that the stability of the amide is due to resonance which through molecular orbital interaction creates almost a double bond between the nitrogen and the carbon.
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 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 long 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.
Are all the bond lengths the same in the carbonate ion, CO 3 2-? Carbonate ion exists as the resonance hybrid of the three resonance forms below. Steven Farmer Sonoma State University. Objective After completing this section, you should be able to use the concept of resonance to explain structural features of certain species; for example, why all of the carbon-oxygen bonds in the carbonate ion are the same length. Example 2. Exercises Questions Q2. Solutions S2. Contributors and Attributions Dr.Several simple rules can help you learn how to properly use them.
These are conventions that help you make sure you draw what you mean, and correctly interpret someone else's drawings. The connection of two structures with the double-headed arrow signifies there is no change in nuclear position between the two structures. This is because resonance structures are separate descriptions of the same molecule. Benzene is the quintessential example. On the other hand, if you intend to show structures that are different, a proper form of notation is the double arrow, meaning an equilibrium which may lie on either side of the arrow.
An example is keto-enol tautomerism. Resonance forms give us information about the degree of delocalization and, by inference, the relative stability of the molecule.
The more structures that can be drawn, and the more favorable the structures, the more stable the molecule. Placing positive charge on an electronegative element is destabilizing and characteristic of a less important contributor to overall structure than other forms. Structures with an incomplete valence shell on any atom is less stable, and thus less important a contributor, than a structure with complete octets on every atom. The structure on the left explains its reactivity, but the structure on the right explains why it has a short C-O bond and why it is more stable than the cyclohexyl cation.
You will have gathered by now that we are devising a short-hand notation to describe the molecular orbitals arising from overlap of p-type atomic orbitals. Because the p orbitals must be parallel to lead to bonding interactions, we can state a geometric restriction:.
If a formal double bond is present in any structure, all four atoms connected to the carbons in that double bond must be approximately coplanar. Alignment of p orbitals. The final rule is a bit obscure, but it does apply in radical chemistry. Proper resonance forms describe the same spin state; different spin states cannot be connected with the resonance arrow. Measure bond lengths Measure bond angles.
Compare the C-O bond distance to a normal C-O single bond 1. Reset structure.
We might assume this is an allyl cation. For the structure on the left, careful examination of the structure shows the double bond has 4 coplanar substiituents. However, look at the substituents on the "double bond" in the structure on the right. They are definitely not coplanar so there can be no double bond between carbons 1 and 2.
The right-hand structure is not a proper resonance form.In chemistryresonance is a way of describing bonding in certain molecules or ions by the combination of several contributing structures or forms also variously known as resonance structures or canonical structures into a resonance hybrid or hybrid structure in valence bond theory.
It has particular value for describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure. Under the framework of valence bond theoryresonance is an extension of the idea that the bonding in a chemical species can be described by a Lewis structure. For many chemical species, a single Lewis structure, consisting of atoms obeying the octet rule, possibly bearing formal charges, and connected by bonds of positive integer order, is sufficient for describing the chemical bonding and rationalizing experimentally determined molecular properties like bond lengths, angles, and dipole moment.
In order to address this type of situation, several contributing structures are considered together as an average, and the molecule is said to be represented by a resonance hybrid in which several Lewis structures are used collectively to describe its true structure. For instance, in NO 2 —nitrite anion, the two N—O bond lengths are equal, even though no single Lewis structure has two N—O bonds with the same formal bond order.
According to the contributing structures, each N—O bond is an average of a formal single and formal double bond, leading to a true bond order of 1. By virtue of this averaging, the Lewis description of the bonding in NO 2 — is reconciled with the experimental fact that the anion has equivalent N—O bonds. The resonance hybrid represents the actual molecule as the "average" of the contributing structures, with bond lengths and partial charges taking on intermediate values compared to those expected for the individual Lewis structures of the contributors, were they to exist as "real" chemical entities.
While contributing structures may differ in formal bond orders and in formal charge assignments, all contributing structures must have the same number of valence electrons and the same spin multiplicity. Because electron delocalization lowers the potential energy of a system, any species represented by a resonance hybrid is more stable than any of the hypothetical contributing structures.
The magnitude of the resonance energy depends on assumptions made about the hypothetical "non-stabilized" species and the computational methods used and does not represent a measurable physical quantity, although comparisons of resonance energies computed under similar assumptions and conditions may be chemically meaningful. Resonance is to be distinguished from isomerism. Isomers are molecules with the same chemical formula but are distinct chemical species with different arrangements of atomic nuclei in space.
Resonance contributors of a molecule, on the other hand, can only differ in the way electrons are formally assigned to atoms in the Lewis structure depictions of the molecule.
Specifically, when a molecular structure is said to be represented by a resonance hybrid, it does not mean that electrons of the molecule are "resonating" or shifting back and forth between several sets of positions, each one represented by a Lewis structure.
Rather, it means that the set of contributing structures represents an intermediate structure a weighted average of the contributorswith a single, well-defined geometry and distribution of electrons. It is incorrect to regard resonance hybrids as rapidly interconverting isomers, even though the term "resonance" might evoke such an image.
A non-chemical analogy is illustrative: one can describe the characteristics of a real animal, the narwhalin terms of the characteristics of two mythical creatures: the unicorna creature with a single horn on its head, and the leviathana large, whale-like creature.
The narwhal is not a creature that goes back and forth between being a unicorn and being a leviathan, nor do the unicorn and leviathan have any physical existence outside the collective human imagination.
Nevertheless, describing the narwhal in terms of these imaginary creatures provides a reasonably good description of its physical characteristics. Due to confusion with the physical meaning of the word resonanceas no entities actually physically "resonate", it has been suggested that the term resonance be abandoned in favor of delocalization  and resonance energy abandoned in favor of delocalization energy.
A resonance structure becomes a contributing structure and the resonance hybrid becomes the hybrid structure. The double headed arrows would be replaced by commas to illustrate a set of structures, as arrows of any type may suggest to beginning students that a chemical change is taking place. All structures together may be enclosed in large square brackets, to indicate they picture one single molecule or ion, not different species in a chemical equilibrium.
Alternatively to the use of contributing structures in diagrams, a hybrid structure can be used. In a hybrid structure, pi bonds that are involved in resonance are usually pictured as curves  or dashed lines, indicating that these are partial rather than normal complete pi bonds.
In benzene and other aromatic rings, the delocalized pi-electrons are sometimes pictured as a solid circle. He proposed that the carbon-carbon bond in benzene is intermediate of a single and double bond. The resonance proposal also helped explain the number of isomers of benzene derivatives.
In reality there are only three dibromobenzene isomers and only one is ortho, in agreement with the idea that there is only one type of carbon-carbon bond, intermediate between a single and a double bond.
The mechanism of resonance was introduced into quantum mechanics by Werner Heisenberg in in a discussion of the quantum states of the helium atom. He compared the structure of the helium atom with the classical system of resonating coupled harmonic oscillators.
Linus Pauling used this mechanism to explain the partial valence of molecules inand developed it further in a series of papers in Ingold inbut did not catch on in the English literature.You can never shift the location of electrons in sigma bonds — if you show a sigma bond forming or breaking, you are showing a chemical reaction taking place.
Likewise, the positions of atoms in the molecule cannot change between two resonance contributors. Because benzene will appear throughout this course, it is important to recognize the stability gained through the resonance delocalization of the six pi electrons throughout the six carbon atoms.
Benzene also illustrates one way to recognize resonance - when it is possible to draw two or more equivalent Lewis structures.
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.
Rules for Using Resonance Structures
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.
Resonance is most useful when it delocalizes charge to stabilize reactive intermediates and products. Recognizing, drawing, and evaluating the relative stability of resonance contributors is essential to understanding organic reaction mechanisms. Rules for drawing and working with resonance contributors.
Learning to draw and interpret resonance structures, there are a few basic guidelines to help avoid drawing nonsensical structures. All of these guidelines make perfect sense as long as we remember that resonance contributors are merely a human-invented convention for depicting the delocalization of pi electrons in conjugated systems.Roger Huisingh, United States Ultimate Journey of Norway, September 2017 Erin Dowty, United States Express Iceland, August 2017 I just wanted to say that everything was fantastic.
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