The Concept of Resonance

Our drawing of the Lewis structure for ozone (O3) satisfied the octet rule for the central atom because we placed a double bond between it and one of the two end O atoms. In fact, we can put the double bond at either end of the molecule, as shown by these two equivalent Lewis strucmres  

However, neither one of these two Lewis structures accounts for the known bond lengths in O3. 

We would expect the O—O bond in O3 to be longer than the 0=0 bond because double bonds are known to be shorter than single bonds. Yet experimental evidence shows that both oxygen-to-oxygen bonds are equal in length (128 pm). We resolve this discrepancy by using both Lewis structures to represent the ozone molecule  

Each of these structures is called a resonance structure. A resonance structure, then, 

The term resonance itself means the use of two or more Lewis structures to represent a particular molecule. Like the medieval European traveler to Africa who described a rhinoceros as a cross between a griffin and a unicorn, two familiar but imaginary animals, we describe ozone, a real molecule, in terms of two familiar but nonexistent structures. 

Electrostatic potential map of O3. The electron density is evenly distributed between the two end O atoms. 

Each of these stmctures is called a resonance stracture. A resonance structure, then, is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure. The double-headed arrow indicates that the stmctures shown are resonance stmctures. 

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Write an essay of approximately 2000 words on the history and the various definitions of the concept of resonance, resonance energy, and aromaticity. 

Diazo coupling follows the rules of orientation of substituents in aromatic systems in accordance with the mechanism of electrophilic aromatic substitution and the concept of resonance. 

In Chapter 7, we used valence bond theory to explain bonding in molecules. It accounts, at least qualitatively, for the stability of the covalent bond in terms of the overlap of atomic orbitals. By invoking hybridization, valence bond theory can account for the molecular geometries predicted by electron-pair repulsion. Where Lewis structures are inadequate, as in S02, the concept of resonance allows us to explain the observed properties. 

We can use the concept of resonance to explain these characteristics of the benzene molecule. There are two Kekule structures with exactly the same energy they differ only in the positions of the double bonds. As a result of resonance... 

The convenience and usefulness of the concept of resonance in the discussion of chemical problems are so great as to make the disadvantage of the element of arbitrariness of little significance. Also, it must not be forgotten that the element of arbitrariness occurs in essentially the same way in the simple structure theory of organic chemistry as in the theory of resonance — there is the same use of idealized, hypothetical structural elements. In the resonance discussion of the benzene molecule the two Kekule structures have to be described as hypothetical it is not possible to synthesize molecules with one or the other of the two Kekule structures. In the same way, however, the concept of the carbon-carbon single bond is an idealization. The benzene molecule has its own structure, which cannot be exactly composed of structural elements from other molecules. The propane molecule also has its own structure, which cannot be composed of structural elements from other molecules — it is not possible to isolate a portion of the propane molecule, involving parts of two carbon atoms... 

It was pointed out in Chapter I that the selection of the primary structures for the discussion of any particular case of quantum-mechanical resonance is arbitrary, but that this arbitrariness (which has an analogue in the classical resonance phenomenon) does not impair the value of the concept of resonance. 

When we compare our present knowledge of structural chemistry with that of ten years ago and become cognizant of the extent to which clarity has been brought into this field of knowledge by the extensive application of the concept of resonance we are tempted to speculate about the future development of this concept and the nature of the further applications of it which may be made. 

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

Recall that we used the analogy of a nectarine (being a hybrid between a peach and plum) to explain the concept of resonance. Now, imagine that we create a new type of fruit that is a hybrid between three fruits a peach, a plum, and a kiwi. Suppose that the hybrid fruit that we produce has the following character 65% peach character, 34% plum character, and 1% kiwi character. This hybrid fruit will look almost exactly like a nectarine, because the amount of kiwi character is too small to have an effect on the nature of the resulting hybrid. Even though this fruit is actually a hybrid of three fruits, nevertheless it will look like a hybrid of only two fruits— because the kiwi character is insignificant. ... 

There are no alternative ways of positioning electrons around the HI molecule. If you missed this question, refer to your textbook on the concept of resonance. 

Note that the equation (74) for k+ can be recast in terms of the probability P-> for the system to continue from the QIC towards products, namely, P—> = k + / (k - + k +), so that k+ = k + P->. This equation can be further worked out in order to include the concept of resonance in a more direct way. To get at the desired result, one has to calculate the rate at which Ci(t) is changing. The differential equation fulfilled by this coefficient can be obtained by using an adiabatic coupling method [47] that, in the limit r ->0 it is given by... 

Aromatic compounds and their reactions are a big part of any Organic 11 course. We introduce you to the aromatic family, including the heterocyclic branch, in Chapter 6. (You may want to brush up on the concept of resonance beforehand.) Then in Chapters 7 and 8, you find out more than you ever wanted to know about aromatic substitution reactions, starring electrophiles and nucleophiles. 

In the following sections of this chapter there are given, after an introductory survey of the types of chemical bonds, discussions of the concept of resonance and of the nature of the one-electron bond and the electron-pair bond. 

There is one fundamental principle of quantum mechanics that finds expression in most of the chemical applications of the theory to problems dealing with the normal states of molecules. This is the principle that underlies the concept of resonance. 

The concept of resonance was introduced into quantum mechanics by Heisenberg16 in connection with the discussion of the quantum states of the helium atom. He pointed out that a quantum-mechanical treatment somewhat analogous to the classical treatment of a system of resonating coupled harmonic oscillators can be applied to many systems. The resonance phenomenon of classical mechanics is observed, for example, for a system of two tuning forks with the same characteristic frequency of oscillation and attached to a common base, hich... 

The discussion in Section 1-3 about the element of arbitrariness in the concept of resonance may be recalled at this point with reference to the nitrous oxide molecule and the other molecules that are described in this chapter as resonating among several valence-bond structures. It is not necessary that the structures A, B, and C be used as the basis of discussion of the nitrous oxide molecule. We might say instead that the molecule cannot be satisfactorily represented by any single valence-bond structure, and abandon the effort to correlate its structure and properties with those of other molecules. By using valence-bond structures as the basis for discussion, howrever, with the aid of the concept of resonance, we are able to account for the properties of the molecule in terms of those of other molecules in a straightforward and simple way. It is for this practical reason that we find it convenient to speak of the resonance of molecules among several electronic structures. 

The concept of resonance provides an obvious explanation of some of the characteristic properties of the carboxyl group, the most striking of which is its acid strength. If the electronic structure of a carboxylic... 

These characteristics of the benzene molecule can be explained by using the concept of resonance. We note that there are in fact two Kekule structures with exactly the same energy they differ only in the positions of the double bonds (18). As a result of resonance between these two structures, the electron density of the C C double bonds is spread over the whole molecule, thereby giving each bond a length intermediate between that of a single and a double bond. Resonance makes all six... 

The discussion of molecular structure in terms of Lewis structures invokes the concept of resonance to explain certain physical properties of molecules, such as why two bond lengths are equal. Molecular orbital theory does not use the concept of resonance. Using the azide ion, N3 , as an example, draw the tr-type molecular orbital that leads to bonding between the three nitrogen atoms. 

Valence bond theory has two main problems (1) For molecules such as 02, valence bond theory makes an incorrect prediction about electronic structure. (2) For molecules such as O3, no single structure is adequate and the concept of resonance involving two or more structures must be added (Section 7.7). The first problem occurs rarely, but the second is much more common. To better deal with resonance, chemists often use a combination of bonding theories in which the <7 bonds in a given molecule are described by valence bond theory and it bonds in the same molecule are described by MO theory. 

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