Delocalized Bonding Resonance

We have assumed up to now that the bonding electrons are localized in the region between two atoms. In some cases, however, this assumption does not fit the experimental data. Suppose, for example, that you try to write an electron-dot formula for ozone, O3. You find that you can write two formulas  

The lengths of the two oxygen-oxygen bonds (that is, the distances between the atomic nuclei) are each 128 pm. 

In formula A, the oxygen-oxygen bond on the left is a double bond and the oxygen-oxygen bond on the right is a single bond. In formula B, the simation is just the opposite. Experiment shows, however, that both bonds in O3 are identical. Therefore, neither formula can be correct.  

A single electron-dot formula cannot properly describe delocalized bonding. Instead, a resonance description is often used. According to the resonance description, you describe the electron structure of a molecule having delocalized bonding by writing all possible electron-dot formulas. These are called the resonance formulas of the molecule. The actual electron distribution of the molecule is a composite of these resonance formulas. 

The electron structure of ozone can be described in terms of the two resonance formulas presented at the start of this section. By convention, we usually write all of the resonance formulas and connect them by double-headed arrows. For ozone we would write 

---

The other C=N systems included in Scheme 8.2 are more stable to aqueous hydrolysis than are the imines. For many of these compounds, the equilibrium constants for formation are high, even in aqueous solution. The additional stability can be attributed to the participation of the atom adjacent to the nitrogen in delocalized bonding. This resonance interaction tends to increase electron density at the sp carbon and reduces its reactivity toward nucleophiles. 

The classic work on delocalized bonding is Wheland, G.W. Resonance in Organic Chemistry, Wiley NY, 1955. 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

The stabilization mechanism operating in aryl-substituted vinyl cations such as 9 can be qualitatively depicted by the Lewis resonance structures A-C (Scheme 4). There is hyperconjugation between the carbon atom C° which is described by the no-bond resonance structure B. In addition, -delocalization between the aryl ring and the C+ carbon atom is indicated by structures such as C. 

Figure 5.36. Schematic representation of the fullerene C60 molecule. Notice its highly symmetric structure (truncated icosahedron) in which all carbon atoms are identical and are located at the connection between two hexagons and one pentagon. The bond lengths are 138.6 pm for the bonds common to two hexagons (having a double-bond resonant structure) and 143.4 pm for the hexagon-pentagon common bonds. The bonding therefore seems to be not completely delocalized as in graphite.

For example, in benzene, all H- and C-atoms are constitutionally equivalent. In chlorobenzene, however, only the o- and m-atoms are pairwise equivalent. Note here that the individual resonance formulas of delocalized bond systems are not distinguishable. 

Problem 16.10 Use the concept of charge delocalization by extended ir bonding (resonance) to explain why... 

In summary, delocalization of electrons enhances stability, and we can visualize delocalized bonding by using the resonance method. In later chapters we will leam more about the effects of resonance on chemical equilibrium and on the kinetics of chemical reactions of organic compounds. 

In contrast to the simple alkoxy radicals, the 0-0 radical is seemingly quite stable against further oxidation by 02. In addition, the odd electron in the 0-O is delocalized by resonance, and can facilitate o- and p-addition of N02 to the benzene ring. The phenolic H—O bond of p-nitrophenol must be formed intermolecularly, whereas an intramolecular H-atom transfer cannot be ruled out for the formation of o-nitrophenol. The detailed mechanism for these final steps is presently unknown. 

Exercise 6-10 Set up an atomic-orbital model of each of the following structures with normal values for the bond angles. Evaluate each model for potential resonance (electron delocalization). If resonance appears to you to be possible, draw a set of reasonable valence-bond structures for each hybrid. ... 

Removal of a proton from Lj gives an anion whose lone pair is orthogonal to the it bond of the carbonyl group by virtue of the rigid geometry of the bicyclic system. Consequently the lone pair cannot overlap with the carbonyl n bond and delocalization via resonance is not possible—it is effectively a localized anion. Removal of a proton from L2 gives rise to a lone pair in a p orbital which can overlap with the carbonyl it bond and thus resonance delocalization is possible. Thus the anion from L2 is resonance stabilized and is thus formed more easily. 

Richard interprets these measurements as implying an increase in delocalization of charge and increase in double bond character at the benzylic carbon atom of the carbocation as the number of electron withdrawing fluorine substituents increases. This is consistent with a changing balance of contributions of the valence bond resonance forms 59 and 60. 

The anomeric effect in terms of a stabilizing effect can be illustrated by the concept of "double-bond - no-bond resonance" (14, 15) shown by the resonance structures 4 and 2 or by the equivalent modern view (16, 17) that this electronic delocalization is due to the overlap of an electron pair orbital of an oxygen atom with the antibonding orbital of a C —OR sigma bond (12). 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

The compounds we shall discuss in this paper have aroused particular interest since 1958, when it was proven that they exemplify a peculiar type of electron delocalization, sometimes referred to as single bond-no bond resonance. This concept refers to a delocalization of a bonds rather similar to the well-known delocalization of -n bonds. 

---