For resonance structures

If the molecule contains multiple bonds, construct the n bonding system using molecular orbital theory, as described in this section and in the remaining pages of Chapter 10. Watch for resonance structures, which signal the presence of delocalized electrons. 

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

Length of single bond = sum of covalent radii, double bonds about 21 pm shorter, triple bonds about 34 pm shorter. Average lengths for resonance structures to get approximate bond length for molecule. 

With the advent of the computer era, it is now possible to reexamine and rethink the resonance theory at the ab initio level. For example, throughout Pauling and Wheland s books, benzene is supposed to be a hybrid of two Kekule structures, by noting that Dewar and other ionic structures make little contribution to the resonance in benzene. However, classical ab initio VB calculations with all possible 175 resonance structures by Norbeck et al. [51] and Tantardini et al. [3], where strictly atomic orbitals are used to construct VB functions, manifested that the five covalent Kekule and Dewar structures make even less contribution to the ground state of benzene than the other 170 ionic structures. This prompts us to reconsider the mathematical formulations for resonance structures [52]. 

The Kekule structures I and II, we now immediately recognize, meet the Cj nditions-for.resonance structures that differ only in the arrangement of elec--trgjisL Benzene is a hybrid of I and II. Since I and II are eScTly equivalent, and hence of exactly the same stability, they make equal contributions to the hybrid. And, also since I and II are exactly equivalent, stabilization due to resonance should be large. 

Problem-Solving Tip Some Guidelines for Resonance Structures... 

The "single line-double arrow" notation, <— , for reversible steps is employed here with apologies to the organic chemist who likes to see it reserved for resonance structures and prefers "double line-double arrow," = , for reversible reactions. The latter notation, however, causes problems in depiction of reversible catalytic cycles Since the arrowheads along the inner and outer circles in the diagram of a cycle point in opposite directions, either all reactants or all products would have to be crowded into the interior of the circle, or cross-overs would occur. For a book in which the distinction between reversible and irreversible steps of cycles is essential and resonance is not an issue, the notation appears more practical. 

For Resonance Structures of Phenol [Fig. 2.5(Z>)] Here, the influence of R at the/>ara position, and the eleetron-donating effect caused due to resonance is more marked, pronounced and significant as compared to the electron-withdrawing influence due to induction. 

As shown in Scheme 6.94, a set of resonance hybrids can be written in which delocalization of the C-H electrons (as is traditional for resonance structures the nuclei do not move) into the ring would cause the incoming electrophile to choose ortho- and para- over meta- substitution. This explanation has variously been called no-bond-resonance and the theory of hyperconjugation. Clearly, the same idea could also be used to account for the stabihty of tertiary carbocations over their secondary and primary analogues. One difficulty with the widespread acceptance of the idea of no-bond-resonance has been the observation that the nitration of other alkyl substituted benzenes such as (l,l-dimethylethyl)benzene (r-butylbenzene) (Figure 6.38), which is incapable of participating in the same way, is also enhanced relative to benzene (CeH ). Indeed, although (l,l-dimethylethyl)benzene (t-... 

It is essential that the tail and head of every arrow be drawn in precisely the proper location. The tail shows where the electrons are coming from, and the head shows where the electrons are going (remember, the electrons aren t really going anywhere, but we treat them as if they were for the purpose of drawing the resonance structures). We will soon learn patterns for drawing proper curved arrows. But, first, we must learn where not to draw curved arrows. There are two rules that must be followed when drawing curved arrows for resonance structures ... 

The Need for Resonance Structures To understand this issue, consider ozone (O3), an air pollntant at ground level bnt an absorber of harmful ultraviolet (UV) radiation in the stratosphere. Two Lewis structures (with lettered O atoms for clarity) are... 

Does the need for resonance structures decrease the value or validity of Lewis (electron dot) theory ... 

It could be argued that the need for resonance structures does not decrease the validity of Lewis theory since it extends the usefulness of the theory and overcomes its limitations. However, if you take the view that Lewis theory means that every molecule and ion should have its own unique structure then you have to accept that the existence of resonance structures diminishes its validity. But then every scientific theory, especially chemical theory, has its limitations. 

The final geometric adjustment to the altered resonance pattern further shifts the adiabatic triplet Z nrt values in the direction expected for resonance structure II ... 

This process shows two radicals (see Solution to Problem 11.3 for resonance structures of these radicals) joining together to form a compound with no unpaired electrons. This step is therefore called a coupling step, and it requires... 

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