Structure resonance

There are some molecules for which satisfactory Lewis structures cannot be made. An example is sulfur trioxide, SO,. Let s begin by making a Lewis structure of the compound  

After completing this portion, you may note that sulfur does not have an octet, which requires you to create multiple bonds. You may also notice that in creating the multiple bonds, there are actually three configurations that are equally possible. 

Of course, now that you re getting the hang of it, you need to know about those ever-present exceptions to the rule. There are some cases in which the octet rule is not obeyed. You should be familiar with the examples, and there are some patterns you can learn to help you memorize this. 

Some molecules and ions have an odd number of electrons. That is, after all electrons are paired up as bonding pairs or lone pairs, there is an extra electron. The most common examples of these compounds contain nitrogen, such as NO and NO,. In these cases, the extra electron should be placed into a multiple bond. 

Molecules with Fewer Than Eight Valence Electrons 

Is the electron density consistent with equal contributions from the two resonance structures for O3 Explain. 

However, this single structure cannot by itself be dominant because it requires that one O — O bond be different from the other, contrary to the observed structure— we would expect the 0=0 double bond to be shorter than the O—O single bond, ooo (Section 8.3) In drawing the Lewis structure, however, we could just as easily have put the 0 = 0 bond on the left  

There is no reason for one of these Lewis structures to be dominant because they are equally vahd representations of the molecule. The placement of the atoms in these two alternative but completely equivalent Lewis structures is the same, but the placement of the electrons is different we call Lewis structures of this sort resonance structures. To describe the structure of ozone properly, we write both resonance structures and use a double-headed arrow to indicate that the real molecule is described by an average of the two  

Notice that the electron density distributed symmetrically across the molecule. 

The 0 — 0 bonds in ozone are often described as one-and-a-half bonds. Is this description consistent with the idea of resonance  

The best way to picture a molecule like SO is as a structure that is a blend of the two resonance structures. A snapshot of the molecule taken on a timescale of a picosecond would show just one of the structures. But, a picture taken on a timescale of a microsecond would show a blur of electrons with equal electron density between the sulfur atom and each of the two oxygen atoms. 

Resonance refers to the idea that the best representation of a molecule or polyatomic ion is a blend of Lewis structures in which multiple bonds are In different positions. 

When the distribution of valence electrons in a molecule cannot be represented adequately by a single Lewis structure, the structure can be approximated by a combination of Lewis structures that differ only in the placement of electrons. Lewis structures that differ only in the placement of electrons are called resonance structures. We use resonance structures to show the delocalization of electrons and to help predict the most likely electron distribution in a molecule. 

A simple method for finding the resonance structures for a given compound or intermediate is to draw one of the resonance structures and then, by using arrows to show the movement of electrons, draw a new structure with a different electron distribution. This movement of electrons is formal only that is, no such electron flow actually takes place in the molecule. The actual molecule is a hybrid of the resonance structures that incorporates some of the characteristics of each resonance structure. Thus, resonance structures themselves are not structures of actual molecules or intermediates but are a formality that helps to predict the electron distribution for the real structures. Resonance structures, and only resonance structures, are separated by a double-headed arrow. 

Note Chemists commonly use the following types of arrows A double-headed arrow links two resonance structures 

A curved arrow indicates the movement of an electron pair in the direction of the arrowhead 

MOLECULES WITH FEWER THAN EIGHT VALENCE ELECTRONS 

There are a few molecules in which an atom will have less than eight valence electrons. The most common examples of these contain H, Be, B, and Al. For example, boron trifluoride, BF3, has a central boron atom surrounded by three fluorine atoms. After filling the octets around the fluorine atoms, there are two possible solutions. One is to leave boron with only six valence electrons, while the second is to draw resonance structures for the molecule. 

Addition to n bonds is a second very common reaction of free radicals. Interaction of die free radical widi die 7r -electron pah causes one of die n electrons to pair up widi die unpaired electron of the free radical to produce a new bond to one of die r-bonded atoms. The remaining n electron is now unpaired and dius forms a new free-radical species. The process is often very favorable since the new a bond (70-90 kcal/mol) formed in die addition process is normally much stronger than die jt bond (60 kcal/mol) which is broken in the reaction. In the above example a new carbon-carbon a bond is formed by free-radical addition to produce a new carbon-centered free radical however, a wide variety of other free-radical species add readily to olefins. 

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Nevertheless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows  

Allyl cations, for example, can be shown nicely while keeping track of charges, electrons, and bonds  

In some molecules there may be a conflict between the theoretical and real 

For bonds between any two atoms, increasing number of bonds decreases the bond length. As the number of bonding electron pairs increases, the attractive force between the atoms gets stronger. Therefore, 

The bonds in the ozone, molecule, 03, are identical and have a length of 128 pm. 

A 0 = 0 bond is shorter than a O — O bond, but studies show that in the 03 molecule both oxygen-oxygen bonds are of equal length. Moreover, this bond length is found to be shorter than a single bond but longer than a double bond. Therefore the structure of the molecule is a hybrid of the two molecular structures shown below. 

A structure midway between the two resonance structures represents the ozone structure best. The bonds in this structure are stronger than a single bond but weaker than a double one. 

OXIDATION NUMBERS, FORMAL CHARGES, AND ACTUAL PARTIAL CHARGES 

Neither oxidation number nor formal charge es an accurate depiction of the actual charges on atoms because oxidation numbers overstate the role of electronegativity and formal charges ignore it. 

M FIGURE 8.11 Oxidation number, formal charge, and electron density distribution for the HCi moiecuie. 

We sometimes encounter molecules and ions in which the experimentally determined arrangement of atoms is not adequately described by a single dominant Lewis structure. Consider ozone, O3, which is a bent molecule with two equal O—O bond lengths ( FIGURE 8.12). Because each oxygen atom contributes 6 valence electrons, the ozone molecule has 18 valence electrons. This means the Lewis structure must have one O — O single bond and one 0 = 0 double bond to attain an octet about each atom  

For some compounds and multi-atom ions, it is possible to draw two or more equivalent arrangements of electrons. As an example, consider the following for sulfur dioxide  

It can he seen that in order to have an octet of electrons around each atom, it is necessary to have a double bond between the S atom and one of the O atoms. However, either 0 atom may be chosen, so that there are two equivalent structures, which are called resonance structures. Resonance structures are conventionally shown with a double arrow between them, as in the example above. They differ only in the locations of bonds and unshared electrons, not in the positions of the atoms. Although the name and structures might lead one to believe that the molecule shifts back and forth between the structures, this is not the case, and the molecule is a hybrid of the two. 

Another compound with different resonance forms is nitrogen dioxide  

Only 7 electrons around each N atom in these resonance structures 

---

Structurally benzene is the simplest stable compound having aromatic character, but a satisfactory graphical representation of its formula proved to be a perplexing problem for chemists. Kekule is usually credited with description of two resonating structures which. 

Dubai H-R and Quack M 1984 Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CFgH J. Chem. Phys. 81 3779-91... 

Segall J, Zare R N, Dubai H R, Lewerenz M and Quack M 1987 Tridiagonal Fermi resonance structure in the vibrational spectrum of the CM chromophore in CHFg. II. Visible spectra J. Chem. Phys. 86 634-46... 

Mandelshtam V A, Taylor H S, Jung C, Bowen H F and Kouri D J 1995 Extraction of dynamics from the resonance structure of H + H2 spectra J. Chem. Phys. 102 7988... 

Clearly such bonding would produce two different carbon-oxygen bond distances (p. 48) but in fact all bonds are found to be identical and intermediate in length between the expected C=0 and C—O bond distances. We conclude, therefore, that the true structure of the carbonate ion cannot be accurately represented by any one diagram of the type shown and a number of resonance structures are suggested (p. 50). 

The carbonate ion is planar and can be regarded as a resonance structure between the three forms given below (see also p. 44) ... 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

Figure 2-52. a) Two semipolar resonance structures are needed in a correct VB representation of the nitro group, b) Representation of a nitro group by a structure having a pentavalent nitrogen atom, c) The RAMSES notation of a nitro group needs no charged resonance structures. One jr-system contains four electrons on three atoms. 

Our first approach took resort in simple resonance theory [36, 37]. For each conjugated nr-system aU resonance structures were generated, such as those shown in Figure 7-5. 

For singlet spin molecules at the equilibrium geometry, RHF and UHF wave functions are almost always identical. RHF wave functions are used for singlets because the calculation takes less CPU time. In a few rare cases, a singlet molecule has biradical resonance structures and UHF will give a better description of the molecule (i.e., ozone). 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

As apparent from the contributing resonance structures, both mesoionic systems contain an azomethinylide contribution, accounting for the reaction with representative dipolarenophiles to give cycioadducts such as 3 or 4 (Scheme 4). The cydoadditions and extrmsion reactions of the adducts have been the mam object of investigation. since previous reviews on me.soionic thiazoles (2.9V Results appearing since 1969 and before June 1976 are reported for each type of compound in this chapter. Tables VIIRl-5 contain all mesoionic thiazoles described before June 1976. 

The rules to be followed when writing resonance structures are summarized m Table 1 5... 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

Atomic positions (connectivity) must be the same in all resonance structures only the electron posi tions may vary among the various contributing structures... 

These are the most important rules to be concerned with at present Additional aspects of electron delocalization as well as additional rules for Its depiction by way of resonance structures will be developed as needed in subsequent chapters... 

In Section 1 9 we introduced curved arrows as a tool to systematically generate resonance structures by moving electrons The mam use of curved arrows however is to show the bonding changes that take place in chemical reactions The acid-base reactions to be discussed in Sections 1 12-1 17 furnish numer ous examples of this and deserve some preliminary comment... 

Show by writing appropriate resonance structures that the two... 

Write a second resonance structure for each of the following ... 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

In general the most stable resonance structure for a polycyclic aromatic hydro carbon is the one with the greatest number of rings that correspond to Kekule formula tions of benzene Naphthalene provides a fairly typical example... 

Write resonance structures for tropylium cation sufficient to... 

Write resonance structures for cyclopentadienyl anion suffi cient to show the delocalization of the negative charge over all five carbons J... 

Each of the following may be represented by at least one alternative resonance structure in which all the six membered nngs correspond to Kekule forms of benzene Write such a resonance form for each... 

Write the principal resonance structures of o methylbenzyl cation and rn methylbenzyl cation Which one has a tertiary carbocation as a contnbuting resonance form ... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

In resonance terms electron delocalization map unsaturated carbonyl compounds IS represented by contributions from three principal resonance structures... 

Principal resonance structures of the anion of a 3 keto ester... 

Protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structures shown... 

Write the most stable resonance structure for the cyclohexa dienyl anion formed by reaction of methoxide ion with o fluoronitrobenzene J... 

The structure of the conjugate base is more like resonance structure B than A because the nega tive charge is on the more electronegative atom (O versus S)... 

---