Benzene resonance forms

Chemists sometimes represent the two benzene resonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds. This hind of representation has to be used carefully, however, because it doesn t indicate the number of tt electrons in the ring. (How many electrons does a circle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond structure. We ll be able to keep count of tt electrons this way but must be aware of the limitations of the drawings. 

Note these resonance forms do not include the simple benzene resonance forms as shown below they are significant, but repetitive, so for simplicity, they are not drawn here.)... 

Ortho- and para-dimethoxybenzene have a redox potential of 4.2 V or more, and their oxidation and reduction reactions are reversible. Therefore, they can be used as overcharge protection additives for lithium-ion batteries. They lose two electrons during the oxidation reaction and are stabilized by the benzene resonance forms. There are fewer resonance forms for meta-dimethoxybenzene, so that its stability is not good enough. 

The two Kekule structures for benzene have the same arrangement of atoms but differ m the placement of electrons Thus they are resonance forms and neither one by Itself correctly describes the bonding m the actual molecule As a hybrid of the two Kekule structures benzene is often represented by a hexagon containing an inscribed circle... 

Because the carbons that are singly bonded m one resonance form are doubly bonded m the other the resonance description is consistent with the observed carbon-carbon bond distances m benzene These distances not only are all identical but also are intermediate between typical single bond and double bond lengths... 

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 a resonance form for guanine in which the six membered ring has an electronic structure analogous to benzene Show all unshared pairs... 

Linus Pauling is portrayed on this 1977 Volta stamp. The chemical formulas depict the two resonance forms of benzene, and the explosion in the background symbolizes Pauling s efforts to limit the testing of nuclear weapons. 

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

Resonance is an extremely useful concept that we ll return to on numerous occasions throughout the rest of this book. We ll see in Chapter 15, for instance, that the six carbon-carbon bonds in so-called aromatic compounds, such as benzene, are equivalent and that benzene is best represented as a hybrid of two resonance forms. Although an individual resonance form seems to imply that benzene has alternating single and double bonds, neither form is correct by itself. The true benzene structure is a hybrid of the two individual forms, and all six carbon-carbon bonds are equivalent. This symmetrical distribution of electrons around the molecule is evident in an electrostatic potential map. 

Rule 1 Individual resonance forms are imaginary, not real. The real structure is a composite, or resonance hybrid, of the different forms. Species such as the acetate ion and benzene are no different from any other. They have single, unchanging structures, and they do not switch back and forth between resonance forms. The only difference between these and other substances is in the way they must be represented on paper. 

In other words, resonance leads to stability. Generally speaking, the larger the number of resonance forms, the more stable a substance is because electrons ate spread out over a larger part of the molecule and are closer to more nuclei. We ll see in Chapter IS, for instance, that a benzene ring is more stable because of resonance than might otherwise be expected. 

Some substances, such as acetate ion and benzene, can t be represented by a single line-bond structure and must be considered as a resonance hybrid of two or more structures, neither of which is correct by itself. The only difference between two resonance forms is in the location of their tt and nonbonding electrons. The nuclei remain in the same places in both structures, and the hybridization of the atoms remains the same. 

Problem 13.18 How many peaks would you expect in the H NMR spectrum of 1,4-dimethyl-benzene (pom-xylene, or p-xylene) What ratio of peak areas would you expect on integration of the spectrum Refer to Table 13.3 for approximate chemical shifts, and sketch what the spectrum would look like. (Remember from Section 2.4 that aromatic rings have two resonance forms.)... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

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. 

The mechanism begins with the attack on the chlorine molecule by aluminum chloride. (This step would be the same if ironGlf) chloride were the catalyst.) The CL ion attracts a pair of electrons from the benzene to form an intermediate species. The presence of resonance in this intermediate stabilizes it and helps the reaction along. 

Thiophene is present in the benzene fraction from the distillation of coal tar. As with pyrrole and furan, the same type of resonance forms contribute to its overall molecular constitution, and the compound is aromatic in character. There is a difference between thiophene and furan, however, because sulfur is less electronegative than oxygen. Thus, the chemistry of thiophene tends to be closer to that of pyrrole than to that of furan. For example, thiophene does not enter easily into [4 + 2] cycloaddition reactions and quite severe conditions, high pressure (15 bar) and a temperature of 100 C, are necessary in order to force a cycloaddition between it and maleic anhydride. 

In substituted benzenes (i.e., benzenes in which hydrogen is substituted by another atom or group of atoms), depending on the type and position of the substituents, the different resonance forms may exhibit somewhat different stabilities. Therefore, their contributions are different. 

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