Resonance form

there will be more than one possible Lewis structure for a molecule. This statement is especially true for charged molecules, but it applies to many neutral species as well. (You may already have encountered this phenomenon in comparing your answers to Problems 1.9 and 1.10 with ours.) How do we decide which Lewis structure is the correct one The answer almost always is that neither Lewis structure is complete all by itself. Instead, the molecule is best described as a combination of all reasonable Lewis structures, that is, as a combination of resonance forms, which are different electronic representations of the same molecule. Whether or not a Lewis structure is reasonable, whether or not it is an important contributor, will be addressed below. The word electronic is emphasized to remind you that the only differences allowed in a set of resonance forms are differences in electron distribution. There can be no change in the position of the atoms in resonance forms. 

PROBLEM 1.12 Draw a Lewis structure for nitric acid (HO—NO2), and verify that the nitrogen is positive and one of the oxygens is negative (see Fig. 1.26). 

WORKED PROBLEM 1.14 Draw another structure for nitromethane in which every atom is neutral. Hint There are only single bonds in this stmcture. 

ANSWER You can arrive at the answer by using arrows to push electron pairs in the following way  

In organic chemistry, we also encounter molecnles for which there are several correct Lewis structures. 

Red arrows Cv denote the movement of electron pairs electron pushing  

The individual resonance forms are connected by donble-headed arrows and are placed within one set of square brackets. They have the characteristic property of being interconvertible by electron-pair movement only, indicated by red arrows, the nuclear positions in the molecule remaining unchanged. Note that, to turn A into B and then into C, we have to shift two electron pairs in each case. Snch movement of electrons can be depicted by curved arrows, a procedure informally called electron pnshing.  

The use of curved arrows to depict electron-pair movement is a nseful technique that will prevent us from making the common mistake of changing the total nnmber of electrons when we draw resonance forms. It is also advantageons in keeping track of electrons when formulating mechanisms (Sections 2-2 and 6-3). 

The resulting picture is called a resonance hybrid. Because A, B, and C are equivalent (i.e., each is composed of the same number of atoms, bonds, and electron pairs), they contribute equally to the true structure of the molecule, but none of them by itself accurately represents it. 

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The term resonance has also been applied in valency. The general idea of resonance in this sense is that if the valency electrons in a molecule are capable of several alternative arrangements which differ by only a small amount in energy and have no geometrical differences, then the actual arrangement will be a hybrid of these various alternatives. See mesomerism. The stabilization of such a system over the non-resonating forms is the resonance energy. 

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

The oxidative coupling of two molecules of butadiene with Pd(0) forms the bis-TT-allylpalladium complex 31, which is the resonance form of 2,5-divinyb palladacyclopentane (30) formed by oxidative cyclization. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

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

It IS good chemical practice to represent molecules by their most stable Lewis structure The ability to write alternative resonance forms and to compare their relative stabilities however can provide insight into both molecular structure and chemical behavior This will become particularly apparent m the last two thirds of this text where the resonance concept will be used regularly... 

Nitromethane is stabilized by electron delocalization more than methyl nitrite is The two most stable resonance forms of nitromethane are equivalent to each other... 

The two most stable resonance forms of methyl nitrite are not equivalent... 

Two Lewis structures (resonance forms) of formamide the atoms are connected in the same order but the arrangment of the electrons is different... 

In each of the following pairs determine whether the two represent resonance forms of a single species or depict different substances If two structures are not resonance forms explain why... 

Among the following four structures one is not a permissible resonance form Identify the wrong structure Why is it incorrect ... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

It must be emphasized that we are not dealing with an equilibrium between two isomeric carbocations There is only one carbocation Its structure is not adequately represented by either of the individual resonance forms but is a hybrid having qualities of both of them The carbocation has more of the character of A than B because resonance struc ture A IS more stable than B Water attacks faster at the tertiary carbon because it bears a greater share of the positive charge... 

Both resonance forms of the allylic radical must be equivalent... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

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... 

Notice that m converting one resonance form to the next electrons are moved m exactly the same way as was done with allyl radical... 

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 ... 

The electrophile in a Friedel-Crafts acylation reaction is an acyl cation (also referred to as an acylium ion) Acyl cations are stabilized by resonance The acyl cation derived from propanoyl chloride is represented by the two resonance forms... 

Most stable resonance form oxygen and carbon have octets of electrons... 

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

The three resonance forms of the intermediate leading to meta substitution are all secondary carbocations... 

None of the three major resonance forms of the intermediate formed by attack at the meta position has a positive charge on the carbon bearing the —CF3 group... 

Sulfonic acid groups are electron withdrawing because sulfur has a formal posi tive charge in several of the resonance forms of benzenesulfomc acid... 

Isomtriles are stable often naturally occumng compounds that contain a divalent carbon An example is axisonitnle 3 which can be isolated from a species of sponge and possesses anti malanal activity Write a resonance form for axisonitnle 3 that satisfies the octet rule Don t for get to include formal charges... 

A particularly stable resonance form satisfies the octet mle for carbon and oxygen... 

Less stable resonance form 6 electrons on positively charged carbon... 

It IS important to recognize that an enol is a real substance capable of mdepen dent existence An enol is not a resonance form of a carbonyl compound the two are constitutional isomers of each other... 

The three most stable resonance forms of this anion are... 

Most stable resonance forms of an O protonated amide... 

Resonance forms illustrating charge delocalization in enolate of a p keto ester... 

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