Stable resonators

Although the abundances of the benzofluoranthene isomers are consistent with their resonance energies, other factors must be considered when comparing other isomer pairs. For example, in comparing benzo[a]pyrene with benzo[e]pyrene, benzo[a]pyrene is the less resonance-stable isomer but is the one that can be formed most easily from the biologically derived chrysene or benz[fl]anthracene. In this case, benzo[fl]pyrene is the most abundant isomer of the two in the coal tar. No universal rules can explain all observations only general trends can be indicated. 

Choosing the right value of 8 makes the resonator stable only for the time interval around the pulse maximum. 

Polythiazyl paste electrodes are also well behaved, and do not influence the kinetics or thermodynamics of solutes undergoing outer sphere electron transfer reactions. They are resonably stable to continuous use and to storage An (SN),-graphite electrode was studied in aqueous solution and has been found to function like a pH electrode in the pH range 0.3 to 5. Below pH 5, oxidative potentials are postulated to result in the formation of (SNO), In both nonaqueous and aqueous media, the paste electrodes prepared using Apiezon RM grease function well in the region —0.5 to 0.9 V versus SCE These electrodes are easily fabricated and the surface is... 

The decay behavior of allenes (1,2-propadienes) is quite different from that of the conjugated 1,3-dienes. Figure 8 shows the decay of ArS in cyclohexane for the reaction with methyl-substituted allenes [46]. By adding allene, the decay of ArS is accelerated even in the degassed solution, suggesting that the reaction proceeds irreversibly. Such irreversibility occurs when the incipient C atom-centered radical becomes a resonance stable allyl-type radical by rotation of the C-C bond, as shown in Scheme 9. In the aerated solution, the decay of ArS is further accelerated, indicating that the irreversibility due to the rotation is not completely established the addition of O2 further shifts the equilibrium to the peroxy radical side by trapping the incipient short-lived C atom-centered radicals. 

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. 

The radical and ions are exceptionally stable due to resonance the free electron or charge is not localized on the methyl carbon atom but is distributed over the benzene rings. 

Figrue BTl 1.1 shows the range of radiolfequencies where resonances may be expected, between 650 and 140 MHz, when Bq = 14.1 T, i.e. when the H resonance frequency is 600 MHz. There is one bar per stable isotope. Its width is the reported chemical shift range (Bl.11.5) for that isotope, and its height corresponds to the log of the sensitivity at the natural abundance of the isotope, covering about six orders of magnitude. The... 

The classical counterpart of resonances is periodic orbits [91, 95, 96, 97 and 98]. For example, a purely classical study of the H+H2 collinear potential surface reveals that near the transition state for the H+H2 H2+H reaction there are several trajectories (in R and r) that are periodic. These trajectories are not stable but they nevertheless affect strongly tire quantum dynamics. A study of tlie resonances in H+H2 scattering as well as many other triatomic systems (see, e.g., [99]) reveals that the scattering peaks are closely related to tlie frequencies of the periodic orbits and the resonance wavefiinctions are large in the regions of space where the periodic orbits reside. 

The reasonable stable products are characterized by an ir-absorption near 1615 cm". The 4-protons resonate near 6.2 ppm in the H NMR spectrum (23). NMR spectra exhibit a carbonyl atom signal near 173 ppm, whereas C-4 resonates near 8 108 these positions are characteristic of other mesoionic ring carbon atoms (24). In the mass spectra, decomposition with loss of CO, rupture of the 1,5 and 2.3 bonds with elimination of R NC2R 0 and cleavage of the 1,2 and 3,4 bonds with elimination of C2R 0S is observed (11)... 

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... 

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... 

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... 

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... 

The extent to which benzene is more stable than either of the Kekule structures is its resonance energy, which is estimated to be 125-150 kJ/mol (30-36 kcal/mol) from heats of hydrogenation data... 

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

An important difference between Fnedel-Crafts alkylations and acylations is that acyl cations do not rearrange The acyl group of the acyl chloride or acid anhydride is transferred to the benzene ring unchanged The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could con ceivably arise from it by a hydride or alkyl group shift... 

C 1 IS more reactive because the intermediate formed by electrophilic attack there IS a relatively stable carbocation A benzene type pattern of bonds is retained m one nng and the positive charge is delocalized by allylic resonance... 

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... 

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

The amide is activated toward nucleophilic attack by protonation of its carbonyl oxygen The cation produced m this step is stabilized by resonance involving the nitro gen lone pair and is more stable than the intermediate m which the amide nitrogen is protonated... 

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

A/-Nitroso amines are stabilized by electron delocalization Write tn tw most stable resonance forms of A/-nitrosodimethylamine... 

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

By analogy to phenols we would expect the isomers with —OH groups on benzene like rings to be more stable This turns out not to be true because the keto forms are also aromatic owing to amide resonance... 

Adenine is a weak base Which one of the three nitrogens designated by arrows in the struc tural formula shown is protonated in acidic solution" A resonance evaluation of the three protonated forms will tell you which one is the most stable... 

Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

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