Carbon atom resonance structures

Anthracene has the formula Cl4Hln. It is similar to benzene but has 3 six-membered rings that share common C—C bonds, as shown below. Complete the structure by drawing in multiple bonds to satisfy the octet rule at each carbon atom. Resonance structures are possible. Draw as many as you can find. 

In a recent 13C-NMR study by Halasa et al. (31), the addition of n-BuLi to 1,3-butadiene at DP = 10-12 (Fig. 7) was studied in the absence of polar modifiers. The vinyl carbon at DP 12 showed the usual (=CH2) olefinic position at 110-114 ppm, structure 16, and a new and unusual absorption at 90-100 ppm appeared. The latter resonance was shown by a gated decoupling technique to be split into a doublet. This observation indicates that this carbon is a methine carbon and could be the y-carbon in structure 15 or the y-carbon in structure 16. Since the methine y-carbon atom in structure 15 is an olefinic carbon, the assignment of the... 

One of the lone pairs of electrons on the oxygen atom of this carbocation is conjugated with the empty p orbital of the positively charged carbon. The resonance structure on the right is more stable than the structure on the left, because the octet rule is satisfied at both C and O. 

The maximum production of the D-fructan, levan, has been shown to occur during the mid-exponential to the late exponential phase of growth of Actinomyces viscosus and is followed by a rapid decline as a result of the production of levan hydrolase activity.The levan produced by Streptococcus mutans OMZ 176 appears to have a structure similar to other known D-fructans and inulin. On the basis of chemical shift displacements of the n.m.r. spectra, resulting from 0-substitution at specific carbon atoms, resonances have been assigned to the carbon atoms of the j3-D-fructofuranosyl residues in levans. A variety of different levans show almost identical n.m.r. spectra in contrast to a wide divergence in the corresponding spectra of dextrans. ... 

The recent development of NMR spectrometry has not contributed signiGcantly to the extermination of high oldin structure either, due to difficulties in separating the carbon atom resonances of main chains and pendant groups. The best results have been obtained Cm ethylene-propylene copolymers (see Section IVA.lb). 

We saw above that the positive charge of an allyhc carbocation is distributed equally at C-1 and C-3. However, the charge is not equally distributed in the 1,1-dimethylaUyhc carbocation. In structure 1, the positive charge is located at a primary carbon atom, in structure 2 the positive charge is at a tertiary carbon atom. Therefore, resonance form 2 is the major contributing structure. 

Protonation gives an allylic carbocation, represented by two contributing resonance structures. In the next step, the allylic carbocation reacts with nucleophilic bromide ion at the secondary carbon atom (resonance form I) to give the 1,2-addition product. Bromide ion also reacts at the primary carbon atom (resonance form II) to give the 1,4-addition product. 

The mechanism for the acid-catalyzed ring opening differs from the base-catalyzed reaction in one important way that explains why the nucleophile now attacks the more hindered carbon atom. The structure of the protonated substrate that reacts with the nucleophile in the rate-determining step can exist in three resonance forms. 

It also forms compounds known as carbonyls with many metals. The best known is nickel tetracarbonyl, Ni(CO)4, a volatile liquid, clearly covalent. Here, donation of two electrons by each carbon atom brings the nickel valency shell up to that of krypton (28 -E 4 x 2) the structure may be written Ni( <- 0=0)4. (The actual structure is more accurately represented as a resonance hybrid of Ni( <- 0=0)4 and Ni(=C=0)4 with the valency shell of nickel further expanded.) Nickel tetracarbonyl has a tetrahedral configuration,... 

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... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

Molecular orbitals are useful tools for identifying reactive sites in a molecule. For example, the positive charge in allyl cation is delocalized over the two terminal carbon atoms, and both atoms can act as electron acceptors. This is normally shown using two resonance structures, but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron-acceptor orbital). Allyl cation s LUMO appear s as four surfaces. Two surfaces are positioned near- each of the terminal car bon atoms, and they identify allyl cation s electron-acceptor sites. 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

The darkest regions in the slices indicate the greatest electron density. The meta form of nitrated chlorobenzene and the para form of nitrated nitrobenzene retain the resonance structure to a much greater degree throughout the extent of the electron density. In contrast, the density in the less-favored conformations becomes more localized on the substituent as one moves outward from the plane of the carbon atoms. 

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