Resonance structures carbon

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) ... 

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

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... 

Notice that the MO picture gives the same qualitative picture of the substituent effects as described by resonance structures. The amino group is pictured by resonance as an electron donor which causes a buildup of electron density at the /3 carbon, whereas the formyl group is an electron acceptor which diminishes electron density at the /3 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. 

The stabilizing role of other functional groups can also be described in resonance terms. Both electron-attracting groups such as carbonyl and cyano and electron-donating groups such as methoxy and dimethylamino have a stabilizing etfect on a radical intermediate at an adjacent carbon. The resonance structures which depict these interactions indicate delocalization of the unpaired electron onto the adjacent substituents ... 

Use Learning By Modeling to view the carbocation represented by resonance structures A and B. How is the positive charge distributed among its carbons ... 

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 car bocation has more of the character of A than B because resonance structure A is more stable than B. Water attacks faster at the tertiary carbon because it bears a greater share of the positive charge. 

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... 

The positive charge is shared by the three carbons indicated in the three most stable 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. 

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. 

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

The negative charge of the cyclohexadienyl anion 5 is delocalized over several carbon centers, as is illustrated by the following resonance structures ... 

The typical reactivity of an enamine 1 results from the nucleophilic character of the /3-carbon center, as indicated by a resonance structure ... 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In the acetate ion, for example, the carbon atom is sp2-hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the r electrons in the C=0 bond and the lone-pair electrons on oxygen differ from one form to another. This movement of electrons from one resonance structure to another can be indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

The 2,4-pentanedione anion has a lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=0 bond on the left. The 0=C-C grouping is a typical one for which two resonance structures can be drawn. 

Active Figure 10.3 An orbital view of the allyl radical. The p orbital on the central carbon can overlap equally well with a p orbital on either neighboring carbon, giving rise to two equivalent resonance structures. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Problem 15.6 Draw the five resonance structures of the cyclopentadienyl anion. Are all carbon-carbon bonds equivalent How many absorption lines would you expect to see in the lH NMR and, 3C NMR spectra of the anion ... 

Look at the three resonance structures of naphthalene shown in Section 15.7, and account for the fact that not all carbon-carbon bonds have the same length. The C1-C2 bond is 136 pm long, whereas the C2-C3 bond is 139 pm long. 

Look at the five resonance structures for phenanthrene (Problem 15.26) and predict which of its carbon-carbon bonds is shortest. 

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