Benzene bond delocalization

In addition to its three sp hybrid orbitals each carbon has a half filled 2p orbital that can participate m tt bonding Figure >b shows the continuous rr system that encompasses all of the carbons that result from overlap of these 2p orbitals The six tt electrons of benzene are delocalized over all six carbons... 

Buckminsterfullerene (Cm or Buckyball ) is structurally related to corannulene. In which molecule would you expect 7U-orbital overlap be more effective Explain. How many chemically unique carbons are there in C6o Measure CC bond distances. How many unique distances are there Is each benzene fully delocalized or is one resonance contributor more important than the other ... 

The cyclic conjugation and orbital phase are both continuous in benzene. Electrons delocalize in a cyclic manner. The cyclic conjugation is discontinuous in borazine in Scheme 33). Electrons cannot delocalize in a cyclic manner, but only between the neighboring pairs of donors and acceptors. There arises a fundamental question how electrons delocalize in the isoelectronic molecules where C=C bonds are replaced with N-B bonds. 

Fragments in compounds 155—157 exhibit aromatic bond delocalization. The lowest aromaticity is calculated for Af-pyridinium cyclopentadienide 157, with the interfragmental C—N bond shorter than the corresponding one in 155 and 158. The phenolate moiety in 159 has a high NICS value (—4.6 ppm), in agreement with the one for deprotonated phenol (—6.2 ppm compared to —9.7 ppm for benzene, as cited),196 while the acceptor pyridinium counterpart has a NICS value of —5.5 ppm, showing aromatic delocalization. 

Many bond structures have been proposed for benzene, and no single one may be accepted as fully satisfactory. Probably the best explanation is that the electrons are delocalized over all the six carbon nuclei. Thus benzene does not contain three carbon-carbon single bonds and three carbon-carbon double bonds. Rather, the benzene molecule contains six identical bonds, each one intermediate between a single and a double bond. This type of bond has been called a hybrid bond, a one and one-half bond, or simply a benzene bond. We normally draw the benzene ring as below. Each of the six comers represents a carbon atom, and each carbon atom is bonded to one hydrogen atom. 

The colossal edifice of evidence has created a very strong cause and effect link between 7r-delocalization and geometry, with a lot of emphasis on the preference of the 7r-electron energy. A consensus seemed to have been reached that the symmetric geometries (with uniform C—C bond lengths) in species such as benzene or allyl result from the inherent tendency of n-electrons to undergo bond delocalization in the absence of constraints by the a-frame.1319 The extent of the consensus may be witnessed from papers94-95 on barriers of identity Sn2 reactions such as Cl- + RC1. The authors reason that since the (Cl—R—Cl) -transition state is isoelectronic to allyl anion, which... 

Scheme 7. Bond-Delocalized n-Electron—zn-Center Species Isoelectronic to the TT-Components of Allyl and Benzene...

We start with some biographical notes on Erich Huckel, in the context of which we also mention the merits of Otto Schmidt, the inventor of the free-electron model. The basic assumptions behind the HMO (Huckel Molecular Orbital) model are discussed, and those aspects of this model are reviewed that make it still a powerful tool in Theoretical Chemistry. We ask whether HMO should be regarded as semiempirical or parameter-free. We present closed solutions for special classes of molecules, review the important concept of alternant hydrocarbons and point out how useful perturbation theory within the HMO model is. We then come to bond alternation and the question whether the pi or the sigma bonds are responsible for bond delocalization in benzene and related molecules. Mobius hydrocarbons and diamagnetic ring currents are other topics. We come to optimistic conclusions as to the further role of the HMO model, not as an approximation for the solution of the Schrodinger equation, but as a way towards the understanding of some aspects of the Chemical Bond. 

We shall describe the Jt electrons in benzene as delocalized, that is, no longer localized in specific double bonds between two particular carbon... 

The classic example of n bond delocalization is found in the cyclic molecule benzene (C6H6), which consists of six carbon atoms bound together in a hexagonal arrangement. Each carbon has a single hydrogen atom attached to it. Earlier, you learned to represent the benzene structure as a composite of two resonance forms. 

Each carbon atom in benzene bonds to three atoms, so their electrons are in three sp orbitals and one p orbital as we ve seen for C2H4. The p orbitals are shown as the shaded shapes below on the left (only the C-C bonds are shown). The p atomic orbitals combine to form molecular orbitals with delocalized electrons as shown in the bonding tt molecular orbital below to the right. 

Unlike the pi bonding molecular orbitals in ethylene, those in benzene form delocalized molecular orbitals, which are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. Therefore, electrons residing in any of these orbitals are free to move around the benzene ring. For this reason, the structure of benzene is sometimes represented as... 

If the carbon-carbon bonds all have the same length, they must also have the same number of electrons between the carbon atoms. This can be so, however, only if the tt electrons of benzene are delocalized around the ring, rather than each pair of tt electrons being localized between two carbon atoms. To better understand the concept of delocalized electrons, we ll now take a close look at the bonding in benzene. 

What are aromatic hydrocarbons Benzene exhibits resonance. Explain. What are the bond angles in benzene Give a detailed description of the bonding in benzene. The TT electrons in benzene are delocalized, while the tt electrons in simple alkenes and alkynes are localized. Explain the difference. 

The symbol on the left shows the pi bond delocalized over the entire molecule. The symbol on the right shows only one of the two resonance stractures of benzene it is an incomplete representation. 

Monosubstituted benzothiadiazinone tautomerism was also investigated. The HF/6-31G level calculations showed a clear preference for the keto tautomers 31 and 33 in the N-1- and N-3-monosubstituted compounds over the respective hydroxy tautomers 32 and 34. The 0-alkyl derivative tautomer 35 is preferred over 36 because of better 7t-bond delocalization in the fused benzene ring <1999T12405>. 

We first need to recognize that aromaticity is not a well-defined physical property various strategies have been used to measure it from its effects on other properties, and numerous attempts have been made to put numbers to it. The situation has been summarized in a review. For our purposes, we can consider the depiction of aromaticity from three theoretical viewpoints, all of which address the main issue that the pi-electrons of benzene are delocalized and not at all like pi-electrons in isolated double bonds. 

The molecular orbital answer to this problem is that all six p orbitals can combine to form (six) new molecular orbitals, and the electrons in these orbitals form a ring of electron density above and below the plane of the molecule. Benzene does not resonate between the two Kekule structures—the electrons are in molecular orbitals spread equally over all the carbon atoms. However, the term resonance is still sometimes used (but not in this book) to describe the averaging effect of this mixing of molecular orbitals. We shall describe the k electrons in benzene as delocalized, that is, no longer localized in specific double bonds between two particular carbon atoms but spread out, or delocalized, over all six atoms in the ring. 

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