Orbital hybridization benzene

According to the orbital hybridization model benzene has six tt elec Irons which are shared by all six sp hybridized carbons Regions of high TT electron density are located above and below the plane of the ring... 

Because all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible lo define three localized tt bonds in which a given p orbital overlaps only one neighboring p orbital. Rather, each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which the six -tt electrons are completely delocalized around the ring. In resonance terms (Sections 2.4 and 2.5), benzene is a hybrid of two equivalent forms. Neither form... 

This leads to modifications of the localized it orbitals. In benzene, for example, a Kekule localization which mixes the a and ir orbitals to form double banana bonds is preferred over the other equivalent ir localizations discussed. 60) In naphthalene a Kekule type structure is found similar to the one presently discussed, but different in that the (jtE2) are hybridized with corresponding o-CC bonding orbitals to form banana bonds, whereas the (ttC2 ) remains a pure jt orbital. 61 > While this is of interest in the discussion of the whole molecule, it is clear that certain intrinsic properties of the ir-electrons are more readily recognized by the localization which has been discussed here. We hope to discuss elsewhere localized orbitals involving a bonds in organic molecules. 

The tail of j/i in the direction of j/2 becomes much less pronounced at the TS (central column of orbitals). This orbital resembles a distorted carbon 2p AO in the rightmost column, but it begins to attain the form of a distorted sp hybrid at the TS. Orbital /6 is reshaped in a similar fashion and becomes less distorted towards /5 and more towards j/i. The changes in j/i and tj/e are mirrored by identical changes in /4 and /5. All of this indicates that the bonds t /i-t /2, t /5 6 and /3- i/4 are breaking up at the TS, in parallel with the formation of new bonds tj/i-tl/g and tl/4-tl/5. The shapes of orbital /2 and its symmetry-related counterpart, /3, are particularly interesting These orbitals appear to be almost equally distorted towards one another (as the reaction proceeds further, these distortions lead to the formation of the cyclohexene n bond) and towards their former partners in the butadiene tz bonds ( /i and /4, respectively). Consequently, j/2 and j/3 closely resemble in form the SC orbitals for benzene [8-10]. 

In the benzene molecule, each of the six carbon atoms undergoes sp hybridization. Benzene has a planar shape and the carbon - carbon bond lengths are 139 pm. The six carbon ring is formed by the overlap of the sp orbitals of the carbon atoms. The resulting p orbitals then overlap to form t bonds. 

Each ring carbon of this system has nonequivalent a orbitals (one sp and two sp orbitals ) as shown in Fig. 1) in contrast to the more usual organic molecules in which the hybridization of a orbitals is equivalent. For instance, methane carbon has four equivalent sp orbitals and benzene-ring carbon has three equivalent sp orbitcds. 

Top) The a hybrids of the carbon atoms of benzene. The n atomic orbitals in benzene (a), and the Kekule pairing schemes (b, c). (Bottom) The n molecular orbitals in benzene (double streamers). 

Figure 15-2 Orbital picture of the bonding in benzene. (A) The a framework is depicted as straight lines except for the bonding to one carbon, in which the p orbital and the sp hybrids are shown explicitly. (B) The six overlapping p orbitals in benzene form a ir-electron cloud located above and below the molecular plane. (C) The electrostatic potential map of benzene shows the relative electron richness of the ring and the even distribution of electron density over the six carbon atoms.

FIGURE 113 (a) The framework of bonds shown in the tube model of benzene are cr bonds (b) Each carbon is sp hybridized and has a 2p orbital perpendicular to the cr framework Overlap of the 2p orbitals generates a tt system encompass mg the entire ring (c) Electrostatic potential map of benzene The red area in the center corresponds to the region above and below the plane of the ring where the tt electrons are concentrated... 

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... 

The picture of benzene as a planar framework of ct bonds with six electrons m a delo cahzed rr orbital is a useful but superficial one Six elecfrons cannof simulfaneously occupy any one orbifal be if an afomic orbifal or a molecular orbifal We can fix fhis wifh the more accurate molecular orbital picture shown m Figure 114 We learned m Section 2 4 that when atomic orbitals (AOs) combine to give molecular orbitals (MOs) the final number of MOs musf equal fhe original number of AOs Thus fhe six 2p AOs of SIX sp hybridized carbons combine fo give six tt MOs of benzene... 

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