Tt bonds in benzene

If the three tt bonds in benzene were independent of each other, i.e., localized, they would have energy 3(2o + 2/3) = 6a + 6/3. Hence the delocalization energy (DE) for benzene is... 

The energy of a single w bond will thus be given by this expression and therefore if the three tt bonds in benzene are localized, then the energy will be... 

We can explain the tt bonding in benzene by constmcting molecular orbitals from linear combinations of these unhybridized 2/ atomic orbitals. Because we start with six atomic orbitals, we will form six molecular orbitals, which are shown in Figure 4.25. 

Because we cannot describe tiie tt bonds in benzene as individual electron-pair bonds between neighboring atoms, we say that the tt bonds are delocalized among tiie six carbon atoms. Delocalization of the electrons in its ir bonds gives benzene a special stability, as will be discussed in Section 25.4. Delocalization of TT bonds is also responsible for the color of many organic molecules. (See tire "Chemistry at Work" box on organic dyes at the end of tiiis chapter.) If you take a course in organic chemistry, you will see many examples of how electron delocalization influences Ihe properties of organic molecules. 

What kind of structure would you predict for benzene if you knew only what the early chemists knew The molecular formula (CsH ) tells us that benzene has eight fewer hydrogens than an acyclic alkane with six carbons (C H2 +2 = C6H14). Benzene, therefore, has a degree of unsaturation of four. In other words, the total number of rings and TT bonds in benzene is four (Section 5.1). 

These are a (sigma) bonds covalent bonds with the pair of electrons found mainly between the nuclei of the atoms bonded to each other. Each carbon atom forms three a bonds. That leaves one electron spare on each of the six carbon atoms. Each carbon atom contributes this one electron to a n (pi) bond (see page 57). However, the n bonds formed are not localised between pairs of carbon atoms as in an alkene C=C bond (see page 193). Instead, the Tt bonds in benzene spread over all six carbon atoms in the hexagonal ring. The six electrons in the 7t bonds are said to be delocalised. 

The Tt bonding in benzene is formed by the overlap of carbon p atomic orbitals, one from each of the six carbon atoms. To achieve maximum overlap, the benzene molecule must be planar. The lobes of the p orbitals overlap to form a ring of delocalised electrons above and below the plane of the carbon atoms in the benzene molecule. This is shown in Figure 25.4. 

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... 

The classic example of resonance is provided by the tt bonding of benzene. This compound was shown in Chapter 1 to have the molecular formula C6H6, to be planar and hexagonal with bond angles of 120°, and to possess six equivalent C-C bonds and six equivalent C-H bonds. Benzene usually is written with a structural formula proposed by Kekule ... 

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. 

Like 1,3,5-hexatriene, benzene has a six-carbon tt system. The six-carbon tt system in benzene, however, is cyclic. The six p atomic orbitals combine to produce six tt molecular orbitals (Figure 7.12). Three of the MOs are bonding (i/>i, i/>2, and 1/ 3) and three are antibonding (i/>4, i/>5, and i/rg). Benzene s six tt electrons occupy the three lowest-energy MOs (the bonding MOs). The two electrons in i/>i are delocalized over the six carbon atoms. The method used to determine the relative energies of the MOs of compounds with cyclic tt systems is described in Section 15.6. 

Dienes are four-electron donors. Alkynes are four-electron donors when they use one tt bond to form a cr bond and the other tt bond to form a ir bond to metal orbitals of the proper symmetry. When only two of the three tt bonds in a benzene ring interact with a metal, the ring is a four-electron donor. 

A special class of cyclic unsaturated hydrocarbons is known as the aromatic hydrocarbons. The simplest of these is benzene (CeHg), which has a planar ring structure, as shown in Fig. 22.11(a). In the localized electron model of the bonding in benzene, resonance structures of the type shown in Fig. 22.11(b) are used to account for the known equivalence of all the carbon-carbon bonds. But as we discussed in Section 9.5, the best description of the benzene molecule assumes that sp hybrid orbitals on each carbon are used to form the C—C and C—H a bonds, while the remaining Ip orbital on each carbon is used to form tt molecular orbitals. The delocalization of these tt electrons is usually indicated by a circle inside the ring [Fig. 22.11(c)]. 

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. 

Be able to explain the existence of delocalized tt bonds in molecules such as benzene. (Section 9.6)... 

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