Pi bond in benzene

Interactive Figure To see an animation of sigma- and pi-bonding in benzene, visit glencoe.com. 

Figure 4.25 Molecular orbitals describing pi bonding in benzene. The three lowest energy molecular orbitals are bonding molecular orbitals and are occupied in the ground state. The three higher unoccupied orbitals in the ground state are antihonding.

A (a) What are resonance stracmres (b) Draw resonance structures for benzene, (c) What do we mean when we say that the electrons associated with the pi bonds in benzene are delocalized over the entire ring ... 

Part 0 shows how the p orbitals of the pi bonds of benzene overlap in a complete cycle around the ring. The hydrogens have been omitted for clarity. Another version is shown in part . 

Benzene is [6]annulene, cyclic, with a continuous ring of overlapping p orbitals. There are six pi electrons in benzene (three double bonds in the classical structure), so it is a (41V+2) system, with N = 1. Hiickel s rule predicts benzene to be aromatic. 

We saw earlier that alkenes react readily with halogens to form addition products, because the pi bond in C=C can be broken easily. The most common reaction of halogens with benzene is the substitution reaction, in which an atom or group of atoms replaces an atom or groups of atoms in another molecule. For example. 

What type of orbital overlaps to form the pi bonding in a benzene ring ... 

Orbital overlap model of the bonding in benzene, (a) The carbon, hydrogen framework. The six 2p orbitals, each with one electron, are shown uncombined, (b) The overlap of parallel 2p orbitals forms a continuous pi cloud, shown by one torus above the plane of the ring and a second below the plane of the ring. 

An orbital representation of the bonding in benzene. Sigma M bonds are formed by the end-on overlap of sp orbitals. In addition, each carbon contributes one electron to the pi (tt) system by lateral overlap of its p orbital with the p orbitals of its two neighbors. 

The symbol with the inscribed circle emphasizes the fact that the electrons are distributed evenly around the ring, and in this sense, it is perhaps the more accurate of the two. The Kekule symbol, however, reminds us very clearly that there are six pi electrons in benzene. For this reason, it is particularly useful in allowing us to keep track of the valence electrons during chemical reactions of benzene. In this book, we will use the Kekule symbol. However, we must keep in mind that the double bonds are not fixed in the positions shown, nor are they really double bonds at all. 

At low temperature N/ 2 orbitals will be occupied by pairs of electrons with opposite spins. The occupied orbitals have more bonding character than antibonding character, like the occupied pi orbitals in benzene. The crystal is therefore held together very strongly, as illustrated by the fact that gold melts at a high temperature (1063 C). 

We can estimate L as three times the length of the aromatic bond in benzene since the middle bond is part of the pi-electron system as 3 (1.4 A) = 4.2 A. Then we can find X... 

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. 

What is the MM3 enthalpy of formation at 298.15 K of styrene Use the option Mark all pi atoms to take into account the conjugated double bonds in styrene. Is the minimum-energy structure planar, or does the ethylene group move out of the plane of the benzene ring ... 

We first consider benzene, 17, the prototypical aromatic molecule. From the entries in Table 3.21 and comparisons analogous to Example 3.15, one can recognize that conjugative stabilizations in benzene are significantly stronger than those of comparable species in Table 3.19. Thus, on a per-pi-bond basis, the estimated stabilizations in benzene are 40.8 kcal mol-1, more than three times those of diene 2 (12.8 kcal mol-1), twice those of the acyclic triene 11 (20.3 kcal mol-1), and about 58% greater than the most strongly stabilized polyene, 16 (25.8 kcal mol-1). 

Benzene s relative lack of reactivity is a consequence of its electronic structure. As shown by the orbital picture in Figure 23.3b, each of the six carbons in benzene is sp2-hybridized and has a p orbital perpendicular to the ring. When these p orbitals overlap to form pi bonds, there are two possibilities, shown in Figure 23.3c. 

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