Bonding in ethene

Is the double bond incorporated into allene significantly shorter, significantly longer or about the same length as the bond in ethene Draw a Lewis structure for allene to justify your conclusion. 

The vinyl H2C=CH radical can be produced by cleavage of a C-H bond in ethene, and has been studied in the gas phase. The unpaired electron clearly occupies a carbon sp hybrid orbital, to lapse into the language of descriptive organic chemistry, but there are regions of space where the, 6-spin electrons have... 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

Figure 3.19 Bent-bond representation of the double bond in ethene. The overlap of sp3 orbitals on each carbon atom produces to bend bond (r) orbitals.

Each carbon atom in ethene uses its three sp hybrid orbitals to form a bonds with two hydrogen atoms and with the other carbon atom. The unhybridised 2p orbitals left on the carbon atoms overlap side-on to form a k bond. The formation of the bonds in ethene is illustrated in the following diagram. 

Of the six bonds in ethene, five are cr bonds (shown in blue) and one is a K bond (shown in pink). 

The bonding in ethene can be expiained in terms of sp hybridisation and a and rr bonds. 

In ethene (C2H4), each carbon atom has three o--bonding electron pairs in its p orbitals to form three a bonds, one with the carhon and the other two with two hydrogen atoms. The tt bond in C2H4 is formed from the sideways overlap of a parallel p orbital on each carbon atom. The C—C bond in ethene is shorter and stronger than in ethane, partly because of the sp -sp overlap being stronger than sp -sp, but especially because of the extra tt bond in ethene. 

Thus, if there were no 7r bonding in ethene and no repulsion between the electrons, the energy of the two electrons (one in each of the two adjacent p orbitals of the carbons) would be twice the Coulomb energy, or 2a. This would be the situation for two carbons such as 3 that are widely separated. 

Another important addition reaction is the one used in the manufacture of ethanol. Ethanol has important uses as a solvent and a fuel (p. 94). It is formed when water (as steam) is added across the double bond in ethene. For this reaction to take place, the reactants have to be passed over a catalyst of phosphoric(v) acid (absorbed on silica pellets) at a temperature of 300 °C and pressure of 60 atmospheres (1 atmosphere =... 

One more example of the CASSCF procedure will be outlined calculating the barrier to rotation around the CC double bond in ethene. Step 2, orbital localization, showed nicely localized orbitals when NBO localization was used, but the orbitals were harder to identify with Boys localization. For a CAS(2,2)/6-31G optimization the active orbitals chosen were the n and 7t MOs, and for a CAS(4,4)/6-31G optimization the n, n, cr and cr MOs. The input structures were the normal planar ethene and perpendicular (90° twisted) ethene. Optimization and frequency calculations gave a minimum for the planar and a transition state for the perpendicular structures. The energies (without ZPE, for comparison with those calculated with the GVB method by Wang and Poirier [71]) were ... 

The Dewar-Chatt-Duncanson model of bonding in ethene complexes is shown in figure below. And is analogous to the... 

On this basis, formally at least, the series 1-3 contain metal-metal triple bonds. As we shall see, the nature of the metal-metal bonding is quite different from that of carbon-carbon bonding in ethyne. Similarly, it is useful to regard the metal-metal bond order in the series 4-6 as double. However, the nature of the M=M bond is quite different from the C=C bond in ethene. The carbonyls are semi-bridging or bridging in 1-6 and are extensively involved in the metal-metal interactions. This has resulted in some differences of opinion as to whether one should really regard the metal-metal bonds as multiple. 

The increased strength of the central bond in ethene is best understood in terms of a simple molecular, rather than an atomic interaction. The central bond is established in the xp-plane by interaction between two CH2 fragments in the valence state C(sp2)1 2xH(s)2-C(pXJ/)1, as shown before in figure... 

X-ray crystallographic analysis indicated that benzene is a planar, regular hexagon in which all the carbon-carbon bond lengths are 139 pm, intermediate between the single C-C bond in ethane (154 pm) and the C=C bond in ethene (134 pm), and therefore all have some double bond character. Thus the representation of benzene by one Kekule structure is unsatisfactory. The picture of benzene according to valence bond theory is a resonance hybrid of the two Kekule or canonical forms 4 and 9, conventionally shown as in Figure 1.2, and so each carbon-carbon bond apparently has a bond order of 1.5. 

When one molecule adds on to another repeatedly with no other substance being formed, the process is called addition polymerisatini. As you will see from Figure 6.2.17 the double bonds in ethene molecules open up as they form new single bonds with each other. 

The a bond framework gives the trigonal planar structure to each end of the ethene molecule. However, this is only part of the story, for the two p. orbitals may interact by overlapping side on, i.e. by overlapping each lobe, one above and the other below the plane of the molecule. This is the second carbon/carbon bond in ethene. This second bond formed between the two pz atomic orbitals,... 

The presence of the k bond confers properties on an alkene that mark it out as different from an alkane. In particular, the n bond, by the nature of its sideways overlap of the constituent p orbitals, is weaker than a a bond. Moreover, the electrons of the n bond are relatively exposed, above and below the plane of the alkene. These electrons are the source of reactivity of the alkene toward electrophiles, as in, say, electrophilic addition of bromine (Chapter 4). The n bond in ethene (and other alkenes) is, however, sufficiently strong that it prevents rotation around the carbon-carbon a bond, which is a well-documented property of the carbon-carbon bond in ethane (Section 1.6). The bonding between sp2... 

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