Bonds of Ethylene

In the 1930s there were no computers and severe approximations were often made to facilitate paper-and-pencil calculations. A simple method due to Hiickel [7] formed a foundation for the notion of molecular orbitals which stretched out across the whole molecule, but the method was limited mostly to the Ip TT orbitals of aromatic hydrocarbons and other 2pj.Tr heteroaromatic 

We introduced the basic ideas of matrix arithmetic in Chapter 14 relative to the Powell d-orbitals and we expand on matrix use here. The determinant f E) is a single number as a function of E in this case and this can be solved for as (a — — (3 = 0. It will be important for later discussion to 

The determinant uses straight vertical bars and is a single number while a matrix has little right-angle extensions on the vertical bars and represents values in a linear system of equations. Note that a Cayley-Hamilton matrix led to a determinant due to the zeros on the right side of the secular equations. We can use these roots to find the coefficients of the 2p TT molecular orbital coefficients. When X = — 1 we can go back to the original system of equations but still use the determinant with x so we have xcj + lc2 = 0 or —cj - - C2 = 0 and ci = C2- Note that we could use either equation and we wiU get the same result However, we need to normalize the molecular orbitals so we set up the 

Sources Streitwieser, A., Molecular Orbital Theory for Organic Chemists, John Wiley Sons LTD., London, U.K., 1961, Chap. 4 Murrell, J.N. et al., Valence Theory, 2nd Edn., John Wiley Sons, LTD., London, U.K., 1970, p. 296. 

It is possible to extend the simple Hiickel method to hetero-atom cases in organic pi-electron systems by adjusting the a and 3 values in terms of the basic carbon parameters. Two additional parameters are introduced as ax = ac + hx c-c and Pc-x = c-xPc-c (Table 16.1). 

---

FIGURE 17 2 Both (a) ethylene and (b) formal dehyde have the same num ber of electrons and carbon IS sp hybridized in both In formaldehyde one of the carbons is replaced by an sp hybridized oxygen Like the carbon-carbon double bond of ethylene the carbon-oxygen double bond of formaldehyde is com posed of a (T component and a TT component... 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

The octet rule must be followed. That is, no second-row atom can be left with ten electrons (or four for hydrogen). If an electron pair moves to an atom that already has an octet (or two for hydrogen), another electron pair must simultaneously move from that atom to maintain the octet. When two electrons move from the C=C bond of ethylene to the hydrogen atom of for... 

I Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small amount of benzoyl peroxide catalyst to break the weak 0-0 bond. A benzoyloxy radical then adds to the C=C bond of ethylene to generate a carbon radical. One electron from the C=C bond pairs up with the odd electron on the benzoyloxy radical to form a C-O bond, and the other election remains on carbon. 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

Figure 9.16. Ethylene hardly adsorbs on clean silver, but it does interact with preadsorbed oxygen atoms. At low coverages, the O atoms preferably interact with the C-H bond of ethylene, leading to its decomposition into fragments that oxidize to CO2 and H2O but at higher coverages the oxygen atoms become electrophilic and interact with the n-system of ethylene to form the epoxide. [After R.A. van Santen and H.P.C.E. Kuipers, Ac/v, Catal. 35 (1987) 265.]...

In contrast to the severe difficulty of cracking a sigma bond, insertion of a transition metal into a pi bond can proceed in facile fashion. This can be illustrated by the attack of Ti on the pi bond of ethylene, which leads to metallacycle formation in the reaction... 

Here, pb is the bond critical point (saddle point in three dimensions, a minimum on the path of the maximum electron density). In Eq. (44), and A.2 are the principal curvatures perpendicular to the bond path. The parameters A and B in Eq. (45) determined using various basis sets are given in Bader et al. [83JA(105)5061]. Convenient parameters in the quantitative analysis of a conjugation effect are the relative 7r-character tj (in %) of the CC formal double or single bonds determined with reference to the bond of ethylene (90MI2) ... 

The CC and CH bonds of ethane (Example 10.1), and the final selection See = 69.633 and 8ch = 106.806 kcal/mol, are used to get the CC and CH bonds found in unsaturated hydrocarbons by retaining both the contribution of Fkh Eq. (11.12), and the effect of charge variations described by Eq. (10.37). The reference CC double bond of ethylene and the reference CC bonds of benzene, however, roughly estimated along the lines described in Example 10.1, are deduced from the appropriate CH bond energies and the energy of atomization of the corresponding molecule, AE, obtained from experimental data. 

Beyond all complications that seem to accompany the multitude of possible carbon-carbon bonds, simple familiar intuition is vindicated it is not false, after all, to consider the C(Ar)—C(Ar) bonds of benzene as a sort of average between 8/ (the double bond of ethylene) and a single CC bond, provided the latter is chosen properly namely, the conjugated sp —sp single bond, Sg, between aromatic carbons (in lieu of the CC single bond of ethane). 

Fig. 10 Energy diagram that illustrates the effect of rotation about the = C bond of ethylene on the -configuration energies. The avoided crossing of Jt2 and 2 configurations is indicated by the dotted lines...

The description of the bonding of unsaturated hydrocarbons to metals was originally developed by Dewar, Chatt and Duncanson and is now known as the well-established DCD model based on a frontier-orbital concept [82]. In this model, the interaction is viewed in terms of a donation of charge from the highest occupied -orbital into the metal and a subsequent backdonation from filled metal-states into the lowest unoccupied -orbital, see Figure 2.33. Contrary to the case of the standard Blyholder model for CO and N2 the DCD frontier-orbital model is supported by experimental XES measurements [83]. In the present section, we will show how we can experimentally identify and quantify the contributions of the different -orbitals involved in the interaction with the surface. The DCD model will be shown to very well describe the chemical bonding of ethylene on Cu and Ni surfaces. Furthermore, the differences in bonding of benzene to Cu and Ni will be discussed. 

The bond of ethylene (and other olefins) is a proper MO, highly localized to the two carbon atoms. It is the linear combination of the two 2p orbitals which is S with respect to reflection in the bisecting plane and A w.r.t. a 180° rotation about the C2 axis which contains that plane. All -type orbitals are A w.r.t. reflection in the nodal plane of the p orbitals themselves. 

The addition polymer polyethylene is formed as electrons from the double bonds of ethylene monomer molecules split away and become unpaired valence electrons. Each unpaired electron then joins with an unpaired electron of a neighboring carbon atom to form a new covalent bond that links two monomer units together. 

Double and triple bonds are represented with bent bonds formed with flexible couplings. Substances that require models with bent bonds normally are found to be much less stable and, therefore, chemically more reactive than molecules which can be constructed with straight sticks. Figure 1-4 shows the double bond of ethylene, the triple bond of acetylene, and the distorted bonds of cyclopropane. 

Two corresponding faces of a molecule (usually but not invariably faces of a double bond) are homotopic when addition of the same reagent to either face gives the same product. For example, addition of HCN to acetone (28) will give the same cyanohydrin 29, no matter to which face addition occurs (Fig. 10) and addition of bromine to ethylene similarly gives BrCH2CH2Br, regardless of the face of approach. The two faces of the C=0 double bond of acetone and of the C=C double bond of ethylene are thus homotopic. 

The qualitative picture of o and k molecular orbitals can be extended to molecules with three or more atoms. Thus the double bond of ethylene, H2C=CH2, and the triple bond of acetylene, HC=CH, can be... 

The classical method for the manufacture of ethylene dichloride is the addition of chlorine to the double bond of ethylene (Fig. 1). 

An illustrative example is the electrophilic attachment of chlorine to the carbon-carbon double bond of ethylene. 

The assumed equilibrium between hypoastatous acid and its protonated form in aqueous solutions—influenced by the H+ -ion concentration—offered a possibility to study the electrophilic addition of astatine to the oleflnic bond of ethylene forming ethylene astato-hydrin(ll). 

---