Double bonds in ethene

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.

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 ... 

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. 

In this example hydrogen and bromine add across the double bond in ethene (Figure 1.2) to give bromoethane, in which a single bond is left between the carbons. Addition reactions are the reverse of elimination reactions. 

Problem 12.3. Describe the orbitals used by carbon in forming the carbon-carbon double bond in ethene, C2H4. 

Ethene, H2C = CH2, serves as the starting material for the synthesis of polyethylene, from which plastic bags and milk jugs are made. Ethyne, H—C=C—H, is used as a fuel for welding torches. The double bond in ethene and the triple bond in ethyne have the same effect on molecular shape as single bonds. Predict the shapes and bond angles of ethene and ethyne. 

The molecular structures of several disilenes in the solid state have been determined by X-ray crystallography. All of them are characterized by very bulky alkyl or aryl substituents, R. The structures of (tert-butyl)(Mes)SiSi(fert-butyl)(Mes), Mes = phenyl-2,4,6-trimethyl (A), and (Mes)2SiSi(Mes)2 (B) are shown in Eig. 14.7. The central C2SiSiC2 framework of the former is planar as expected for an ethene analogue. The SiSi bond distance is 214 pm, about 9% shorter than the single bond distance in Me3SiSiMe3, 234 pm. (By comparison the CC double bond in ethene is 13% shorter than the single bond in ethane.)... 

Carbon atoms can also share more than one electron pair with another atom to form a multiple covalent bond. Consider the examples of a carbon-carbon double bond in ethene (ethylene) and a carbon-carbon triple bond in ethyne (acetylene). 

Fig. 6 Two pairs of electrons either forming a double bond in ethene using the classical Lewis description or two dative bonds. The latter is observed in analogues of ethene for the heavier group 14 elements. The geometric consequences of the different bonding modes result in the progressive folding of the molecule [76]...

Unsaturated aliphatic hydrocarbons presented few problems in terms of structure and bonding Crum Brown had shown a double bond in ethene in 1864. However, the structure of benzene presented a serious difficulty, and it was here that Kekule was to make another vital contribution to the development of organic chemistry. 

Number the carbon chain from the end nearer the double or triple bond. The number of the first carbon in the double bond is used to give the location of the double bond. Alkenes or alkynes with two or three carbons do not need numbers. For example, the double bond in ethene or propene must be between carbon 1 and carbon 2. 

Figure 1-21 The double bond in ethene (ethylene) and the triple bond in ethyne (acetylene).

Figure 11-2 An orbital picture of the double bond in ethene. The (T carbon-carbon bond is made by sp -sp overlap. The pair of p orbitals perpendicular to the ethene molecular plane overlap to form the additional tt bond. For clarity, this overlap is indicated in (A) by the dashed green lines the orbital lobes are shown artificially separated. Another way of presenting the TT bond is depicted in (B), in which the V-electron cloud is above and below the molecular plane.

On pages 193-4 we saw how the double bond in ethene is formed from a a bond and a n bond. There are four electrons in total in these two bonds. So although ethene is a non-polar molecule, there is an area of high electron density around the C=C bond. This makes the alkenes open to attack by electrophiles (see page 197). 

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... 

Alkenes or alkynes with two or three carbons do not need numbers. For example, the double bond in ethene or propene must be between carbon 1 and carbon 2. 

A consideration of the reactions in Figure 27-24 indicates that one possibility is the addition of water to the double bond in ethene in the presence of sulfuric acid as a catalyst. Thus, we write... 

---