Tt-Bonded ethylene

The TT bond in ethylene generated by overlap of p orbitals of adjacent carbons... 

FIGURE 5 1 (a) The planar framework of u bonds in ethylene showing bond distances and angles (b) and (c) The p orbitals of two sp hybridized carbons overlap to produce a tt bond (d) The electrostatic potential map shows a region of high negative potential due to the tt elec trons above and below the plane of the atoms... 

The simplest arithmetic ap proach subtracts the C—C (j bond energy of ethane (368 kj/mol 88 kcal/mol) from the C=C bond energy of ethylene (605 kJ/mol 144 5 kcal/mol) This gives a value of 237 kJ/mol (56 5 kcal/mol) for the tt bond energy... 

FIGURE 9 3 Electro static potential maps of eth yiene and acetylene The region of highest negative charge (red) is associated with the TT bonds and lies between the two carbons in both This electron rich re gion IS above and below the plane of the molecule in ethylene Because acetylene has two TT bonds a band of high electron density encir cles the molecule... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

Section 2.20 Carbon is sp -hybridized in ethylene, and the double bond has a a component and a tt component. The sp hybridization state is derived by mixing the 2s and two of the three 2p orbitals. Three equivalent sp orbitals result, and their axes are coplanai. Overlap of an sp orbital of one carbon with an sp orbital of another produces a a bond between them. Each carbon still has one unhybridized p orbital available for bonding, and side-by-side overlap of the p orbitals of adjacent carbons gives a tt bond between them. 

The double bond in ethylene is stronger than the C—C single bond in ethane, but it is not twice as strong. Chemists do not agree on exactly how to apportion the total C=C bond energy between its a and tt components, but all agree that the tt bond is weaker than the a bond. 

The activation energy for rotation about a typical carbon-carbon double bond is very high—on the order of 250 kj/mol (about 60 kcal/ mol). This quantity may be taken as a measure of the tt bond contribution to the total C=C bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated by manipulation of thermochemical data on page 191. 

An sp hybridization model for the caibon-caibon triple bond was developed in Section 2.21 and is reviewed for acetylene in Figure 9.2. Figure 9.3 compares the electrostatic potential maps of ethylene and acetylene and shows how the second tt bond in acetylene causes a band of high electron density to encircle the molecule. 

Figure 1.18 A molecular orbital description of the C=C tt bond in ethylene. The lower-energy, tt bonding MO results from a combination of p orbital lobes with the same algebraic sign and is filled. The higher-energy, -tt antibonding MO results from a combination of p orbital lobes with the opposite algebraic signs and is unfilled.

As the reaction proceeds, ethylene and HBr must approach each other, the ethylene tt bond and the H—Br bond must break, a new C—H bond must form in the first step, and a new C—Br bond must form in the second step. 

Along the bond axis itself, the electron density is zero. The electron pair of a pi (tt) bond occupies a pi bonding orbital. There is one tt bond in the C2H4 molecule, two in QH The geometries of the bonding orbitals in ethylene and acetylene are shown in Figure 7.13. 

The relative rate constants (kr) do not account for the fact that approach of the nitrile oxide to the tt-bond can occur from both olefinic diastereofaces with two regioisomeric modes of reaction (Scheme 6.14). In the case of achiral 1-alkenes, only one regioisomer is formed. With chiral dipolarophiles, preference for one of the two is usually found (diastereodifferentiation). The relative diastereofacial reactivity (kTn) is used to evaluate this effect (121). With ethylene, there are four possibilities of attack (two for each face corresponding to the different regio-isomers), and the kTn of each is set as 0.25. In diastereodifferentiating cycloadditions, such as those with a-chiral alkenes, the major isomer generally results... 

---