Ethylene a bonds

Fig. 48. Formation of a bond in ethylene a bonds omitted). If the two 2p orbitals are parallel there is substantial lateral overlap (for clarity not shown in the Fig.) giving a 71 bond rotation of the CIL groups lessens the bonding.

Overlap of carbon sp2 hybrid orbitals with hydrogen Is orbitals to produce the ethylene a bonded framework. (The axes have been changed for the sake of clarity—i.e. the z axis would be out of the plane of the paper.)... 

The LUMO of ethylene and the HOMO of 1,3-butadiene have the proper symmetry to allow cycloaddition. The two molecules approach each other in parallel planes, and electrons flow from the HOMO of 1,3-butadiene to the LUMO of ethylene, a Bonds form when orbitals of the same symmetry (blue-to-blue and red-to-red) overlap. 

The highest occupied molecular orbital (HOMO) of ethylene, a bonding n orbital, can be approximated reasonably well by a symmetric combination of carbon Ip orbitals centered on carbon atoms 1 and 2 and oriented so that their z-axes are parallel ... 

One measure of the strength of a bond is its bond dissoa ation energy This topic will be introduced in Section 4 16 and applied to ethylene in Section 5 2... 

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... 

Alternatively a bonded poly(ethylene glycol) capillary column held at 35°C for 5 min and programmed to 190°C at 8°C/min may be employed to determine all components but water. The Kad-Eischer method for water gives inaccurate results. 

Organoaluminum Compounds. Apphcation of aluminum compounds in organic chemistry came of age in the 1950s when the direct synthesis of trialkylalurninum compounds, particularly triethylalurninum and triisobutylalurninum from metallic aluminum, hydrogen, and the olefins ethylene and isobutylene, made available economic organoalurninum raw materials for a wide variety of chemical reactions (see a-BONDED alkyls and aryls). 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

Ethylene is planar- with bond angles close to 120° (Figure 2.15) therefore, some hybridization state other than sp is required. The hybridization scheme is determined by the number of atoms to which carbon is directly attached. In sp hybridization, four atoms are attached to carbon by a bonds, and so four equivalent sp hybrid orbitals are required. In ethylene, three atoms are attached to each carbon, so three equivalent hybrid orbitals... 

Each carbon of ethylene uses two of its sp hybrid orbitals to form a bonds to two hydrogen atoms, as illustrated in the first par-f of Figure 2.17. The remaining sp orbitals, one on each carbon, overlap along the internuclear- axis to give a a bond connecting the two carbons. 

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. 

Figure 10.12 shows the interaction between the FIOMO of one ethylene molecule and the LUMO of another. In particular, notice that two of the carbons that are to become a-bonded to each other in the product experience an antibonding interaction during the cycloaddition process. This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry-forbidden reaction. Reaction, were it to occur, would take place slowly and by a mechanism in which the two new a bonds are fonned in separate steps rather than by way of a concerted process involving a single transition state. 

With electrons flowing from ethylene to zirconium, the Zr—CH3 bond weakens, the carbons of ethylene become positively polarized, and the methyl group migrates from zirconium to one of the carbons of ethylene. Cleavage of the Zr—CH3 bond is accompanied by formation of a a bond between zirconium and one of the car bons of ethylene in Step 3. The product of this step is a chain-extended form of the active catalyst, ready to accept another ethylene ligand and repeat the chain extending steps. 

Step 3 The methyl group migrates from zirconium to one of the carbons of the ethylene ligand. At the sane time, the tt electrons of the ethylene ligand are used to fonn a a bond between the other carbon and zirconium. 

Section 14.15 Coordination polymerization of ethylene and propene has the biggest economic impact of any organic chemical process. Ziegler-Natta polymerization is canied out using catalysts derived from transition metals such as titanium and zirconium. ir-Bonded and a-bonded organometallic compounds aie intennediates in coordination polymerization. 

In ethylene, both the HOMO and LUMO are formed primarily from p orbitals from the two carbons. The carbons lie in the YZ-plane, and so the p,j orbitals lie above and below the C-C bond. In the HOMO, the orbitals have like signs, and so they combine to form a bonding n molecular orbital. In contrast, in the LUMO, they have opposite signs, indicating that they combine to form an antibonding Tt molecular orbital. 

Orbitals 7 and 9 (the latter is the LUMO) of formaldehyde exhibit this same character. Orbital 7 is a bonding 7t orbital, and orbital 9 is a Tt . However, the n orbital formed of the pj orbitals from the carbon and the oxygen (which also lie in the YZ plane) is not the HOMO. Instead, an orbital formed from Pj, orbitals from the carbon and the oxygen and from the s orbitals on the hydrogens is the highest occupied orbital. The contributions from the carbon and oxygen are situated along the double bond while the HOMO in ethylene was perpendicular to this bond. 

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. 

Historically, ethylene potymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved half-arrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

Recently several examples of diolefin crystals in which the reaction behaviour deviates from the topochemical rule have been observed. For example, in the photoreaction of methyl a-cyano-4-[2-(4-pyridyl)-ethenyljcinnamate (2 OMe), the first reaction occurs exclusively at the pyridyl side although the distance between the ethylenic double bonds on the pyridyl side is exactly the same as that between the ethylenic double bonds on the ester side (4.049 A), as shown in Fig. 5 (Maekawa et al., 1991a). A few other unsymmetrical diolefin compounds display the same regioselective behaviour (Hatada, 1989). 

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