Ethene three-electron bond

The Three-electron Bond of the Ethene Anion Radical... 

The structure of the ethene anion radical remains unknown, but a simple HMO picture of this species (Scheme 64) reveals a three electron bond loosely analogous to those found in some radicals, such as the nitroxyl radicals. 

Scheme 64. The three electron bond of the ethene anion radical.

Double and triple covalent bonds can be formed between elements by the sharing of two or three electron pairs respectively. Consider the formation of ethene (ethylene), C2H4 ... 

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. 

There exist, however, compounds such as ethene (ethylene), C2H4, in which two electrons from each of the carbon atoms are mutually shared, thereby producing two two-electron bonds, an arrangement which is called a double bond. Each carbon in ethene is attached to only three other atoms ... 

Alternatively, the 2s and two 2p orbitals may be hybridized to give a planar sp system accommodating three electrons from the carbon, one in each hybrid orbital. Three bonds may then be formed with other atoms (see 1.3). The remaining electron, which is in a p orbital at 90° to the plane of the sp system, may overlap with a comparable p orbital from a second atom to form a Ji-bond. leading to a double bond between the carbon and this atom as in ethene (1.4). 

The stabilization of benzene can be placed on a semiquantitative scale. If we define the stabilization of one electron in the pi orbital of ethene as P, then the stabilization of two electrons in a double bond is just 2p. The stabilization of six pi electrons in three double bonds is 6p. Benzene s MO energy levels are at 2p, ip, ip, and -Ip,-ip, -2P the first three are filled with two electrons each for a stabilization of 8p total (2 x 2p + 4 X ip). The six electrons of benzene pi loop are then 2p more stabilized than the six electrons in three isolated double bonds. 

Butadiene dimerizes to 4-vinylcyclohexene (4-ethenylcyclohexene, 1) (reaction 7.1). The absence of intermediates suggests a cyclic movement of three electron pairs, (which could equally well have been written in the opposite direction, or as single electron movements). The transition state would involve partial bond-making and breaking in the six-membered transition state as shown. Reactions involving such cyclic transition states are known as pericyclie reactions. However, ethene does not dimerize to cyclobutane (reaction 7.2) under thermal conditions, even though a cyclic movement of two pairs of electrons could have been invoked. 

Since each carbon atom in ethene is bonded to three other atoms, we need three O bonds. We hybridize carbon by mixing a 2s orbital and two 2p orbitals to obtain three sp hybrid orbitals (pronounced s-p-two ). The third 2p orbital remains unchanged. The three sp hybrid orbitals have the same shapes and energies. The orbitals differ only in their position in space. They he in a plane and are directed to the comers of an equilateral triangle—therefore separated by 120°—to achieve maximum separation of the electrons (Figure 1.15). 

Carbon can also form double bonds between its atoms in organic molecules, as well as forming single bonds. A C=C double bond, as found in alkenes such as ethene, is made up of a a bond and a pi (n) bond. The carbon atoms involved in the double bond will each form three a bonds an example of sp hybridisation (see page 57). This leaves each carbon atom with one spare outer electron in a 2p orbital. When these two p orbitals overlap they form a Tt bond. Figure 14.14 shows how the Tt bond is formed in ethene. 

In a doubly bonded molecule like O2 or ethene, two electron pairs are shared between the two atoms, 0 0 or H2C CH2. In a triply bonded molecule, three electron pairs are shared, N N or HC CH. 

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

The use of lithium amides to metalate the a-position of the N-substituent of imines generates 2-azaallyl anions, typically stabilized by two or three aryl groups (Scheme 11.2) (48-62), a process pioneered by Kauffmann in 1970 (49). Although these reactive anionic species may be regarded as N-lithiated azomethine ylides if the lithium metal is covalently bonded to the imine nitrogen, they have consistently been discussed as 2-azaallyl anions. Their cyclization reactions are characterized by their enhanced reactivity toward relatively unactivated alkenes such as ethene, styrenes, stilbenes, acenaphtylene, 1,3-butadienes, diphenylacetylene, and related derivatives. Accordingly, these cycloaddition reactions are called anionic [3+2] cycloadditions. Reactions with the electron-poor alkenes are rare (54,57). Such reactivity makes a striking contrast with that of N-metalated azomethine ylides, which will be discussed below (Section 11.1.4). 

In our second example we look at the reduction of chlorinated ethenes at a nickel electrode and at the surfaces of two zero-valent metals [Fe(0), Zn(0)]. To gain insight into the rate-limiting process(es) in these cases, we consider how the relative overall reduction rates (relative to PCE) of PCE, TCE, and the three DCE isomers (see Fig. 14.15 for structures) vary as a function of two common descriptors used in QSARs, the one-electron reduction potential (EJ Fig- 14.17a) and the bond dissociation energy (DR X Fig. 14.176). In all these systems, the reduction rates were found to be significantly slower than diffusion of the compounds to the respective surfaces. Therefore, the large differences in the relative reactivities of the compounds between the systems reflect differences in the actual reaction at the metal surface. 

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