Octet, electron

Achter, m., Achter-figur, /, figure-of-eight, figure 8, -schale, /, shell of eight electrons, octet shell. 

The electrons needed to fill up the electron octet at X are supplied by the more electropositive element M. An M atom has e(M) valence electrons. 

For example, the bonding in B4C14 can be interpreted in the following way every boron atom takes part in four bonds, one 2c2e B-Cl bond and three 3c2< Cl bonds on the faces of the B4 tetrahedron. In this way every boron atom attains an electron octet. Eight of the valence electrons take part in the multicenter bonds the other eight are needed for the B-Cl bonds. 

There is no question that, indirectly or directly, Kirrmann and Prevost were influenced by Lowry s theories for explanation of reaction mechanisms. Another important influence was Dupont, with whom they talked at length in the laboratory and who published a paper in 1927 in which he attempted to combine the electron octet theory of valence and Bohr s hydrogen electron model with classical concepts of stereochemistry. Dupont also adopted without reservation Lowry s application of ionic radicals in hydrocarbon chemistry. 66... 

When looking at the compounds formed by two elements near the end of the periods, viz. the negative, non-metallic elements preceding the rare gases, it is found that a very common type of bond is that in which a complete electron octet is formed, as in combinations of two equal atoms. In a molecule Cl2, one electron pair is formed by the two unpaired electrons, and each Cl atom has still three pairs of... 

If, in these formulations, the aluminum, with its electron sextet and its tendency to form an electron octet, is decisive for the reaction course, the question naturally arises whether such a formulation also applies to the other metals which are catalytically effective. The answer is positive, returning to the starting point of the considerations, that is, to the existence of the complex Na(Al(OH)4). It becomes evident that similar addition compounds actually exist with other metals which are catalytically effective. Literature (70, 77, 18) on the subject describes among others, HZn(OH)3, H2Pb(OH)6, nd H2Sn(OH)6. 

Ylid. A compound in which adjacent, covalently bonded atoms, both having an electronic octet, have opposite charges. 

Group 3A elements, such as boron, have three valence electrons and can therefore form three electron-pair bonds in neutral molecules such as borane, BH3. The boron atom in the resultant molecule has only three bonding pairs of electrons and cannot reach an electron octet. (The bonding situation in BH3 is actually more complicated than suggested here we ll deal with it in Section 19.4.)... 

Because valence electron octets are so common, particularly for second-row elements, the atoms in a great many molecules have shapes based on the tetrahedron. Methane, for example, has a tetrahedral shape, with H-C-H bond angles of 109.5°. In NH3, the nitrogen atom has a tetrahedral arrangement of its four charge clouds, but one corner of the tetrahedron is occupied by a lone pair, resulting in a trigonal pyramidal shape for the molecule. Similarly, H20 has two corners of the tetrahedron occupied by lone pairs and thus has a bent shape. 

Because the electron octet is exceeded, the existence of bridged radical species 72 is unacceptable to some (Cadogan and Perkins, 1964) whereas it is taken for granted by others (Readio and Skell, 1966). In a preparative study, Eberhardt (1967) considered that the product distribution as well as the delayed appearance of 2-phenyl-l-iodoethane in (161) was... 

In monoborane (BH3), monoalkylboranes RBH2, or dialkylboranes R2BII there is only an electron sextet at the boron atom. In comparison to the more stable electron octet, the boron atom thus lacks two valence electrons. It obtains them by bonding with a suitable electron pair donor. When no better donor is available, the bonding electron pair of the B—H bond of a second borane molecule acts as the donor so that a two-electron, three-center bond is produced. Under these conditions, boranes are consequently present as dimers BH3, for example, as B2H6. Still, small fractions of the monomers appear as minor components in the dissociation equilibrium of the dimer B2H6, for example, thus contains some BH3. 

In covalent organolithium compounds and covalent Grignard reagents neither the lithium nor the magnesium possesses a valence electron octet. This is energetically disadvantageous. In principle, the same mechanism can be used to stabilize these metals that monomeric boranes BH3 n Rb use to attain a valence electron octet at the boron atom (Section 3.3.3) the formation either of oligomers or, with suitable electron pair donors, of Lewis acid/Lewis base complexes. 

From the viewpoint of polar, yet covalent Li—O and Li—N bonds, lithium would be unable to reach a valence electron octet in the absence of bonding partners besides the heteroatom. The lithium thus has to surround itself by other donors in much the same way as has been seen in the case of the organolithium compounds (cf. Section 10.1). 

Figure 15.20 shows the multistep mechanism of the [4+2]-cycloaddition between 1-(dimethylamino)-l,3-butadiene and cis-dicyanoethenedicarboxylic acid diester. The reaction proceeds via an intermediate, which must be zwitterion conformer B. The anionic moiety of this zwitterion is well stabilized because it represents the conjugate base of a carbon-acidic compound (Section 13.1.2). The cationic moiety of zwitterion B also is well stabilized. It is an iminium ion (i.e., a species with valence electron octet) rather than a carbenium ion (which is a species with valence electron sextet). Moreover, the iminium ion is stabilized by conjugation to a C=C double bond. 

At the so-called radical center an organic radical R- has an electron septet, which is an electron deficiency in comparison to the electron octet of valence-saturated compounds. Carbon atoms are the most frequently found radical centers and most often have three neighbors (see below). Carbon-centered radicals with their electron septet occupy an intermediate position between the carbenium ions, which have one electron less (electron sextet at the valence-unsaturated C atom), and the carbanions, which have one electron more (electron octet at the valence-unsaturated C atom). Since there is an electron deficiency present both in C radicals and in carbenium ions, the latter are more closely related to each other than C radicals are related to carbanions. Because of this, C radicals and carbenium ions are also stabilized or destabilized by the same substituents. 

The N atom of the ketone-associated heterocycle B of the Corey-Itsuno reduction (Figure 8.21) is a Lewis base Other than in the ketone-free heterocycle A, its lone electron pair is not shared with the boron atom because the latter has already acquired a valence electron octet. Therefore, the electron pair is available for binding a molecule of the Lewis acid BH3, whereby the ternary complex C is produced. 

The ionic representation of the ylides in Figure 9.1 shows only one of two conceivable resonance forms of such species. In contrast to the N atom in the center of the N ylides, the P or S atoms in the centers of the P and S ylides (Figure 9.1) may exceed their valence electron octets and share a fifth electron pair. For P and S ylides one can therefore also write a resonance form with a C=P or C=S double bond, respectively these are resonance forms free of formal charges (Figure 9.1). For the sulfoxonium ylide there is a second resonance form in which the S atom exceeds its valence electron octet however, this does contain formal charges. Resonance forms of ylides in which the heteroatom exceeds its valence electron octet are called ylene resonance forms. The ene part of the designation ylene refers to the double bond between the heteroatom and the deprotonated alkyl group. 

Protonation of the radical anion C of Figure 14.71 results in the radical D. This radical regains its valence electron octet by capturing another solvated electron to form the carbanion E. Carbanions of this type are protonated regioselectively by the second equivalent of alcohol. The regioselectivity is independent of the substituents protonation forms a 1,4-dihydroaromatic compound rather than a 1,2-dihydroaromatic compound. Hence, the 1,4-dihydrobenzene A is the reduction product in the example of Figure 14.71. 

Formula II presents a pentacovalent nitrogen atom surrounded by 10 electrons. All five valences are the ordinary, nonpolar ones. Compounds of pentacovalent nitrogen such as the type N(CH3)5 are not known. On the other hand formulae III present nitrogen as a tetracovalent atom, surrounded by an electron octet. Here, one of the nitrogen atoms is bonded with an oxygen atom by a coordinate (semi-polar) link. 

The rule mentioned, according to which the atoms are associating themselves to form a configuration with the electronic octet in the outermost sphere, has its exceptions, as systems are known with outer shells comprising ten or twelve electrons. 

The last resonance structure is a minor contributor because its carbon lacks a complete electron octet. 

BF3 is likely to be an electrophile because the electrostatic potential map indicates that it is electron-poor (blue). The Lewis structure shows that BF3 lacks a complete electron octet and can accept an electron pair from a nucleophile. 

Aluminum alkyl halides of the types R2A1X and RA1X2 are halide-bridged dimers in which, unlike Al2Me6, Al is surrounded by an electron octet. They are stronger Lewis acids than the trialkyls. The sterically very crowded compounds mes MX2 are monomeric they fail to give complexes with Et20 (M = Al, Ga, In X = Cl, Br).27... 

The carbonyl oxide structure is usually drawn as C+—O—O, and so this structure will be most generally used in this chapter. In our opinion, however, C=0+—O-, with its smaller charge separation and carbon electron octet, is preferable. 

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