Carbon electrons

There are many compounds which do not conduct electricity when solid or fused indicating that the bonding is neither metallic nor ionic. Lewis, in 1916. suggested that in such cases bonding resulted from a sharing of electrons. In the formation of methane CH4 for example, carbon, electronic configuration l.s 2.s 2p. uses the tour electrons in the second quantum level to form four equivalent... 

H3P—CH2 where it can be seen that the electron distribution is highly polarized m the direction that makes carbon electron rich The carbon has much of the character of a car banion and can act as a nucleophile toward C=0... 

Branching at Isolating Carbons Electronic effects through 7C-bonds... 

Extension of aromaticity. The extension of the aromatic ring system through fusion (as in naphthalene) or direct substitution (as in biphenyl) appears to increase log P, especially if the heteroaromatic atom is next to the juncture. If the ring-joining carbons are attached only to other aromatic carbons, electron delocalization is minimal the correction is -rO.lO for each 1C. If the ICs are also attached... 

Fig. 23 Entropy effects on intramolecular reactions of polymethylene chains. Plot of 9AS (e.u.) against number of single bonds for (O) nucleophilic substitutions at saturated carbon ( ) electron-exchange reactions (A) quenching of benzophenone phosphorescence. The straight line has intercept +30 e.u. and slope —4.0 e.u. per rotor. The right-hand ordinate reports the purely entropic EM s calculated as exp(0AS /J )...

Bimolecular reactions of aniline with /V-acyloxy-/V-alkoxyamides are model Sn2 processes in which reactivity is dictated by a transition state that resembles normal Sn2 processes at carbon. Electronic influences of substituents support a non-synchronous process which has strong charge separation at the transition state and which is subject to steric effects around the reactive centre, at the nucleophile but not on the leaving group. The sp3 character of nitrogen and disconnection between the amino group and the amide carbonyl renders these reactions analogous to the displacement of halides in a-haloketones. 

Shifts for meta carbon atoms remain almost unaffected by all kinds of substituents, unlike shifts for ortho and para carbons. Electron-releasing substituents (electron donors) increase tt electron densities in ortho and para positions and thereby induce a shielding relative to benzene (Sc(o,p) < 128.5 ppm)22. Electron-withdrawing groups (electron acceptors), on the other hand, decrease the ortho and para jr electron densities and lead to a deshielding relative to benzene (Sc(o,p) > 128.5 ppm). 

As can be seen in the different boundary conditions, the main effects of having ribs are electronic conductivity and transport of oxygen and water, especially in the liquid phase. In terms of electronic conductivity, the diffusion media are mainly carbon, a material that is fairly conductive. However, for very hydro-phobic or porous gas-diffusion layers that have a small volume fraction of carbon, electronic conductivity can become important. Because the electrons leave the fuel cell through the ribs, hot spots can develop with large gradients in electron flux density next to the channel. " Furthermore, if the conductivity of the gas-diffusion layer becomes too small, a... 

Differences in electronegativities (see Section 2.7) between carbon and the leaving group atom lead to bond polarity. This confers a partial positive charge on the carbon and facilitates attack of the nucleophile. As the nucleophile electrons are used to make a new bond to the carbon, electrons must be transferred away to a suitable acceptor in order to maintain carbon s octet. The suitable acceptor is the electronegative leaving group. 

In Section 6-5 an atomic model of benzene was discussed in some detail. Each carbon in the ring was considered to form three coplanar s/A-hybrid a bonds at 120° angles. These carbon-carbon and carbon-hydrogen a- bonds use three of the four valence electrons of each carbon. The remaining six carbon electrons are in parallel p orbitals, one on each of the six carbons. Each of... 

The bond polarity makes the carbon atom electron deficient and capable of accepting electrons from carbon electron donors (carbon-centered nucleophiles) into the a antibonding orbital. Population of die a orbital by electron donation weakens the bond to the leaving group. Ultimately the leaving group is cleaved from the... 

The chain retains its zigzag shape except at the double bond site and triple bond site, where the chain straightens out. To form these bonds, carbon electrons must be stretched into new locations. 

There was much affinity between Coulson and Barriol, not only because of the many subjects they shared, but also because of their similar way of proceeding and thinking. They both conceded a high value, in many respects, to the determination of dipole moments. Both worked on methane (CH4) and more particularly on the dipole moment of the C-H group, for which Coulson gave a direction when Barriol s simple model could not. [25] It is highly interesting to compare the way how the two authors express themselves to show that experience or physical and chemical evidence had to correct the false inferences or deductions that square in no way with reality the description of the carbon electronic structure fails to account for four equivalent bonds. We have to admit that the C-orbitals that are... 

In the Tm case, resonant photoemission proves that there is no significant hybridisation between the rare-earth 5d levels and the carbon electronic states thus the Tm ions have an essentially ionic interaction with their fullerene host. In the Gd case, the magnitude of the resonant enhancement of emission from the 4f levels signals the presence of hybridisation between the Gd valence levels and those of the carbon cage. However, this does not alter the conclusion as regards the Gd s trivalency and the transfer of essentially three electrons to the fullerene MOs. 

The chemical shifts of the carbon atom bonded to the nitro group qualitatively correlate with the carbon electron density. When several nitro groups are introduced a considerably worse additivity of 13C chemical shifts compared with benzene derivatives is observed. The experimental 13C shielding in dinitroimidazoles is smaller than that calculated by the additive scheme, and this points to a less clearly displayed conjugation between the nitro group and the imidazole ring compared with the benzene ring [339],... 

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. 

Our next example is CH4, methane. The 4 ontermost carbon electrons plus the 4 electrons from the hydrogen atoms total 8. These are distributed in four pairs as far apart as possible. In this case, the distribution is toward the comers of a tetrahedron (Figure 13.5a). Note that the electrons are not limited to a single plane. The addition of the hydrogen atoms produces a tetrahedral molecule (Figure 13.5b and c). 

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