Bonding in Organic Molecules

In many organic molecules, carbon uses only three or two of its four valence orbitals for a bonding. This leaves one or two Ip orbitals for T bonding. The main purpose of this chapter is to describe bonding in some of the important atomic groupings containing carbon with v valence orbitals. 

The structure of ethylene, C2H4, is shown in Fig. 8-2. The molecule is planar, and each carbon is bonded to two hydrogens and to the other carbon, With three groups attached to each carbon, we use a set of s-f hybrid orbitals for v bonding. 

Hybrid-Orbital Picture for cr Bonding of Carbon In Organic Molecules 

This leaves each carbon with a 2p orbital, which is perpendicular to the plane of the molecule. We form bonding and antibonding molecular orbitals with the Ipx valence orbitals, as follows  

Thus we have the same type of energy-level scheme for the t molecular orbitals of ethylene as we had for the c molecular orbitals of the hydrogen molecule. The diagram for C2H4 is shown in Fig. 8-4. 

The unique properties of carbon relate to its position in the periodic table. As a second-period element, carbon atoms are relatively small. Therefore, it can easily form the double and triple bonds that are rare in the compounds of related elements, such as silicon. As a Group IV element, carbon can form four bonds, which is more than the other second-period elements this characteristic gives it wide 

One simple and unusual hydrocarbon is cubane (CgHs), in which the eight carbon atoms are arranged at the corners of a cube. Recently, a derivative was made in which all eight hydrogen atoms were replaced by — NO2 groups. 

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Of the various geometric parameters associated with molecular shape, the one most nearly constant from molecule to molecule and most nearly independent of substituent effects is bond length. Bond lengths to carbon depend strongly on the hybridization of the carbon involved but are little influenced by other factors. Table 1.2 lists the interatomic distances for some of the most common bonds in organic molecules. The near constancy of bond lengths from molecule to molecule reflects the fact that the properties of individual bonds are, to a good approximation, independent of the remainder of the molecule. 

When the difference in electronegativities is great, the orbital may be so far over to one side that it barely covers the other nucleus. This is an ionic bond, which is seen to arise naturally out of the previous discussion, leaving us with basically only one type of bond in organic molecules. Most bonds can be considered intermediate between ionic and covalent. We speak of percent ionic character of a bond, which indicates the extent of electron-cloud distortion. There is a continuous gradation from ionic to covalent bonds. 

Most bonds in organic molecules, except C—H and C—C, may be hydrogenated or cleaved upon reduction by eam and by eh. 

This leads to modifications of the localized it orbitals. In benzene, for example, a Kekule localization which mixes the a and ir orbitals to form double banana bonds is preferred over the other equivalent ir localizations discussed. 60) In naphthalene a Kekule type structure is found similar to the one presently discussed, but different in that the (jtE2) are hybridized with corresponding o-CC bonding orbitals to form banana bonds, whereas the (ttC2 ) remains a pure jt orbital. 61 > While this is of interest in the discussion of the whole molecule, it is clear that certain intrinsic properties of the ir-electrons are more readily recognized by the localization which has been discussed here. We hope to discuss elsewhere localized orbitals involving a bonds in organic molecules. 

Stemhell, S. Rotation about Single Bond in Organic Molecules." In Dynamic Nuclear Magnetic Resonance Spectroscopy, Jackman, L. M. Cotton, F. A., Eds. Academic Press New York, 1975 pp. 163-201. 

Preferably, the new solvents are also expected to possess better stability or ability in interfacial chemistry on both anode and cathode materials so that the new electrolyte formulation can rely less on EC or they are expected to be less inflammable, as a major shortcoming of the linear carbonates is their low flash points (Tf) (Table 1). In the search for new solvents, fluorination has been adopted as a favorable approach to achieve improvements in these two aspects because the presence of C—F bonds in organic molecules is found to affect interfacial chemistry on carbonaceous anodes in a positive manner,and... 

B-68MI20400 R. L. Flurry, Jr. Molecular Orbital Theories of Bonding in Organic Molecules ,... 

BONDING IN ORGANIC MOLECULES. ATOMIC-ORBITAL MODELS... 

Bonding in Organic Molecules. Atomic-Orbital Models... 

Most bonds in organic molecules, however, are covalent bonds in which electrons are shared between two atoms. Sharing electrons is a way to enable each... 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

Stemhell, S. Rotation about Single Bonds in Organic Molecules, in ref. 79... 

The predominant interaction for a 2H spin system is the quadrupolar interaction, which couples the electric quadrupole moment of the 2H nucleus to its electronic surrounding. This interaction is a second-rank tensor Hq which lies approximately along the C-2H bond in organic molecules. Thus, in practice, 2H nuclei may be considered to be isolated. It shows that the 2H NMR formalism is similar to that of an isolated proton pair [8] ... 

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