Saturated molecules

Any application of Eqs. (10.37)-(10.40) requires a solid knowledge of the appropriate set of reference bond energies sa, of the bond energy parameters a/ i and, finally, of the appropriate atomic charges. 

The data of Tables 10.2 and 10.3 give access to the parameters listed in Table 10.4. Concerning the sa energies, we ate henceforth in a position to benefit from detailed solutions obtained for the alkane molecules, but for the details regarding nitrogen- and oxygen-containing molecules we must refer to Chapters 15 and 16, respectively. 

With the second-order derivatives being usually omitted in Eq. (10.32) owing to the smallness of the values, the a i terms can be treated as constants. 

Calculation of Reference Bond Energies [Eq. (10.39)]. The parameters indicated in Table 10.4 are ready for use in the bond energy formula, Eq. (10.39). The following examples, in part based on detailed results given in Chapters 15 and 16 for nitrogen- and oxygen-containing molecules, illustrate the procedure and report the input data. 

TABLE 10.4. Reference Bond Energies (kcal/mol) and a/a Parameters 

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Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

Hydrocracking. Hydrogenolysis of a carbon—carbon bond in an organic molecule produces smaller, hydrogen-saturated molecules and is termed hydrocracking when 50% or more of the feedstock is converted into smaller molecules. 

One or both of tlie radicals formed in the initiation step abstracts a hydrogen atom from the parent compound to form a small saturated molecule and a new free radical. 

An elimination reaction is, in a sense, the reverse of an addition reaction. It involves the elimination of two groups from adjacent carbon atoms, converting a saturated molecule into one that is unsaturated. An example is the dehydration of ethanol, which occurs when it is heated with sulfuric acid ... 

The delocalization of molecular orbitals lies at the heart of modern chemistry. The concept that the tt orbitals of benzene or naphthalene cover the entire carbon skeleton promoted the successful understanding of conjugated molecules. The work of R. Hoffmann and others has proven that in saturated molecules [Pg.1]

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

Typical chain transfer reactions involve the abstraction of an atom from a neutral saturated molecule, which may be solvent or a chain transfer agent added to the polymerisation mixture specifically to control the final size and distribution of molar masses in the polymer product. The chain transfer reaction may be represented as in Reaction 2.7. 

Three- and four-membered ring molecules of nitrogen and phosphorus are investigated (Scheme 32) [80]. The unsaturated four-membered ring molecule, tetracyclo-phosphene 79 is less strained than the saturated molecule, tetracyclophosphane 15... 

If the above comparison of the properties of metal atoms with those of hydrogen deposited on the surface of a solid body (semiconductor) is correct, the effect of their adsorption on electric properties of semiconductor oxide films will be similar to features accompanying adsorption of hydrogen atoms. The atoms of hydrogen are very mobile and, in contrast to metal atoms, are capable of surface recombination resulting in formation of saturated molecules with strong covalent bond. 

Our studies [46] of interaction of hydroxyl radicals with the surface of oxide semiconductors show that our reasoning on other radicals is also applicable to these particles as their chemical activity is sufficiently high. With radicals possessing low chemical activity the situation changes drastically becoming close to the adsorption of valence-saturated molecules. 

A method which is similar to the Pariser-Parr-Pople method for the n electron system and is applicable to common, saturated molecules has been proposed by Pople 28>. This method is called the CNDO complete neglect of differential overlap) SCF calculation. Katagiri and Sandorfy 29> and Imamura et al. °) have used hybridized orbitals as basis of the Pariser-Parr-Pople type semiempirical SCF calculation. 

The frontier-electron density was used for discussing the reactivity within a molecule, while the superdelocalizability was employed in comparing the reactivity of different molecules 44>. Afterwards, the applicability of the frontier-electron theory was extended to saturated compounds 50>. The new theoretical quantity "delocalizability was introduced for discussing the reactivity of saturated molecules 60>. These indices satisfactorily reflected experimental results of various chemical reactions. In addition to this, the conspicuous behavior of HO and LU in determining the steric course of organic reactions was disclosed 44.51). 

The reactivity index is the conventional theoretical quantity which is used as a measure of the relative rate of reactions of similar sort occurring in different positions in a molecule or in different molecules. As has already been mentioned in Chap. 2, most reactivity indices have been derived from LCAO MO calculations for unicentric reactions of planar n electron systems as). The theoretical indices for saturated molecules have also been put to use B0>. In the present section the discussion is limited to the indices derived from the theory developed in the preceding sections, since the other reactivity indices are presented in more detail than the frontier-electron theory in the usual textbooks 65,86) jn this field. 

In some cases it is necessary to hydrogenate allylic compounds to saturated molecules without hydrogenolysis. It was found that nickel boride is a good catalyst for this purpose.46... 

A fourth success concerns the high degree of unsaturation found in the observed list of molecules. Very few highly saturated molecules are detected, and those that are saturated tend to be found in highly localized sources known as hot cores, where they are probably formed via H-atom hydrogenation on grain surfaces.54 The reason that ion-molecule reactions do not produce more saturated polyatomic species is, as discussed above, the small number of reactions between hydrocarbon ions and H2 that can occur rapidly. 

Substitution reactions usually occur with saturated molecules. A typical case is the reaction of chlorine and methane in which the hydrogen atoms of methane are replaced by chlorine in sequence—for example,... 

It is interesting to note that even a "saturated" molecule such as CH4 has a significant attraction for a proton. This demonstrates clearly that even a pair of electrons that is shared in a bond can be a binding site for H+. In general, the more acidic (or less basic) a compound is, the lower the value for its proton affinity. For example, the proton affinity for NH3 is 866kJ/mol, whereas that for PH3 is 774, in keeping with the fact that PH3 is the weaker base. 

A free atom or a radical possesses an odd number of electrons, except atoms of noble gases. T ypically, a valence-saturated molecule has an even number of electrons. Therefore, the reaction of a radical or an atom with a molecule will inevitably give rise to another atom or radical [1-3] ... 

Thus, free valence persists whenever an atom or a radical undergoes a unimolecular reaction or interacts with valence-saturated molecules (possessing an even number of electrons). This is a natural consequence of conservation of the number of electrons in chemical reactions. Therefore, free valence cannot persist when a radical reacts with a radical. Both reactants have an odd numbers of electrons, and the product formed has an even number of electrons, for example,... 

Table 8 presents structures observed for monocyclic dienes and polyenes with rings large enough to accommodate trans C=C double bonds. In a cyclodecadiene molecule strain-free carbon skeletons can only be derived when two double bonds are diametrically placed and have the same configuration (as, cis or trans,trans). Cw,cis-Cyclodeca-1,6-diene (1,6-CDD) may exist in twelve different conformations, and it is therefore noteworthy that it almost exclusively prefers one of these, namely the one indicated in Table 8. This conformer does not have the repulsive transannular HH interactions that destabilize the corresponding saturated molecule in all conceivable conformers.

Quoted, without citation, in Russell, "Specialism and Its Hazards," 11. To explain the combination of apparently saturated molecules, Alfred Werner introduced the theory of "coordination number" to supersede the old idea of "molecular compounds." Werner intended his theory to apply equally to organic and inorganic chemistry. Ibid., 15, 17. 

This approach has also been applied to saturated C(i) atoms. Due to the much lower polarizabilities of C-H and C-C single bonds, however, the effects are clearly smaller (82), though still detectable. Thus, some effects of substituents X on S-positioned carbon atoms in cyclohexanes and cholestanes were attributed to LEF effects of the C-X dipoles by Schneider and colleagues (76,77,85). The finding that only minor effects are to be expected in saturated molecules was confirmed by INDO-SCF calculations of electric-field effects on l3C chemical shifts of some model compounds performed by Seidman and Maciel (86). These authors conclude, further, that conformational studies on such systems are not promising (86). 

Ehnholt et al.8 produced a broad paper covering raw materials, and in-process and final-product measurements. While the uses are primarily in the food industry, the rancidity was often caused by microorganisms. One case involved off-flavor materials being produced in drying and curing ovens. Marker compounds (concomitant) released during the breakdown process (of saturated and unsaturated compounds) were nonenal, decenal, and octenone for the unsaturated aldehydes and ketones, and nonanal, decanal, and octanone for the saturated molecules. A 10-m folded path gas cell was used with an FT-IR for measurements down to 1 Lig/m3. 

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