Univalent atoms

Additions include the attachment of two univalent atoms or groups (called addends) to an unsaturated system, e. g., to olefins, carbonyl groups, aromatic systems, carbenes, etc. (Rule 2.1). For example, the addition of hydrocyanic acid to the car-... 

The second important condition in C=C group is that each of the doubly linked carbon atoms is attached to two different univalent atoms or groups. 

In general, first bond the multicovalent atoms to each other and then, to achieve their normal covalences, bond them to the univalent atoms (H, Cl, Br, I, and F). If the number of univalent atoms is insufficient for this purpose, use multiple bonds or form rings. In their bonded state, the second-period elements (C, N, O, and F) should have eight (an octet) electrons but not more. Furthermore, the number of electrons shown in the Lewis structure should equal the sum of all the valence electrons of the individual atoms in the molecule. Each bond represents a shared pair of electrons. 

The missing electron can be provided by the formation of a covalent bond with a univalent atom, e.g. hydrogen or a halogen, in compounds like ClCo(CO)4 and HCo(CO)4. 

The principal reason for this is steric. A univalent atom is much more exposed to attack by the incoming radical than an atom with a higher valence. Another reason is that in many cases abstraction of a univalent atom is energetically more favored. For example, in the reaction given above, a C2Hs—H bond is broken (D = 100 kcal/mol, 419 kJ/mol, from Table 5.3) whichever pathway is taken, but in the former case an H—Cl bond is formed (D = 103 kcal/mol, 432 kJ/mol) while in the latter case it is a C2H5—Cl bond (D = 82 kcal/mol, 343 kJ/mol). Thus the first reaction is favored because it is exothermic by 3 kcal/mol (100 - 103) [13 kJ/mol (419 - 432)], while the latter is endothermic by 18 kcal/mol (100 - 82) [76 kJ/mol (419 - 343)].35 However, the steric reason is clearly more important, because even in cases where AW is not very different for the two possibilities, the univalent atom is chosen. 

The two propagation steps, 2 and 3, are SH2 substitutions. Note that the substitutions occur by attack of the radical on a terminal, univalent atom, in one case H, in the other halogen. This feature is characteristic of bimolecular radical substitution steps attack at multiply bonded sites tends to be by addition (Equation 9.65), and attack at saturated carbon occurs only in highly strained molecules. Thus since terminal singly bonded centers in organic compounds are nearly always hydrogen or halogen, it is at these atoms that most SH2 substitutions occur. 

Since most SH2 displacements occur at univalent atoms, tests of stereochemistry at the reaction center are ordinarily not possible. Substitutions do nevertheless occur at saturated carbons in highly strained rings.131 Halogen atoms attack cyclopropane (Equation 9.76) and other strained cyclic compounds,... 

The fact that the co-ordination number for so many elements is six, and is generally independent of the nature of the co-ordinated groups, has made A. Werner suggest that the number is decided by available space rather than affinity, and that six is usually the maximum number which can be fitted about the central atom to form a stable system. Consequently, the co-ordination number represents a property of the atom which enables the constitution of molecular compounds to be referred back to actual linkings between definite atoms. A molecular compound is primarily formed through the agency of secondary valencies and, just as primary valencies determine the number of univalent atoms or their equivalent which can be linked to a central atom, so secondary valencies determine the number of mols. which can be attached to the central atom. The secondary valency is often active only towards definite mol. complexes, and hence the formation of additive compounds with other mol. complexes does not occur. Accordingly, the number of secondary valencies which are active towards different molecules is not always the same. 

If H = number of univalent atoms (H, halogen), N = number of trivalent atoms (N, P), and C = number of tetravalent atoms, then... 

The first proposal that the valences of carbon were arranged tetrahedrally was made by Aleksandr Mikhailovich Butlerov (1828-1886) in 1862. In an attempt to explain the isomerism (now known to be illusory) of C2H5.H and CH3.CH3, he proposed as a model a tetrahedral carbon atom, each face of which was capable of attaching a univalent atom or group. He proceeded to calculate the number of isomers to be expected in the case of methane and its substitution products if two, three, or four of the valences of carbon (even if all bonded to hydrogen) were different in character. By assuming differences in carbon affinities he was able to explain the isomerism between methyl and ethyl hydride mentioned above. 

First General Principle, Let us consider a molecule of a chemical compound having the formula M A4 M being a simple or complex radical combined with four univalent atoms A, capable of being replaced by substitution. Let us replace three of them by simple or complex univalent radicals differing from one another and from M the body obtained will be asymmetric. 

Halogens (particularly chlorine) can be replaced by other electron-attracting functions snch as trifluoromethyl or cyano groups. In the antibiotic chloramphenicol, both the chlorine atoms of the dichloroacetic moiety and of the para-nitro-phenyl group yielded productive isosteric replacements (Table 15.6). Many other examples of univalent atoms or groups replacements are found in the chapter dealing with substituent effects (Chapter 20) and with quantitative structure-activity relationships (Chapter 23). 

A free radical, or univalent atom, is a chemical system like any other x and ri can be found for it, and Table 3.11 shows a listing of such data for a number of important radicals. The acid-base character of free radicals has been recognized for some time. It is common to speak of electrophilic radicals, such as Cl, and nucleophilic radicals, such as (CH3)C. Table 3.11 is a quantitative ordering of these descriptions. The alkali metal atoms could also be added to the list. These would be the most nucleophilic, or best electron donors. 

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