Odd electron species

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

Weak interactions may occur between molecules (intermolecular association) as well as within a molecule (intramolecular) for chalcogen-nitrogen ring systems. This behaviour is especially significant for odd electron species, e.g., [EsNa]" (4.14, E = S, Se) and [PhCNaEa] (4.15, E = S, Se), both of which are seven r-electron molecules. As mentioned in the previous section, it also occurs for the eight r-electron dithiatriazines 4.10... 

For such odd electron species (sometimes called free radicals) it is impossible to write Lewis structures in which each atom obeys the octet rule. In the NO molecule, the unpaired electron is put on the nitrogen atom, giving both atoms a formal charge of zero ... 

Fig. 4. Schematic drawing of the structure of the odd-electron species t-Bu3SiGaGa Si(t-Bu)3 [27] without H atoms...

In most cases, the direct reaction of N2 with O2 is slow under ambient conditions. It is the presence of numerous odd electron species (for example, OH, HO2, and RO2 radicals) that are photochemically produced and responsible for most of the oxidizing reactions of nitrogen species in the atmosphere. Some of the important reactions are shown below ... 

Wedekind and Stauwe" studied the oxidation of 3-substituted formazans and concluded that ease of oxidation depended on the steric effects of the 3-substituent. More recently, Hegoraty et al. 100 studied the reaction of formazans with bromine. It proceeds via an odd-electron species such as 52 favoring an electronic substituent effect (Scheme 5). The rate of reaction increases with electron-donating substituents. Similar conclusions have been reached using thalium(III) as the oxidant.101,102... 

As seen in the previous section, one characteristic of the triplet state is its paramagnetism. This alone would of course not suffice as a definition of the triplet since there are many odd-electron species that also exhibit paramagnetism but do not exist as triplets. Thus we might state that a triplet is a paramagnetic even-electron species. This still does not constitute a limiting definition since compounds containing even numbers of electrons may exhibit two, three, or even five distinct electronic levels. For example, when in a biradical the radical centers are separated by several carbon atoms as below, no interaction between the electron spins occurs and the radicals appear as two doublet states ... 

The naphthalene anion radical spectrum (Figure 2.2) provided several surprises when Samuel Weissman and his associates1 first obtained it in the early 1950s at Washington University in St. Louis. It was a surprise that such an odd-electron species would be stable, but in the absence of air or other oxidants, [CioHg]- is stable virtually indefinitely. A second surprise was the appearance of hyperfine coupling to the two sets of four equivalent protons. The odd electron was presumed (correctly) to occupy a it molecular orbital... 

The r + d algorithm produces integers for odd-electron ions and molecules, but non-integers for even-electron ions that have to be rounded to the next lower integer, thereby allowing to distinguish even- from odd-electron species. 

A somewhat similar example52, in equation 19, appears to be partially intermolecular since there is evidence for some extrusion of a p-methoxycumenyl free radical. The evidence suggested that ca 12% of the reaction is occurring by dissociation-recombination. This example, we note, is not typical since the p-methoxycumenyl group is much more subject to extrusion as an odd-electron species than (e.g.) methyl. 

Many reactions occur in which one organic compound is convened into another. The molecular details of the intemiediate steps by which compounds are converted into new products are called reaction mechanisms. The four broad classes of reaction mechanisms are cationic, anionic, free radical, and multicenter processes in which neither charged species nor odd electron species is involved. Examples of each type will be given, but many variations can exist within each type. Also, varying degrees of sophistication exist in our knowledge of the exact reaction pathways that organic compounds follow. The examples discussed show only the major steps involved. 

One of the relatively few simple odd electron species, nitric oxide is an intriguing heteronuclear diatomic and the parent member of the oxides of nitrogen. Like carbon monoxide, nitric oxide has a long and distinguished coordination chemistry, but unlike CO, it forms very few binary metal... 

In dimethylsulfoxide, the two starting cation radicals of Scheme 1-35 have pKa values of -20 and -25, respectively (Bordwell Cheng 1989). It is clear that both species give rise to the stabilized carboradicals after deprotonation. Electron-donating substituents increase the stability of the arene cation radical and render the odd-electron species less acidic for example, the cation radical of hexamethylbenzene has a pKa value of only 2 in AN (Ama-tore Kochi 1991). The cation radical of tris(bicyclopentyl)annelated benzene is not prone to proton loss, due entirely to the spin-charge location more or less in the aromatic (nodal) phase (Rathore Lindeman et al. 1998), Scheme 1-36. 

The CS, CR, and CSh processes in general may involve even or odd electron species (in the former case, the states may be either closed or open shell in character). Furthermore, the initial state can be either a ground (e.g., DA) or an electronically excited state e.g., a locally excited (LE) state (D A or DA ), as depicted for CS in Equations (3.71) ... 

Transition metal (TM) chemistry stands in contrast to this. Many compounds involve metal centres with partially filled d shells, and/or with one or several unpaired electrons. Therefore, it is not always straightforward to predict the orbital occupation pattern of a given stable compound. For intermediates on a reactive pathway, this is an even greater problem. This is also true for organometallic chemistry, despite the fact that many compounds obey the 18-electron rule and have closed-shell singlet ground states. Thus, there are many 16- or even 14-electron intermediates, odd-electron species [1], and polymetallic clusters and complexes for which the spin state is not readily predicted. 

The mechanisms for these reactions are not well understood, but there is evidence that some actually follow an Sn 1 pathway. Many others are known to involve radicals (odd electron species). We will not be concerned with the details of the various mechanisms here. 

The product of a reaction of a radical, an odd electron species, with a normal molecule, an even electron species, must produce an odd electron species, a radical, as one of the products. Although this type of reaction is not as energetically favorable as the reaction of two radicals, it is quite common because the collision of a radical with a normal molecule is more probable than the collision of two radicals. 

Limited electrochemical data for Mo(CO)(RC=CR)L2X2 complexes indicate a reversible reduction at -1.18 V versus SSCE for four 2-butyne derivatives while the one phenylacetylene complex studied exhibited a reversible reduction at -1.00 V (Table IX). These results are consistent with the model developed for Mo(CO)(RC=CR)(S2CNEt2)2 in that the more electron-rich dialkylalkyne would be expected to push the LUMO to higher energy than PhC=CH. These same complexes were characterized by an irreversible oxidation around +0.9 V (46). A preliminary report that [CpMo[P(OMe)3]2(MeC=CMe)]+ undergoes a reversible one-electron reduction at -1.04 V versus SSCE has been used to support the possibility of odd-electron species as reactive intermediates in this system (74). 

Finally, it should be stressed that organic electron transfers only rarely occur as isolated steps because of the high chemical reactivity of odd-electron species. Normally, they are part of multi-step mechanisms together with other types of elementary reaction, such as bond forming and breaking. In organic electrochemistry a useful shorthand nomenclature for electrode mechanisms denotes electrochemical (= electron transfer) steps by E and chemical ones by C, and it is appropriate to use the same notation for homogeneous electron-transfer mechanisms too. Thus, an example of a very common mechanism would be the ECEC sequence illustrated below by the Ce(IV) oxidation of an alkylaromatic compound (14-17) (Baciocchi et al., 1976,... 

Many reactions start slowly at first and then speed up, as reagents are consumed and products are made. This is particularly true of chain reactions, in which products are made, and some reactive intermediate is regenerated to "keep the chain going." Polymerizations, explosions, and nuclear bombs are examples of chain reactions. These chain reactions have precise components that must be identified in a successful reaction mechanism (1) chain initiation, (2) chain propagation, (3) chain termination. The propagation step in chemical reactions usually involves the formation of very reactive free radicals (odd-electron species, while the chain termination steps may involve radical-radical reactions, which shut off the supply of reactive intermediates. We return to the gaseous hydrogen-bromine reaction discussed above ... 

Addition of acid in acetonitrile to the hexa-anion gives only traces of [Ni32H(C)6(CO)36]5 (396), although the penta-anion is stable in acetone. Reduction (sodium/naphthalene, dmf) affords the hepta- and octa-anions, which are stable for hours in solution, while oxidation (C7H7+) regenerates the hexa-anion and ultimately gives the unstable penta-anion.577 The odd-electron species do not give ESR spectra. 

Woodward-Hoffmann orbital symmetry rules (Section 30.9) a series of rules for predicting the stereochemistry of pericyclic reactions. Even-electron species react thermally through either antarafacial or conrotatory pathways, whereas odd-electron species react thermally through either suprafacial or disrotatory pathways. 

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