Atom attack

Nitrite ion, an ainbident nucleophile in aprotic solvents, favors nitrogen atom attack on the double bond of various fluoro(halo)olefins. 2-Monohydroperfluoro-nitroalkanes can thus be produced [5] (equation 5)... 

In the reactions of nucleophilic addition to diacetylene, monoalkylhydrazines behave in two ways (71AKZ743). In an anhydrous medium at 40-50°C, the reaction with methyl- and ethylhydrazines proceeds in such a way that a more nucleophilic disubstituted nitrogen atom attacks the terminal carbon atom of diacetylene to form l-alkyl-3-methylpyrazoles (17), the content of isomeric 1-alkyl-5-methylpyrazoles being 15% according to GLC (71AKZ743 73DIS 77AKZ332). 

The Cl atom attacks methane and forms a methyl free radical plus HCI. The methyl radical reacts in a subsequent step with a chlorine molecule, forming methyl chloride and a Cl atom ... 

FIGURE 13.17 An alternative, two-step mechanism for the decomposition of ozone. In the first step, an energized ozone molecule shakes off an oxygen atom. In the second step, that oxygen atom attacks another ozone molecule. 

Two-step mechanism In the first step, an C)3 molecule is energized by solar radiation and dissociates into an O atom and an 02 molecule. In the second step, the O atom attacks another 03 molecule to produce two more 02 molecules (Fig. 13.17) ... 

After the reaction has been initiated, two new radicals are formed when one hydrogen atom attacks an oxygen molecule ... 

The oxygen atom, with valence electron configuration 2s12px12pv 12p J, has two electrons with unpaired spins (its Lewis symbol is -O-, which we abbreviate to -0-). Two radicals are also produced when the oxygen atom attacks a hydrogen molecule ... 

In any heterolytic reaction in which a new carbon-carbon bond is formed one carbon atoms attacks as a nucleophile and the other as an electrophile. The classification of a given reaction as nucleophilic or electrophilic is a matter of convention and is usually based on analogy. Although not discussed in this chapter, 11-12-11-28 and 12-14-12-19 are nucleophilic substitutions with respect to one reactant, though, following convention, we classify them with respect to the other. Similarly, all the reactions in this section (10-93-10-123) would be called electrophilic substitution (aromatic or aliphatic) if we were to consider the reagent as the substrate. 

The carbon atom with the unpaired eiectron is another free radical, so the stage is set for propagation. The unpaired eiectron on the carbon atom attacks the n bond of another molecule of ethylene, making a new C—C a bond and ieaving yet another carbon free radical ... 

Stationary concentration of adsorbed acceptor particles of O- and N-atoms on a film of zinc oxide is attained for the most part due to the competition between the chemisorbtion of particles and their interaction, i. e. mutual recombination on the adsorbent surface, and with free atoms attacking the adsorbed layer of the adsorbent from outside. 

When a radical or atom attacks a polar O—H or N—H bond, the reactant Y forms a hydrogen bond of the type O—H Y or N—H Y in polar solvents. The hydrogen bond shields the reactant and slows down the reaction regardless of of the type of radical (polar or nonpolar) attacking it (see Chapters 12 and 13). 

The key issues reduce to these (a) given that a phosphine sulfide is the product, at what positions do phosphine atoms attack in the transition state (b) What is the role for pyridine (c) Although 21 is formed from 20, with excess phosphine 23 results given that 21 -23 is slower than 20 23 and 23 -21, is 21 an intermediate along the pathway of the 20— 23 reaction ... 

Sn2 and SNAr Reactions In these reactions the metal atom attacks aliphatic or aromatic carbon bonded to X, respectively. A stronger nucleophilic metal as well as a better leaving group X (I>Br>Cl>F) facilitates, whereas steric hindrance in R slows these types of oxidative addition [193, 194]. SNAr reactions are favored by electron-withdrawing substituents Y in the case of the substrates 4-YQH4X [2], Sn2 [27, 29, 89, 117, 180, 181] and SNAr [31, 33, 62-67, 95, 100, 107-109] mechanisms have been suggested frequently for zerovalent d10 complexes such as [L M] (M = Ni, Pd, Pt L=tertiary phosphine =2,3,4). For example ... 

Hegedus proposed the probable course of the cyclization reaction, which follows a Wacker-type reaction mechanism. Coordination of the olefin to Pd(II) results in precipitate 110, which upon treatment with Et3N undergoes intramolecular amination to afford intermediate 111. As expected, the nitrogen atom attack occurs in a 5-exo-trig fashion to afford 112. Hydride... 

In analogy with reactions discussed with the examples of Schemes 6.12 and 6.13, 74 and 82 were trapped by enolates. Without exception, the enolate /3-carbon atom attacked the central carbon atom of the allene moiety with eventual formation of 3-methylenecyclobutanol derivatives as major products in most cases [64, 77]. This type of reaction is illustrated in Scheme 6.23 by two examples. 

The (phosphino)(silyl)carbene 2a readily and cleanly adds to benzalde-hyde and cinnamaldehyde, affording the oxiranes 27 and 28, as single diaste-reomers.40 These results strongly suggest a concerted mechanism, since the formation of a zwitterionic intermediate, such as 29, would result in the formation of a phosphoryl alkene via oxygen atom attack at the phosphorus center. Note that 2a does not react with ketones, which is in line with its nucleophilic character. 

Rate constants comparing reactions 10.1 and 10.4,10.7 and 10.8 or 10.3 andl0.5 illustrate an interesting point. When the hydrogen (or deuterium) atom attacks the... 

ZPEhh — ZPEHh) — (ZPEh — ZPEd)]reactants] = — [(ZPEHhh — ZPEdhh) ] because the atom attack is on a common diatomic (H2 in this case) and the zero point... 

Fig. 10.1 Zero point energy diagrams, (a) An H or D atom attacking an H2 molecule. The TST isotope effect is negative (inverse, kn > kn) because there is no zero point isotope effect in the ground state, and tunneling is ignored in the TST approximation, (b) An H atom attacking either an H2 or D2 molecule. The isotope effect calculated in the TST approximation is positive (normal, kH > kn) because the zero point isotope effect in the ground state is larger than that in the transition state.

Fig. 10.1 (continued) (c) An H atom attacking D2 or a D atom attacking HD. The TST isotope effect is negative (inverse) because the zero point isotope effect in the ground state is negative)... 

The chiral substrate trans- stilbene oxide (10.121) behaved differently, yielding meso-l,2-diphenylethane-l,2-diol (meso-10.122) [183], This means that, in both enantiomeric substrates, the enzyme does not discriminate between the two oxirane C-atoms, bringing about inversion of configuration at the C-atom attacked. Interestingly, the various stereoisomers of 1,2-diphenylethane-l, 2-diol can be interconverted metabolically by alcohol/ketone equilibria catalyzed by alcohol dehydrogenases. 

As is undoubtedly apparent, the kinetic route of NO formation is not the attack of an oxygen molecule on a nitrogen molecule. Mechanistically, as described in Chapter 3, oxygen atoms form from the H2—02 radical pool, or possibly from the dissociation of 02, and these oxygen atoms attack nitrogen molecules to start the simple chain shown by reactions (8.49) and (8.50) ... 

The N atoms could form NO, in part at least, by reactions (8.50) and (8.51), and the CN could yield NO by oxygen or oxygen atom attack. It is well known that CH exists in flames and indeed, as stated in Chapter 4, is the molecule that gives the deep violet color to a Bunsen flame. 

The H-Br molecule is the electrophile and is already polarised. Its atom attacks the double bond in propene, forming an intermediate carbocation. At the same time, the bond in the H-Br molecule breaks heterolytically and a Br" ion is generated. 

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