Dissociation, reverse

Chemical Properties. The chemistry of the sulfur chlorides has been reviewed (141,142). Sulfur monochloride is stable at ambient temperature but undergoes exchange with dissolved sulfur at 100°C, indicating reversible dissociation. When distilled at its atmospheric boiling point, it undergoes some decomposition to the dichloride, but decomposition is avoided with distillation at ca 6.7 kPa (50 mm Hg). At above 300°C, substantial dissociation to S2 and CI2 occurs. Sulfur monochloride is noncombustible at ambient temperature, but at elevated temperatures it decomposes to chlorine and sulfur (137). The sulfur then is capable of burning to sulfur dioxide and a small proportion of sulfur trioxide. 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

BrNO dissociates reversibly into NO and Br, the extent of dissociation being 7% at room temperature and 1 atm pressure. A similar reversible dissociation occurs with CINO at higher temperatures. 

An equilibrium involving reversible dissociation of the N-N bond in N-nitrobenzotriazole 299a has not been directly registered, although occurrence of the ring-opened form 299b was inferred on the basis of isolation of its derivative 300 (Scheme 112) [96CHEC-II(4)1]. 

The general equation for the reversible dissociation of a carboxylic acid is... 

Accumulatory pressure measurements have been used to study the kinetics of more complicated reactions. In the low temperature decomposition of ammonium perchlorate, the rate measurements depend on the constancy of composition of the non-condensable components of the product mixture [120], The kinetics of the high temperature decomposition [ 59] of this compound have been studied by accumulatory pressure measurements in the presence of an inert gas to suppress sublimation of the solid reactant. Reversible dissociations are not, however, appropriately studied in a closed system, where product readsorption and diffusion effects within the product layer may control, or exert perceptible influence on, the rate of gas release [121]. 

The first and rate-determining step involves carbon monoxide dissociation from the initial pentacarbonyl carbene complex A to yield the coordinatively unsaturated tetracarbonyl carbene complex B (Scheme 3). The decarbonyla-tion and consequently the benzannulation reaction may be induced thermally, photochemically [2], sonochemically [3], or even under microwave-assisted conditions [4]. A detailed kinetic study by Dotz et al. proved that the initial reaction step proceeds via a reversible dissociative mechanism [5]. More recently, density functional studies on the preactivation scenario by Sola et al. tried to propose alkyne addition as the first step [6],but it was shown that this... 

Since the synthesis temperatures are higher than the dissociation temperatures of the phases that are formed (at a pressure of lO N m ), it is necessary to react the alkali metal with boron under metal pressure in excess of that defined by Eq. (a), in sealed vessels. The alkali metal is present as a liquid in equilibrium with the vapor phase, the pressure of which is determined by the T of the coldest point. This pressure (greater the more volatile the metal) favors the synthetic reaction relative to the reverse dissociation reaction. 

These dehydrogenases use nicotinamide adenine dinucleotide (NAD ) or nicotinamide adenine dinucleotide phosphate (NADP )—or both—and are formed in the body from the vitamin niacin (Chapter 45). The coenzymes are reduced by the specific substrate of the dehydrogenase and reoxidized by a suitable electron acceptor (Figure 11-4). They may freely and reversibly dissociate from their respective apoenzymes. 

Unlike such unstable intermediates, the first, rare example of reversible dissociation of a carbon-carbon a bond into a stable carbocation and carbanion was reported for a nitro-dicyano compound (20) prepared from trimethyl- and triphenyl-cyclopropenylium tetrafluoroborate ([4" ]BF4 and [5 JBFJ) with the potassium salt of p-substituted-phenylmalononitrile anions (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985). Other ionically dissociative malononitrile derivatives have been prepared from such carbocations as the tropylium [S ] (Arnett and Troughton, 1983) and the tris(p-methoxyphenyl)methylium [93 j (Arnett and Troughton, 1983) ions. 

Spectrophotometric studies revealed that these hydrocarbons reversibly dissociate in DMSO and in 5 95 3-methylsulfolane-sulfolane (Miyabo et al.. 

Similarly, three dissociative hydrocarbons [25-2], [99-2] and [100-2], whose structures were determined from their nmr spectra, have been prepared from substituted cyclopropenylium ions phenyl- [25 ], m-methylphenyl- [99 ] and m,m -dimethylphenyl- [100 ] dicyclopropylcyelo-propenylium ions (Seheme 2). These hydrocarbons reversibly dissociate (24) into their ionic moieties in DMSO (Takeuchi et al., 1993). 

To mimic the square-pyramidal coordination of iron bleomycin, a series of iron (Il)complexes with pyridine-containing macrocycles 4 was synthesized and used for the epoxidation of alkenes with H2O2 (Scheme 4) [35]. These macrocycles bear an aminopropyl pendant arm and in presence of poorly coordinating acids like triflic acid a reversible dissociation of the arm is possible and the catalytic active species is formed. These complexes perform well in alkene epoxidations (66-89% yield with 90-98% selectivity in 5 min at room temperature). Furthermore, recyclable terpyridines 5 lead to highly active Fe -complexes, which show good to excellent results (up to 96% yield) for the epoxidation with oxone at room temperature (Scheme 4) [36]. 

Co (nh3)5 H2 + Cl [Co (nh3)5 cif- + H2 O A proposed mechanism for this displacement starts with rapid reversible dissociation of water from the complex ... 

Observations on the polymerization of readily polymerizable vinyl monomers such as styrene, vinyl chloride, and butadiene date back approximately to the first recorded isolation of the monomer in each case. Simon 2 reported in 1839 the conversion of styrene to a gelatinous mass, and Berthelot applied the term polymerization to the process in 1866. Bouchardat polymerized isoprene to a rubberlike substance. Depolymerization of a vinyl polymer to its monomer (and other products as well) by heating at elevated temperatures was frequently noted. Lemoine thought that these transformations of styrene could be likened to a reversible dissociation, a commonly held view. While the terms polymerization and depolymerization were quite generally applied in this sense, the constitution of the polymers was almost completely unknown. 

Another possibility to reach high storage capacities is the utilization of reversible chemical reactions. An ideal reaction scheme is a reversible dissociation of a solid or liquid compound AB to a solid or liquid component A and a gaseous component B. 

The rate equation for the reversible reaction of E and I must reflect both the forward (association) and reverse (dissociation) reactions ... 

For other electrolytes, now termed weak, factor i has non-integral values depending on the overall electrolyte concentration. This fact was explained by Arrhenius in terms of a reversible dissociation reaction, whose equilibrium state is described by the law of mass action. 

A recent discovery that RNA will act as a self-catalyst, called a ribozyme, leads to a simple three-step model for self-replication - this might include a surface. In the model (Figure 8.18), the template molecule T is self-complementary and is able to act as an autocatalyst. In the first step, it reversibly binds with its constituents A and B, forming the termolecular complex M. The termolecular complex undergoes irreversible polymerisation and becomes the duplex molecule D. Reversible dissociation of D gives two template molecules T, which can initiate new replication. The model preserves the order of the moieties on the template (the direction of the arrow) and the backbone, which may be on the surface... 

The mechanism is dominated by the remarkable stability of the Fe( 72-H2) bond, which is one of the most stable 72-H2 complexes reported in the literature [8, 10]. Remarkably, the free coordination site for the incoming alkyne is provided by the reversible dissociation of one of the phosphine moieties of the PP3 ligand rather than dissociation of the dihydrogen ligand (see Scheme 14.1). The coordinated alkyne subsequently inserts into the Fe-H bond and the emerging Fe-vinyl bond is... 

Kinetic studies of the hydrogenations of BT to DHBT catalyzed by [Rh(PPh3)2(cod)]PF6 [56] and [Ir(PPh3)2(cod)]PF6 [57] indicated the hydride migration yielding the dihydrobenzothienyl intermediate as the rate-determining step. In contrast, the rate-determining step of the reaction catalyzed by [RuH(tri-phos)]+ was shown to be the reversible dissociation of DHBT from the metal center [59],... 

---