Carbon structure factors influencing rates

MgCOj has the calcite structure [2]. Britton et al. [30] concluded that the decomposition of magnesite (MgCOj) was an inter ce process, initiated at boundary surfaces and thereafter advancing inwards. The value of E found (150 kJ mol between 813 and 873 K) was appreciably greater than the enthalpy of dissociation (101 kJ mol ). They considered the possible influences on the reaction rates of factors such as self-cooling, the recombination process at the reaction interface, the restriction of escape of carbon dioxide, and the rate of the nucleation step. 

Many factors influence the ability of reinforced concrete to resist carbonation induced corrosion. As the carbonation rate is a function of thickness, good cover is essential to resist carbonation. As the process is one of neutralizing the alkalinity of the concrete, good reserves of alkali are needed, that is, a high cement content. The diffusion process is made easier if the concrete has an open pore structure. On the macroscopic scale this means that there should be good compaction. On a microscopic scale well cured concrete has small pores and lower connectivity of pores to the CO2 has a harder job moving through the concrete. Microsilica and other additives can block pores or reduce pores sizes. 

Tatlow and his co-workers conducted an extremely comprehensive programme of syntheses and structure derivations of a series of fluorinated cycloalkanes [24], and concluded that the reactivity of the system, as well as the orientation of the cycloalkene produced, are similarly influenced by electronic factors which have been outlined in the preceding sections of this chapter. Anti elimination is generally the more favourable process but conformational effects may make the synjanti rates nearly comparable. Elimination from the cyclohexanes 6.16A and 6.16B illustrates the balance between electronic and conformational effects [25]. Anti elimination is possible from 6.16A, involving removal of fluoride from >CHF rather than >Cp2 since in this case electronic (the carbon-fluorine bond in CFH is weaker than in CF2) and conformational effects (H and F are anf -periplanar) are in concert (Figure 6.16). In contrast, anti elimination from 6.16B can only occur with elimination of fluoride from the more stable >CF2 position and therefore anti and syn eliminations occur together. 

The presence of natural solids can significantly modify the rates of transformation in aqueous systems—relative to the rates observed in homogeneous solution—for many pesticide compounds, but may have little effect on others (Barbash and Resek, 1996). Factors that can influence the rates and mechanisms of transformation of pesticide compounds at the water/solid interface include the structure of the compound of interest (e.g.. Torrents and Stone, 1991 Baldwin et al., 2001), the composition and surface structure of the mineral phase (e.g., Kriegman-King and Reinhard, 1992 Wei et al., 2001 Carlson et al., 2002), the solid-phase organic-carbon content (e.g., Wolfe and Macalady, 1992), and the characteristics, health, and size of the resident microbial community. 

The mechanism of the reaction is well-known. The first step is formation of a carbanion, followed by nucleophile addition to the carbonyl carbon atom halo-hydrin alcoholates are produced finally, ring-closure takes place by intramolecular substitution. The stereochemistry of the reaction is much disputed the reason why a unified viewpoint has not emerged is that the configuration of the end-product is influenced by the structure of the starting compound (including steric hindrance), the base employed, and solvation by the solvent, sometimes in an unclear manner. The stereochemical course of the reaction is controlled by the kinetic and thermodynamic factors in the second step the structure of the oxirane formed is decided by the reversibility of the aldolization and the reaction rate of the ring-closure. 

Some characteristics of initiators used for thermal initiation arc summarized in Table 3.1. These provide some general guidelines for initiator selection. In general, initiators which afford carbon-ccntcrcd radicals e.g. dialkyldiazcncs, aliphatic diacyl peroxides) have lower efficiencies for initiation of polymerization than those that produce oxygen-centered radicals. Exact values of efficiency depend on the particular initiators, monomers, and reaction conditions. Further details of initiator chemistry are summarized in Sections 3.3.1 (azo-compounds) and 3.3.2 (peroxides) as indicated in Table 3,1. In these sections, we detail the factors which influence the rate of decomposition i.e. initiator structure, solvent, complexing agents), the nature of the radicals formed, the susceptibility of the initiator to induced decomposition, and the importance of transfer to initiator and other side reactions of the initiator or initiation system. The reactions of radicals produced from the initiator arc given detailed treatment in Section 3.4. 

In the early years, service-life models for reinforcement corrosion basically followed the square-root-of-time approach or shght modifications, such as the example illustrated in Figure 11.2. Based on empirical data on the rate of carbonation or chloride ingress, a minimum cover depth was determined that was expected to delay the onset of corrosion for a required period. Following this approach, more and more data were collected from existing structures and exposure sites, and the influence of various factors such as cement type and local environment became more clear [1,13). 

---