Unsaturated sites

Other terms that mean the same thing as the index of hydrogen deficiency include elements of unsaturation sites of unsaturation and the sum of double bonds and rings... 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

Most solvents for hydroperoxides are not completely inert to radical attack and, consequendy, react with radicals from the hydroperoxide to form solvent-derived radicals, either by addition to unsaturated sites or by hydrogen- or chlorine-atom abstraction. In equation 15, S—H represents solvent and S is a solvent radical. 

Cross-linking agents or oligomers with multifunctional unsaturation sites include the following ... 

Butyl rubber and other isobutylene polymers of technological importance iaclude various homopolymers and isobutylene copolymers containing unsaturation achieved by copolymerization with isoprene. Bromination or chlorination of the unsaturated site is practiced commercially, and other modifications are beiag iavestigated. 

Recenl work has defined more carefully ihe nature of active sites. Metal surfaces are thought to contain three main types of sites terraces, ledges (or steps) and kinks, which correspond to one, two. and three coordinatively unsaturated sites of organometallic chemistry. These sites display differing activities toward saturation, isomerization, and CKChiingQ 7 J0,68 JO 1.103,104,105). 

PVC has been shown to have a head-to-tail structure. Typical experimental evidence for this is that when dissolved in dioxan and treated with zinc dust, it undergoes a Wurtz-type reaction to yield a product containing a small amount of chlorine and no detectable unsaturation. The alternative possible structure, the head-to-head arrangement, would yield unsaturated sites where adjacent chlorine atoms had been removed (Reaction 1.4). 

CoSx-MoSx/NaY exhibited doublet bands at 1867 and 1807 cm, accompanying a weak shoulder peak at ca. 1880 cm. These signals are apparently assigned to those of NO molecules adsorbed on Co sulfides. No peaks ascribable to e NO adsorption on Mo sulfide sites were detected at all. What is important in Fig.7 is that in CoSx-MoSx/NaY, coordinative unsaturation sites are present only on the Co sites in spite of the coexistence of the same amount of Mo sulfide species in the zeolite cavities. These results clearly support that the Co sites in CoSx-MoSx/NaY play major roles in the HYD and HDS reactions. 

Fundamental is that the atoms in the surface pha.se are not fully co-ordinated. These sites are often called Co-ordinatively Unsaturated Sites (CUS) . These sites chemisorb molecules because upon formation of bonds with the adsorbing molecules the Gibbs free energy is lowered. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

Additional information that can be derived from the molecular formula is the number of unsaturated sites. The total number in a molecule includes... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

We next contemplated whether the unsaturation site could be encompassed in the context of a properly positioned benzo linkage. We were intrigued to discover that excellent diastereoface selectivity was obtained in the aldol condensation of the Z-lithium enolate with the benzyl-substituted formyl moiety, entry g. 

Previous studies in conventional reactor setups at Philip Morris USA have demonstrated the significant effectiveness of nanoparticle iron oxide on the oxidation of carbon monoxide when compared to the conventional, micron-sized iron oxide, " as well as its effect on the combustion and pyrolysis of biomass and biomass model compounds.These effects are derived from a higher reactivity of nanoparticles that are attributed to a higher BET surface area as well as the coordination of unsaturated sites on the surfaces. The chemical and electronic properties of nanoparticle iron oxide could also contribute to its higher reactivity. In this work, we present the possibility of using nanoparticle iron oxide as a catalyst for the decomposition of phenolic compounds. 

The fact that surface structure, in particular steps and coordinatively unsaturated sites, has an influence on the state and reactivity of carbon monoxide is entirely in keeping with the empirical correlation (Fig. 6) between heat of adsorption, electron binding energies, and molecular state. Elegant studies by Mason, Somorjai, and their colleagues (32, 33) have established that with Pt(lll) surfaces, dissociation occurs at the step sites only, and once these are filled carbon monoxide is adsorbed molecularly (Fig. 7). The implications of the facile dissociation of carbon monoxide by such metals as iron, molybdenum, and tungsten for the conversion of carbon monoxide into hydrocarbons (the Fischer-Tropsch process) have been emphasized and discussed by a number of people (32,34). 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

As discussed in Chapter 9, there are a variety of low-volatility organics, some of which are unsaturated, which are found in or on particles in the atmosphere, and these would be expected to undergo heterogeneous reactions with O,. Consistent with this, de Gouw and Lovejoy (1998) report reaction probabilities of — 10 2—10 3 for O, on organic liquids containing unsaturated sites. 

Crosslinks can be controlled by the number of unsaturated sites in the polyester prepolymer. Theoretically if each molecule has only two reaction sites, then infinite, almost linear, chains could be obtained. Hence, average functionability and molecular weight distribution in the prepolymer are extremely important. Plasticizers can be used to advantage in adjusting the average properties of the binder as obtained in the solid propellant formulation. 

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