Basic Site Requirements

This chapter is brief but is nonetheless important for stakeholders, residents, and regulators. Numerous examples of the types of actual recommendation documents are included in Part II. These examples are more useful than just listing the types of considerations because the formats can be examined for applicability to the reader s site. 

It is important for the stakeholders to make detailed recommendations for the remediation of a chemical or explosive weapons site. Obviously, these must be tailored by the archival research and facts known about the particular site. However, a few routine recommendations follow. 

The groundwater should likewise be sampled by strategically placed monitoring wells. Surface water and sediments should also be sampled. Sediments in bogs or intermittent streams may provide useful data. Finally, a health survey of nearby residents should be conducted to determine if there is any elevated incidence of any disease processes related to the hazardous substances found in the sampling. 

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Mesoporous materials can also be used as base catalysts— for example, when the negative charge on a mesoporous aluminosilicate is compensated by metal ions such as sodium (Na) or cesium (Cs). Amines anchored on mesoporous silica can also be used as base catalysts. Successful utilization of basic sites requires the total absence of acid sites since the two functions tendto drive reactions through different pathways. There are, however, some cases where adjacent acid/base sites are desirable. 

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

Lewis acid catalysis enormously enriches the scope of Diels-Alder reactions, but it is limited to reagents containing Lewis basic sites, i.e. functional groups with lone pairs such as carbonyl, amino, ether or nitro close to the reaction centre. As we have seen in the discussion about the FMO aspects of Lewis acids, the major reason for catalysis is the reduction of the HOMO-LUMO gap. In case of Diels-Alder reactions with normal electron demand, it follows that the coordination of the Lewis acid lowers the LUMO energy of the dienophile. Such interactions are only possible if there is a spatial proximity or an electronic conjugation between the coordinated Lewis basic site and the reaction centre. Fortunately, in nearly every Diels-Alder reaction one of the reagents, mostly the dienophile, meets this requirement. 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

The pre-treatment temperature of MgO to give the maximum activity is higher by about lOOK for the double bond isomerization of VBH than for but-1-cue isomerization. Taking into account that the allylic hydrogen bonded to a tertiary carbon atom in VBH is more difficult to abstract than a secondary allylic hydrogen in but-1-ene, we infer that the results suggest that stronger basic sites are required for double bond isomerizaton of VBH than for double bond isomerization of but-1-cue. 

The oxonium ylide mechanism requires a bifunctional acid-base catalyst. The validity of the oxonium ylide mechanism on zeolites was questioned459,461,464 because zeolites do not necessarily possess sufficiently strong basic sites to abstract a proton from the trimethyloxonium ion to form an ylide. It should, however, be pointed out, as emphasized by Olah,447,465 that over solid acid-base catalysts (including zeolites) the initial coordination of an electron-deficient (i.e., Lewis acidic) site of the catalysts allows formation of a catalyst-coordinated dimethyl ether complex. It then can act as an oxonium ion forming the catalyst-coordinated oxonium ylide complex (10) with the participation of surface bound CH30 ions ... 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

These relations seem to be valid for the dehydration of primary alcohols, but secondary and tertiary alcohols may need other combinations of acidic and basic sites. It has been observed that the dehydration of tert-butanol was more sensitive to the presence of strongly acidic sites than the reaction of methanol, but both processes required basic sites [8]. All this is in accordance with the dynamic model of elimination mechanisms presented in Sect. 2.1, which allows transition from El to E2 or further to ElcB according to the structure of the reactant and the nature of the catalyst. 

Alumina is a catalyst which shows intermediate behaviour and over which the concerted E2 mechanism is accepted [66] with slight transition either to the E2cA or E2cB mechanisms according to the structure of the reactant. Salts of strong acids and bases also show similar intermediate properties. The concerted or partly concerted mechanisms require two-site adsorption and because the mechanisms are ionic, the active centre must consist of a pair of an acidic and a basic site. Metal salts fulfil this... 

As one of the characteristics of t t[CH2NH] replacement is the introduction of a new basic site, protection of the secondary amine can be required during peptide elongation. However, it has been shown by Coy and co-workers that the tp[CH2NH] surrogate is generally resistant to further acylation, except for the incorporation of the Gly residue, probably due to a steric factor J7 8 In the case of pseudopeptide libraries, protection of the secondary amine seems to be important for affording products in better yields and purity.[9]... 

Some zeolitic and non-zeolitic molecular sieve catalysts are claimed to be capable for ortho- and para-selective alkylation using olefin as alkylating agent (refs. 1,2). Zeolite catalysts are less active and selective in the methylation of aniline by methanol (refs. 3,4). Reaction is usually carried out with a large excess of methanol since a large fraction of the alcohol decomposes without participating in the alkylation. Numerous N- and C-alkylated aniline derivatives appear in the reaction product. It was found that N-alkylation requires basic sites while C-alkylation occurs mainly on acidic sites (refs. 5-7). 

Unless a substrate or reagent contains an acidic or basic site, the conditions for most radical reactions are neutral. Thus, ionic side reactions such as base-catalyzed epimerization are rarely a problem. While radical reactions are typically conducted at temperatures above ambient, this is often solely for experimental convenience most commercially available initiators require heating to generate radicals. Many radical reactions should succeed at lower temperatures provided that the chain is maintained (in chain methods) or that the rate of generation of radicals is sufficiently rapid (in nonchain methods). Low temperature initiators are available.30,31... 

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