Sensitivity to preparation conditions

Overall, the metalloid aluminium and gallium clusters bear some relationship to fragments of the bulk metals, protected and stabilised by the ligands on the surface, and as such can be considered intermediate between molecules and metals. They show a marked sensitivity to preparative conditions and the steric influence of the ligands on the surface. Spectroscopic studies and theoretical calculations on the clusters are not discussed here but are summarised in recent review articles, and these additional studies provide a... 

A number of main group and transition metal dithiocarboxylates have been described. They are commonly prepared by metathesis between alkali metal dithiocarboxylates and metal halides. Heavy metal derivatives usually precipitate from aqueous solutions, but are soluble in organic solvents and can be solvent extracted. The alkali metal, ammonium, phosphonium, and arsonium salts are water soluble.300 The composition of metal complexes is sensitive to preparation conditions and is frequently pH dependent. 

Due to the number of experiments necessary in such studies which take typically times of several hours to several days, there is a strong incentive to work on acceleration of synthesis experiments in zeolite science. On the other hand, there are also major obstacles to overcome in such an endeavour. Zeolite syntheses are often carried out at elevated pressure, reaction media are typically corrosive, precursor solutions and the synthesis gel itself may be highly viscous and difficult to handle, the synthesis is very sensitive to preparation conditions, such as sequence of reagent addition, stirring (difficult for small volumes) may be necessary for some formulations, aging conditions may differ from batch to batch, if automated sequential preparation is chosen, the work-up involving many steps is complex, and the resulting materials are often not phase-pure and difficult to characterize. Nevertheless, in spite of these problems some of the earliest examples for the synthesis of catalytically relevant solids in parallelized and - some points -automated equipment were reported for zeolites [4]. 

In Section 1.3 it was noted that the energy of adsorption even for a perfect crystal differs from one face to another. An actual specimen of solid will tend to be microcrystalline, and the proportion of the various faces exposed will depend not only on the lattice itself but also on the crystal habit this may well vary amongst the crystallites, since it is highly sensitive to the conditions prevailing during the preparation of the specimen. Thus the overall behaviour of the solid as an adsorbent will be determined not only by its chemical nature but also by the way in which it was prepared. 

The method of preparation of a support material has a tremendous effect on its properties (11). For example, zeoHtes, which are highly stmctured aluminosihcates, are known to be extremely sensitive to the conditions employed both during and after crystallization (12). Also, when siUca—titania is precipitated by a coprecipitation method using ammonia, in which localized hydroxide ion gradients are estabUshed by the precipitation process itself, the product is much more acidic than when it is precipitated using urea, which suppHes hydroxide ion slowly and uniformly during precipitation (13). 

Convenient syntheses of vinyl fluorides are of synthetic interest, fhe conjugate base of fluoromethyl phenyl sulfone reacts with carbonyl compounds to provide P-tluoro alcohols, which are used to prepare terminal vinyl fluorides [25] (equation 23) (Table 9) This reaction offers an alternative to the Winig reaction, which may be very sensitive to reaction conditions. 

The photocatalytic decomposition of TCE was carried out and the results were shown in Fig. 3. Photocatalytic reaction has been found to be less sensitive to the conditions such as the concentration of trichloroethylene, and the stoichiometric decomposition (Cl2C=CHCl + 3/2O2 + H2O 2CO2+ 3HC1) proceeds with fairly good reproducibility by prepared photocatalysts. 

Ethylbenzoic acid was converted to the acid chloride, which was treated with AICI3 and benzene to give 4-ethylbenzophenone in 90% yield overall. Condensation with ethyl cyanoacetate afforded ethyl 4-ethyl-a-cyano-B-phenylcinnamate as an essentially 50/50 mixture of the Z- and E-isomers. The yield of the condensation was highly sensitive to reaction conditions, and was optimized at 75% with portionwise addition of the ammonium acetate catalyst. Bromination and dehydrobromination as described earlier then completed the preparation. The overall yield of ethyl 4-vinyl-a-cyano-p-phenylcinnamate was 20%. 

Bromide 280 (derived by bromination of silyl enol ether 270) undergoes both zinc- and cerium-mediated cleavage under mild and essentially neutral conditions, and was used to prepare the nucleoside-containing C-glycoside 282 (Scheme 73) [ 112,113], The aldehyde 281 used in this transformation was exceptionally sensitive to basic conditions which completely precluded use of a conventional enolate obtained by deprotonation of ketone 265 (Sect. 4.3.1). 

Thioamide enolates are also interesting substrates for the stereoselective aldol-type reactions. The aldol stereochemistry is very sensitive to the conditions of preparation of magnesium thioamide enolates and it generally gives different results depending on the procedure used. Illustrations of some aspects of the reactivity are provided in the examples presented below. 

This procedure is general for the conversion of epoxides to dichlorides with inversion of configuration at each of the two carbons and, in effect, provides a method for the cis-addition of chlorine to a double bond.2 ci.v-1,2-Dichlorocyclohcxane has also been prepared from 1,2-epoxycyclohexane and sulfuryl chloride,3 but the stereospecificity of the reaction appears to be extremely sensitive to reaction conditions, and the yield is lower than that obtained by the method described here. Other methods give cis-1,2-dichlorocyclohexane contaminated with considerable amounts of the trans -isomer. This method has been used to convert cis- and trans-4,5-epoxyoctanes to meso- and d/-4,5-dichlorooctanes, respectively, and trans-7,8-epoxyoctadecane to fhreo-7,8-dichloro-octadecane. These conversions were carried out on smaller amounts of material, and the products were purified by column chromatography on silica gel. Yields were 51-63%. 

The materials used in most current research are irregular mats of highly crystalline fibrils with diameters of around 10 nm, so that the films are characterised by a very high surface area (around 60 m2 g-1), a problem in some potential applications and an asset in others. The morphology of polyacetylene is sensitive to the conditions of preparation and to ageing and was the subject of much heated discussion in the early development of polyacetylene. 

Synthesis of complex oxides from metal alkoxides includes the preparation of the initial solution, hydrolysis, drying, and thermal treatment of the product. Despite the apparent simplicity and versatility of the scheme, the properties of oxides are extremely sensitive to the conditions of their preparation. The commonly used precursor for oxides preparation is a solution of M(OR) in organic solvents. The heterogeneous precursor usually affects the homogeneity of the future oxide phase. 

It has been known for many years that the adsorptive and catalytic properties of chromium oxide gels are very sensitive to the conditions of preparation, storage and heat treatment (Burwell et al., 1960 Deren et al., 1963 Carruthers and Sing, 1967 Baker etal., 1970,1971). 

Molecular complexes, such as the complex formed between poly(N-vinylcaibazole) and 2,4,7-trinitro-9-fluorenone, and dye-polymer aggregates were widely used as generation materials in many early applications. Since these materials are not infrared sensitive, there has been increasing emphasis on pigments. The more widely studied are various azo, phthalocyanine, squaraine, and peiylene diimide derivatives. A common feature of all of these materials is that they are polymorphic and exist in many different crystal forms. The properties are thus very sensitive to the conditions used in their preparation. Further, the sensitivity of these materials is strongly field dependent as well as dependent on the transport material. For a review of generation materials, see Law (1993). 

---