Rare preparation

Pure hydroxylamine is a crystalline solid of low melting point (306 K) but is rarely prepared because it decomposes above 288 K... 

Iodine is rarely prepared in the laboratory the method used is the oxidation of an iodide by manganese(IV) oxide and sulphuric acid, for example with sodium iodide ... 

Camphene is extremely difficult to separate in the solid condition from essential oils, and it may therefore be taken for granted that natural camphene is rarely prepared in the pure condition. The figure given above for the specific rotation is for a sample artificially prepared from pinene hydrochloride. 

This is an extremely important area. Commercial polymers are very rarely prepared or used without additives and they often contain a combination of additives. These improve processability of the polymer, its durability, service life under adverse conditions (e.g., temperature, UV light and various chemical environments), strength (e.g., using reinforcing fillers), appearance (colorants), etc. A recent use of additives is to facilitate reprocessing or recycling or, alternatively, to promote decomposition (which is often difficult because the stability is inherent in the polymer s chemical structure). (The analysis of additives in polymers is the subject of Chapter 14.)... 

Olah and Lin reported obtaining pure TNB (2) in 50 % yield from the nitration of m-dinitrobenzene (32) with nitronium tetrafluoroborate in fluorosulfuric acid at 150 °C higher yields of TNB can be obtained by reducing reaction time but the product then contains some unreacted m-dinitrobenzene. A solution of dinitrogen pentoxide in sulfuric acid at 160 °C is also reported to effect the conversion of m-dinitrobenzene to TNB. TNB is rarely prepared via these routes and is best obtained from TNT via an indirect route (Section 4.9). 

Ethanol is commonly obtained in a 95 per cent and an absolute grade. The former has the composition of the azeotrope with water (bp 78.2°) and except for the water is quite pure. If absolute ethanol (bp 78.3°) is required, it may be prepared by heating the 95 per cent ethanol to reflux with calcium oxide for several hours and then distilling. However, absolute ethanol is available at a reasonable cost and is rarely prepared in a laboratory. The commercial absolute ethanol often contains a small amount of benzene, since it is prepared from 95 per cent ethanol by removing the water through the ternary azeotrope of benzene-ethanol-water (bp 65°). The absolute ethanol is therefore not suitable for use as a solvent for ultraviolet spectroscopy, and the 95 per cent ethanol is usually used. 

Despite their ubiquitous distribution in plants, manmade glucosidic derivatives of alcohols or phenols are rarely prepared in medicinal chemistry. An old-timer is the sedative-hypnotic a-chloralose (Figure 38.24) which is presently used only as a surgical anesthetic for laboratory animals. 

Mixed condensations of two ketones are rarely preparatively useful. The factors that determine which of the four possible products will predominate include the relative concentrations and relative acidities of the two ketones, the relative rates of addition of the two carbonyl groups, and the relative rates of dehydration of the intermediate aldols. The examples shown in equations (35) and (36) demonstrate that only minor structural changes can greatly affect the product distribution. 

Studies on low and high mass cholesteric phases have shown that the compound may form a focal-conic texture if no displacement is made on the cover slip when the sample is heated up or cooled down to the cholesteric phase. The focal-conic texture is similar to that of a smectic A, but the particles are usually too small to identify except for rare preparations. If the cover slip is displaced the focal-conic texture will give way to a homeotropic one with the optic axis normal to the plane of the preparation. The cholesteric homeotropic film reflects vivid colors having wavelength A decided by the De Vries (1951) expression A = pn where p is the pitch... 

Carboxylic acids are rarely prepared in practice from carbonyl chlorides. This hydrolysis occurs when the acid halide is dropped with vigorous stirring into ice-cold water or ice-cold sodium carbonate solution alcoholic potassium hydroxide is required only in exceptional cases. Aromatic acid halides are more stable than their aliphatic counterparts. 

Precocene II (125) and other chromenes have been synthesized and screened for their effect on the cloth moth. Precocene II has larvicidal properties. A trifluoro-compound (126), related to precocene, has been synthesized from 3,4-dimethoxyphenol, trifluoroacetone, and allylmagnesium bromide. Several natural chromenes, for example 6-acetyl-2,2-dimethyl-5-hydroxy-8-methoxy-2//-chromene, have been synthesized from the appropriate substituted 2-hydroxyacetophenone and either 2-methylbut-3-en-2-ol or 3-chloro-3-methylbut-l-yne. 2//-Chromenes are rarely prepared from chromanones, but a direct conversion of substituted 2,2-dimethylchromanones, e.g. (127), in high yield, into 2H-chromenes, e.g. (128), has been achieved by the action of PCls. °... 

Unlike their C-2 and C-4-substituted counterparts, pyrimidines with fluorinated alkyl at C-5 were rarely prepared using reaction of NCN binucleophiles and CCC bis-electrophiles. Several examples of such transformations were already mentioned in previous section (reactions with 710 leading to 4,5-bis-perfluoroalkylpyrimidines, see Table 40, Entries 6,7). Analogous reactions with alkene 817, as well as its precursor 816 (a stable adduct of methanol and 2-(trifluoromethyl)-l,l,3,3,3-pentafluoropropene), led to the formation of 5-trifluo-romethylpyrimidines 818 (Scheme 163) [505, 506], Pyrimidines 822 and 825 were also prepared from 816. Reaction of 816 with triethylamine resulted in formation of enolate 819 (Schane 163) [507]. When in situ generated 819 was treated with HCl, acyl fluoride 820 was formed. Reaction of 820 with AdV -dimethyl(thio)urea resulted in the formation of adduct 821, which underwent cyclization to 822 upon... 

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