Quenching colour

Now take another batch of sulphur flowers, but this time heat it well past its melting point. The liquid sulphur gets darker in colour and becomes more and more viscous. Just before the liquid becomes completely unpourable it is decanted into a dish of cold water, quenching it. When we test the properties of this quenched sulphur we find that we have produced a tough and rubbery substance. We have, in fact, produced an amorphous form of sulphur with radically altered properties. 

The intensity and colour of the fluorescence of many substances depend upon the pH of the solution indeed, some substances are so sensitive to pH that they can be used as pH indicators. These are termed fluorescent or luminescent indicators. Those substances which fluoresce in ultraviolet light and change in colour or have their fluorescence quenched with change in pH can be used as fluorescent indicators in acid-base titrations. The merit of such indicators is that they can be employed in the titration of coloured (and sometimes of intensely coloured) solutions in which the colour changes of the usual indicators would... 

By means of water fire may be extinguished, and utterly quenched. If much water be poured upon a little fire, the fire is overcome, and compelled to yield up the victory to the water. In the same way our fiery sulphur must be overcome by means of our prepared water. But, after the water has vanished, the fiery life of our sulphurous vapour must triumph, and again obtain the victory. But no such triumph can take place unless the King imparts great strength and potency to his water and tinges it with his own colour, that... 

Diaryl tellurides (generalprocedure). A mixture of powdered Te (0.128 g, 1.0 mmol), NaH (0.053 g, 2.2 mmol, 60% suspension in oil, washed with hexane) and dry DMF (3 mL) is heated at 140°C for 1 h. Within 0.5 h the initial deep red colour is lost, and a pale yellow snspension is obtained. After cooling at room temperature, the aryl iodide (2.0 mmol) in dry DMF (3 mL) is added and the mixture heated at 130°C for 24 h. After cooling at room temperatnre, the mixture is quenched with 10% aqueous NH4HSO4 (10 mL) and extracted with ether (10 mL). The ethereal extract is washed with HjO, dried (Na2S04) and evaporated, giving the teUuride, which is purified by column chromatography on SiOj (elnent/hexane). 

TeClf catalysed Friedel-Crafts aromatic alkylation (typical procedure. To a solution of 1-phenylethanol (3.7 g, 30 mmol) in toluene (30 mL) is added slowly TeCl4 (9.7 g, 36 mmol), keeping the temperature at 25°C (exothermic reaction). Small amounts of white precipitate appear immediately, and after a few minutes the colour of the mixture becomes dark brown. The mixture is stirred for 3 h and then quenched with HgO (20 mL). The organic layer is separated, washed with brine (2x20 mL) and dried (MgS04). Evaporation of the solvent leaves an oily residue which is distilled under vacuum, giving a mixture of 1-phenyl-1-tolylethanes (4.9 g (83.3%) b.p. 117-128°C/1 torr). GLC analysis (silicone OV-101, 0.24 mm X 30 m capillary column at 100-260°C, 4°C min i) reveals an ortholpara ratio of 12 88. 

Sulfur-selenium phases can be prepared by cooling molten mixtures of the elements either slowly or by quenching followed by extraction with carbon disulfide, carbon tetrachloride or benzene. The crystals are obtained upon evaporation or cooling of the resulting solutions. Their colour deepens from yellow to ruby red with increasing selenium content In the older literature there has been some confusion whether to consider these phases as mixed crystals of discrete Sg and SCg molecules or as binary compounds containing SeS bonds. 

It is possible to make the colour of a cholesteric LC independent of temperature by locking it covalently into a polymer matrix. This can be achieved by cross-linking parts of the sample at different temperatures or by quenching locally at temperatures below the glass transition temperature. 

Finally, the reaction is quenched after a short period of time by addition of a strong acid, which destroys the enzyme. The intensity of the colour in the tube is then read using a colorimeter. 

Methyl 3-hydroxybutanedithioate (R = Me) (0.6 g 4 mmol) was added dropwise to a solution of LDA (2 eq) in THF cooled at -78°C. The mixture was stirred for 20 min. The yellow dithioester colour disappeared. Allyl bromide (1 eq) was added. Stirring was maintained for 45 min. The solution was quenched by an aqueous solution of ammonium chloride. The mixture was partitioned between ether and brine. The organic layer was washed with brine, dried with magnesium sulfate and concentrated. The crude ketene dithioacetal was left at ambient temperature for X d to achieve the rearrangement. A yellow oil was isolated by flash chromatography (elution with a 90 10 mixture of cyclohexane/ethyl acetate), yield 74%, as a 90 10 syn/anti mixture of diastereoisomers. 

There is no standard, universal, procedure for the Birch reduction. Experiment 7.19 illustrates some of the variants which have been reported in the literature. The original Birch procedure is to add small pieces of sodium metal to a solution of the aromatic compound in a mixture of liquid ammonia and the proton source (ethanol).18 After completion of the reaction, which is usually indicated by the disappearance of the blue colour, it is quenched by the addition of ammonium chloride and the ammonia allowed to evaporate before the cautious addition of water, and isolation of the product by ether extraction. In a modified procedure a co-solvent (ether, tetrahydrofuran, etc.) is initially added to the solution of aromatic compound/liquid ammonia prior to the addition of metal lithium metal is often used in place of sodium.19a,b In general these latter procedures are used for substrates which are more difficult to reduce. Redistilled liquid ammonia is found to be beneficial since the common contaminant iron, in collodial form or in the form of its salts, has a deleterious effect on the reaction.20 A representative selection of procedures is given in Expt 7.19 for the reduction of o-xylene, anisole, benzoic acid, and 3,4,5-trimethoxybenzoic acid. 

As far as chemical sensors are concerned, colorimetric chemosensors for anions based on calix[4]pyrrole (16-18, 22-31) showed strong binding to the fluoride anion. Receptors (29-31) are the first naked-eye detectable chemosensors that are able to discriminate between different anionic substrates as a result of detectable colour changes. On the other hand, the fluorescence of the receptors (16-18, 22-28) is quenched significantly in the presence of anionic guests. 

However, coloured or uncoloured materials may be finished in a great variety of ways. For certain purposes (even for natural ), quenching or annealing immediately after processing are necessary in order to obtain the molecular arrangements and therefore the mechanical or other properties required. In other words, the finishing is an important factor in deciding the suitability of the object for its purpose, and whether or not it will meet a specification. 

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