Reagents, excess

In industrial reactions the components are seldom fed to the reactor in exact stoichiometric proportions. A reagent may be supplied in excess to promote the desired reaction to maximise the use of an expensive reagent or to ensure complete reaction of a reagent, as in combustion. 

The percentage excess reagent is defined by the following equation  

It is necessary to state clearly to which reagent the excess refers. This is often termed the limiting reagent. 

To ensure complete combustion, 20 per cent excess air is supplied to a furnace burning natural gas. The gas composition (by volume) is methane 95 per cent, ethane 5 per cent. Calculate the mols of air required per mol of fuel. 

C2H6 + 3 02 2CO2 + 3H2O Stoichiometric mols O2 required = 95x2 + 5x3 = 207.5 

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Combinatorial chemistry has significantly increased the nurnjjers of molecules that can be synthesised in a modern chemical laboratory. The classic approach to combinatorial synthesis involves the use of a solid support (e.g. polystyrene beads) together with a scheme called split-mix. Solid-phase chemistry is particularly appealing because it permits excess reagent to be used, so ensuring that the reaction proceeds to completion. The excess... 

Step 3 The resin bound C terminal ammo acid IS coupled to an N protected ammo acid by using N N dicyclohexylcarbodiimide Excess reagent and N N dicyclohexylurea are washed away from the resin after coupling is complete... 

A titration in which a reagent is added to a solution containing the analyte, and the excess reagent remaining after its reaction with the analyte is determined by a titration. 

In the laboratory, excess reagent in a product should be destroyed before workup. Addition of diluted aqueous ammonia is the most effective practice, if ammonia is otherwise acceptable. Combustibility is a minor problem. The open-cup flash point of 116°C for dimethyl sulfate is well above normal handling temperatures. Elammable, toxic vapors are given off at elevated temperatures. 

The ether should be reagent grade but not sodium-dried. The traces of water present destroy excess reagent, leading to a cleaner product. 

Steroids The applied steroids are reduced by means of a mixture of 10% ethanolic sodium borohydnde solution and 0 1 N sodium hydroxide solution (1-1-1) The excess reagent is neutralized with aad after 30 min [3]... 

Apply extracts of cereals or fungal cultures apply [66] 50 pi pyridine — acetic anhydride (1 -I-1) on top remove the excess reagent in a stream of cold air and chromatograph. The reagents can also be applied via gas phase... 

Apply sample solution followed by acetyl chloride [70] and then remove the excess reagent in a stream of hot air. 

Apply 10% 3,5-dinitrobenzoyl chloride solution [72] in p-xylene — tetrahydrofuran (15-1-2), followed by sample solution allow to react destroy excess reagent with 10% sodium hydroxide solution. 

Prechromatographic dansylation has the advantage that chromatography separates excess reagent and also the fluorescent by-products (e g dansyl hydroxide) from the reaction products of the substances to be determined In the case of postchromatographic dansylation the whole of the plate baekground fluoresees blue, so that in situ analysis is made more diflicult... 

This material in pyridine (120 ml) is benzoylated by addition of benzoyl chloride (12 ml). After 2 hr, water (20 ml) is added to destroy excess reagent, and the product is precipitated after a further 2 hr by addition of more water (200 ml). Addition of boiling methanol (135 ml) to a solution of the substance in boiling chloroform (45 ml) gives 11-ketotigogenin benzoate as lances (11.5g), mp 228-232° [aJo, —32°. A further 1.05 g is obtained from the mother liquors by evaporation and repeated crystallization. The total yield is 12.6 g, 83%. 

A solution of 3jS-hydroxy-5a-androstan-17-one tosylate (193, 60 mg) in tetrahydrofuran (10 ml, freshly distilled from lithium aluminum hydride) is added dropwise to a boiling suspension of lithium aluminum deuteride (60 mg) in tetrahydrofuran (10 ml). The resulting suspension is heated under reflux for 30 min and after cooling the excess reagent is decomposed by the careful addition of a few drops of water. The heating is continued for a few minutes to coagulate the inorganic salts which are removed by filtration... 

A solution of 17-cyanoandrosta-5,16-dien-3jS-ol acetate (46 g) and anhydrous potassium acetate (0.46 g) in methylene dichloride (310 ml) is treated with a mixture of 40% peracetic acid (37 ml) and anhydrous potassium acetate (1.84 g) in methylene dichloride (46 ml), the temperature of the solution being maintained below 25°. The mixture is stored at room temperature for 4 hr and then washed successively with water, 5% sodium bicarbonate solution (aqueous sodium bisulfite, 10g/150g water, has been used to decompose excess reagent before workup) and water until neutral. Evaporation of the dried solution and addition of ether gives 24.1 g of 5oc,6a-epoxy-17-cyanoandrost-16-en-3 -ol acetate mp 187-190°. One recrystallization from methanol gives 20.4 g of oxirane melting at 191-194°. 

The mixture is distilled until most of the ether has been removed and then refluxed for 8 hr. Ethyl acetate is added to decompose the excess reagent and concentrated aqueous sodium sulfate is then added until the precipitate begins to adhere to the sides of the flask. Finally ca. 100 g of solid sodium sulfate is added, the salts are removed by filtration and washed well with dioxane. Evaporation of the solvent gives a solid residue which is dissolved in 60 ml of chloroform and shaken with 3.5 g of manganese dioxide for 16 hr. Subsequently another 3.5 g of manganese dioxide is added and shaking continued for a further 16 hr. The solid is removed by filtration and washed well with hot chloroform. Evaporation of solvent and crystallization of the residue from acetone-hexane affords 0.51 g (72%) of 17a-hydroxy-17jff-ethylandrost-4-en-3-one mp 145-148°. 

During methylation of 6-azauracil with a theoretical amount of diazomcthane, the 3-methyl derivative (63) was obtained in very good yield. Excess reagent produces the dimethyl derivative (64). During none of the alkylation reactions was it possible to observe the formation of 0-alkyl derivatives of 6-azauracil. This can be taken as evidence that 6-azauracil does not react in the lactim form (e.g., Section II,B, b). 

A comparison of ortho vs. para direct deactivation by a methoxy group has been made by Karmas and Spoerri in 2,3-dibromo-5,6-dimethyl- and 2,5-dibromo-3,6-dimethyl-pyrazine. The former gives monomethoxy-debromination with one equivalent of methanolic methoxide (65°, 6 hr) and disubstitution via 198 with excess reagent for a longer time (10 hr). In contrast, the isomeric 2,5-dibromo compound gave only monosubstitution, forming 199, under the latter conditions. 

Since there is a high percentage of acetonitrile in the derivatization solution, the latter must be diluted in order to decrease the retention of the derivative in the first column and so that the derivatives can be separated from the excess reagent. The three compounds can be determined at a level of 1 p.g 1 with the sample throughput being at least 40 samples per day. 

The acid (0.1 to 0.5 g) is dissolved or suspended in ether, and the ethereal diazomethane solution is added in small portions with swirling until the yellow color of diazomethane persists and nitrogen gas is no longer evolved. The solution is then warmed on a steam bath briefly to expel the excess reagent and the ether is evaporated to give the desired methyl ester. Examples are given in Table 7.2. 

The advantages of this method are a short reaction time and the nonfluorescence of the OPA reagent. Therefore, excess reagent must not be removed before the chromatography stage. Using this method, it is possible to measure tryptophan, but not secondary amino acids such as proline or hydroxyproline. Cysteine and cystine can be measured, but because of the low fluorescence of their derivatives, they must be detected using an UV system, or alternatively oxidized to cysteic acid before reaction. 

The polymer-bonded amino acid is washed free of excess reagent and then treated with trifluoroacetic acid to remove the Boc group. 

A second Boc-protected amino acid is coupled to the first by reaction with DCC. Excess reagents are removed by washing them from the insoluble polymer. 

Pre-column off-line derivatisation requires no modification to the instrument and, compared with the post-column techniques, imposes fewer limitations on the reaction conditions. Disadvantages are that the presence of excess reagent and by-products may interfere with the separation, whilst the group introduced into the molecules may change the chromatographic properties of the sample. 

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