Reagents large excesses

The above Cl reactions will occur if they are exothennic. In order for these reactions to occur with high efficiency, the pressure in the ion source must be raised to the milliTorr level. Also, the reagent species are often introduced in large excess so that they are preferentially ionized by the electron beam. 

A condition of equilibrium is reached (70-90 per cent, of bisulphite compound with equivalent quantities of the reagents in 1 hour), but by using a large excess of bisulphite almost complete conversion into the... 

If the compound to be tested is insoluble in water, it should be brought into solution by the addition of a little dioxan. Alcohols and some methyl ketones frequently react slowly in such cases it is advisable to employ a large excess (4-5 fold) of the relatively unstable reagent (3NaOI -> NaI03 -f- 2NaI). Quinones and hydroquinones also give the iodoform reaction. 

Vote 2. A large excess of R MgCl is required, as the subsequent metallation of the enyne is rapid. In the case of C2H5 the Grignard reagent was prepared from ethyl bromide. 

Flooding and Pseudo-First-Order Conditions For an example, consider a reaction that is independent of product concentrations and has three reagents. If a large excess of [BJ and [CJ are used, and the disappearance of a lesser amount of A is measured, such flooding of the system with all components butM permits the rate law to be integrated with the assumption that all concentrations are constant except A. Consequentiy, simple expressions are derived for the time variation of A. Under flooding conditions and using equation 8, if x happens to be 1, the time-dependent concentration... 

It should be noted that the highest possible absorption rates will occur under conditions in which the hquid-phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. Such situations would exist, for instance, for NH3 absorption into an acid solution, for SO9 absorption into an alkali solution, for vaporization of water into air, and for H9S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, wrien both the hquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pres-... 

As discussed later, the reaction-enhancement factor ( ) will be large for all extremely fast pseudo-first-order reac tions and will be large tor extremely fast second-order irreversible reaction systems in which there is a sufficiently large excess of liquid-phase reagent. When the rate of an extremely fast second-order irreversible reaction system A -t-VB produc ts is limited by the availabihty of the liquid-phase reagent B, then the reac tion-enhancement factor may be estimated by the formula ( ) = 1 -t- B /VCj. In systems for which this formula is applicable, it can be shown that the interface concentration yj will be equal to zero whenever the ratio k yV/k B is less than or equal to unity. 

Figure 14-10 illustrates the gas-film and liquid-film concentration profiles one might find in an extremely fast (gas-phase mass-transfer limited) second-order irreversible reaction system. The solid curve for reagent B represents the case in which there is a large excess of bulk-liquid reagent B. The dashed curve in Fig. 14-10 represents the case in which the bulk concentration B is not sufficiently large to prevent the depletion of B near the liquid interface and for which the equation ( ) = I -t- B /vCj is applicable. 

Reagent grade potassium cyanide was purchased from Matheson, Coleman and Bell, and dried at IIB C (0.5 itm) for 24 hr. The checkers found it necessary to use newly purchased potassium cyanide. The use of potassium cyanide which was several years old gave incomplete reaction even at extended reaction times. The large excess of potassium cyanide is used simply to obtain convenient reaction times. For comparison, use of 1.5 equiv of KCN gave 38% conversion under conditions where 3 equiv produced 100% conversion. 

The flask should be full of chlorine gas when the tare weight is taken. The success of this preparation depends in large measure on the use of stoichiometric quantities of all reagents. An excess or dehdency of any one will lead to an impure product and will greatly complicate the problem of purification. ... 

Reactions of polyhydroxyl compounds such as carbohydrates with DAST lead to replacement of one or two hydroxyl groups by fluorine, more fluorine atoms are not introduced even when a large excess of the reagent is used [132, 139, 147] Although diethylaminosulfur tnfluonde (DAST) is the most popular, other dialkylaminosulfuranes, such as diisopropylamino- [95] pyrrolidino [95 109 /27], dimethylamino- [148], piperidino- [148] and particularly morpholinosulfur trifluonde [148,149, ISO], are also used as fluonnating agents to convert alcohols into fluorides... 

Since the ethyl Grignard reagent is used in large excess, no special precautions need to be taken in the transfer to prevent the loss of small amounts. 

Of course, the usual equilibrium considerations apply. For example, if we add the substance methanol, equilibrium conditions will shift, consuming the added reagent (methanol) and acetic acid to produce more methyl acetate and water, in accord with Le Chatelier s Principle. Thus a large excess of methanol causes most of the acetic acid to be converted to methyl acetate. 

Discussion. When a solution of an orthophosphate is treated with a large excess of ammonium molybdate solution in the presence of nitric acid at a temperature of 20-45 °C, a precipitate is obtained, which after washing is converted into ammonium molybdophosphate with the composition (NH4)3[P04,12Mo03]. This may be titrated with standard sodium hydroxide solution using phenolph-thalein as indicator, but the end point is rather poor due to the liberation of ammonia. If, however, the ammonium molybdate is replaced by a reagent containing sodium molybdate and quinoline, then quinoline molybdophosphate is precipitated which can be isolated and titrated with standard sodium hydroxide ... 

A large excess of the reagent should be avoided as there is a danger that the oxine itself may precipitate. 

Dimethylglyoxime is almost insoluble in water, and is added in the form of a 1 per cent solution in 90% ethanol (rectified spirit) or absolute ethanol 1 mL of this solution is sufficient for the precipitation of 0.0025 g of nickel. As already pointed out, the reagent is added to a hot feebly acid solution of a nickel salt, and the solution is then rendered faintly ammoniacal. This procedure gives a more easily filterable precipitate than does direct precipitation from cold or from ammoniacal solutions. Only a slight excess of the reagent should be used, since dimethylglyoxime is not very soluble in water or in very dilute ethanol and may precipitate if a very large excess is added (such that the alcohol content of the solution exceeds 50 per cent), some of the precipitate may dissolve. 

The relative change of conductance of the solution during the reaction and upon the addition of an excess of reagent largely determines the accuracy of the titration under optimum conditions this is about 0.5 per cent. Large amounts of foreign electrolytes, which do not take part in the reaction, must be absent, since these have a considerable effect upon the accuracy. In consequence, the conductimetric method has much more limited application than visual, potentiometric, or amperometric procedures. 

Diffusion control can be particularly important in reactions in which two aromatic substances of differing reactivity are reacting with a deficiency of reagent. The more reactive aromatic will react first and since diffusion is slow compared with the rate of reaction it becomes impoverished in the reaction zone, and ensuing reaction will occur mainly with the less reactive aromatic which is now in large excess. The observed relative reaction rate then comes out to be less than it would otherwise be. It follows that this may also be true even when the aromatics are reacting at considerably less than the encounter rate. 

With two of the concentrations in large excess, the fourth-order kinetic expression has been reduced to a first-order one, with considerable mathematical simplification. The experimental design in which all the concentrations save one are set much higher, so that they can be treated as approximate constants, is termed the method of flooding (or the method of isolation, since the dependence on one reagent is thereby isolated). We shall consider the method of flooding further in Section 2.7. Here our concern is with the data analysis it should be evident that the same treatment suffices for first-order and pseudo-first-order kinetics. 

A plot of the first-order rate constant for equilibration in reaction (3-23) is shown as a function of [Co(edta)2 ]. the reagent present in large excess. The plot is linear as expected from Eq. (3-23). Data, from Ref. 1. are given in Table 3-1. 

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