Multiple addition

The products of an addition-elimination path very often undergo a second attack by the nucleophile (Ad] j + Ep then a second Adj ) because the product of the first addition-elimination is also a reactive electron sink. If the addition-elimination product is more reactive than the original starting material, a second addition almost certainly will occur. A ranking of the reactivity of electron sinks and sources is very important for path selection. For each reaction, the useful limit must be found for each reactivity spectrum. How far down the reactivity trend for the electron source can we go and still have the reaction with a particular electron sink occur at a useful rate There are four ways that the reaction may stop after the initial AdN -t- Ep  

Unfavorable Equilibrium The Nu is such a weak base that the second addition easily reverses and the equilibrium favors the more stable carbonyl. Heteroatom lone pair sources almost never add twice. 

Low Temperature One equivalent or less of the electron source was added to the sink at a low enough temperature (usually -78°C). Since the addition step usually has a lower barrier than the elimination step, the loss of the leaving group from the tetrahedral intermediate by elimination path Ep does not occur to a significant extent before the electron source is used up. Tetrahydrofuran is the preferred solvent. 

With few exceptions, organometalUcs add only once to nitriles (Section 8.6.4). Only if there is a very strong Lewis acid present to activate the anionic intermediate of the first nucleophilic attack will a second addition take place. An example of this second addition is the reduction of nitriles with LiAlH4, which proceeds all the way to the amine (Section 8.6.3). The AIH3 formed from the initial reduction acts as a Lewis acid to catalyze the second addition. This second addition can be prevented by using one equivalent of a less reactive aluminum hydride like LiAlH(OEt)3 or diisobutylaluminum hydride, [(CH3)2CHCH2l2AlH. The reactions in the acidic water workup are the reverse of imine formation (Section 10.5.2). 

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Addition Point. The flocculant addition point in a continuous system can also have a significant effect on flocculant performance. The turbulence as the flocculant is mixed in and the floes travel toward the point where they enter the thickener or filter causes both the formation and breakup of floes. Usually there is an optimal addition point or points which have to be determined empirically. In cases where the same polymer is being added at two or more points, the relative amounts added at each point may also affect performance. Thus providing multiple addition points in the design of new installations is recommended (56). 

Addition Reactions. The addition of nucleophiles to quinones is often an acid-catalyzed, Michael-type reductive process (7,43,44). The addition of benzenethiol to 1,4-benzoquinone (2) was studied by A. Michael for a better understanding of valence in organic chemistry (45). The presence of the reduced product thiophenyUiydroquinone (52), the cross-oxidation product 2-thiophenyl-1,4-benzoquinone [18232-03-6] (53), and multiple-addition products such as 2,5-(bis(thiophenyl)-l,4-benzoquinone [17058-53-6] (54) and 2,6-bis(thiophenyl)-l,4-benzoquinone [121194-11-4] (55), is typical ofmany such transformations. 

Dual-Enzyme Processes. In some cases, especially in symp production in Europe, a Hquefaction process is used that incorporates both a thermostable enzyme and a high temperature heat treatment. This type of process provides better hydrolyzate tilterabiHty than that attained in an acid Hquefaction process (9). Consequendy, dual-enzyme processes were developed that utilized multiple additions of either B. licheniformis or B. stearothermophilus a-amylase and a heat treatment step (see Eig. 1). 

It is possible to use a graph to calculate the addition of decibels, even in the case of multiple additions (Figure 42.2). The graph is used in the following way ... 

In the case of multiple additions. If there are more levels to be added, the first two levels are added using the graph and then the third is added to the resultant using the same method. 

By diluting HF by means of multiple additions of water, followed by the evaporation of a solution initially prepared by the dissolution of NaF and Nb205 in HF, Stomberg [58] obtained Na3NbOF6 in the form of small single crystals that were suitable for X-ray structure investigations. 

PUR are a broad class of highly cross-linked plastics prepared by multiple additions of poly-functional hydroxyl or amino compounds. Typical reactants are polyisocyanates [toluene diisocyanate (TDI)] and polyhydroxyl molecules such as polyols, glycols, polyesters, and polyethers. The cyanate group can also combine with water this reaction is the basis for hardening of the one-part foam formulations. 

Known concentrations of Sn solutions were spiked Into acid digested samples of Ps. 244 which had been exposed to 10 ppm solutions of Bu Sn (x = 0,1,2,3). Atomic absorption analysis of these samples allowed for the calculation of the amount of tin accumulated on the cell by the method of multiple additions Q6). 

The use of the particle-beam interface for introduction of samples into a mass spectrometer (PB-MS), without chromatographic separation, was shown by Bonilla [55] to be a useful method for analysis of semi-volatile and nonvolatile additives in PC and PC/PBT blends. The method uses the full power of mass spectrometry to identify multiple additives in a single matrix. The usefulness, speed and simplicity of this approach were illustrated for AOs, UVAs, FRs, slip agents and other additives. 

Add a quantity of succinic anhydride to the reaction medium to provide at least a 5-10 molar excess of reagent over the amount of amines to be modified. Even greater molar excess may be required for total blocking of all the amines of some proteins. When adding solid succinic anhydride, multiple additions may be done to maintain solubility of the reagent in the reaction solution. The anhydride also may be dissolved in dry dioxane before addition to aid in dissolution. 

Hirsch, A., Lamparth, I., and Grdsscr, T. (1994) Regiochemistry of multiple additions to the fullerene core Synthesis of a T -symmetric hexakisadduct of Ca with bis(ethoxycarbonyl)methylene. J. Am. Chem. Soc. 116, 9385-9386. 

In 2003, Efskind and Undheim reported dienyne and triyne domino RCMs of appropriately functionalized substrates with Grubbs type II or I catalysts (Scheme 6.71, reactions a and b, respectively) [151]. While the thermal processes (toluene, 85 °C) required multiple addition of fresh catalyst (3 x 10 mol%) over a period of 9 h to furnish a 92% yield of product, microwave irradiation for 10 min at 160 °C (5 mol% catalyst, toluene) led to full conversion. The authors ascribe the dramatic rate enhancement to rapid and uniform heating of the reaction mixture and increased catalyst lifetime through the elimination of wall effects. In some instances, use of the Grubbs I catalyst was more efficient than use of the more common Grubbs II equivalent. 

A mathematical group, by definition, consists of a set of distinct elements G — E, A, B,C, D,..., endowed with a law of composition (such as multiplication, addition, or some other operation), such that the following properties are satisfied ... 

The chemical reactivity of cobalt cluster anions, Co (n = 2-8), toward 02, N2, and CO have been investigated using a flow tube reactor (226). The reactivity was found to be in the order 02 > CO > N2 the least reactive ligand N2 only reacted with C07 and Cog. The primary reaction of oxygen was the removal of one or two cobalt atoms from the cluster. Carbon monoxide reacts by multiple additions giving saturation limits shown in Table V. 

Addition polymerization is a reaction in which monomers with double bonds are joined together through multiple addition reactions to form a polymer. Figure 2.12 illustrates the addition polymerization reaction of ethene to form polyethene. Table 2.1, on the next page, gives the names, structures, and uses of some common polymers that are formed by addition polymerization. 

Gran plot A commonly employed multiple-addition method, used to correct for unknown amounts of contaminant and for dilution errors (see Section 4.3.2). 

Cx = (CsFo//xF3)(1-10- ) = Ac (1-10- ) (ASM). (5.20) The double and multiple addition methods are introduced in an attempt further to improve the measuring precision, because with three or more experimental potential values the slope value S need not be knowa Under the same assumptions and with the same symbols as above, provided the same volumes are always added, it holds for the nth addition of the determinand standard solution that... 

Double addition methods do not significantly improve the precision of the determination and thus multiple addition methods should be preferred. 

Potentiometric titration is actually a form of the multiple known subtraction method. The main advantage of titration procedures, similar to multiple addition techniques in general, is the improved precision, especially at high determinand concentrations. ISEs are suitable for end-point indication in all combination titrations (acid-base, precipitation, complexometric), provided that either the titrand or the titrant is sensed by an ISE. If both the titrant and the titrand are electro-inactive, an electrometric indicator must be added (for example Fe ion can be titrated with EDTA using the fluoride ISE when a small amount of fluoride is added to the sample solution [126]). 

In potentiometry with ISEs, however, the second Gran method [46] has found especially wide use, not only in titrations, but also in multiple addition methods in general. In these methods, the concentration of the test substance is plotted against the volume of the titrant or of the standard solution and thus the curve is linearized. The end-point in the titration or the determinand concentration in a multiple addition method is found as the intercept of the straight line with the volume axis. Linearization is attained by taking the antilogarithm of the Nernst equation ... 

In the Gran linearization of a multiple addition method, (5.21) is rearranged to give... 

Fig. 5.3. The use of the Gran method in the multiple addition method. For explanation see the text.

Continuous flocculation in stirred tanks shows similar trends with regard to agitation/mixing conditions as for batch systems. The overall performance of continuous flocculation processes tends to be inferior to that which can be obtained in batch systems. This is attributed to a closer approach to equilibrium in the continuous case. In-line flocculation, by injection of polymer into turbulent flow in a pipe, is widely practised in industry and offers certain advantages. Multiple addition at several points along the length of a pipe is preferred to single-point addition. 

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