Incomplete reactions

In the determination of free formaldehyde in solution, eg, commercial reagents and pad bath formulation, the conditions of analysis allow hydrolysis of the /V-methy1o1 groups, usually between <1% and several percent. The NaOH formed is titrated with hydrochloric acid (82). Because of an incomplete reaction of sulfite with free formaldehyde, these low temperature methods (83) detect only 80—90% of the free formaldehyde present. Skill is important for correct results. 

The three T s of combustion—time, temperature, and turbulence—govern the speed and completeness of the combustion reaction. For complete combustion, the oxygen must come into intimate contact with the combustible molecule at sufficient temperature and for a sufficient length of time for the reaction to be completed. Incomplete reactions may result in the generation of aldehydes, organic acids, carbon, and carbon monoxide. 

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. 

Carry out an incomplete reaction with an enantiomerically pure reagent... 

Entry 4 deals with the unprotected hydrazine (incomplete reaction with ClCOOCH3). 

Lithium and zinc tert-butyl phenylmethyl sulfoxide (1) and A-phenyl imines 2, in which the substituent R is alkenyl or aryl, react at —78 °C over 2 hours with high anti diastereoselection (d.r. >98.5 1.5)6. Shorter reaction times result in poorer yields, due to incomplete reaction. In contrast, the reaction of the sulfoxide anion with benzaldehyde is complete after 5 seconds, but shows poor diastereoselection. When the substituent R1 or R2 of the imine 2 is aliphatic, the substrates exhibit poor chemical reactivity and diastereoselection. The high anti diastereoselection suggests that if a chelated cyclic transition state is involved (E configuration of the imine), then the boat transition state 4 is favored over its chair counterpart 5. 

An excellent overview of the problems that students experience in learning the notions underlying chemical equilibrium is available (van Driel Graber, 2002). Research shows that conceptual problems arose when students, who had been introduced to chemical reactions through examples that evidently go to completion , first met examples of incomplete reactions . In this situation, they... 

Morphology of the cured samples was analyzed by SEM of the fractured samples etched with tetrohydrofuran (THE), which is a solvent for the rubber. Figure 11.24 shows the fracture surfaces of the PWE and PNE specimens. Whereas the PWE fracture surface presents an essentially homogeneous surface with only a few small voids present, small yet uniformly distributed cavities are seen in PNE samples. The PWE morphology is consistent with the high degree of intermolecular link between rubber and epoxy macromolecules. The PNE morphology indicates incomplete reaction between epoxy and rubber. 

For the adiabatic condition in which RHL is suppressed, the flame response exhibits the conventional upper and middle branches of the characteristic ignition-extinction curve, with the upper branch representing the physically realistic solutions. It can be noted that the effective Le of this lean methane/air mixture is sub-unity. It can be seen from Figure 6.3.1 that, with increasing stretch rate, first increases owing to the nonequidiffusion effects (S > 0), and then decreases as the extinction state is approached, owing to incomplete reaction. Furthermore, is also expected to degenerate to the adiabatic flame temperature, when v = 0. 

In the more general case of incomplete reaction (p l) and nonequivalence of the reactants all three of the above species are... 

A deficiency of water in the cement liquid has the same effect and this occurs when the H3PO4 content exceeds 60%. Wilson Mesley (1968) noted that in a cement formed from a solution of 65 % H3PO4 there was evidence of incomplete reaction even after 6 hours. We have noted in Section 6.5.3 that there is a sharp decline in the rate of reaction when the orthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14). The avidity of cements to absorb water from humid surroundings also increases sharply when the phosphoric acid in the cement-forming liquid exceeds 60%. It is difficult to avoid the conclusion that these two phenomena are related and that a deficiency of water retards the cementforming reaction. 

Figure 9 shows the electronic absorption spectrum of a PTTB film which has undergone extensive but incomplete reaction with bromine in a non-in-situ experiment. The absorption spectrum is that expected for a one-dimensional conjugated polymer. The sharpest absorption edge is at about 1490 nm (o.83 eV) and the absorption maximum is located at 1240 nm (1.0 eV). Thus, this material has a bandgap of about 0.83 eV. Note that two small... 

A failure by one of us to take fully into account the presence of inactivated xanthine oxidase, leading to misinterpretation of incomplete reaction of enzyme with iodoacetamide and hence to the apparently erroneous conclusion, that the two FAD molecules in the enzyme were non-equivalent (72), may serve as a warning to others. This reagent has since been shown to alkylate the flavin of reduced xanthine oxidase molecules, whether these are of the active or inactivated forms (73). Thus, under conditions where little of the inactivated form is reduced, the reagent becomes a specific one for the active enzyme (20). In the original experiments (59, 72) the content of active enzyme was, by coincidence, rather close to half of the total enzyme present. Thus, the presence of inactivated enzyme, rather than a lack of reactivity of one... 

Measurement caused by concurrent reactions or incomplete reaction processes in the case of chemical principles, and by instrumental deviations and wrong adjustment in the case of physical methods. A frequently encountered reason for the occurrence of systematic deviations is erroneous calibration due to unsuitable calibration standards, matrix effects, or insufficient methodical or theoretical foundation. 

Highly non-linear homopolymers can even be synthesized exclusively from monomers of functionality higher than 2. Dendrimers (from ancient Greek "SavSpov" — tree) are obtained when all the reaction sites of each branch links to another one and when all branches exhibit the same length. Whereas, hyperbranched (HB) polymers result from incomplete reaction of each multifunctional monomer, see Figure 9. 

Using an equimolar quantity of allyl methyl sulphide and ds-pent-2-ene resulted in incomplete reaction of the allyl sulphide and some self-metathesis of the sulphide substrate. When an excess (4 equiv) of but-2-ene was used, however, the desired but-2-enyl sulphide was formed in a good yield at ambient temperature. In this case, the large quantities of unwanted hydrocarbon starting material and self-metathesis products were gaseous alkenes and therefore easily removed. Using a large excess of one alkene to improve the yield of the desired cross-metathesis product in this way is obviously only viable if this alkene is inexpensive and both it and its self-metathesis product are easily removed. 

For the incomplete reaction, (2), the molar ratios of the incomplete products are given as... 

An extensive study of the reactions of the bare transition metal ions with phosphine (PH3) has been undertaken (111). Although a few metals formed addition complexes [M(PH3) ]+, most transition metal ions first reacted by a dehydrogenation reaction. As an example of addition reactions, Cu+ and Ag+ reacted very slowly with PH3 (incomplete reactions after 50 s with PH3 at a pressure of 1 x 10 5 Pa in a FT-ICR) sequentially forming [M(PH3)]+ and [M(PH3)2]+. 

The coupling of Naphtol AS or its phenyl-substituted derivatives with diazonium salts from variously substituted anilines in aqueous alkaline solution (section 4-11) gave incomplete reactions and impure products in some instances, probably because these coupling components have inadequate solubility in aqueous media. Pure dyes in ca. 90% yields were obtained by reaction in dimethylformamide in the presence of sodium acetate. Metallisation of these o,o -dihydroxyazo ligands with sodium chromium salicylate or a cobalt(II) salt gave metal-complex dyes in 80-100% yields [22]. Specific structural isomers of these complexes were identified by i.r., n.m.r., Raman and UV/visible spectroscopy [23]. 

Normally no observable effect occurs upon this addition. Gas evolution with a slight exotherm indicates incomplete reaction. 

In other words even with the most rigorous experimental procedure the amount of residual water could not be reduced significantly below [H2()]f at temperatures below -60°, so that in that temperature range only comparatively few incomplete reactions were achieved. Reactions which had stopped by consumption of water could always be re-started by the admission of moist air or the breaking of a phial containing water. It was found that at -60° the yield obeyed the relation... 

Moreover, B 2(OH) 22 can also perbenzylated with benzylchloride in the presence of EtNPrl2 under reflux in acetonitrile. The alkylation requires 6 days [see Eq. (72)]. Shorter reaction times result in incomplete reaction, longer ones in the formation of larger quantities of the purple radical anion [B 2(OCH2Ph) 2. This radical can also be generated by a one electron oxidation with Fe3+, and it is even possible to further oxidize the radical anion to the dark orange neutral hyper-doso-B12(OCH2Ph)i2 [121] ... 

---