Quantitative reactivity studies

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

A quantitative study has been made on the effect of a methyl group in the 2-position of five-membered heteroaromatic compounds on the reactivity of position 5 in the formylation and trifluoroacetylation reaction. The order of sensitivity to the activating effect of the substituent is furan > tellurophene >selenophene = thiophene (77AHC(2l)ll9). 

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of iV-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive... 

The halogens of halothiophenes are more labile than those of the corresponding benzenes in accordance with theoretical considera-tions which indicate that thiophenes should also undergo nucleophilic substitutions more rapidly than benzenes. Hurd and Kreuz" found that in qualitative experiments 3,5-dinitro-2-chlorothiophene was more reactive toward piperidine and methanolic potassium hydroxide than 2,4-dinitrochlorobenzene. A quantitative study on the reaction of the six isomeric bromonitrothiophenes with piperidine (Table V) shows that the thiophenes react about one thousand times... 

A -(l-Haloalkyl)pyridinium halides have been advantageously employed in the Hantzsch multicomponent synthesis, yielding alkyl 1,4-dihydropyri-dine-3,5-dicarboxylates, which are a well-known class of calcium channel modulators (81AGE762). Tire halides readily interact with an excess of an ethyl 3-aminobut-2-enoate 82 (R = H) in dichloromethane at room temperature to afford the heterocycles 83 (R = H) in good to excellent yields (65-95%) (92T1263). Tliis observation has been exploited to perform a quantitative study of the reactivity of the salts (93CB1251).Tlie results have... 

The first quantitative study of the reaction was carried out with anthracene-9-carboxylic acid (which possesses the necessary steric requirement by virtue of the peri-hydrogen atoms, and is very reactive at the 9 position towards electrophilic substitution). Schenkel632 found that the decarboxylation rate was increased in the presence of acid, and the first-order rate coefficients (believed to be in sec-1) are given in Table 205. It was subsequently concluded that in the absence of acid,... 

An early, qualitative observation of the exceptional reactivity of phenyl and naphthyl acetates having proximate formyl groups was made in 1946 (Vavon and Scandel, 1946). A quantitative study of the alkaline hydrolysis of the 2-, 3- and 4-formylphenyl acetates was made by Holleck et al. (1958). The 2-formyl ester [39] was very rapidly hydrolysed compared with the 3- and... 

Quantitative studies of the photolysis of 9a or 9c-g in the presence of alkenes provided evidence for the participation of a second reactive intermediate, in addition to carbenes 10a or lOc-g, because the addn/rearr ratio was not linearly dependent on [alkene] cf. Section III.A and Scheme 2. 

Carbon-13 NMR was utilized to study different aspects of the reactivity of the metal complexes as a function of certain structural features in the selected oxocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) as depicted in Scheme 1 and illustrated in Figs. 1-4. The NMR spectral properties were similar to those obtained from 13C NMR in general, i.e., very sharp lines indicative of fairly long relaxation times in the order of a few seconds. The large quadrupolar moment ofTc-99 (7 = 9/2, 100% abundance) led to a very broad bound 13C signal (Fig. 5), thus excluding the quantitative study of the cyanide exchange by 13C NMR. However, 16N NMR was successfully used instead. 

The concentration of the spin trap is usually not critical, although care must be exercised in quantitative studies (next Section). When reactive radicals are being trapped in competition with attack on substrate, the scavenger concentration may have to be adjusted in order to detect substrate-derived radicals. In these experiments the variation of the scavenger concentrations can give useful information, as in the example of alcohol oxidation discussed earlier. 

Platts and Tomkinson reported a detailed quantitative study of the iminium ion catalysed Diels-Alder reaction between cyclopentadiene and cinnamaldehyde catalysed by trifluoromethyl pyrrolidine salt 182-HPFg [227]. This combination of secondary amine, co-acid and dienophile allowed the isolation and structural elucidation of the reactive iminium ion intermediate. As a result it was possible to... 

Temperature, pH, ionic strength, concentration of a metal ion, and other environmental parameters influence a chemical reaction and varying their values can signify drastic changes, depending on the case, on the rate, mechanism, or direction of the reaction. For this reason quantitative studies on the effects of physical parameters on reactivity often take a very long time in this kind of research. 

A Co(IH) complex is inert in ligand-substitution reactions, and its uniform structure is thus maintained even in an aqueous solution. The reaction mechanism of a Co(III) complex in solution is well known, so that a pendant-type polymer-Co(IU) complex, e.g. 17,19, is one of the most suitable compounds for a quantitative study of the effects of a polymer ligand on the reactivity of a metal complex. The reactivities of the polymer-Co(III) complexes are discussed here kinetically and compared with those of the monomeric Co(III) complexes in the following reactions electron-transfer reactions between the polymer complexes and Fe(II) [Eqs. (5) and (6)], and the ligand-substitution reaction of the polymer-Co(III) complex with hydroxy ions or water [Eqs. (7) and (8)J. One of the electron-transfer reactions proceeds via... 

The above consideration indicates that at present, the researchers face large difficulties in the physical interpretation of the effective reaction rate constant associated with the absence of detailed quantitative information on the thermodynamics of formation of H-complexes and their reactivities. Further studies of the donor-acceptor interactions in the epoxy-amine systems will shed more light on this problem. 

By comparison with the extensive kinetic results available for the quatemization of six-membered rings, especially where substituent effects on reactivity are concerned, few quantitative studies have involved five-membered heterocyclic rings. These are considered next. 

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of Af-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive to hydroxide addition at the carbonyl carbon than A-methylpyrrole-2-carbaldehyde (76JOC1952). Pyrrole-2-aldehydes fail to undergo Cannizzaro and benzoin reactions, which is attributed to mesomerism involving the ring nitrogen (see 366). They yield 2-hydroxymethylpyrroles (by NaBH4 reduction) and 2-methylpyrroles (Wolff-Kishner reduction). The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

The activated halogenothiophene is more reactive than a similarly activated halogeno-benzene. A quantitative study of the rate of piperidino-debromination of six isomeric bromonitrothiophenes reveals that these substrates react considerably faster than the corresponding bromonitrobenzenes. As in electrophilic substitution, the 2,3-, 2,4- and 2,5-relationships have been equated to ortho, meta and para substitution in benzene derivatives. Although there is some validity in this, a few inexplicable results have been encountered in the above study thus 2-bromo-4-nitrothiophene reacts much faster than either 2-bromo-3-nitro- or 2-bromo-5-nitro-thiophene (64AHC(3)285>. 

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