Color-Forming Reaction Sequences

This reaction has been used to determine the amount of cinnamaldehyde units in native lignin (see Table 2 11) 

Following Adler s identification of comferaldehyde units as the structural type giving rise to color in the Wiesner reaction, many other color reactions were re-examined and found attributable to the same structural unit These include the reaction of ligmfied tissue with methanol-hydrochloric acid (Brauns 

The steps comprising the Maule reaction may be portrayed as follows (Meshitsuka and Nakano 1978, Iiyama and Pant 1988)  

The syringyl nucleus (4) in hardwood lignin is converted to the methoxycatechol structure (5) by treatment with potassium permanganate and hydrochloric acid and thence to a methoxy-o-quinone (6) following reaction with ammonium hydroxide. 

The steps in the Cross and Bevan reaction may be depicted as follows (Migita and Nakano 1955)  

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The color-forming reaction is interesting because most a-amino acids give the same color irrespective of their structure.4 The sequence of steps that leads to the color is as follows ... 

The examples that are treated below are those sequences where all steps - except the last — are preparations for a color or fluorescence derivatization reaction which is carried out in the last step, i. e. they can be regarded as a sort of selective in situ pretreatment for a final detection reaction. Such reaction sequences are frequently necessary because all the reagents cannot be mixed together in a single solvent, or because it is necessary to dry, heat or irradiate with UV light between the individual reaction steps. The detection of aromatics by the reaction sequence nitration — reduction — diazotization - couple to form an azo dye is an example of this type (Fig 21). 

This reaction sequence, although consistent with the observed drop in pH [5], does not provide an explanation for the unusual color of the product. Careful monitoring of pH and development of color during reaction of [PtCl4]2 with acetonitrile later clearly revealed that release of H+ is complete long before the blue is fully formed [6]. Obviously, formation of... 

Reaction Sequence A 100 mL of a 1C % aqueous solution of KI are placed in each Erlenmeyer flask. 100 ml, of chlorine water are added to one flask, 100 mL of bromine water to the other in both cases the solution immediately turns brown. The contents of the two flasks are now transferred to the five large test tubes, to which the following substances are added test tube 1, freshly pre pared starch solution test tube 2, a few grams of solid KI test tube 3, diethyl ether test tube 4, toluene test tube 5, -hcxanc (or -hcptanc). The contents of the first test tube turn to a deep blue-black, while the brown color in the second becomes much deeper. In the other test tubes two layers are formed in each case the lower aqueous layer is much lighter in color, while the upper organic layer.s arc colored yellow(test tube 3), wine-red (test tube 4) and violet (test tube 5) respectively. 

Under elevated temperature conditions and acidic catalysis, hexoses eliminate 3 moles of water to form HMF (see [21] for possible reaction mechanisms). But in aqueous solution, reaction does not stop at this stage (Fig. 4). The dehydration reaction is subsequently followed by a rehydration step leading to levulinic and formic acid. In addition, this reaction sequence is accompanied by intermediates, side products, and at least the formation of colored soluble or insoluble polymeric compounds [23, 24]. 

In order to confirm this reaction route and at the same time to keep the reaction process as simple as possible, the symmetrically substituted tetraaryldisilene 9 [8] was prepared by the method of West et al. [9] and allowed to react with an excess of lithium. The observed color changes, first to dark green and then to brown-red, suggest the initial transfer of an electron to an aromatic residue [10], followed by extrusion of LiR and formation of the postulated disilenyllithium compound. In a second step of the reaction sequence, 10 was allowed to react with bromomesitylene in anticipation that the poor solubility of the aryllithium compound would favor the halogenation over the competing transarylation. It appears that the bromodisilene 11 is indeed formed and then reacts further with 10 by intermolecular cleavage of lithium bromide to furnish the isolated hexaaryltetrasilabuta-1,3-diene 12 (Scheme 5) [II]. 

A solution of potassium dichromate is made basic with sodium hydroxide the color changes from red to yellow. Addition of silver nitrate to the yellow solution gives a precipitate. This precipitate dissolves in concentrated ammonia but re-forms when nitric acid is added. Write balanced net ionic equations for all the reactions in this sequence. 

This reaction undergoes conversion in one sequence of consecutive elementary reaction steps and so only one propagating front is formed in a spatially distributed system [68]. Depending on the initial ratio of reactants, iodine as colored and iodide as uncolored product, or both, are formed [145]. 

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

Both iron (II) and iron (III) form complexes with mercaptoacetic acid, SRSH2 (5, 11). The ferrous complexes, Fe(II) (RS)2-2 and Fe(II) (OH) (RS) , are highly air-sensitive and are rapidly oxidized to the intense red ferric complex, Fe(III)OH(RS)2 2 (5). Under air-free conditions the color of this latter complex is observed to fade at moderate to fast rates because of a redox reaction in which the iron is reduced to the ferrous state and the mercaptoacetate is oxidized to the disulfide. Michaelis and Schubert (9) proposed that the catalysis takes place through the alternate oxidation and reduction of iron ions in a sequence similar to that just described, but Lamfrom and Nielsen (4) were able to show that under mildly acid conditions the rate of oxygen uptake of solutions containing iron and... 

Perhaps one of the best known syntheses of a heterocyclic polymer via the modification method is the generation of nitrogen-containing ladder polymers by pyrolysis of polyacrylonitrile) (77MI11109). The thermolysis is known to take place in discrete steps. The first step in the sequence, which can take place with explosive violence if the heating rate is not sufficiently slow, occurs at about 150 °C and can be detected by the onset of intense color formation. The product of this reaction (Scheme 101) is the cyclic tetrahydropyridine ladder structure (209). The next step, which is conducted in the presence of air at ca. 250 °C, involves the thermooxidation of polymer (209) to form what is best described as terpolymer (210) containing dihydropyridine, pyridone and pyridine units. 

Mechanisms of catalase and peroxidase catalysis. Attention has been focused on a series of strikingly colored intermediates formed in the presence of substrates. When a slight excess of H202 is added to a solution of horseradish peroxidase, the dark brown enzyme first turns olive green as compound I is formed, and then pale red as it turns into compound II. The latter reacts slowly with substrate AH2 or with another H202 molecule to regenerate the original enzyme. This sequence of reactions is indicated by the colored arrows in Fig. 16-14, steps a-d. 

Hydrocarbon oxidation may also be considered a free radical chain-type reaction. At elevated temperatures, hydrocarbon free radicals (R) are formed which react with oxygen lo form peroxy radicals (R(X These, in turn, take up a hydrogen atom from the hydrocarbon to form a hydroperoxide (ROOH) and another hydrocarbon free radical. The cycle repeals itself with the addition of oxygen. The unstable hydroperoxides remaining are the major points for degradation and lead to rancidity and color development in oils, fats, and waxes decomposition and gum formation in gasolines sludging in lubricants and breakdown of plastics and rubber products. Antioxidants, such as amines and phenols, are often introduced into hydrocarbon systems in order lo prevent this free radical oxidation sequence. 

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