Reaction Conditions and Mechanisms

In the early part of this project, zinc chloride was used as the catalyst. The reactions were carried out in laboratory glass reactors. The insertion was monitored 

When the reaction time was plotted against the logarithm of acid concentration, log [acid], a straight line was obtained, suggesting that the insertion reaction is first-order with respect to the acid (or anhydride). 

The reaction time also depends on the concentration of the catalyst. At the high-concentration end (1.5%), the reaction is completed within 7h. At the very low level (0.5 %), it usually takes about 15 to 20 h. Using the high-catalyst level, however, is complicated by two other factors. The color of the product becomes very dark with more catalyst and also the amount of byproducts increases rapidly. Because of these considerations, the catalyst level was fixed between 0.7 and 1 % of the total weight. 

The reaction temperature is limited between 170 and 200 °C. At the high-temperature end, the byproduct generation again became a concern. In addition, we have noticed a minor exotherm at around 200 °C. Since polyols can undergo depolymerization at temperatures above 230 °C, for safety reasons, it is therefore 

According to Ganem and Small [5], the reaction mechanism is an acylation at the ether oxygen followed by a dissociation step, either of an SaU or Sat2 nature. With carboxylate as the nucleophile, both the Sjvl and S v2 routes are feasible. Regardless of the true mechanism, the net effect of this ether-to-diester reaction is an insertion of esters to ether bonds. 

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The C-nitrosation of aromatic compounds is characterized by similar reaction conditions and mechanisms to those discussed earlier in this section. The reaction is normally carried out in a strongly acidic solution, and in most cases it is the nitrosyl ion which attacks the aromatic ring in the manner of an electrophilic aromatic substitution, i. e., via a a-complex as steady-state intermediate (see review by Williams, 1988, p. 58). We mention C-nitrosation here because it may interfere with diazotization of strongly basic aromatic amines if the reaction is carried out in concentrated sulfuric acid. Little information on such unwanted C-nitrosations of aromatic amines has been published (Blangey, 1938 see Sec. 2.2). 

The well-known reaction of Ni(II) with dimethylglyoxime (H Dm) in alkaline medium under the influence of such oxidants as persulphate and iodine is widely used for the photometric determination of nickel. The red product (RP) of this reaction is used for this purpose. However, the nature of this red compound has not been defined yet. Using of peroxyacids makes it possible to obtain additional data concerning the conditions and mechanism of generation of RP as well as to improve the metrological pai ameters of the method. 

This mechanism is supported by the finding, that esters containing just one a-hydrogen do not undergo that reaction, unless much stronger bases are used, since the condensation product 9 cannot be stabilized under the usual reaction conditions and the equilibrium lies to the left ... 

These examples indicate that the (Z)-syn,(E)-antt correlation should be considered to be a rule with many exceptions. Two explanations may be given in order to rationalize the manifold stereochemical results in aldol additions. Firstly, it seems plausible that the many different reaction conditions and starting materials (e.g., various types of enolates, counterions, etc.) may cause the aldol addition to follow different reaction mechanisms, so that different types of transition states are involved. Secondly, in a single type of transition state model, the reactants may have different orientations to each other, so that the formation of different stereoisomers may result even for one and the same transition state model. 

The processes described in this section should be contrasted with RAFT polymerization (Section 9.5.3), which can involve the use of similar thioearbonylthio compounds. A. A -dialkyl dithiocarbamates have very low transfer constants in polymerizations of S and (mctb)acrylatcs and arc not effective in RAFT polymerization of these monomers. However, /V,A -dialkyl dithiocarbamates have been successfully used in RAFT polymerization of VAc. Certain O-alkyl xanthates have been successfully used to control RAFT polymerizations of VAc, acrylates and S. The failure of the earlier experiments using these reagents and monomers to provide narrow molecular weight distributions by a RAFT mechanism can he attributed to the use of non-ideal reaction conditions and reagent choice. A two part photo-initiator system comprising a mixture of a benzyl dithiocarhamate and a dithiuram disulfide has also been described and provides better control (narrower molecular weight distributions).43... 

Anthraquinone is widely use in the manufacture of a range of dyes. Two possible routes for manufacturing anthraquinone are (1) from the reaction of 1,4-naphthoquinone with butadiene and (2) reaction of benzene with phthalic anhydride. Describe mechanisms for both these reactions and identify likely reaction conditions and any other reagents required. Compare the atom economy of the two routes. Identify three factors for each route that may influence the commercial viability. 

The surface transformations of propylene, allyl alcohol and acrylic acid in the presence or absence of NHs over V-antimonate catalysts were studied by IR spectroscopy. The results show the existence of various possible pathways of surface transformation in the mechanism of propane ammoxidation, depending on the reaction condition and the surface coverage with chemisorbed NH3. A surface reaction network is proposed and used to explain the catalytic behavior observed in flow reactor conditions. 

Selenium dioxide is a useful reagent for allylic oxidation of alkenes. The products can include enones, allylic alcohols, or allylic esters, depending on the reaction conditions. The mechanism consists of three essential steps (a) an electrophilic ene reaction with Se02, (b) a [2,3]-sigmatropic rearrangement that restores the original location of the double bond, and (c) solvolysis of the resulting selenium ester.183... 

Synthesis and characterization of an Al69 cluster which can be regarded as a slightly modified Al77 cluster show the extreme sensitivity of the cluster geometry— even with the same ligand—to reaction conditions and number of ligands. Many more clusters will have to be synthesized to get a consistent picture of their formation and, consequently, the mechanism of metal formation which represents, as already mentioned, one of the oldest chemical processes in history. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

The bromination of ethylenic compounds is in most cases a very fast reaction. Half-lives of typical olefins are given in Table 1. Most of them are very short. In order to obtain extended and meaningful kinetic data, it has been necessary to find suitable reaction conditions and to design specific kinetic techniques. This was not done until 1960-1970. As a consequence, kinetic approaches to the bromination mechanism are relatively recent as compared with those to solvolytic reactions, for example. 

The reaction mechanisms available to the organic chemist today are both useful and self-consistent. They are aids to the prediction of by-products and in the improvement of reaction conditions. And yet they very often assume intermediates which have not been isolable for direct experimental study. The physical reality of such intermediates depends not only on the self-consistency of the mechanisms which use them but also on their relation to similar substances that do happen to be stable enough to study directly. It is the additional relationships to things directly accessible that constitute our reasons for believing in the physical reality of the unstable intermediates.1... 

The thiolsulfinate ArS(0)SR also formed in (117) is not stable under the reaction conditions and decomposes to yield a mixture of ArSOzSR, ArSSR, and ArS02SAr. Although the exact mechanism of this process was not determined by Kice and Morkved (1964), the mechanisms of other acid-catalyzed reactions of thiolsulfinates discussed earlier, and the known behavior of dithiosulfonium ions (Smallcombe and Caserio, 1971), suggest the pathway shown in Scheme 1. 

The results of the present work are summarized in Figure 10. As far as the initial part of the reaction is concerned, we have found that the associative mechanism is slightly more favored than the dissociative one. However, the small energy difference found between the two mechanisms and the possible effect of the solvent, which has not been included in the present study, precludes formulation of a definitive conclusion on the most effective reaction pathway. Most likely depending on the reaction conditions and the initial reactants the two mechanisms can be operative. In fact, experimental evidence in favor of both the dissociative [37, 38] and associative [41] mechanisms has been provided. 

Burton K (1956) A study of the conditions and mechanism of the diphenylamine reaction for the colorimetric estimation of deoxyribonucleic acid. Biochem. 62 315-323. 

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