Branched intermediates

In the oxidation of PX and MX, formaldehyde is a degenerate branching intermediate, whereas phthalan is formed from OX ... 

The re-oxidation of lead was expected to occur rapidly at 600 K the surface of the lead oxide would then remain unchanged in the presence of oxygen. The authors concluded that, as a consequence, general hydrocarbon combustion in which formaldehyde is a degenerate branching intermediate is inhibited in the presence of PbO by the rapid removal of formaldehyde. 

In Schemes A and B and those following, the symbols in boxes represent reactive intermediates, starting materials, termination products (T), or branching intermediates (B) single arrows represent elementary reactions consuming one molecule of intermediate double arrows represent elementary reactions consuming two molecules of intermediate ... 

Figure 6 shows the variation of peroxide concentration in methyl ethyl ketone slow combustion, and similar results, but with no peracid formed, have been found for acetone and diethyl ketone. The concentrations of the organic peroxy compounds run parallel to the rate of reaction, but the hydrogen peroxide concentration increases to a steady value. There thus seems little doubt that the degenerate branching intermediates at low temperatures are the alkyl hydroperoxides, and with methyl ethyl ketone, peracetic acid also. The tvfo types of cool flames given by methyl ethyl ketone may arise from the twin branching intermediates (1) observed in its combustion. 

Now values of < are already obtained from plots of log Ap vs. time, and if we assume that kd is negligible and that all the main chain steps yield the branching intermediate, and hence that a = 1 and is independent of [RH], then a plot of vs. [RH] should yield a straight line with an intercept equal to kb. 

The increased temperature results in an increased rate of destruction of the branching intermediate (methyl hydroperoxide) with a consequent further increase of the rate, but also a decreased rate of formation of fresh hydroperoxide since Equilibrium 5 is displaced to the left, and the alternative reactions of methylperoxy increase in rate faster than that leading to formation of hydroperoxide. Consequently the quasi-stationary concentration of methyl hydroperoxide falls, and the rate of reaction declines since the new product of methyl oxidation—formaldehyde— cannot bring about branching at these temperatures. The temperature of the reaction mixture falls (because the rate has fallen), and when it has fallen sufficiently, provided sufficient of the reactants remain, the whole process may be repeated, and several further flames may be observed. 

Figure 1.4 A 6 X 6 square lattice site model. The dots correspond to multifunctional monomers. (A) The encircled neighboring occupied sites are clusters (branched intermediate polymers). (B) The entire network of the polymer is shown as a cluster that percolates through the lattice from left to right.

The origins of percolation theory are usually attributed to Flory and Stock-mayer [5-8], who published the first studies of polymerization of multifunctional units (monomers). The polymerization process of the multifunctional monomers leads to a continuous formation of bonds between the monomers, and the final ensemble of the branched polymer is a network of chemical bonds. The polymerization reaction is usually considered in terms of a lattice, where each site (square) represents a monomer and the branched intermediate polymers represent clusters (neighboring occupied sites), Figure 1.4 A. When the entire network of the polymer, i.e., the cluster, spans two opposite sides of the lattice, it is called a percolating cluster, Figure 1.4 B. 

Fig. 2. Continued) chain of Cys+i (N-terminal amino acid of the C-extein) on the thioester results in the formation of a branched intermediate. Excision of the intein occurs by peptide bond cleavage coupled to succinimide formation at the C-terminal asparagine of the intein. The ligated exteins undergo a spontaneous S-N acyl rearrangement to create a stable amide bond.

Figure 4 Mechanism of trans-protein splicing, (a) Initial association of the intein halves to form a functional intein. (b) Activation of the N-terminal splice-junction via an N-S acyl shift, (c) Formation of a branched intermediate upon transthioesterification. (d) Branch resolution and intein release by succinimide formation. Spontaneous S-N acyl rearrangement yields the processed product with a native peptide backbone.

Aldehydes are often intermediates in the oxidation of other fuels [1—4, 29], and the ease with which they themselves oxidize and give rise to peroxidic materials or active radicals means that their role in these systems is likely to be important. For example, formaldehyde is produced during the oxidation of most hydrocarbons, and is known to behave as a branching intermediate during the high temperature combustion of methane [1—6], However, in certain systems, and particularly at lower temperatures, formaldehyde may behave as a retarder [7—9, 57]. Acetaldehyde is an intermediate in the oxidation of propene [10] and other olefins [11, 12], and its addition to these systems reduces the induction period or enhances the maximum rate. Many other examples are known both of the occurrence of aldehydes amongst the combustion products and of the ability of aldehydes to influence the oxidation of systems in which they occur [1, 13—19]. 

A prolonged debate prevailed in the 1950s and 1960s regarding the role of aldehydes versus organic peroxides as the important branching intermediates in alkane oxidation at low-temperatures [125]. It would appear that there is a synergy of the two, and both classes of compounds play an important part in the low-temperature autocatalytic process. 

It is this exothermic step that probably is the source of the preference for linear hydroformylation products over branched ones. The structure of the comparable 18-electron branched intermediate 7 is about 2 kcal/mol less stable than 7, according to Jiao s calculations. This difference leads ultimately to the anti-Markovnikov, linear aldehyde over the branched-chain isomer. Although -elimination is possible now, the high partial pressure of CO present in the reaction vessel tends to stabilize 7 and prevent loss of CO that would generate the vacant site necessary for elimination to occur. 

The utility criterion supposes the selection (flow analysis) of the reaction set the most important for the particular behavior of the system, or for the particular product formation and transformations. This approach is very useful for the segregation of reactions, in which a particular substance (target product, leading radical, or key branching intermediate) forms or transforms. As a result, the analysis of the process under different conditions, or at different stages (when the ratio of various elementary reactions is changing) becomes available. 

The reactions of isoparaffins are analogous to those of normal paraffins, but in general the rates of reaction are much more rapid for isoparaffins than for a normal paraffin of the same carbon number. [Exceptions to this are branched paraffin reactants in which all of the branching is on quaternary carbon atoms (2).] The comparatively rapid rates of cracking of isoparaffins explain why only small amounts of branched intermediates with the same molecular weight as the reactant are found in the products from hydrocracking of n-paraflfins. 

The transposases of E. coli Tn5, Tn7, and TnlO act by hydrolytically cutting both strands of duplex DNA at the transposon ends leaving the phospho groups attached to the 5 cut ends, as is depicted in detail in Eq. 27-14, step a. The two 3 ends then carry out transesterification reactions, as in Eq. 27-14, step b. These two steps are used to nick both strands of the DNA carrying the transposon and to join them to a target DNA sequence to give a branched intermediate (Eq. 

Thr252Ala/Asp251 Asn, in which the stoichiometric ratio of the two pathways is equal at room temj>erature . As primary proton transfer is also visible at low temperature in this mutant construct, prior to decay to the resting state, one is able to definitely assess that the protonated peroxo intermediate serves as the branching intermediate of these two pathways. 

An alternative description of the transfer to polymer process has been suggested by Goto et al. [35]. They also assume the transfer to polymer rate, eq (4.6-9), to be proportional to the chain length of the reacting polymer. By introducing the P-scission step, eq (4.6-10), for branched intermediate macroradicals Rq, the steep rise in toward high conversion is avoided. 

Figure 13.19 Proposed protein splicing mechanisms. Two possible mechanisms for protein splicing of spUce jnnctions with Cys as the N-terminal residnes of intein and C-extein. The initial step involves either N-S shift (or N-O shift for Ser as the N-terminal residue of intein) at the N-intein jnnction or nncleophUic attack by the N-terminal residne of C-extein. The function of the pennltimate His is not explicitly indicated, however it may acts as general base-acid to assist this and snbseqnent step. Both mechanisms involve the formation of the branched intermediate which nndergoes splicing to remove the intein as the succinimide derivative and the extein prodnct after the N-S (or N-O) shift...

Wallace, P.A., D.E. Minnikin, K. McCrudden, and A. Pizzarello, Synthesis of (R,5)-10-Methyloctadecanoic Acid (Tuberculostearic Acid) and Key Chiral 2-Methyl Branched Intermediates, Chem. Phys. Lipids 71 145-162 (1994). 

Studies of this type have been used by Smit and co-workers to explain the so-called inverse shape selectivity observed in the conversion of long chain w-alkanes over acid zeolites. In such reactions, product distributions are found to depend on the pore structure, particularly for medium-pore zeolites such as ZSM-5. In some cases branched alkanes are favoured over linear alkanes in the products of medium-pore zeolites compared to the reaction selectivities of large-pore zeolites such as zeolite Y. For example, doubly branched isomers are favoured over ZSM-5. This is in contrast with what would be expected from dilfusion rates and is attributed to the enhanced thermodynamic stability of some branched intermediates in the medium-pore zeolites that is predicted by configurational bias GCMC. 

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