Reactions, slowed

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

No intennolecular reaction of malonate or /3-keto esters with halides has been reported, but the intramolecular reaction of /3-diketones such as 790 and malonates proceeds smoothly[652,653]. Even the simple ketone 791 can be arylated or alkenylated intramolecularly. In this reaction, slow addition of a base is important to prevent alkyne formation from the vinyl iodide by elim-ination[654]. 

The concentrations of both A and B, hence the reaction slows down as the conversion to polymer progresses, and. ... 

The rate of reaction slows down as the conversion to tertiary amine increases and primary amine concentrations drop below 1%. Conversion to 100% tertiary amine is difficult. 

In a 2-1. round-bottom flask, fitted with an efficient reflux condenser, is placed 500 g. (5.7 moles) of ethyl acetate (Note i), and 50 g. (2.2 moles) of clean sodium wire or finely sliced sodium (Note 2) is added. The reaction is at first quite slow, and must be started by warming on a water bath (Note 3). After the reaction is once started it proceeds vigorously and cooling is then necessary in order to avoid loss of material through the condenser. When the rapid reaction slows down, the reaction mixture is heated on a water bath until the sodium has completely dissolved. This usually requires about one and one-half hours. At this stage the reaction mixture should be a clear red liquid with a green fluorescence. 

The left-hand end of the activated monomer is sealed off by the OH terminator, but the right-hand end (with the star) is aggressively reactive and now attacks another ethylene molecule, as we illustrated earlier in Fig. 22.1. The process continues, forming a longer and longer molecule by a sort of chain reaction. The —OH used to start a chain will, of course, terminate one just as effectively, so excess initiator leads to short chains. As the monomer is exhausted the reaction slows down and finally stops. The DP depends not only on the amount of initiator, but on the pressure and temperature as well. 

Physiological response glycogen breakdown visual excitation histamine secretion in all allergic reactions slowing of pacemaker activity that controls the rate of the heartbeat... 

Steensma, M., and K. R. Westererp (1990). Thermally Safe Operation of a Semibatch Reactor for Liquid-Liquid Reactions. Slow Reactions. Ind. Eng. Chem. Res. 29, 1259-70. 

IS further extended by the replacement of X by other anionic or neutral ligands. The inertness of the compounds makes such substitution reactions slow (taking hours or days to attain equilibrium) and, being therefore amenable to examination by conventional analytical techniques, they have provided a continuing focus for kinetic studies. The forward (aquation) and backward (anation) reactions of the pentaammines ... 

Overall, the partial pressure of N204 drops, and the forward reaction slows down. Conversely, the partial pressure of N02 increases, so the rate of the reverse reaction increases. Soon these rates become equal. A dynamic equilibrium has been established. 

The actual result is this The chains are broken when Cu24 is added. The reaction slows considerably but does not come to a halt. Kinetic studies show that the order with respect to [RM] drops from f to 1 with Cu2+ added and that the order with respect to [02] rises from 0 to 1. One term shows an inverse first-order dependence on [Cu2+], The rate under these conditions became... 

Most reactions slow down as the reactants are used up. In other words, the average reaction rate decreases as the reaction proceeds. The reaction rate may also change over the time interval during which the change in concentration is being measured. 

The explanation for autoacceleration is as follows. As polymerisation proceeds there is an increase in the viscosity of the reaction mixture which reduces the mobility of the reacting species. Growing polymer molecules are more affected by this than either the molecules of monomer or the fragments arising from decomposition of the initiator. Hence termination reactions slow down and eventually stop, while initiation and propagation reactions still continue. Such a decrease in the rate of the termination steps thus leads to the observed increase in the overall rate of polymerisation. 

In perchlorate media the latter reaction was found to be rapid (100 % exchange in < 60 sec) and the former reaction slow (half times of > 200 hours at 100 °C). The isotopic method was used ( Am). 

Arrhenius plots of both 1u( h) and ln(A D) versus T were found to be curved significantly, exhibiting decreasing dependence on temperature at lower temperatures. As described in the Introduction, such nonlinear Arrhenius plots are telling indicators of QMT as temperature is lowered, the classical overbarrier reaction slows more significantly than does one that proceeds via tunneling. Later work showed that the abstraction reaction of 2 to 3 persisted at a measurable rate at least down to 28 K in frozen media, where the decay became nearly temperature independent. 

The results of the kinetic study and model discrimination show that insertion of SM is rate-controlling. Two reasons may explain why this step is ratecontrolling. First, the protection group in om SM is very bulky, making the reaction slow, which is consistent with literature data (8) showing the size effect on reactivity. Second, a free aniline group in the SM could bond with Rh and reduce the catalyst reactivity. 

If the aqueous phase containing the ertzyme is recycled, the reaction slowed down. Presumably, this is caused by the build-up of an organic impurity in the aqueous... 

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

Figure 1 compares the conversion predicted for any reduced time I = k t with the use of Keller s theory and the above values of k /k and k2/k with experimental results obtained when polyacrylamide was exposed to 0.2N NaOH at 53 C. It may be seen that the reaction slows down at large x much more than predicted by Keller s model. In fact, this decrease of the reaction rate is even more pronounced than predicted by Keller s equations for the case where a single reacted nearest neighbor completely inhibits amide hydrolysis. We believe that this discrepancy is due to the repulsion of the catalyzing hydroxyl ions from amide residues by non-neighboring carboxylate groups. 

Hydrolysis appears to be the most important abiotic degradative mechanism for organophosphate esters under basic pH conditions. Under neutral and acidic conditions, the reaction slows considerably and could become an insignificant removal mechanism. The hydrolysis proceeds by a stepwise mechanism in which one alcohol group is removed at a time. The first step is cleavage of a P-OR bond (where "R" is an aryl or alkyl group) to produce a diester of phosphoric acid, which, under basic conditions, becomes an anion. 

During step growth polymerization, the rate of reaction slows down as the reaction progresses. What factors are responsible for this observation ... 

In aqueous solutions, ROOH can react with p-methoxyphenol at pH > 8 however, at pH >10 the reaction slows down due to ROOH dissociation and a weak oxidative ability of the ROz anion. 

---