Induction slow reaction

It has been well known for a long time that some reactions taking place at a very slow rate may be markedly accelerated by the simultaneous occurrence of another reaction of measurable velocity. On the suggestion of Kessler this phenomenon is called chemical induction and it is said that the reaction of measurable velocity induces the other slow reaction. 

In consideration of external effects, it is essential to emphasize that under some conditions the thermal induction period could persist for a very long period of time, even hours. This condition arises when the vessel walls are thermally insulated. In this case, even with a very low initial temperature, the heat of the corresponding slow reaction remains in the system and gradually self-heats the reactive components until ignition (explosion) takes place. If the vessel is not insulated and heat is transferred to the external atmosphere, equilibrium is rapidly reached between the heat release and heat loss, so thermal explosion is not likely. This point will be refined in Section C.2.a. 

Figure 5. Regions of ignition, slow reaction with and% without the pic darret in propane oxidation at 429°C. Bars on the normal flames boundary represent experimental points and numbers on this curve signify duration of induction periods (in seconds). Numbered regions are defined in the text...

Where 1,3-dioxolane is the nucleophile then the kinetic product equivalent to 15 is sterieally unfavoured, and so a slow reaction to thermodynamic products 17 equivalent to 16, seems most likely to account for the observed induction period... 

Kach method suffers from one or more inherent sources of error. Method 1 is not readily adaptable to the determination of second explosion limits. If temperature equilibrium is reached very quickly by the gas flowing into the vessel, as the continued flow causes the pressure to increase, the system must first intersect the lower explosion limit. Method 2 can lead to large errors if explosion is preceded by an induction period. In the carbon monoxide-oxygen reaction, for example, it was found that the heating rate could considerably affect the results owing to the existence of a zone of slow reaction adjacent to the second limit and inhibition of the reaction by the product, carbon dioxide... 

In our earlier work [13] we studied the ethanolysis of TMOS. It was noticed that in slow reactions that were catalyzed by carboxylic acids, a change in rate occurred. After an initial induction period, the rate would increase. It was postulated that an initial equilibrium between the alcohol and the catalyst to form some sort of complex was required to occur before the rate of the alcoholysis reaction would become rate determining ... 

Kinetic curves obtained for a sufficiently slow reaction (low initiator concentration, low temperature, copolymerization in solution) show a sigmoidal shape 35,36,45, 52> 74> (Fig. 6) with an induction period. 

Some ketones such as /3-dicarbonyls contain substantial amounts of the enol at equilibrium. For example, acetylacetone in aqueous solutions contains 13% of 4-hydroxypent-3-en-2-one, which is stabilized both by an intramolecular hydrogen bond and the inductive effect of the remaining carbonyl group.17 When bromine is added to such a solution, a portion is initially consumed very rapidly by the enol that is already present at equilibrium. The ketone remaining after consumption of the enol reacts more slowly via rate-determining enolization. The slow consumption of bromine is readily measured by optical absorption. In acidic solutions containing a large excess of the ketone the slow reaction follows a zero-order rate law the rate is independent of bromine concentration, because any enol formed is rapidly trapped by bromine (Scheme 1). In this case, the amount of enol present at equilibrium may be determined as the difference between the amount of bromine added and that determined by extrapolation of the observed rate law to time zero, as is shown schematically in Fig. 2. 

Kinetic Order of the Reaction. The decomposition of the hydroperoxide had an initial slow reaction or induction period followed by a faster main reaction. The induction period was unaffected by the addition of dilauryl sulfinyl dipropionate or by carrying out the reaction in an atmosphere of nitrogen but was eliminated by the addition of acetic acid. The length of the induction period decreased as the initial concentration of both hydroperoxide or sulfur compound increased. 

In Fig. 8 the amounts of A, B and C at any time are given by the three curves A, B and C. A decreases exponentially as a simple first order reaction. If there were no intermedia1 e step such as B, C would accumulate at the same rate that A decomposed. It would follow the formula 1 — e °-u as given by the dotted line D. However, A decomposes first into B and the decomposition of B is slower than its formation so that there is an accumulation of B increasing up to a maximum and then decreasing as the supply from A becomes less. The actual curve of production of C does not follow a simple law and the induction period of slow reaction is plainly evident at the beginning. If only B is being determined by chemical analysis, the reaction rate will not be easily understood since the concentration will first increase and then decrease. 

Fig. 5.13 General form of curves relating the fraetion of C3S consumed to time in a paste. AB initial reaetion. BC induction period. CD acceleratory period. DE deceleratory period and continuing, slow reaction. Taylor et al. (T25).

Polymerisations proceeded without an induction period. The monomer concentration vs. time plots indicated extremly slow reactions with half lives of the order of 50 h, but unfortunately the conditions of these experiments were not specified. 

A more recent study of the Dj + O2 reaction by Baldwin et al. [246] has involved measurements of the second limits, and the induction periods and maximum rates of the slow reaction in an aged boric acid coated vessel of 52 mm diameter. Maximum concentrations of D2 O2 in the slow reaction were also determined. The kinetic parameters of the oxidation process were then determined by a computer optimization treatment similar to that described in Sect. 4.3.3 for the H2 + O2 reaction. Excluding the primary initiation rate 6 which is necessary for the calculation of induction periods, but which needs to be only approximately defined, there are a minimum of seven significant parameters (cf. Table 18). 

First, there is an inhibiting effect of the additive on the low pressure explosions, so that the second limit pressure is reduced and the first limit is raised on addition of the hydrocarbon [329—331]. Secondly, there may be an increase in the maximum rate (of decrease of pressure) in the slow reaction [330]. Thirdly, induced explosions may occur in some cases (not with methane) at pressures outside the + O2 explosion peninsula. In most cases such induced explosions appear as one sharp explosive reaction. However, they are sometimes characterized (e.g. with CaHg at 560 °C) by an induction period during which there is a rapid pressure increase, and this is followed immediately by a very rapid pressure decrease in the system. It is probable that all the induced explosions follow this two-stage pattern. This type of explosion does not occur in H2—Oj—CH4 mixtures because methane is not as reactive as propane in... 

Polymerization and Cyclization of Alkynes (163). In 1940 Reppe and Schweckendieck 163) discovered that the substitution derivatives of tetracarbonylnickel with phosphines, and particularly those of the type Ni(CO)2(PR3)2, catalyze the linear polymerization and the cyclization to benzene derivatives of alkynes. Schrauzer 167) more recently showed that stoichiometric amounts of bisacrylonitrilenickel give the cyclization reaction. The reaction becomes, on the contrary, catalytic in the presence of a tertiary phosphine. Meriwether and co-workers (57, 136, 137, I40) studied the possible mechanism of Reppe s reaction. They showed that both the polymerization and the cyclization reactions are particularly effective with monosubstituted acetylenes, and that the cyclization reaction is infiuenced by steric and electronic effects. They then proposed a mechanism by which the induction time is ascribed to this rather slow reaction ... 

Much faster reaction can be achieved with strong bases (/07). The chain is started by an N-acylimide which may be N-caproyl-caprolactam produced in a slow reaction from the monomer during an induction period or an N-acylamid produced by action of a cocatalyst like for example carboxylic acid chlorides or anhydrides on caprolactam. The cocatalyst action speeds up the reaction such, that fast polymerization below the melting point of the polymer becomes possible. The strong base, such as alkali metal, metal hydride, metal amid, or oiganometallic compound, activates the monomer by lactam anion formation ... 

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