Seasoned reaction vessels

The first significant attempt to study the kinetics of the thermal decomposition of chlorinated and brominated hydrocarbons and related compounds was by Lessig in 1932. These studies were complicated by the catalytic activity of the pyrex or quartz glass walls of the vessels in which the gas phase reactions were carried out. It was later found that the heterogeneous reactions could be suppressed by using seasoned reaction vessels in which a fine coating of carbonised material had formed on the walls from previous decompositions of similar compounds. Substances such as allyl bromide and 3-chloro-2-methyl-propene have been found particularly effective in producing carbonised films. 

It is found that in clean reaction vessels, rates often tend to be both fast and irreproducible. This led to the technique of working in seasoned reaction vessels (Brearley et al., 1936 Ogg and Polanyi, 1935), that is, vessels that had had a large number of runs done in them, or where... 

The reaction is carried out in a 150-mL heavy-wall Monel pressure vessel rated to 350 bar, equipped with an 0.25-in. NPT opening and corresponding adapter with a Monel high-pressure valve with PTFE stem packing, rated to 700 bar. About 2 g of pulverized anhydrous nickel(II) fluoride is put into the vessel. The vessel is prefluorinated once or twice with about 500 mbar of fluorine and subsequently once or twice with about 500 mbar of chlorine trifluoride for an hour under occasional heating up to 300-400°C. Into the seasoned reaction vessel, cooled to - I96°C, about 40 mmol of chlorine trifluoride and about 400 mmol of fluorine are condensed. 

It is found that in clean reaction vessels, rates often tend to he both fast and irreproducible. This led to the technique of working in seasoned reaction vessels (Brearley et al., 1936 Ogg and Polanyi, 1935), that is, vessels that had had a large number of runs done in them, or where surfaces had been rendered inactive by a carbonaceous layer such as is deposited by decomposing allyl bromide (MaccoU, 1955). When such vessels are used, it is found that the rate is usuaUy much smaller than in clean vessels and also that the kinetic behaviour becomes reproducible. If packing the vessels with glass and re-seasoning them has no effect upon the rate, then it is reasonable to assume that the reaction is homogeneous if it is molecular, or, if it is of a radical chain character, that the chains are both initiated and terminated upon the walls. 

The trans -> cis isomerisation of octafluorobut-2-ene is claimed by Schlag and Kaiser " to be a clean unimolecular reversible reaction. Studies by the static method using a seasoned quartz reaction vessel at 430-477 °C yielded the Arrhenius parameters, A = 3.4x 10 sec E = 56.4 kcal.mole L The E value is 6.4 kcal.mole lower than for the corresponding hydrocarbon - . Craig and Entemann have compared the standard enthalpy change for the cis trans isomerisation of 1,2-difluoroethylene with those for related halogenated ethylenes. The reported values are 4-928 cal.mole for difluoroethylene, 4-500 cal.mole for dichloroethylene, nearly zero for dibromoethylene, and —2000 cal.mole for diiodoethylene. 

Class 1. Characteristics (i) The decomposition in seasoned vessels exhibits first-order kinetics and there are no apparent induction periods (i7) the rate is unaffected by packing the reaction vessel or by the addition of known radical-chain inhibitors such as propene iii) the Arrhenius pre-exponential factor is of the order of 10 sec L This behaviour is consistent with a unimolecular mechanism for the decomposition. Among reactions in this class are included the dehydrohalogena-tion of monochlorinated saturated hydrocarbons containing /S-hydrogen atoms e.g., chloropropane 2-chloropropane f-butyl chloride) and of most of the secondary and tertiary monobrominated saturated hydrocarbons. 

The reaction was studied at nitric oxide and hydrogen iodide pressures of 40-500 and 120-750 torr, respectively. The reaction rate was determined by analyzing for NO and I2 at various times and, in some cases, for ammonium iodide and hydrogen iodide. In clean Pyrex vessels results were not reproducible, but in vessels seasoned with a carbonaceous deposit reproducibility was satisfactory and the reaction was observed to be homogeneous. The rate law was determined to be second order... 

Heterogeneous or surface effects have been found to complicate the interpretation of kinetic experiments, which lead to erroneous Arrhenius parameters. However, with special precautions involving the use of seasoned vessels and the presence of a free-radical suppressor, the errors are minimized. Consequently, the present chapter will cover mostly homogeneous gas-phase processes. Studies on chemical activation, the use of catalysts, the bimolecular gas phase and heterogeneous reactions are not included. As an attempt to describe important pyrolyses data from 1972 to 1992, this review does not pretend to offer a complete coverage of the literature. 

The pyrolysis of 2-bromo-2-butene in a static system, with seasoned vessels, and even in the presence of a free radical inhibitor, was autocatalyzed by the HBr product109. However, under maximum catalysis with HBr gas, the reaction is molecular in nature and follows first-order kinetics. The overall rate coefficient was given by the following Arrhenius equation log kx (s-1) = (13.57 0.56)-(200.4 6.8) kJmol-1 (2.303R7)-1. The mechanism was suggested to involve a six-membered cyclic transition state as described in equation 22. 

Ethyl chloroformate was pyrolyzed in the gas phase at 286-353 °C190. The reaction, in a seasoned vessel and in the presence of an inhibitor, was found to be homogeneous and... 

The thermal decomposition is essentially homogeneous in a seasoned vessel, the heterogeneous component is not more than a few percent On the other hand, the reaction rate is different with a clean surface. The same applies to the reaction order and, to a certain extent, to the nature of the products as well ° . Only the investigations carried out in seasoned vessels will be dealt with. 

The thermal decomposition of propionaldehyde is a homogeneous reaction in a seasoned vessel - however, in a clean one, the reaction proceeds more slowly. 

Static system no surface effect was observed in seasoned vessels. First order to only 45 % reaction. 

If packing the vessels with glass and re-seasoning them has no effect upon the rate, then it is reasonable to assume that the reaction is homogeneous if it is molecular, or, if it is of a radical chain character, that the chains are both initiated and terminated upon the walls. 

From a kinetic standpoint, this field has been extensively studied and reviewed by MaccolE - . Heterogeneous reactions, which give irreproducible kinetics, can be minimised by the use of seasoned vessels and the rigid ex-... 

Another complication with gas phase pyrolyses is that many possible nonconcerted reaction pathways are possible. Alkyl halides xmdergo elimination in the gas phase, and some compounds, such as ethyl chloride, appear to undergo unimolecular elimination. Their unimolecular decompositions may involve transition structures with significant carbocation character. For example, p5u-olysis of (-l- )-2-chlorooctane in the gas phase at 325-385°C was found to produce racemization of the starting material as well as elimination of HCl. Some compounds appear to react by radical chain mechanisms, and heterogeneous radical reactions often complicate studies that are not carried out in "well-seasoned" (i.e., coated with a layer of organic material) vessels. Furthermore, there appears to be a significant radical (but not radical chain) component to the pyrolysis of sulfoxides. These complications mean that many control studies are necessary to clarify the mechanism of gas phase elimination reactions. 

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