Reactions Accompanied by Gas Evolution

When a homogeneous or heterogeneous reaction produces a gaseous product, the reaction can be followed by measuring or recording the volume of gas as a function of time. 

When the rate of gas formation is not very fast, an apparatus depicted in Fig.2.13 can be used. One reactant solution is pipetted into the thermostatted funnel A and the other reactant is placed in flask B. When both are thermally equilibrated by water pumped from a thermostat into their jackets, by opening the funnel s stopcock, the solution in A is mixed with the contents of B while magnetically stirring throughout the run. The gas evolved is collected in the gas burette C and its volume measured at timed intervals after adjusting the level of liquid in the tube D with that in C and simultaneously noting the temperature of the thermometer attached between C and D and the atmospheric pressure on a barometer in the laboratory. For precise data, the atmospheric pressure is corrected for the water vapour pressure at the same temperature as C and D. In this case, the volumes measured are adjusted to give values at the same temperature and pressure. 

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It should be mentioned that such kinetic behavior of gas evolution (Figure 10.8) is typical for multistage successive reactions which are the first order ones with decreasing rate constants for the each subsequent stage [38]. At constant temperature the dependence of conversion on time (t) = a (t)/ a ( infinity ) in coordinates [- ln(l - tj ), t is linear plots following one by one. Indeed, if there are a few successive reactions accompanied by gas evolution on each stage (Eq. 10.2). 

Figure 2.13 Apparatus for studying a reaction accompanied by gas evolution.

A series of 2- and 4-nitroaniline derivatives and analogues when heated with cone, sulfuric acid to above 200°C undergo, after an induction period, a vigorous reaction. This is accompanied by gas evolution which produces up to a 150-fold increase in volume of a solid foam, and is rapid enough to be potentially hazardous if confined. 

A. N,N-Dimethylchloromethylenammonium chloride. A 500-mL, three-necked, round-bottomed flask is equipped with a magnetic stirring bar, thermometer (Note 1), and a three way stopcock fitted with a drying tube containing anhydrous calcium chloride and a rubber septum. The flask is charged with 50 mL of dichloromethane (Note 2) and 3.07 g (0.042 mol) of N,N-dimethylformamide (Note 3) added throu i the septum from a syringe, and cooled in an ice bath. To the cooled mixture is slowly added 5.23 mL (0.06 mol) of oxalyl chloride (Note 4) by means of a syringe. The addition 1s accompanied by gas evolution and formation of a white precipitate. The reaction mixture is stirred for an... 

Ariga et al. [48] have investigated the behavior of the monolith reactor in which Echerichia coli with P-galactosidase or Saccharomyces cerevisiae was immobilized within a thin film of K-carragcenan gel deposited on the channel wall. The effects of mass transfer resistance and axial dispersion on the conversion were studied. Those authors found that the monolith reactor behaved like the plug-flow reactor. The residence-time distribution in this reactor was comparable to four ideally mixed tanks in series. The influence of gas evolution on liquid film resistance in the monolith reactor was also investigated. It was shown that at low superficial gas velocities, the gas bubble may adhere to the wall, which decreases the effective surface area available for the reaction. The authors concluded that the reactor was very effective in the reaction systems accompanied by gas evolution, such as fermentations. 

The soluble uranium species produced was stable at 200°C for at least three hours, at which time the temperature was gradually increased from 200°C to 300°C over a period of one hour. Suspended solids were first noted at 300°C. The temperature was increased to 350°C for one hour, then to 400°C for 0.5 hour. The solids produced through the thermal decomposition of the soluble uranium species were accompanied by gas evolution. We did not determine if this gas evolution was simply removal of excess nitric acid from the melt or a decomposition product. The solids were formed uniformly throughout the melt and had a gelatinous appearance. These solids settled very slowly to the bottom of the reaction tube. The final product of this second test was also recovered by aqueous dissolution of... 

Typical procedure. 2,9-Dicyano-l,W-phenanthroline 1391 [1058] To DMF (200 mL), oxalyl chloride (5.7 mL, 0.066 mol) was added with stirring at 0 °C under argon atmosphere. A white precipitate formed immediately, which was accompanied by gas evolution. When the gas evolution had ceased, a solution of the diamide 1390 (7.0 g, 0.026 mol) in DMF (150 mL) was added to the reaction flask. The resulting yellow mixture was stirred for 6 h at 0 °C. Pyridine (9.4 mL, 0.116 mol) was then added and, after stirring for a further 30 min, the mixture was neutralized with saturated aq. Na2C03 solution (500 mL). Some precipitate was formed, and precipitation was completed by adding water (700 mL). After filtration, the pale-yellow... 

Figure 2.14 A sketch of an apparatus for studying reactions accompanied by fast evolution of gas.

Several explosions or violent decompositions dining distillation of aldoximes may be attributable to presence of peroxides arising from autoxidation. The peroxides may form on the -C=NOH system (both aldehydes and hydroxylamines perox-idise [1]) or perhaps arise from unreacted aldehyde. Attention has been drawn to an explosion hazard inherent to ketoximes and many of their derivatives (and not limited to them). The hazard is attributed to inadvertent occurence of acidic conditions leading to highly exothermic Beckmann rearrangement reactions accompanied by potentially catastrophic gas evolution. Presence of acidic salts (iron(III)... 

The kinetics of the reaction need to be known or measured, in particular the rate constant and how it may be affected by temperature. Many gas-liquid reactions, like chemical reactions generally, are accompanied by the evolution or absorption of heat. Even if there are arrangements within the reactor for the removal of heat (e.g. cooling coils in a stirred tank reactor), it is unlikely that the temperature will be maintained constant at all stages in the process. Experimental methods for measuring the kinetics of reactions are considered in a later section. 

The most striking example of these reactions is the Chichibabin amination performed in its classical variant under heterogeneous conditions. The reaction is accompanied by the evolution of hydrogen gas that is conveniently monitored. Presumably, on elimination, the hydride ion is combined with an acidic proton, likely from an amino group 4, to form a hydrogen molecule the reaction mechanism without details is shown in Scheme 3. 

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