| dc.description.abstract | The thesis pertains to investigations relating to the synthesis and oxidation of substituted hydrazines. The contents of this thesis can be categorised into two parts. The first part concerns the synthesis of hydrazines and the second part comprises the studies pertaining to the oxidation of methyl?substituted hydrazines by gaseous oxygen.
Hydrazine and substituted hydrazines, particularly monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH), have been extensively used as rocket fuels. The various derivatives of hydrazine have also found immense application in agriculture, pharmaceuticals, metallurgical and plastic industries. However, in spite of their wide applications, most of the methods available in the literature for the synthesis of hydrazines produce them only as dilute (1–2%) aqueous solutions. The preparation of anhydrous hydrazines from dilute aqueous solutions involves multiple distillation, extraction etc., which are cumbersome and expensive processes. Other reported methods are multistep processes requiring relatively expensive or inaccessible starting materials. Apart from the cost, some of the methods involve carcinogenic substances as intermediates. Alternate methods for the synthesis of hydrazines are, therefore, needed. One of the reactions which has been investigated as the basis for an alternate process in recent years is the chloramination of an amine. The reaction of anhydrous chloramine (NH?Cl) with amines dissolved in a non?aqueous solvent may provide an attractive method for the synthesis and also for easy recovery of the corresponding hydrazines. The research reported herein explores the possibility of adopting this method for the synthesis of substituted hydrazines.
The synthetic methods reported for preparing the tri? and tetra?substituted hydrazines are even more complex. However, using the chloramine method it could be envisaged that these hydrazines could also be synthesised by making use of the substituted chloramines instead of chloramine (reverse chloramine method). Substituted chloramines could be produced by the reaction of chlorine with an amine. These chloramines, on reacting with amines, could possibly give the corresponding hydrazine. It is further believed that investigations on the reverse chloramine process may provide an insight into the mechanism of the chloramine–amine reaction.
The redox systems involving hydrazines are highly interesting since a variety of products are formed depending on the hydrazine and oxidising agent chosen. Of the various oxidations, the oxidation by gaseous oxygen is important. Although due to their rapid reaction with oxygen, hydrazines pose no serious health hazards in case of their accidental spillage, this reaction is absolutely undesirable during the storage of hydrazines as it leads to the deterioration of their chemical properties. For example, an exposure of UDMH to oxygen is known to form formaldehydedimethylhydrazone (FDMH), a product which, once formed, is difficult to separate. It is therefore necessary to understand the mechanism of this reaction in detail, to effectively store, handle and utilise hydrazines. Since only little information is available, an exploration to this effect has been carried out by studying the oxidation of liquid MMH and UDMH by gaseous oxygen.
The work presented in this thesis has been divided into five chapters. Chapter 1 gives an introduction to the present work and incorporates a pertinent literature survey. The various synthetic methods reported for the synthesis of hydrazines have been reviewed in the light of their merits and demerits. This chapter also includes the available important oxidation studies on hydrazines.
Chapter 2 is devoted to the chloramine generation studies. Chloramine has been produced by the reaction of gaseous chlorine and ammonia using a modified generator. The effect of various parameters such as the temperature of the reactor wall, the introduction of excess nitrogen as carrier gas, the introduction of excess ammonia over the stoichiometric equivalent of chlorine, the time of the experimental run, the variation of chlorine input on the yield of chloramine has been evaluated. Various designs of the generator have been conceived and fabricated. The effect of the design parameters on the yield of chloramine generation has been studied. The yield of chloramine has been optimised by varying the various parameters.
Chapter 3 comprises the studies on synthesis of hydrazines by the anhydrous chloramine and the reverse chloramine processes. Various substituted hydrazines such as monomethylhydrazine, symmetrical and unsymmetrical dimethylhydrazine, phenylhydrazine, p?tolylhydrazine, 2?aminoethylhydrazine, N?aminopyrrolidine and N?aminopiperidine have been synthesised in non?aqueous solvents such as ethylene glycol, methanol, diglyme and dichloromethane. The mechanisms of chloramine and reverse chloramine processes have been investigated in detail. The role of the fixed base in the chloramine process has been determined. The effect of amine/chlorine and base/chlorine ratios on hydrazine yields has been evaluated.
Chapter 4 deals with the studies on the kinetics of oxidation of liquid MMH by gaseous oxygen. Nitrogen, methane, methanol, formaldehydemonomethylhydrazone and water have been identified as products of oxidation. The oxidation study has been conducted in specially designed glass vessels, monitoring the kinetics on a gas chromatograph. Based on the products formed and the kinetics of the reaction, a suitable mechanism has been proposed for the oxidation process. The rate law expression derived indicates that it follows second?order reaction kinetics.
In Chapter 5, investigations on the kinetics of the oxidation of UDMH liquid by molecular oxygen are reported. Here again, the kinetics was followed using a gas chromatograph. Based on kinetic evidence and the products formed, two mechanisms for the oxidation process have been proposed and rate laws derived. It appears that the oxidation process follows first?order kinetics with respect to oxygen, and not zero order as postulated earlier. | |
