Accepted Manuscript Title: Pyrolysis of hydrazine derivatives and related compounds with N-N single bonds Author: Abd-El Aal M. Gaber Curt Wentrup PII: S0165-2370(17)30020-7 DOI: http://dx.doi.org/doi:10.1016/j.jaap.2017.03.016 Reference: JAAP 4003 To appear in: J. Anal. Appl. Pyrolysis Received date: 7-1-2017 Revised date: 10-3-2017 Accepted date: 15-3-2017 Please cite this article as: A.-E.A.M. Gaber, C. Wentrup, Pyrolysis of hydrazine derivatives and related compounds with N-N single bonds, Journal of Analytical and Applied Pyrolysis (2017), http://dx.doi.org/10.1016/j.jaap.2017.03.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 1 Pyrolysis of hydrazine derivatives and related compounds with N-N single 2 bonds 3 4 Abd-El Aal M. Gabera,* and Curt Wentrupb,* 5 6 a Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt 7 b School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, 8 Queensland 4072, Australia 9 10 ∗Corresponding authors, E-mail addresses: [email protected] (Gaber), [email protected] (Wentrup). 11 12 Highlights 13 Pyrolysis of all types of hydrazine derivatives are reviewed 14 N-aminoheterocycles, N-aminoisocyanides, and N-aminoisocyanates are included 15 Aminimides, azimines, and tri-, tetra-, penta- and hexazenes are covered 16 Pyrolyses in solution, in the solid state, and under flash vacuum pyrolysis conditions 17 18 ABSTRACT 19 Pyrolysis of hydrazines and their derivatives often results in homolytic cleavage of the N-N 20 bonds, but molecular rearrangements and eliminations are also observed in many cases. 21 Thermal reactions in solution, in the solid state, and in the gas phase under static, flow, and 22 flash vacuum pyrolysis conditions are considered in this review. Arrhenius or Eyring 23 activation parameters are reported whenever available, but the original literature should 24 always be consulted, as these parameters are often derived under widely different 25 experimental conditions. The compound classes investigated include open-chain and cyclic 26 hydrazine derivatives and their relationship with azomethine imines, N-amino-heterocycles, 27 N-isocyanoamines and N-isocyanatoamines, N-aminocarbodiimides, azines, hydrazones, 28 semicarbazidesAccepted and semicarbazones, hydrazides, Manuscript aminimides, amidrazones, azimines, 29 triazenes, tetrazadienes, pentazadienes and hexazatrienes, and triazines. Understanding of 30 reaction mechanisms is emphasized whenever possible. Reactive intermediates involved in 31 these pyrolysis reactions include ketenes, ketenimines and other cumulenes, free radicals and 32 diradicals, zwitterions, carbenes, nitrenes, nitrile ylides, nitrile imines, arynes and azetes. 33 34 Contents 1 Page 1 of 41 35 1. Introduction 36 2. Hydrazines 37 3. Cyclic hydrazine derivatives and azomethine imines 38 4. N-Amino-heterocycles 39 5. Azines 40 6. Hydrazones 41 7. Isocyanoamines R2N-NC from heterocyclic hydrazones and semicarbazones 42 8. N-Isocyanatoamines (amino isocyanates) R2N-N=C=O 43 9. N-Aminocarbodiimides R2N-N=C=NR’ 44 10. Semicarbazides 45 11. Hydrazides 46 12. Aminimides 47 13. Amidrazones 48 14. Triazane and triazenes 49 15. Tetrazenes, pentazenes and hexazenes 50 16. Azimines (azo imides) 51 17. 1,2,3-Triazines 52 18. Conclusion 53 54 Keywords: Aminyls; N-aminoheterocycles; Aminimides; N-isocyanoamines; N- 55 isocyanatoamines; Azimines 56 57 1. Introduction 58 59 Hydrazine and its derivatives are ubiquitous in organic synthesis, in medicinal 60 chemistry, and, of course, as rocket fuels. The relatively weak N-N single bond lends itself to 61 investigation of thermal rearrangements and fragmentations, and such reactions have indeed 62 been investigated for over 125 years. Yet, there has been no comprehensive review dedicated 63 to the pyrolysisAccepted chemistry of hydrazine and its derivatives Manuscript since Hurd’s classic 1929 book on 64 pyrolysis of carbon compounds[1]. Hurd’s book is a treasure trove of reactions worthy of 65 reinvestigation using modern methods, but these reactions will not be repeated in full here. A 66 treatise on the chemistry of the hydrazo, azo and azoxy groups[2] includes a chapter on 67 thermochemistry of hydrazines[3]. The benzidine rearrangement is usually carried out under 68 acid catalysis, but the purely thermal benzidine rearrangement is also known. These reactions 69 have been reviewed extensively[4,5,6] and will not be repeated here. A review on short- 70 contact time, radical-forming gas-phase pyrolyses, which includes some hydrazine 71 derivatives, has been published[7]. We will deal primarily with pyrolysis reactions of 72 hydrazine derivatives understood in the widest sense. This will include some triazenes, 2 Page 2 of 41 73 azimines, and 1,2,3-triazines inasmuch as N-N single bond cleavage is involved, but 74 fragmentation of the aromatic pyrazoles[8], triazoles[10] and tetrazoles[10] will be excluded, 75 as pyrolyses of these compounds have been described elsewhere. Likewise, azides are 76 excluded, as they have been covered recently [10]. The formation of diazoalkanes and hence 77 carbenes by thermolysis of salts of tosylhydrazones (Bamford-Stevens reaction)[11] will not 78 be covered in detail, as numerous reviews and books are available. It will be mentioned 79 briefly in Section 6 together with other pyrolysis reactions of hydrazones. The highly 80 interesting thermal chemistry of nitrile imines RCNNR’ will also not be covered here, as 81 recent reviews are available[10,12]. Nitrosamines R2N-NO and nitramines R2N-NO2 are 82 outside the scope of this review, but it is noted that the environmentally important assay of 83 nitrosamines is often performed by pyrolysis-GC-TEA, which involves thermal cleavage to 84 aminyl and NO radicals[13]. Nitramines are important explosives of the HMX and RDX 85 type, whose decomposition is initiated by homolysis of the weak N-NO2 bonds[14], but 86 several other free radical and molecular decompositions, e.g. forming CH2=N-NO2, occur 87 too[15,16]. In this review thermal reactions in the condensed phases, in the gas phase, under 88 conditions of flash vacuum pyrolysis (FVP), and in shock tubes will be included where 89 appropriate. 90 91 2. Hydrazines 92 93 Hydrazine decomposes at 200-300 oC in a sealed titanium vessel in an apparently 94 zero-order reaction as determined using accelerating rate calorimetry, where the overall 95 reaction corresponds to N2H4 1/3 N2 + 4/3 NH3[17]. The activation energy for the thermal 96 decomposition of hydrazine is not well defined, with values between 7 and 37 kcal mol-1 97 under conventional conditions having been reported[18,19]. In a shock-tube investigation 12 -1 98 with N2 as the bath gas the reaction rate was determined as k = 1.9 10 exp(-45500/RT) s , 99 and the rate was dependent on the bath gas. The initiation step was considered to be the 100 dissociation into NH2 radicals[20]. The N-N bond dissociation energy (BDE) has been given 101 as 65.5 kcal mol-1[19], and the calculated BDE at the G2 level is 64.3 kcal mol-1[21]. 102 103 In earlier shock-tube experiments at 1100-2000 K the rate and molecularity was found 104 to be pressure dependent, but by using hydrazine highly diluted in Ar, essentially first-order 105 kinetics were observed. The rate was independent of the hydrazine concentration, but 106 dependent on the total pressure of the gas (mainly Ar) with k = 1012 exp(-48000/RT) s-1 at a 107 total gas density of 2.5 10-5 mol/cm3, and k = 1012.8 exp(52000/RT) s-1 for 7.5 10-5 3 108 mol/cm [22,23]. The initial reaction was considered to be N2H4 2 NH2, followed by NH2 + 109 N2H4 NH3 + N2H3,N2H3 H + HNNH, 2 NH2 HNNH + H2, and other reactions. At 110 1000 K the ratio of NH3 formed to N2H4 decomposed was ~1, but it decreased to ~0 at 2000 111 K. 112 113 DecompositionAccepted of hydrazine using an adiabatic Manuscript flow reactor over the temperature 114 range 750-1000 K yielded a first order reaction with k = 1010.33 exp(-36170/RT) s-1 with the 115 stoichiometry N2H4 0.9 NH3 + 0.5 N2 + 0.6 H2[24]. In the same apparatus methylhydrazine 116 and 1,1-dimethylhydrazine also showed first-order kinetics with A-factors of 1013.4 and 109.29 117 and activation energies of 47 and 28.8 kcal/mol, respectively. These Arrhenius data indicate 118 the presence of a strong compensation effect. 119 120 Investigation of the pyrolysis of methylhydrazine using the very low pressure 121 pyrolysis (VLPP) technique revealed a molecular decomposition to methanimine CH2=NH 122 and NH3[25]. Similarly, 1,1-dimethylhydrazine N-methylmethanimine CH2=N-CH3 + 3 Page 3 of 41 123 NH3, 1,2-dimethylhydrazine CH2=NH + CH3NH2, trimethylhydrazine CH2=NCH3 + 124 CH3NH2, and tetramethylhydrazine CH2=NCH3 + (CH3)2NH. In addition, 125 methylhydrazine also yielded methyldiazene CH3N=NH + H2. These reactions have 126 activation energies of 54-63 kcal/mol and A factors of 1013.2-1017.4. In the case of 1,1- 127 dimethylhydrazine, homolysis was indicated by the appearance of a peak at m/z 45 ((CH3)2 128 NH) in the mass spectrum, which reached maximum intensity at 694 oC. This was explained 129 by formation of the aminyl (CH3)2N, which in the lower temperature range abstracted a 130 hydrogen atom, but at higher temperatures eliminated a hydrogen atom to for the imine 131 CH2=N-CH3.