"Front Matter". In: Organosilanes in Radical Chemistry - Index of

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20 Thermochemistry R 3Si + (2.21) R 3 SiH (2.16) (2.16) R 3Si (2.1) R 3 SiH (2.13) (2.11) R 3Si − R 3 SiH (2.14) Scheme 2.1 Relationships between R3SiH and its related radicals, ions, and radical ions (The numbers refer to reaction numbers as they appear in the text) absence of hydroxylic or acidic bonds). This is true also in organosilanes since good agreement between the gas-phase and the liquid-phase DHs has been observed. 2.2 BOND DISSOCIATION ENTHALPIES Knowledge of bond dissociation enthalpies (DH) has always been considered fundamental for understanding kinetics and mechanisms of free radicals. DHs offer an interesting window through which to view stability of radicals. Indeed, based on Reaction (2.1) the bond dissociation enthalpy of silanes DH(R3Si H) w is related to enthalpy of formation of silyl radicals, DHf (R3Si:), by Equation (2.2). R3Si H !R3Si: þ H: (2:1) w DH(R3Si w H) ¼ DH f (R3Si:) þ DH f (H:) DH f (R3SiH) (2:2) Several techniques have been used to determine silane bond enthalpies: radical kinetics, gas-phase acidity cycles and photoacoustic calorimetry. However, the following sections concentrate on the results rather than the details of experimental techniques. DHs of silanes have changed substantially over time. For example, the DH(Me3Si w H) has changed significantly over the last 60 years. Indeed, values of 314, 339, 377 and 397 kJ/mol were reported in 1942, 1971, 1982, 1994, respectively [1]. Consequently, many bond dissociation enthalpies that were derived using these reference values have changed as well. Nevertheless, it is worth recalling that today the majority of data are consistent within the different experimental approaches. For consistency, all the DH and DH f values are normally extrapolated to standard state conditions (i.e., 258C in the gas phase). 2.2.1 RADICAL KINETICS Studies of the kinetics of chemical equilibrium (Reactions 2:3= 2:3) have provided very accurate thermodynamic information from the series
Bond Dissociation Enthalpies 21 Table 2.1 Bond dissociation enthalpies and standard enthalpies of formation of silanes, and enthalpies of associated silyl radicals (kJ=mol) a Silane (R3SiH) DH(R3Si w H) b DHf8(R3SiH) c DHf8(R3Si:) d H3Si w H 384:1 2 34.3 2 200.5 2.5 MeSiH2 w H 388 5 29:1 4 141 6 Me2SiH w H 392 5 94:7 4 79 6 Me3Si w H 397:4 2 163:4 4 16 6 a At 25 8C b From References [2–4] c From Reference [5] d Calculated from Equation (2.2) using DHf 8(H:) ¼ 218:0kJ=mol; rounded to the nearest 0.5 kJ/mol. Me3 nSiHnþ1 (with n having values from 0 to 3) [2–4]. In particular, the rate constants k2:3 and k 2:3 obtained by time-resolved experiments allow the determination of the reaction enthalpy (DHr) by either second or third law methods. DH(R3Si w H) is obtained by Equation (2.4) and then DH f (R3Si:) from Equation (2.2). The values are collected in Table 2.1. R3SiH þ Br: k2:3 Ðk 2:3 R3Si: þ HBr (2:3= 2:3) DH(R3Si w H) ¼ DHr þ DH(H w Br) (2:4) A similar value has been obtained for DH f (Me3Si:) from an independent kinetic study on the very low pressure pyrolysis of hexamethyldisilane (Reaction 2.5) [6]. A bond dissociation enthalpy DH(Me3Si w SiMe3) ¼ 332 12 kJ=mol was obtained which is related to DH f (Me3Si:) by Equation (2.6). Me3Si w SiMe3 !2Me3Si: (2:5) DH(Me3Si w SiMe3) ¼ 2 DH f (Me3Si:) DH f (Me3SiSiMe3) (2:6) From Table 2.1 it emerges that, when the Me group progressively replaces the H atom, the Si w H bond strength in the silane significantly increases (ca 4 kJ/mol), the effect being cumulative. It is worth mentioning that this effect is opposite to that for hydrocarbons, where DH(R3C w H) is 438.5, 423.0, 412.5 and 404.0 kJ/mol for H3C w H, MeCH2 w H, Me2CH w H and Me3C w H, respectively [7]. A rationalization is based on the fact that C is more electronegative than Si. It is suggested that the electron-deficient central C atom is stabilized by electron donation from the methyl groups, whereas the central Si atom is destabilized by electron withdrawal by the methyl groups (inductive effect). Table 2.1 also brings to light that DH f (R3Si:) decreases by a fixed amount, which is approximately of 60 kJ/mol, by replacements of H atoms with methyl groups.
Page 1 and 2: Organosilanes in Radical Chemistry
Page 3 and 4: DEDICATION To my parents XrZstoB an
Page 5 and 6: viii Contents 3.6 Hydrogen Atom: An
Page 7 and 8: PREFACE A large number of papers de
Page 9 and 10: ACKNOWLEDGEMENTS I thank Keith U. I
Page 11 and 12: 2 Formation and Structures of Silyl
Page 19 and 20: 10 Formation and Structures of Sily
Page 27: 2 Thermochemistry 2.1 GENERAL CONSI
Page 31 and 32: Bond Dissociation Enthalpies 23 2.2
Page 33 and 34: Ion Thermochemistry 25 Table 2.4 Re
Page 35 and 36: Ion Thermochemistry 27 Table 2.5 El
Page 37 and 38: References 29 3. Goumri, A., Yuan,
Page 39 and 40: 32 Hydrogen Donor Abilities of Sili
Page 55 and 56: 4 Reducing Agents 4.1 GENERAL ASPEC
Page 57 and 58: General Aspects of Radical Chain Re
Page 59 and 60: Tris(trimethylsilyl)silane 53 Therm
Page 61 and 62: Tris(trimethylsilyl)silane 55 4.3.1
Page 63 and 64: Tris(trimethylsilyl)silane 57 A goo
Page 65 and 66: Tris(trimethylsilyl)silane 59 C(O)C
Page 67 and 68: Tris(trimethylsilyl)silane 61 radic
Page 69 and 70: Tris(trimethylsilyl)silane 63 86% y
Page 71 and 72: Tris(trimethylsilyl)silane 65 RO RO
Page 73 and 74: Tris(trimethylsilyl)silane 67 O AcO
Page 75 and 76: Tris(trimethylsilyl)silane 69 Tris(
Page 77 and 78: Other Silicon Hydrides 71 The reduc
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Other Silicon Hydrides 73 The decre
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Other Silicon Hydrides 75 Ph MeS O
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Other Silicon Hydrides 77 Table 4.6
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Silicon Hydride / Thiol Mixture 79
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Silylated Cyclohexadienes 81 and (4
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References 83 34. Kawashima, E., Uc
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References 85 104. Gimisis, T., Bal
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88 Addition to Unsaturated Bonds te
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90 Addition to Unsaturated Bonds ap
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92 Addition to Unsaturated Bonds 5.
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94 Addition to Unsaturated Bonds R
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96 Addition to Unsaturated Bonds Ph
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98 Addition to Unsaturated Bonds EP
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100 Addition to Unsaturated Bonds 5
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102 Addition to Unsaturated Bonds T
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104 Addition to Unsaturated Bonds R
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106 Addition to Unsaturated Bonds a
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110 Addition to Unsaturated Bonds S
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112 Addition to Unsaturated Bonds T
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114 Addition to Unsaturated Bonds I
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120 Unimolecular Reactions 1 Si(H)M
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122 Unimolecular Reactions t-Bu t-B
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124 Unimolecular Reactions MeO Si O
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126 Unimolecular Reactions t-Bu t-B
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128 Unimolecular Reactions R 1 R 2
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130 Unimolecular Reactions From the
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132 Unimolecular Reactions rearrang
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134 Unimolecular Reactions H 3 C +
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136 Unimolecular Reactions Br O Si(
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138 Unimolecular Reactions R3Si C C
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140 Unimolecular Reactions Me3Si O
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142 Unimolecular Reactions 43. Albe
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144 Consecutive Radical Reactions c
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146 Consecutive Radical Reactions O
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148 Consecutive Radical Reactions a
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150 Consecutive Radical Reactions d
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152 Consecutive Radical Reactions T
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154 Consecutive Radical Reactions M
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156 Consecutive Radical Reactions F
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162 Consecutive Radical Reactions i
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164 Consecutive Radical Reactions A
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168 Consecutive Radical Reactions r
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170 Consecutive Radical Reactions E
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174 Consecutive Radical Reactions A
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176 Consecutive Radical Reactions P
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178 Consecutive Radical Reactions f
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182 Consecutive Radical Reactions 1
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184 Consecutive Radical Reactions 8
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186 Silyl Radicals in Polymers and
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220 List of Abbreviations PED photo
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222 Index Carbonyl sulfide 111 Carb
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224 Index Organosilicon boranes 2,
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226 Index Triethylsilane as mediato
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