Date of Award

1990

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Supervisor

John Warkentin

Abstract

α-Hydroxyalkyl diazenes and -hydroperoxyallcyl diaz.enes are known for a long time
as initiators for fn:e radical polymerization. Their application as suitable radical precursors for kinetic studies such as the radical-molecule and radical-radical reactions are not well-exploited. This thesis deals mainly with the rate constants for a number of radical-molecule reactions studied by generating radicals in solution from suitable radical
precursors mentioned above. The initiation mechanism for the decomposition of
a-hydroxyalkyl diazenes was also investigated.

Alkyl(1-hydroperoxy-1-methylethyl)diazenes (154) [(CH₃)₂C(OOH)N=NR] a, R = c-C₃H₅-CH₂; b,R = c-C₃H₅-CD₂; c, R = CH₂=CH-(CH₂)₃-CH₂; d, R =CH₃(CH₂)₂-CH₂ and phenyl(1-hydroperoxy-1-methylethyl)diazene (l54e) were prepared in solution by autoxidation of the corresponding hydrazones of acetone. They (154a-e) were converted to the corresponding alkyl(1-hydroxy-1-methylethyl)diazene (155a-d) and
phenyl(1-hydroxy-1-methylethyl)diazene (155e) by reduction with biphenyl phosphine.

The radical chain decomposition of 5-hexenyl(1-hydroxy-1-methylethyl)diazene
(155c) in carbon tetrachloride and product analysis gave the rate constant for chlorine
abstraction by the S-hexenyl radical. The rate constant was calculated from the product
composition and the known rate constant for the cyclization of the S-hexenyl radical. For the
temperature range 274-353 K, the rate constant is given by log(kᴄʟ/M⁻¹s⁻¹) = (8.4 ± 0.3) - (6.2 ± 0.4)/θ, where θ = 2.3 kca/mol, which leads to kᴄʟ(²⁵°c) = 7.2 x 10³ M⁻¹s⁻¹.

Rearrangement of cyclopropylmethyl radical to the but-3-enyl radical was used to
clock bromine and iodine abstraction reactions from a number of substrates. Cyclopropylmethyl(1-hydroxy-1-methylethyl)diazene (155a) was used as the source for cyclopropylmcthyl radical.

Decomposition of 155a in hexafluorobenzene or in dichloromethane containing bromotrichloromethane, and product analysis enabled the calculation of the rate constant (kʙᵣ) for bromine abstraction by cyclopropylmethyl radicals from bromotrichloromethane. The rate constant was calculated from the product composition and from the known rate constant for the isomerization of the cydopropylmethyl radical. For the temperature range 253-341 K, the rate constant is given by log (kʙᵣ/M⁻¹ s⁻¹) = 9.75 - 2.11/θ, where θ = 2.3 RT kcal/mol, which gives kʙᵣ =2 x 10⁸ M⁻¹ s⁻¹ at 25°C.

Rate constants (kʙᵣ) for bromine abstractions and the rate constants (kɪ) for iodine
abstractions from various substrates were also determined by the use of the
cyclopropylmethyl clock. The rate constants (in M⁻¹ s⁻¹ units) at 80ºC are: kʙᵣ(CHBᵣ₃) = 2.86 x 10⁷; kʙᵣ(CHCl₂ʙᵣ) = 1.07 x 10⁷; kɪ(CH₂I₂) = 2.63 x 10⁷; kɪ(CH₃I) = 3.9 x 10⁶; kɪ(C₆H₅CH₂I) = 1/2 x 10⁸, kɪ((CH₃)₂CHI) = 1.4 x 10⁷; kɪ(CF₃CH₂I) = 6.6 x 10⁶; kɪ((CH₃)₃CI) = 6.5 x 10⁷.

Non-chain decomposition of 155d and 155e in solutions containing bromonitrichloromethane and I,I,3,3-tetramethylisoindolin-2-yloxyl (86) afforded butyl
bromide and bromobenzene, in yields determined by the concenlrations of 86 and BrCCl₃. From product yields and from the known rate constants for coupling of radicals with 86, the rate constants for the attack (at 80ºC) of butyl (kʙᵣ(ʙᵤ) = 0.26 x 10⁹ M⁻¹ s⁻¹) and phenyl (kʙᵣ(ph) = 1.55 x 10⁹ M⁻¹ s⁻¹) radicals were determined.

Generation of the deuterium-labelled cyclopropylmethyl radical (c-C₃H₅-CD₂) from 155b in a solution containing me spin trap, 1-methyl-4-nitroso-3,5-diphenylpyrazole (123),
resulted in the formation of the spin adducts, [1-methyl-3,5-diphenyl]-4-pyrazolyl-[1'-cyclopropyl-1',1'-dideuterio]methyl nitroxyl (190) and [1-methyl-3,5-diphenyl]-4-pyrazolyl-[4',4'-dideuterio-but-3-enyl] nitroxyl (191) radicals. From the relative
concentrations of the two spin adducts determined from esr spectral measurements, the rate
constant (kτ) for spin trapping was determined. For the temperature range 283-333 K, the rate
constant was calculated as log (kτ) =(10.4 ± 0.4) - (3.6 ± 0.5)/θ, where θ =2.3 RT kcal/mol, which gives kτ = 7.7 X 10⁷ M⁻¹ s⁻¹ at 40ºC.

The initiation mechanism for the decomposition of α-hydroxyalkyl diazene was
investigated using phenyl(1-hydroxy-1-methylethyl)diazene (155e). The results of various
kinetic studies strongly suggested that azocarbinols decompose by the reversible formation of
acetone and the 1-substituted-1-H diazene (197).


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