National metrology institute - its value to Canadian economy and society

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A case for a national metrology agenda for Canada

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Reaction of organotin hydrides with acid chlorides. Mechanism of aldehyde and ester formation

Tri-n-butyltin hydride reacts with acid chloride, RCOCl, spontaneously at ambient temperatures to form n-Bu 3SnCl, RCHO, RC(O)OCH 2R, and a number of minor products. The reaction is not a radical chain process, nor are radicals involved as intermediates. The initial products are n-Bu 3SnCl and RCHO; it is not known whether these are formed in a direct reaction between n-Bu 3SnH and RCOCl or via an unstable chloroalkoxytin species, n-Bu 3SnOCHClR. The remaining products are formed by subsequent reactions of the aldehyde. Thus, the alkoxytin species, n-Bu 3SnOCH 2R, is formed from aldehyde and tin hydride. This can react further with RCOCl to form the ester RC(O)OCH 2R, with RCHO to form n-Bu 3SnOCH(R)OCH 2R, and with n-Bu 3SnH to form RCH 2OH. The aldehyde can also react with RCOCl to form the \u3b1\u2032-chloro ester, RC(O)OCHClR.Peer reviewed: YesNRC publication: Ye

Reaction of acyl halides with organotin hydrides. Mechanism of aldehyde formation

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Spectroscopic and kinetic characteristics of aroyloxyl radicals. 1. The 4-methoxybenzoyloxyl radical

A detailed analysis of the time-resolved, UV-visible spectrum obtained by 308-nm laser flash photolysis of bis-(4-methoxybenzoyl) peroxide proves that the broad, structureless absorption in the 500-800-nm region is due to the 4-methoxybenzoyloxyl radical. This radical also has an absorption at 320 nm. The long-wavelength absorption, for which there is less interference from other light-absorbing transients, has been used to measure absolute rate constants, k, for the reaction of 4-methoxybenzoyloxyl with a wide variety of organic substrates at ambient temperatures, e.g., cyclohexane, benzene, triethylsilane, cyclohexene, and 1,3-cyclohexadiene for which k in CCl 4 = (5.3 \ub1 3.0) 7 10 5, (2.3 \ub1 0.2) 7 10 6, (4.8 \ub1 0.1) 7 10 6, (6.4 \ub1 0.3) 7 10 7, and (4.8 \ub1 0.2) 7 10 8 M -1 s -1, respectively. Compared with the tert-butoxyl radical the 4-methoxybenzoyloxyl radical is about as reactive in hydrogen atom abstractions but is very much more reactive in additions to multiple bonds. The rate constant for decarboxylation of 4-methoxybenzoyloxyl at 24\ub0C is (3.4 \ub1 0.1) 7 10 5 s -1 in CCl 4 but is reduced to 642 7 10 4 s -1 in CH 3CN. The 4-methoxybenzoyloxyl radical can also be photodecarboxylated by using 700-nm light from a second laser or by using high power levels in the primary laser.Peer reviewed: YesNRC publication: Ye

First absolute rate constants for some hydrogen atom abstractions and addition reactions of an aroyloxyl radical: 4-Methoxybenzoyloxyl

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Photoinduced electron transfer from dialkyl nitroxides to halogenated solvents

Laser flash photolysis (LFP) at wavelengths within the charge-transfer absorption present in CCl 4 solutions of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) yields the oxoammonium chloride of TEMPO, 1 (\u3bb max = 460 nm), and the trichloromethyl radical in an essentially instantaneous ( 6418 ps) process. The primary photochemical event is an electron transfer from TEMPO to CCl 4, and this is followed by immediate decomposition of the CCl 4 \u2022- radical anion to Cl - and Cl 3C \u2022. An independent synthesis of 1 confirmed that the absorption attributed to this species had been correctly assigned. The formation of Cl 3C \u2022 was inferred by its trapping by molecular oxygen. LFP of TEMPO in other halogenated solvents and of other nitroxides in halogenated solvents has confirmed the generality of these photoreactions. Published 1990 by the American Chemical Society.Peer reviewed: YesNRC publication: Ye

Spectroscopic and kinetic characteristics of aroyloxyl radicals. 2. Benzoyloxyl and ring-substituted aroyloxyl radicals

The 308-nm laser flash photolysis of dibenzoyl peroxide and some ring-substituted derivatives yields broad, structureless absorptions in the range 500-800 nm. These are assigned to the corresponding aroyloxyl radical, in part by analogy with the previously studied 3 4-methoxybenzoyloxyl radical. Absolute rate constants for reaction of four aroyloxyls with their parent peroxide and with six organic substrates have been measured at ambient temperatures. In general, the rate constants increase along the series 4-CH 3OC 6H 4CO 2\u2022 < 4-CH 3C 6H 4CO 2\u2022 72 C 6H 5CO 2\u2022 < 4-ClC 6H 4CO 2\u2022 both for hydrogen atom abstractions (e.g., with cyclohexane: 5.3 7 10 5, 2.1 7 10 6, 1.4 7 10 6, and 1.2 7 10 7 M -1 s -1, respectively) and for additions (e.g., with benzene: 2.3 7 10 6, 2.2 7 10 7, 7.8 7 10 7, and 2.2 7 10 8 M -1 s -1, respectively). The rates of decarboxylation of aroyloxyl radicals increase along the series (4-FC 6H 4CO 2\u2022 64) 4-CH 3OC 6H 4CO 2\u2022 < 4-CH 3C 6H 4CO 2\u2022 3c 4-ClC 6H 4CO 2\u2022 < C 6H 5CO 2\u2022 < 3-ClC 6H 4CO 2\u2022. Rate constants, k 2, for some of these decarboxylations have been determined over a range of temperatures; e g for C 6H 5CO 2\u2022, log (k 2/s -1) = 12.6 - 8.6/\u3b8, where \u3b8 = 2.3RT kcal/mol. The structure of aroyloxyl radicals is considered and it is concluded that the long-wavelength absorption (\u3b5 720nm 3c 290 M -1 cm -1 for 4-CH 3OC 6H 4CO 2\u2022) 3 is most probably due to a transition from the 2B 2 ground state to the 2A 1 potential energy surface. The effects of ring substituents on intermolecular reactivity and decarboxylation rates are rationalized in terms of an aroyloxyl structure in which the aromatic ring and carboxyl group are probably coplanar or nearly so and of the contributing valence-bond canonical structures. Some spin-trapping experiments using C 6H 6 and C 6F 6 have also been performed. The production of some phenyl radicals in the direct photolysis of dibenzoyl peroxide is indicated by an enhanced yield, relative to the thermal decomposition, of the geminate cage product, phenyl benzoate. However, it is concluded that the yield of phenyl radicals formed in the photolysis is probably considerably less than has been presumed previously.Peer reviewed: YesNRC publication: Ye

Autoxidation of alkylnaphthalenes. 1. Self-inhibition during the autoxidation of 1- and 2-methylnaphthalenes puts a limit on the maximum possible kinetic chain length

The kinetics of the azobis(isobutyronitrile) initiated autoxidation of 1- and 2-methylnaphthalene have been studied from 30 to 60 \ub0C in the pretence of sufficient tert-butyl hydroperoxide (1.0 M) to ensure that the tert-butylperoxyl radical totally dominates chain propagation and bimolecular peroxyl/peroxyl chain termination. Initial rate measurements demonstrate that these oxidations do not follow the kinetic rate law which applies to alkylbenzenes and other hydrocarbons. Specifically, in addition to the usual second-order peroxyl/peroxyl termination there is a kinetically first-order chain termination reaction, the relative importance of which increases as the oxygen partial pressure is reduced. As a consequence, these autoxidations are self-inhibiting. The major reaction between tert-butylperoxyl and the methylnapbthalenes is hydrogen abstraction from the methyl group (rate constant, K p BR). Self-inhibition is a consequence of a competing, peroxyl radical addition to the aromatic ring, reaction 8. This is a very minor process, e.g., K 8 BR/K p BR = 0.0036 at 30 \ub0C, and it does not entirely lead to chain termination. That is, the adduct radical formed in reaction 8 may react with O 2, a process which leads to chain propagation, or it may decompose to yield tert-butyl alcohol and a methylnaphthoxyl radical (reaction 9). It is the methylnaphthoxyl radical that is responsible for the kinetically first-order termination process (via its reaction with a second peroxyl). Reaction 9 is also responsible for the existence of a maximum chain length, \u3bd max, in these autoxidations. That is, at a given temperature and oxygen partial pressure \u3bd cannot be increased indefinitely by reducing the rate of chain initiation as is the case for most hydrocarbons. Thus, at 60 \ub0C in the presence of 1.0 M tert-butyl hydroperoxide \u3bd max 48 88 \ub1 4 and 161 \ub1 23 at 160 and 760 Torr of O 2, respectively, while in the absence of the hydroperoxide \u3bd maxis only ca. 12 and does not depend on the oxygen pressure. Our detailed kinetic results and mechanistic conclusions explain why alkylnaphthalenes are so much more resistant to autoxidation than the corresponding alkylbenzenes.Peer reviewed: YesNRC publication: Ye

Selectivity in the photochlorination of 2,3-dimethylbutane with molecular chlorine in noncomplexing solvents

The factors affecting changes in the measured selectivity for the photochlorination of 2,3-dimethylbutane (DMB) in alkane solvents and in CCl 4 have been identified. At low chlorine concentrations in CCl 4 as solvent, the selectivities, S m = [2-ClDMB]/[1-ClDMB], are dramatically enhanced because of CCl 4 and Cl 3C \u2022 participation in the overall chain process. In the absence of CCl 4, the selectivities may also increase at very low chlorine concentrations because residual O 2 and peroxyl radicals participate in the overall chain process. In the absence of either of the above phenomena, S m was calculated to be 0.64, a value that compares well with a value of 0.62, which we have measured in the gas phase.Peer reviewed: YesNRC publication: Ye