Using ESR dimer-radical equilibrium techniques the heat of formation, △Hf0, of the pentamethylcyclopentadienyl radical was determined as 7.4 kcal mol−1, and hence SEMeH was calculated to be 37.9 kcal mol−1. It was attempted to determine △Hf0 for the pentaethyl- and pentamethoxycarbonylcyclopentadienyl radicals, but this proved unsuccessful. From appearance energy measurements, △Hf0 (Pentadienyl) was determined to be 49.6 kcal mol−1 Using photoacoustic calorimetry, the C3-H bond dissociation energy of penta−1,4-diene and several 3-substituted pentadienes were determined; i.e. for penta−1,4-diene (DH° (C3-H) = 76.6±0.6 kcal mol−1), 3-methyl- penta−1,4-diene (DH°(C3-H) = 76.6±0.6 kcal mol−1) and 3-hydroxypenta−1,4-diene (DH°(C3-H) = 69.0±0.6 kcal mol−1). ESR exchange broadening methods were used to show that SEESR (3-methylpentadienyl) is virtually the same as that of the parent pentadienyl radical, (i.e. 25 kcal mol−1). The 3-hydroxypentadienyl, l-hydroxy-3-methylpentadienyl and l-trimethylsilyloxy-3-methylpentadienyl radicals were generated, but exchange broadening was not observed in the accessible temperature range. Sulphonate esters were examined as a potential source of free radicals. The reaction of simple alkyl alkanesulphonates with photochemically generated trimethyltin radicals in the cavity of an ESR spectrometer gave the corresponding alkanesulphonyl radicals. Good ESR spectra were obtained when sulphonyl radicals, generated from sulphonate esters, were used to form adduct radicals with alkenes of the type CH2=CHR where R is an electron releasing substituent, (R = OMe, OBun, SPh, SiME3). Delocalised radicals were generated from sulphonate esters which contained a terminal alkenyl or alkynyl substituent. For example, the reaction of allyl methanesulphonate with photochemically generated trimethyltin radicals in the cavity of an ESR spectrometer gave good spectra of the allyl radical. The pent-4-ene-1-sulphonyl radical was generated from the corresponding sulphonyl chloride using several different radical initiator systems (tri-n-butyltin hydride, triphenylsilane, hexa-n-butylditin, 9-trimethylstannyl-9,10-dihydroanthracene, copper (II) chloride and tris (triphenylphosphine) ruthenium (II) chloride). The radical was found to cyclise in a predominantly endo-fashion to give 3-chlorotetrahydrothiopyran-1,1-dioxide, the exact proportion of endo- and exo-products depending on the temperature. The hex-5-ene-1-sulphonyl radical was found to give thiepane-1,1- dioxide, also the product of endo-cyclisation. The inclusion of a bulky ring substituent forced the cyclohexenylethanesulphonyl radical to cyclise in the exo-mode to give 2-chloro-9-thiabicyclo[4.3.0]nonane-9,9-dioxide. Dialkyltellurides and dialkylditellurides gave the corresponding alkyl radical on photolysis in the cavity of an ESR spectrometer. For example, diallyltelluride gave good spectra of the allyl radical. Under thermolysis conditions, a nitrosodurene spin trap was employed to observe the trapped alkyl radicals. It is believed that certain of the trapped radicals were tellurium-centred species.

The first chapter is concerned with an investigation into the kinetics and mechanism of the base-catalysed hydrolysis of some substituted phenyl chloroformates. A study of kinetic isotope effects and activation parameters for the hydrolysis of phenyl chloroformate indicates that acetate ion and pyridine act as nucleophilic catalysts. Both the spontaneous and pyridine catalysed hydrolyses of substituted phenyl chloroformates fit a Hammett plot. The effect of solvent polarity and added salt indicates that there is little charge separation in the transition state for hydrolysis. The mechanism of reaction is discussed. The results of an investigation into the role of nucleophilic and general base catalysis by pyridine and methylpyridines in the hydrolysis of aryl acetates are presented in Chapter 2. Activation parameters for several of these reactions have been determined. The size of the kinetic isotope effect indicates nucleophilic catalysis. Substitution at the 2-position in pyridine prevents nucleophilic catalysis, but a much slower general base catalysed pathway was detected, and it is argued that this occurs even when nucleophilic catalysis appears to be the mechanism. The third chapter is concerned with the study of micelle effects in the base-catalysed hydrolysis of aryl acetates. It was thought that the base 2(2-hydroxyethyl)-pyridine, in which nucleophilic catalysis due to the nitrogen is effectively blocked by substitution at the two position, would have a tendency to form intramolecular hydrogen bonds within the hydrophobic micelle environment, and hence to enhance the nucleophilicity of the hydroxyl group. No evidence was found to suggest that such a process occurs in the micelle solutions used, but the hydrogen-bonded configuration is shown to be formed in carbon tetrachloride solution. Substrate substituent effects in the enzymatic hydrolysis of a series of aryl acetates are investigated in Chapter 4. It is found that substituent effects can change the observed enzymatic mechanism. The possession of a 4-nitro substituent by the substrate is also shown to be of mechanistic importance. An analogue computer was used to simulate the kinetic results, and factors involved in the change of mechanism are discussed.

Various aldehydes have been reacted with methyl acrylate under Baylis-Hillman conditions, using DABCO as a catalyst, to afford a range of α-substituted acrylic esters containing an allylic hydroxy group. Selected Baylis-Hillman products have been brominated, hydrolysed and acetylated to afford substrates for the synthesis of necic acid analogues. The diastereo- and regioselectivity of nucleophilic attack, using sodium methylmercaptan, on the Baylis-Hillman products and selected brominated derivatives was investigated. The allylic hydroxy compounds favour conjugate addition with the generation of a new chiral centre, while the allylic bromo derivatives favour substitution (SN and SN') (S[subscript N] and S[subscript N]') with consequent loss of chirality. (E)-2-Isopropylcrotonic acid, a vital precursor in the synthesis of all stereoisomers of trachelanthic and viridifloric acid, was synthesised in an attempt to obtain the necic acid components required for total alkaloid synthesis of lycopsamine and its derivatives. This precursor and salicylic acid were then used to prepare esters of retronecine, a dihydroxy necine base obtained via extraction and consequent hydrolysis of retrorsine.

The main investigation consisted of determining corrosion rates of mild steel (A lSI 1 020) exposed to de-aerated stagnant solutions of fatty acids in water at different acid numbers and temperatures, with the aid of mass loss tests. The temperature range investigated varied from 25°C to 250°C. Results showed that corrosion rates initially increased with temperature and at certain elevated temperatures thick multi-laminated corrosion product films formed on the specimen surfaces which .led to decreases in corrosion rates with further increases in temperature. Corrosion product films influenced the control mechanism of the corrosion process and it was seen that the characteristics of these films were dependent on the temperature as well as fatty acid type. The acid number range investigated was from 0 to 25 mg KOH/g solution. Corrosion rates increased with an increase in acid number irrespective of the control mechanism. Results indicated that the corrosivity of acid solutions decreased with an increase in chain length of the organic acid molecule, when no corrosion product films were present. The presence of corrosion product films rendered complex relationships. Electrochemical tests were done on specific fatty acid systems at 25 °C. Corrosion rates evaluated from electrochemical observations correlated well with mass loss results and it was concluded that the corrosion mechanism was electrochemical. Supplementary investigations were conducted on specific fatty acid systems and included the following: Turbulence did not influence the corrosion process. This is typical for an activation-controlled system where no corrosion product films are present on the specimen surfaces. Mixing of acids showed a corrosion rate of approximately an average of the two individual corrosion rates of the pure systems. Monitoring studies showed that results obtained with the Corrator correlated better with mass loss and electrochemical corrosion rates than Corrosometer results. Corrosion rates increased with an increase in dissolved oxygen content level of the solution and with additions of chloride, but sulphur additions rendered lower corrosion rates. A water phase process stream received from SASOL showed that the corrosion rate increased with temperature and levelled out towards 200°C The corrosion characteristics of the received stream can be simulated by acetic acid with the same acid number. / Die hoofondersoek het bestaan· uit die bepaling van korrosietempo's van laekoolstofstaal ( A lSI 1020 ) wat blootgestel word a an ontlugte stagnante oplossings van organiese sure in water by verskillende temperature en suurgetalle. Die korrosietempo's is bepaal deur gebruik te maak van massaverliestoetse. Die temperatuurgebied wat ondersoek is, was van 25 °C tot 250°C. Dit is waargeneem dat die korrosietempo aanvanklik toeneem met 'n toename in temperatuur en daarna is 'n afname in die korrosietempo waargeneem. Die afname word toegeskryf a an 'n dik multi-laminere-korrosieprodukfilm wat op die monsteroppervlak gevorm het. Hier is waargeneem dat die beheermeganisme be"invloed word deur die teenwoordigheid van korrosieprodukfilms terwyl eienskappe van die films afhanklik was van die tipe suur, asook die temperatuur waarby eksperimente gedoen is. Die suurgetalgebied wat ondersoek is, was vanaf 0 tot 25 mg KOH/g oplossing. Korrosietempo's het toegeneem met 'n toename in suurgetal ongeag die beheermeganisme wat bestaan het. Die korrosiwiteit van die suuroplossings het afgeneem met 'n toename in organiese molekuullengte, waar geen korrosieprodukfilm teenwoordig was nie. Die teenwoordigheid van korrosieprodukfilms het komplekse korrosietempo-tipe suur verhoudings by konstante temperature opgelewer. Elektrochemiese toetse is gedoen op sekere sistema by 'n temperatuur van 25 °C. Korrosietempo's verkry deur gebruik te maak van elektrochemiese toetsmetodes vergelyk baie gunstig met massaverliestoetsresultate. Die gevolgtrekking wat dus gemaak is, was dat 'n elektrochemiese korrosiemeganisme bestaan. Newe-ondersoeke is gedoen op sekere .organiese suursisteme en het die volgende ingesluit: Turbulensie het geen effek op die korrosieproses gehad nie. Dit is tipies wat verwag word van 'n aktiveringsbeheerde sisteem waar geen korrosieprodukfilms teenwoordig is op die monsteroppervlak nie. Mengsels van die sure het getoon dat die korrosietempo's 'n gemiddelde waarde van die individuele korrosietempo's van die suiwer suursisteme oplewer. In die moniteringstudies het die Corrator resultate beter gekorreleer met massaverlies en elektrochemiese toetsresultate as Corrosometer resultate. Die korrosietempo het verhoog met 'n toename in opgeloste suurstofinhoud van die oplossing asook met die byvoeging van chloriede, terwyl 'n afname in korrosietempo waargeneem is met die byvoeging van swael. 'n SASOL waterfase produkstroom het getoon dat die korrosietempo toeneem soos die temperatuur toeneem tot ongeveer 200°C waarna 'n afplatting in korrosietempo waargeneem is. Die korrosiekarakteristiek van die SASOL waterfase produkstroom word redelik goed verteenwoordig deur 'n asynsuur sisteem met dieselfde suurgetal. / Dissertation (MEng)--University of Pretoria, 1994. / SASOL / Chemical Engineering / MEng / unrestricted

Call number: LD2668 .T4 1959 W49

Digitized by Kansas Correctional Industries

No description available.

No description available.

Type I cells of the carotid body play a major role in acid chemoreception. Extracellular acidosis causes membrane depolarisation, Ca²⁺ influx and neurosecretion in the type I cell. A previous study has shown that pH_i in the type I cell is very sensitive to changes of pH_o, and intracellular acidification is a key step in the signalling pathway for acid chemoreception. This thesis investigates the mechanism responsible for mediating acid influx during isocapnic extracellular acidosis. Type I cells were enzymically isolated from carotid bodies obtained from neonatal rats. pH_i was determined by microspectrofluorimetry, using pH-sensitive fluorescent dye carboxy-SNARF-1. My results show that there are two acid influx pathways. At resting pHi, Cl^--HCO₃^- exchange accounts for over 70% of acid influx in response to extracellular acidosis. The remaining 30% of acid influx is mediated by an unidentified mechanism, which does not require either Cl^- or HCO₃^-. I have also demonstrated that, the second pathway is an acid loading mechanism enhanced by a fall in pH_o, rather than an existing background acid loading unmasked by the inhibition of acid extruder. Although 200 µM DIDS inhibited Cl^-₀-free induced acid efflux mediated by reversed mode of Cl^--HCO₃^- exchange as well as the acid influx induced by alkali load, it had no effect on the acid influx in response to acid challenge. The difference in DIDS effect to block acid influx is probably due to difference in Cl^--HCO₃^- exchangers. I proposed that the Cl^--HCO₃^- exchange system in the type I cell comprises two distinct exchanger populations. One of them is DIDS sensitive and activated by high pH_i, while the other is DIDS insensitive and activated by low pH_o. The pH_i and pH_o sensitivity of both acid influx pathways have also been characterised. It is found that the unidentified HCO₃^--independent acid loading mechanism is activated by H₀⁺ while virtually pH_i-independent. In addition, the activity of Cl^--HCO₃^- exchange system is very sensitive to pH_i and pH_o, with pK_aⁱ and pK_a^o values close to resting pH_i and pH_o. Thus any shift of pH_i or pH_o from the normal resting range will produce significant changes in exchange activity, leading to changes in acid flux into the cell. In this way, the Cl^--HCO₃^- exchange system serves as a link for transducing acidic pH₀ into parallel acidification in pH_i in the type I cell.

Thesis (doctoral)--Würzburg, 1954. / Includes bibliographical references (p. I-V).