Abstract
A number of different types of organic reactions (for each of which the traditional chemistry is known) were investigated to establish whether ultrasonic irradiation could be used to affect their rates of reaction and/or product distributions.An investigation was performed into methods of improving nucleophilic fluorination with potassium fluoride. A new process has been developed for the fluorination of benzyl halides by reaction with powdered KF at 89℃ in acetonitrile using polyethylene glycol (PEG 200) and potassium iodide as catalysts. When the above reaction was subjected to ultrasonic irradiation at lower temperatures then some enhancement in yield was observed compared with that obtained in control (non-ultrasonic) experiments.
When sodium naphthoxide was alkylated with benzyl bromide in a range of aqueous ethanolic solvents both O- and C- alkylated products were observed. The ratio of the O/C products depended upon the composition of this highly structured solvent mixture. In water C-alkylation predominated but the amount of O-alkylation increased with the proportion of ethanol. These results are consistent with the view that the increase in the proportion of O-alkylation obtained is inversely related to the ability of the solvent to hydrogen bond (so shield) to the oxygen anion. With an increasing proportion of ethanol more of the O-alkylated product is observed since ethanol is a less efficient hydrogen bonding solvent than water. The effect of ultrasound was observed on the O/C ratio in the solvents 0%, 25%, 50%, 75% and 100% (m/m) aqueous ethanol. In only 50% and 75% ethanol does sonication significantly disturb the product distribution to give an increase of 18% and 19% respectively in C-alkylated product as compared to that obtained in a control reaction. The difference in O/C ratio in these solvents was rationalised in terms of ultrasonic disruption of the bulk solvent structure. In the solvents were ethanol is the major source of solvation any increase in the proportion of “free” water molecules as a result of sonic solvent structural breakdown would be significant and lead to an increase in the amount of C-alkylation.
In the case of 5-hydroxychromones, the rate of reaction for O-alkylation was improved by ultrasonic irradiation. The reaction required the use of the aprotic solvent N-methyl pyrrolidinone and investigation revealed optimum reagent conditions of: a 2-fold excess of alkyl halide and potassium carbonate over substrate and a reaction temperature of 65℃. In this way the alkylation of 5-hydroxychromone-2-carboxylic acid ethyl ester and 5-hydroxy-2-methylchromone was successfully achieved using a number of different alkyl halides. The explanation of sonochemical enhancement in
this reaction is believed to lie in a combination of particle size reduction of potassium carbonate, which increases solubilisation of the base, and an improved mixing of reagents.
| Date of Award | 1988 |
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| Original language | English |
| Supervisor | Timothy Mason (Supervisor) & j Philip Lorimer (Supervisor) |