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Abstract

This thesis is concerned with five aspects of aromatic nitration in aqueous sulphuric acid.

1. The effect of ipso-substitution and encounter control for substrates activated by substituents having an oxygen atom adjacent to the nucleus. The rates for o-cresol, m-cresol and 3,5-dimethylphenol have been determined and these substrates are nitrated at the encounter rate. The proportion of substitution para to hydroxy has been found to increase with increasing acidity from 38 to 55%, 33 to 52% and 30 to 62% respectively, and these changes in the product ratios may be due to competition between Wheland intermediate formation, either directly from the hydrogen-bonded encounter pair or subsequent to loss of the hydronium ion.

2-Methyl-6-nitrophenol is established as an important product of nitration of o-methylanisole and a mechanism involving demethoxylation of a Wheland intermediate formed by ipso-substitution at C-Me and a subsequent specific 1,3-rearrangement is proposed.

4-Isopropyl-2-nitrophenol and p-nitrophenol are established as important products of nitration of p-isopropylanisole.

2. The nitrations of substrates having a halogen atom adjacent to the nucleus, namely p-bromophenol, p-chlorophenol, o-bromophenol and 2,4-dibromophenol, are reported and the yields of the products formed have been determined. Nitrodehalogenation was not detected with p-chlorophenol and o-bromophenol, but was the major outcome of the nitration of p-bromophenol and 2,4-dibromophenol, the degree of nitrodebromination decreasing with increasing acidity.

3. The rates of nitration of phloroglucinol at various temperatures have been determined and, after correction for C-protonation, the rate profile for the nitration of this compound was very close to that for mesitylene, and it is concluded that in as low as 40% sulphuric acid the nitronium ion remains the effective electrophile. The nitration of phenol under the “zeroth-order” conditions has been determined and it has been estimated that a pre-association mechanism should apply to encounter-controlled nitrations in below 40% sulphuric acid.

4. Some nitrocyclohexadienones have been synthesised and the rates of rearomatisation of 2,4,6-trimethyl- and 4-methyl-4-nitrocyclohexadienones in sulphuric acid have been determined. Above 50% sulphuric acid the rate profiles (ψs − H0) are parallel with slope ~0.64 where the oxygen-protonated conjugate acid is the reactant, and below 50% sulphuric acid the slope decreases gradually to zero and a homolytic dissociation-recombination mechanism is suggested. 3,5-Dimethyl-4-hydroxybenzaldehyde and m-nitromesitol are the main products of reaction of the former dienone, whereas in the latter case 4-methyl-2-nitrophenol is formed quantitatively.

5. The nitration of some molecules having two benzenoid nuclei. For the homogeneous nitration of diphenylmethane six nitro products have been identified and the anomalous ratio of mono- to disubstitution is discussed. For the nitration of diphenyl ether two stages were observed in below 66% sulphuric acid, corresponding to mono- and dinitration, and in above 66% sulphuric acid the rate profile for mononitration had zero slope, suggesting that the substrate was C-protonated.

Publication Type:	Thesis (Masters)
Subjects:	Q Science Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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