City Research Online

Abstract

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

1. The effect of encounter control and ipsosubstitution for substrates activated by substituents having an oxygen atom adjacent to the nucleus. The rates for anisole, phenol, p-methylanisole, p-cresol and veratrole (1, 2-dimethoxybenzene) and the proportions of the products formed have been determined. These substrates are nitrated at the encounter rate and for anisole and phenol in 56 - 82% sulphuric acid the o:p-ratios vary from 1.8 to 0.7 and from 2.4 to 0.88 respectively. It is suggested that the rate limiting step is the formation of an encounter pair between a nitronium ion and a nyareeen bonded substrate molecule. The change in the o:p-ratio may be due to competition between Wheland intermediate formation either directly from the hydrogen-bonded encounter pair or subsequently to loss of the hydronium ion. With p-methylanisole 4-methyl-2-nitroanisole and 4-methyl-2-nitrophenol were formed quantitatively in constant ratio in 46 - 80% sulphuric acid. The latter is considered to result from ipso-attack by nitronium ion at C-Me and subsequent 1,3- rearrangement of the nitrogroup. For p-cresol quantitative yields of 4-methy1-2- nitrophenol were obtained in 56 - 78% sulphuric acid. It is suggested that ipso-attack again occurs to give transiently 4-methyl-4-nitrocyclohexa-2,5-dienone. In the case of veratrole quantitative yields of 4-nitroveratrole were obtained in 60 - 74% sulphuric acid and this anomaly is discussed.

2. The investigation of the nitration of some very reactive aromatic compounds at low acidity. The rates for 1,3,5-trimethoxybenzene, 1,3-dimethoxybenzene, and mesitylene at various temperatures have been determined. After correction for C-protonation where necessary, the rate profiles for these reactions were approximately parallel and it is concluded that in as low as 44% sulphuric acid the nitronium ion remains the effective electrophile.

3. The nitration of some molecules having two benzanoid nuclei. For the homogeneous nitration of biphenyl in aqueous sulphuric acid, a high o:p-ratio (~ 3.6) has been confirmed, and no evidence for ipsosubstitution has been found. For the nitration of diphenylmethane good first-order kinetics were obtained and six nitro-compounds were identified. The ratio of mono- to di-substitution is discussed.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	Doctoral Theses

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