John H. Ridd

Diffusion Control and Pre-association in Nitrosation, Nitration and Halogenation

Publisher Summary A number of the common reactions of organic chemistry involve small concentrations of highly reactive intermediates and that, in principle, the reactions of these intermediates can be subject to diffusion control even though the overall reaction is relatively slow. However, the description of these reaction paths is complicated by the very short lifetime of some of the reactive intermediates for there may be insufficient time to permit the intermediates to come together by diffusion. This has implications concerning the mechanisms that can be written for these processes and for the products and relative reactivities that are observed. This chapter summarizes the results for a number of reactions that raise questions concerning diffusion control, considers the mechanistic points that arise thereby, and formulates some generalizations concerning the implications of diffusion control on mechanistic pathways and relative reactivities.

The electronic effect of positively charged substituents on an aromatic ring: a 13cmr and theoretical study

Abstract The effects of the substituents -N + Me 3 , -CH 2 N + Me 3 and -P + Me 3 on the relative 13 C chemical shifts at meta and para positions accord with the σ r ° values obtained from IR intensities provided the correlation equation of Syrova et al. is used. SCF-MO calculations using a minimal STO-3G basis set suggest that the -N + H 3 substituent acts as a weak π-electron donor and the -P + H 3 substituent as a π-electron acceptor in accord with the σ r ° constants for the methylated poles. In these calculations, the action of -PH 3 + as a π-electron acceptor can be rationalised in terms of an hyperconjugative effect involving the empty π-type group MO, that in -PH 3 + is located at a significantly lower energy than in -NH 3 + . This π-type group MO is formed by suitable combinations of the 3p AOs of P with the 1s AOs of the hydrogens and therefore there is no need to invoke 3d-orbitals participation in -PH 3 + to explain this differential behaviour.

Reactions of cerium(IV) ammonium nitrate with aromatic compounds in acetonitrile. Part 2. Nitration; comparison with reactions of nitric acid

The nitration of benzene and a number of alkylbenzenes by cerium(IV) ammonium nitrate in acetonitrile shows the same intra- and inter-molecular selectivity as nitration with nitric acid under the same conditions but the extent of the other products formed in the two sets of reactions is very different. Nitration, by cerium(IV) ammonium nitrate (but not side-chain substitution) is suppressed by the addition of water. The results suggest that these nitration reactions by cerium(IV) ammonium nitrate occur through the intermediate formation of a nitronium ion from the cerium(IV) complex but the order with respect to the aromatic compound shows that this nitronium ion must then be formed in the presence of the aromatic substrate as a ‘ spectator’. The relative reactivities with respect to benzene in the nitration of anisole and naphthalene are much greater than those observed in nitration by nitric acid and, with both, the isomer proportions are also anomalous.

Reactions of cerium(IV) ammonium nitrate with aromatic compounds in acetonitrile. Part 1. The mechanism of side-chain substitution

Benzene, alkylbenzenes, and phenolic ethers react with cerium(IV) ammonium nitrate in acetonitrile to give nitrocompounds and (if α-hydrogen atoms are present) also the products of side-chain substitution. With many substrates, the major products of side-chain substitution are benzyl nitrates but compounds which can give rise to particularly stable benzyl cations give mainly products from reaction with the solvent. Relative rates of side-chain substitution have been determined by the competition method: the results indicate reaction via a radical cation and the isotope effect kH/kD= 2.3 observed in reactions with [2H10]-p-xylene suggests that proton loss from this radical cation is, at least partly, rate-determining.

Evidence for a reversible nitro–nitrito rearrangement following ipso-attack in nitration

1 H and 15N n.m.r. spectra indicate that the reactions of 2,6-dichloro-4-methyl-4-NO2-cyclohexa-2,5-dienone in chloroform are accompanied by a reversible nitro–nitrito rearrangement.

The reactions of nitrogen dioxide with dienes

The reaction of nitrogen dioxide with conjugated dienes (2,5-dimethylhexa-2,4-diene, hexa-2,4-diene and 2,3-dimethylbuta-1,3-diene) in organic solvents (mainly hexane, benzene and diethyl ether) results mainly in 1,4-addition giving, with the first two substrates, a mixture of 1,4-dinitro compounds and 1,4-nitro alcohols. The latter are presumably formed from hydrolysis of the intermediate nitronitrites. With 2,3-dimethylbuta-1,3-diene, only the cis and trans 1,4-dinitro compounds were isolated. The reactions effectively stop after the addition of two nitrogen dioxide radicals, only traces of tetrasubstituted products being obtained. With the unconjugated hepta-1,6-diene, addition can be made to occur to both double bonds but, when two equivalents of nitrogen dioxide are added very slowly, the main products derive from addition to one double bond and cyclisation to form cis and trans 1,2-bis(nitromethyl)cyclopentane. All the above reactions give a very complex mixture of products and the product composition depends markedly on the reaction conditions; however, with the conjugated dienes, the 1,4-dinitro compounds can be obtained in yields of ca. 50%. The crystal structure of trans-1,4-dinitro-2,3-dimethylbut-2-ene was determined.

Reactions of nitrogen dioxide with hexenes. The mechanistic and structural factors controlling the product composition

The reaction of nitrogen dioxide with a number of hexenes has been investigated using several solvents and the major products have been identified. The heterolytic reaction path has been effectively eliminated by the use of hexane as solvent. The mechanism of the heterolytic reaction path is discussed with the aid of ab initio molecular orbital calculations on possible nitrosating agents with particular reference to the syn and anti forms of nitrosyl nitrate. A new mechanism is proposed for the formation of certain trisubstituted derivatives.

A reassessment of the isoinversion principle

A maximum in a plot of In P vs. 1/T does not necessarily imply that the values of ΔΔH‡ for the two possible rate determining steps are of opposite sign.

Polymerisation and related reactions involving nucleophilic aromatic substitution. Part 3. Mathematical models of the polycondensation reactions of halogenobenzophenones

Linear free energy relations have been used to calculate rate coefficients for the stages in the polycondensation reaction of the potassium salt of 4-fluoro-4′-hydroxybenzophenone and for the polycondensation reactions of 4,4′-difluorobenzophenone with the dipotassium salt of 4,4′-dihydroxybenzophenone and the dipotassium salt of hydroquinone. These rate coefficients have been used in a kinetic model of the polycondensations to calculate how the concentrations of the individual molecular species formed vary with time. The kinetic model is used to show that the molecular composition of the solutions, when considered as a function of the percentage reaction, should be relatively insensitive to the initial monomer concentration and to medium effects provided that all of the species concerned are fully soluble.

The kinetics of hydrogen-isotope exchange at the nitrogen atom of substituted anilinium ions. Part 2. The nitrosonium-ion-catalysed reaction

Nitrosonium ions catalyse hydrogen-isotope exchange between the N–H protons of NN-dimethylanilinium ions and aqueous sulphuric acid (83–93%). At the acidities used, the subsequent chemical reactions between the amines and nitrosonium ions occur much more slowly. The rate of the catalysed exchange is proportional to the concentration of nitrosonium ions and inversely proportional to the appropriate acidity function (h0‴). A mechanism is put forward for the catalysed exchange based on the formation of a loose complex between the nitrosonium ion and the anilinium ion followed by proton loss from the nitrogen pole. This process is shown to be related to the direct diazotisation of some anilinium ions and to the nitrous-acid-catalysed nitration and oxidation of the NN-dimethylanilinium ion. Substituent effects on the nitrosonium-ion-catalysed exchange are much less than those on the acid-catalysed exchange and resemble those on the acid-catalysed mechanism of diazotisation.