The O-nitrations of methyl alcohol, pnitrobenzyl alcohol, ethylene glycol, trimethylene glycol, and also glycerol with respect to its primary hydroxyl groups, with nitric acid in constant excess in solvent nitromethane of nottoo-high water-content, are of zeroth-order and identical in absolute rate with one another and with the N-nitrations of N-methyl-di-or -tri-nitroaniline and with the C-nitrations of benzenoid hydrocarbons under the same conditions. For the nitration of methyl alcohol small concentrations of sulphuric acid increase, of nitrate ions decrease, and of water do not change the zeroth-order rate, whilst larger concentrations of water increase it, and still larger concentrations cause a switch from zeroth-t o first-order kinetics with reduction in the absolute rate. I n the presence of sufficient water to secure first-order kinetics, the rates of attack of nitronium ion on the unsaturated or unshared electrons of carbon, nitrogen, and oxygen in the substances mentioned were compared.In conditions similar to those of these zeroth-order nitrations, neopentyl alcohol is nitrated in a kinetic form between zeroth and first order, and a t a smaller absolute rate, whilst the secondary hydroxyl group, remaining after the nitration of glycerol has completed its zeroth-order course, is nitrated in first-order form, and a t a still smaller rate.AS NOTED in the preceding paper, we expect that, for any one general reaction within the natural family of electrophilic substitutions, for instance, nitration, nitrosation, or chlorination, much the same pattern of available mechanisms will apply to C-, N-, and O-substitutions. In particular, we expect that, since in C-nitration the nitronium-ion mechanism is outstandingly important, this mechanism will probably dominate the picture of Nand O-nitrations.We are here concerned with O-nitrations, particularly with the conversions of alcohols into alkyl nitrates. Until recently, such esterifications with nitric acid seem not to have been generally regarded, at least not automatically, as nitrations. In 1950 Israelashvili suggested that the conversion of starch into its nitrate might be an O-nitrati0n.l In 1951, KIein and Mentser proved, by the use of isotopically labelled oxygen, that the conversion of cellulose into its nitrate was indeed an O-nitration: what was shown was that, for each group C-0-H converted, the replacement was of H by NO,, with two oxygen atoms from the nitrating medium, not of OH by NO,, with three oxygen atoms from that source.2 This work firmly classified the esterifications of alcohols with nitric acid as nitrations, but left untouched the question of nitration mechanism.The nitronium-ion mechanism of the O-nitration of alcohols was first identified in this research by the kinetic method originally developed (refs. 1 and 2 of preceding paper) for the study of aromatic C-nitration, the principle of which is to show that a conjugate acid of nitric acid is dehydrated to a nitrating entity, which, according to how we adjust the competitive conditions, ...
The AT-nitration of N-methyl-2 : 4 : 6-trinitroaniline by nitric acid in constant excess in nitromethane of not-too-high water-content is of zeroth order and, at not-too-high concentrations of nitric acid, the absolute rate of the nitration is identical with the common rate of nitration of aromatic hydrocarbons under the same conditions. Small additions of sulphuric acid increase, of nitrate ions reduce, and of water do not change, the zeroth-order rate, but larger additions of water increase rate, with maintenance of the zeroth-order form, and still larger ones change the kinetics to first-order form, with reduction in the absolute rate during the kinetic transition, and in the first-order rate-constant after it is completed. These effects fully establish the nitronium-ion mechanism for this N-nitration.Two successive steps of nitration of N-methyl-2 : 4-dinitroaniline occur in similar conditions, The first step is an N-nitration, the kinetic characteristics of which, as far as studied, are practically identical with those of the nitration just described. The second kinetic step is stoicheiometrically a C-nitration, and is observed only in first-order form. Almost certainly the rate-controlling process here is an intramolecular rearrangement, in which the N-nitro-group moves into the aromatic ring, this slow reaction then being very quickly succeeded by another N-nitration. ELECTROPHILIC substitution has hitherto been studied largely in the form of aromatic substitution, where the attack by the substituting agent is on the unsaturation electrons, i.e., conjugated carbon 29 electrons. Since non-bonding 29 electrons of nitrogen and oxygen can participate deeply in such conjugation, we should expect them to share many properties with the unsaturation 29 electrons of carbon, including general vulnerability to electrophilic substituting agents. Indeed, for a given substitution of this class, e.g., nitration, nitrosation, or chlorination, we should expect the pattern of mechanisms to be much the same for C-, N-, and O-substitutions.Nitration has proved to be one of the simpler forms of electrophilic aromatic substitution, because it is dominated by one outstanding mechanism, that of the nitronium ion. Aromatic nitration through attack of the nitronium ion was kinetically demonstrated, at first in non-hydroxylic, and in hydroxylic but non-aqueous solvents,lS and, more recently, even in solvent water.3 +1
Trimethylene and 1 : 3-butylene sulphites are hydrolysed in alkaline solution a t rates which can be followed by conventional techniques at 0". The reactions are of the second kinetic order, and have the same rates whether measured by the uptake of alkali or the liberation of sulphur dioxide. The possible intermediate (e.g., HO*CH,*CH,*CH,*O-SO,H or its anion) does not build up to high concentration during reaction. Alkyl substitution in the carbon chain seems to have little effect on the rate of alkaline hydrolysis, but 1 : 2-sulphites are in this process very much more rapidly hydrolysed. These results, in conjunction with studies of bond-fission (Part I) allow comparison with the corresponding data for acid-catalysed hydrolysis (Part 11). In the latter reaction also, the possible intermediate does not build up in concentration; and alkyl substitution in the carbon chain has little influence on the rate, which now is little changed when the ring-size is altered.THE detailed courses of hydrolyses of cyclic sulphites, and the kinetic effects of structural changes, are considered. In contrast to the acid-catalysed hydrolyses of cyclic sulphites discussed in Part 11, those in alkaline solution are very rapid. Possible stages in the reaction (e.g., of trimethylene sulphite) are as follows:* Parts I and 11, preceding papers.
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