The reduction of nitrobenzene by sodium sulphide in aqueous ethanol has been examined. Hydrosulphide and hydrodisulphide ions give rise to quite similar, though slow, rates of reduction. Sulphide ion appears to be two to three times as effective as the hydrosulphide ion. Both the sulphide and hydrosulphide species are considered to be responsible for the initial, very slow reduction with possibly some contribution from traces of polysulphide ions. The reduction is markedly accelerated by the addition of elemental sulphur to form di-and polysulphide, a much more reactive reducing species. The formation of "active" elemental sulphur during the course of the reduction produces disulphide and thus gives rise to an autocatalytic reaction. Some of this elemental sulphur is lost a s thios~~lphate by reaction with hydroxyl ions produced by the reduction itself, and by hydrolysis of sulphide and disulphide ions. The addition of base increases the rate of reduction due to a shift in the equilibria S= + Hz0 e HS-+ OH-and S2= + H 2 0 e HSz-+ OH-, but this is offset somewhat by greater loss of the active elemental sulphur as thiosulphate, thus decreasing the autocatalytic acceleration of rate.Although, from its absorption a t 265 rnp, nitrobenzene concentration could be accurately measured, no satisfactory method was found for estimating the concentration of disulphide ions during the course of the reduction. INTRODUCTIONIn order to understand the variable results obtained in this laboratory in the alkaline sulphide reduction of nitro compounds and to rationalize the well-known selectivity observed in the sulphide reduction of mono-and poly-nitro compounds (1-4), a detailed study of the sulphide reduction of nitrobenzene was undertaken.Reduction of aromatic nitro coinpounds by such reagents as titanous chloride or stannous chloride under acidic conditions has been followed by measurement of the consumption of reducing agent (5-7). Since the disappearance of the nitro compounds was not followed directly, one must either determine or assume that certain definite changes occur in the reducible substance and that all the reducing agent has been consumed by this process and no other. This approach has revealed that the reaction is first order with respect to both nitro coinpound and reducing agent, and agrees with the generally accepted course of the reaction suggested by Haber (8) and shown as follows.There is a considerably greater difficulty in followiilg the reduction by measuring change in concentration of reducing agent when sulphide ions are employed, since the composition of the reduction mixture is more complex than it is in the case of stannous chloride. I t is known that a solution of sodium sulphide contains a number of species including sulphide, hydrosulphide, disulphide, hydrodisulphide, and thiosulphate ions, the relative composition of which depends upon the source of the sodium sulphide used for its preparation and the conditions under which the solution is made. The particular species responsible for th...
Uepc~rt~~le?lt of Cliet~tzstry, G1~loers~t31 of illbefta, Edlno~tto?~, iliberte Received June 11, 19G'S i2BSrI'R.-1 CTT h e retluctior~ of nitrobenzenc by sodium hydrosulphide and sodiiim hpdrotlisulphide in aqueoils media a t 50' has bee11 examined. Goltlschmiclt's report oE lirst-order dependence Llporl both ~~itrobelizenc arid hyclrosrilphide is corroboratetl. 'I'he action oE hydrosulphide on nitrobcrrzel~e procluces phenylhydrosyla~nilli~~e, which is reduced by hydrosulphitie m~~c l i tilore slowly than is l~itrobenzene. rls reaction progresses, nitrobenzene reduction protluces yello\v liydroclisulpl~icle, which is responsible for the observecl atrtocatalytic eiEect. Flyclrodisulphitlc reduces phenylliydroxylarnine more rapidly than it tloes the original nitroberizene, and lience as reaction time is extended, p l~e n~~l h y d r o s y l a n~i l~e disappears niore rapidly than does tiitrobenzei~e, yielding only aniline and some ~inreactetl nitrobenzene to\vards the eilcl of the reaction. Hydrosrilpliide retluction of plier~ylhydroxyla~i~ii~e t o aniline produces thiosulphate but apparently no hydrodis~~lphidc. Hyclrodis~ilphide reduction of the phenylhydrosylanline leads only to conversion oE the yello\v hydrodisulphidc to a colorless species, apparelitiy thiosiilphate. On the other liancl hytlrosulphide or hyclrodisulphide reduction of nitrobenzene is accolnpanied by thiosulphate formation ancl sorne increase in hydrodisulpiiicle. ri comparison of the rate constants sho~vs that ~~i i d e r the co~iclitions of the reaction, hyclrotlis~llphide ion reduces nitrobenzene about se\.en times more rapidly than does hydrosulphicle ion while phe~~ylhydrosylar~ii~ie is reti~lced tmo to three timcs more rapidly by hydroclisulphide than is nitrobenzene.The use of sodium hydrosulphide for the selective I-eduction of nitro groups in the aromatic nucleus is lvell ltnown. This reagent has been prepared for this purpose by treatnlent of sodium monosulphide with 1 equivalent of sodiu~n bicarbonate (I), or, in the for111 of lnore acidic systems, by bubbling hydrogen sulphide through a solutioll of the reducible substance contailling a catalytic a~n o u n t of ammo~lia (2). The addition of excess a~lllnoniu~n salts to a solutio~l of hydrosulphide has also been elnployed to produce hydrosulphide under co~lditio~ls designed t o maintain a pI-I of 7.5 to 8.0 (3).The only reported kinetic study of the hydrosulphide reduction is that of Goldschmidt and Larsen (A), who investigated the reduction in aqueous solution a t 25' C of various nitro compounds by sodiunl I~ydrosulpl~ide saturated with hydrogen sulphide. Tlle conditions of their reaction resulted in conversion of the nitrobenzene to the corresponding N-pl~enylhydroxylamine along with co~nplete precipitation of the sulphur formed according to equation (a). The rate of reductiorl was found to be first order in nitrobenzene, first order in hydrosulpl~ide ion, and illdepellde~lt of hydl-ogen sulphide concentration.In a previous paper ( 5 ) we have shown that the S,= species reduces ...
By measuring the initial rates of production of hydrodisulphide during reductions by sodium hydrosulphide in aqueous ethanol, it has been shown that the relative rates of reduction of a number of para-alkylated nitrobenzenes are in the order p-H-> p-t-butyl-> Pisopropyl-= p-cyclohexyl-> @-ethyl-> 9-methyl-nitrobenzene. This is in accord with the "hyperconjugative" order of electron release by these alkyl groups.
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