POI,AIRCX~KAIPHIC BEHA4TIOR OF ARO3LITIC S I T R O C‘O3IPOITDS. I 3 I O S O S I T R O C O l l P O L A-DS
.I. 1
l’-IGk;, J. IT.SlIITH,
AND
J. G . ITT’XLLI:R1
G/tr.ro I,rihorrcto? I C S , Lid , Greenfaid, Jlnddleses, a n d Chemzstri/ Dcpn?tnu ni, Batterscn PolytechnLc, L o n d o n S I T - 11, Ercglnid
H C C Fed~ .Uay ~ 18, 1948
The electroreduction of aromatic nitro compounds is of particular interest in relation to the electron distribution in such compounds. The nitro group j; :i pon erful electron-seeking group; consequently it is to be espected that the electron density in its vicinity will be affected by the nature and position of other electron-donating or electron-seeking substituents in the aromatic ring. This in turn vi11 be reflected in the ease of electron capture a t the electrode. We have therefore studied the polarographic behavior of several simple mononiti o aromatic compounds nith a vien to throwing light on the mechanism of their reduction. Sitrobenzene uas one of the first organic compounds t o be studied polhrographically. Shikata (13, 14) measured the ‘(reduction potential” of nitrobenzene at different p H values, and subsequently, n-ith hi5 collaborators (15, 1 2 , studied the nitrophenols, dinitrobenzenes, and dinitrophenols. (L‘Keduction potential” is an empirical magnitude only, being the value of the potential at the foot of the current-voltage curve, whereas the half-ware potential i, the potential at that point on the current-roltage curve at n h k h the current i q equal to one-half of its limiting value.) Unfortunately, ihe Japanese authors did not record wave heights, although they calculated the number of electrons involved in the reactions by means of the Sernst equation
E’
=
- 0.0591
K
~
11
log [HI [RSO.]
\\-here E’ is the “reduction potential”, n the number of electrons involi-ed in the reaction, K a constant, and [HI and [RSO?]the concentrations of hydrogen ion md nitro compound, respectively. The values of n so obtained are open to -riticism, since equation 1 is not valid for irreversible systems. More recently, is part of a study of hydrogen bonding, ;Istle and his coworkers (1, 2, 3) have letermined by modern techniques the polarographic characteristics of certain iromatic nitro compounds, including o- and p-nitrophenols. The reduction of he nitro--\: ,S‘-diphenylacetamidineshas been similarly investigated by Runner, Glpatrick, and Wagner (12). Winkel and Proske (22) investigated nitrobenzene )olarographically and found that its “reduction potential” increased from -0.1 v. a t about p H 1.0 to -0.28 I-. a t p H 4.0 and that at higher pH values it emained constant a t -0.75 These authors reported their results graphically nd did not state whether their solutions were buffered or not. The \\-ell-defined polarographic step formed by nitrobenzene has proved of 17.
546
less rapidly.
J-it mbc ti 20 ic n cids 'lYie po1:irographic t)elia~-iurof the nitruhcnzuic acid> I \ - L I ~studied in ordcr con1p:ti.e the effect of the electron-seeking carhosyl group with that of the ecti,on-tIonating hydros>-l group. The significmit (lata for the reduction of ui-. and p-nitrobenzoic. acids are , G e t out in tahles 8 and 9. The influence of pH T-nlue on the reduction of the nitrobenzoic acids is much .eater than on that of tlie nitrophenols and nitroitnisoles. All three nitrom z o i c ac4tls gave doutile \vaves a t high 1213 lxlues;. The total step height \\.:is )nsttint for the o- arid p-isonier. o-, but above pH 3.0, \there the molecules exist dmost completely a s negatively charged ions, the acids are reduced less readily han nitrobenzene. I t is noteworthy that at pH values below 3.0 the constant
558
J. E. PAGE, J. W. SMITH AIUD J. G. WALLER
\
+ 0.25
I
2.Q
I
\I
I
4.0
6.0
I 1
8.0
10.0
12.0
PH FI(;. 3 . I:if'wt of hydrogeri-ion co~ice~itrution 011 the half-wave potcriti:iis o f u - . v - , a n d p-nitrophenols and 0 - ,? T I - , ant1 p-nitroanisoles, T h e diffcrencc 1)ctn.c.cri t h c h 11alf-w:iy~~ potential for e:icli compound and tlic' corresponding pot entia1 for nitrol)t~nzeric:it the scrnicx hydrogen-ion concentration is plotted against pH valuc: A , o-nitrophenol: 11. VI-nitrop k i r ~ r i o l :c', p-riitrophcnol; D, o-riitroanisole; F:> wnitroanisole; 1:. p-nit ro:iniscrl(~;G , i i i t r o henzene.
+0.151
I
I
I
,
,
,
,
20
4.0
6.0
8.0
,
,
1
10.0 120
PH
FIG 4 . Effect of hydrogen-ion concentration on the half-wavc potentials of o-, m-, and r-nitrobenzoic acids and methrl 0 - ,m - , and p-nitrobenzoates. The difference betn-een he half-wave potential for each compound and the corresponding potential for nitrobenene at the same hydrogen-ion concentration is plotted against pH value. A , o-riitrobenzoic cid; B, m-nitrobenzoic acid; C , p-nitrobenzoic acid; D, methyl o-nitrobenzoate; E , methyl i-nitrobenzoate; F, methvl p-nitrobenzoate; G, nitrobenzene. 559
560
J . E. PAGE,
J. W. SMITH AND J . G. T A L L E R
a (equation 2 ) approximates to unity (vide supra). The reduction of the methyl nitrobenzoates is not complicated by dissociation and over the full p H range t h e three esters are reduced more readily than nitrobenzene. Their half-wave potentials bear a relation t o those of the corresponding nitrobensoic acids similar t o that between the half-ware potentials of the nitroanisoles and the nitrophenols.
DijJusio,i curruits The diffusion current in amperes, id, is related to the diffusion coefficient, D , by the IlkoviE equation (8), which can be expressed in the form
id
=
0 . ~ 0 5 r d' ~ m 3t1 '
(3)
where n is the number of electrons involved in the reaction, C the concentration of the reducible compound in moles per liter, m the mass of mercury flowing from the capillary in milligrams per second, and t the drop time in seconds (9). The value of D can therefore be calculated from polarographic results, provided the number of electrons taking part in the reduction is linen-n. Conversely, if the diffusion coefficient is obtained by an independent method, the value of n can be calculated. The diffusion currents for 3 x lo-' ;I1neutral solutions of all the compounds studied were ahout 5.8 pa.; for alkaline solutions of 0- and p-nitrophenols and m-nitrobenzoic acid the diffusion currents rose to about 7.4 pa. For acid 5 X lo-' M solutions, the total wave heights for most of the compounds Irere between 7.5 and 8.5 pa. Since for the reduction of the nitro group the value of n must be either 2, 4, or G, it appears that for a 5 x lop4 M solution, diffusion currents of 5.8 and 7.4 pa. correspond to values of n of 4 and 6, respectively. Thus, in alkaline solutions the reductions of 0- and p-nitrophenols and m-nitrobenzoic acid involve six electrons each, the reduction products being the corresponding amino compounds; reduction of the other compounds in alkaline solutions, and of all the compounds in neutral solutions, involves four electrons, suggesting the formation of intermediate reduction products. The total height of the two steps formed below p H 4.0 corresponds to six electron reductions. The diffusion coefficient of the o-nitrobenzoate ion in neutral solution calculated from these results has a value of about G.0 X lopGem.? set.-' a t 2.5"C. The approximate diffusion coefficient for an ionizable substance can he calculated from its electrical conductivity a t infinite dilution by means of the equation
nhere D is the diffusion coefficient, .io the equivalent conductivity a t infinite for the o-nitrobendilution, and z the charge on the ion (9). The value of zoate ion is 28.5 (B), and gives a value for D of 7.G x cm.? sec.-l, which agrees approximately with the figure of 6.0 x cm.* sec.-I calculated from our polarographic results.
POLAROGRAPHIC BEHAVIOR O F AROMATIC NITRO COMPOUXDS.
I
561
SUMMARY
The effect of pH value and concentration on the half-wave potential and diffusion current of nitrobenzene, 0-,m-,and p-nitrophenols, 0-,m-,and p-nitroanisoles, 0-,m-, and p-nitrobenzoic acids, and methyl 0-,m-,and p-nitrobenzoates, and the effect of ethyl alcohol concentration on the reduction of nitrobenzene have been studied. The half-wave potentials of all the substances studied varied linearly with hydrogen-ion concentration oyer a limited pH range, the slope of the curve being 0.039 T'. per p H unit, and the diffusion current of each compound was proportional to concentration over the range 0.1 to 1.0 X loM3 41; at greater concentrations the diffusion current increased less rapidly than the concentration.
One of the mthors (J. G . K.) wishes t o thank G h ~ Laboratories o Ltd. for generous provision of special laboratory facilities. .IDDESDLX
Since this paper \vas submitted for put)lication, I j r , .T. Pearson lins published a scries of pnpers (Trans. Faraday 8oc. 44, ti8.3, ti92 (1948): 46, 199 (1949)) on the polnrogixphic bchnrior of the mono-. di-, and tri-nitrobenzenes, -nitrotoluciiw. -nitrophenols, and -nitroresorcinols. Dr. Pearson's clatn for nitrotienzcnc :ind the 0-, m-,xiid p-nitrophenols ai'c in good agreement \vith those ohtainod by 11,s. It is of interest, ho\wver, that the pH nlue-linlf-n.are potential slope for many of the other aul)stances examined hy Dr. Pcarson difl'erd grcatly from 0.059, and that the slope in acid solution i w s nl\vays grc:itc.r than in al1i:iline solution. IiI
