Electroanalytical study of isomeric trifluoromethyl-nitrosobenzene intermediates

Awlyticrc Cltirnicu Acta. 71 ( 1974) 433-441 0 Elsevicr Scientific Publishing Company,

ELECTROANALYTICAL NITROSOBENZENE

R. L. DICKERSON Depccrtttwttt

(Received

of

433 Amsterdam

STUDY OF INTERMEDIATES

- Printed

in The Netherlands

ISOMERIC

TRIFLUOROMETHYL-

and J. W. ROGERS*

Chemistry,

17th January

M iriwescerr~ Uniaersity,

Wiclritcr

Fdls,

TLXIS

76308

( U.S.A.)

1974)

The nitro group of various nitroaromatic compounds is electrochemically reduced, in one polarographic step, to the corresponding phenylhydroxylamine in basic aqueous media, and to the corresponding amine in acidic media’e2. In purified nonaqueous solvents such as acetonitrile, dimethylsulfoxide and N,Ndimethylformamide (DMF) most nitroaromatics are reduced, to hydroxylamines and other products, only at the higher potential step of two polarographic steps3. In general, the lower potential step represents the production of a long-lived anion x-radica14*‘. If intermolecular coupling reactions are ignored, the reduction of the nitro function in an aprotic medium at the second polarographic step may be cohveniently represented by the following scheme: RNO,’

-

+C?+2

RN0

El+

-

+2e-

RNHOH

+2H+

Kemula and Sioda6, who used rapid-sweep cyclic voltammetry with oscilloscopic recording, found evidence of a very transient nitrosobenzene anion radical in the electrochemical reduction of nitrobenzene. However, the nitroso group is known to couple readily with hydroxylamines as well as dimerize to form azoxy compounds ‘-’ . Also under the influence of sufficiently negative potentials, electron transfer to the Aitroso intermediate may compete effectively with intermolecular coupling reactions to produce hydroxylamine and amino compoundslO. Consequently, th5 nitrosobenzene intermediate is not commonly detected in the reduction of nitroaromatics. A notable exception to this is found in the electrochemical reduction of o- and m-trifluoromethylnitrobenzene in DMF’ I. These compounds are reduced to abnormally long-lived trifluoromethylnitrosobenzene anion n-radicals at the more cathodic of two polarographic steps via a mechanism in which a chemical reaction is coupled between two reversible electron transfers (e.c.e. mechanism). The ortho isomer exhibits the greatest stability, both being stable enough to be detected in cyclic voltammetric experiments conducted at the lower limits of potential sweep rate. Synthesis of p-trifluoromethylnitrobenzene has made it possible to investigate the relative stabilities of the nitroso radical reduction intermediates of all three isomers in aprotic media. A very significant and interesting trend in stability of the o-, nt- and p-trifluoromethylnitrosobenzene radical is observed. Potential step * To whom correspond&cc

and

requests

for reprints

should

be directed.

434

II.

L. DICKERSON,

J. W. ROGERS

experiments with concomitant cyclic sweep voltammetry measurements at a planar platinum disc electrode (p.p.d.e.) were employed to compare the lifetimes of the three species. Auxiliary techniques employed were chronoamperometry, and mass electrolysis with spectrophotornetric identification of products. EXPERIMENTAL

The p.p.d.e. employed in the cyclic voltammetric experiments was a Beckmann platinum button electrode with an area of 80 mm2. The reference electrode employed in all experiments, saturated calomel (s.c.e.), made contact with the solution through an agar plug behind a Pyrex frit. Water leakage through this tip was found to be negligible. The cyclic voltammetric and chromoamperometric data were recorded on a Bolt-Barnak-Newman X-Y plotter with a PAR Model 173 potentiostat and Exact wave form source for potential variation and control. The mass electrolytic experiments were performed w it11 a Wenking Potentiostat in a nitrogenstirred solution at a platinum gauze electrode. The platinum foil anode and s.c.e. reference were isolated from the cathode compartment with fine porosity glass frits. The U.V. spectra of the electrolysis products were recorded on a Perkin-Elmer Model 202 scanning spectrophotometer in quartz cells with pure DMF solvent as reference. All solutions were 1 mM in electroactive compound and 0.1 M in the supporting electrolyte, tetra-N-propylammonium perchlorate. Spectroquality DMF containing ca. 0.030/, water was obtained from Eastman Organic Chemicals. The solvent was vacuum-distilled from anhydrous copper(H) sulphate befdre use. The preparation and purilication of TPAP has been described previously16. o- and nz-Trifluoiomethylnitrobenzene were purchased from K and K Chemicals. The para isomer was prepared by diazotization of cc,cc,a-trifluoro-p-toluidine after the method of StarkeyI’. RESULTS

Cyclic

voltammetry. of For purposes of comparison, typical multi-sweep cyclic voltammograms o-, it- and p-trifluoromethylnitrobenzene are shown in Fig. 1. Pertinent electrochemical data from cyclic voltammetric experiments conducted on the three isomers at a p.p.d.e. in dry DMF are given in Table I. These criteria include variations of the cathodic peak potential, (E,),, the ratio of anodic to cathodic current for a single peak, (i,)4/(i,)c. and the current function, ($,&/uf, where u denotes the potential sweep rate 12* ’ 3. Data presented previously l I from similar experiments ‘on the ortho and meta isomers were taken at a hangitig mercury drop electrode. Owing to peak potential differences at the two electrodes and for completeness, some data are represented herein. Each of the three parent compounds exhibits an initial voltammetric current peak at which it is reduced to a radical anion via a one-electron reversible

TRIFL ~UOROMETHYLNITROSOBENZENE

0.0

-2.0

-1.0

E

YS

435

ISOMERS

SCE (V)

Fig. 1. Muhi-sweep cyclic voltammograms at a p.p.d.e. at 3,O V Inin-‘, (a) o- (b) w- and (c) p-trifluoromcthylnitrobcnz~n~.

of 1 mM DMF

solutions

of

transfer12. The lifetime of these species is long relative to the potential sweep rate as evidenced by data in Table 1. and Fig, 1. Each isomer exhibits a second cathodic current response at higher potentials on the initial voltage excursion (Fig. I). As previously discussed, application of the Nicholson and Shain diagnostic criteria’3 to data for the orrho and tnetu isomers suggests that each is primarily reduced via a chemical reaction coupled between two reversible electron transfers, The anion radical of the corresponding nitroso compound is a product of the electrochemical step. RN02’ RN0,2-f2 RNO-i-e-

-i-e-

r=t RNO;H-+ -+ RNO-#-H20

r=t RNO-

436

R. L. DICKERSON,

TABLE CYCLIC SOLVENT

;I”

I VOLTAMMETRIC AT A PLANAR Firsr

twirl -

J. W. ROGERS

lvut:c

DATA FOR TRIFLUOROMETHYLNITROBENZENES PLATINUM DISC ELECTRODE“ Secord

-_(E,),.

-.-L---__~___-(E,),-(I:‘,,),, (i,),l(i,,)?

(i, A/t) i

( L’)

(Ill I’)

(pA

’)

p- ~~rijl~toror~~e~h~lr~itrohrrl:erIe 16.06 0.88 60 10.71 0.88 60 5.35 0.88 60 2.14 0.87 60 l.Gl 0.87 60

IN

DMF

1vuve

--_ V-+

-_(E,)c tniri-4)

f

V)

1.95

( i,Jp/u4” (pl

1.0 1.0 1.0 1.0 1.0

33.54 33.64 33.55 33.60 35.43

1.94 1.93 1.90

78.55 81.04 88.3 I 94.24 98.43

1.0 1.0 1.0 1.0 1.0

42.16 42.88 44.24 44.45 41.35

2.21 2.20 2.15 2.09 2.07

77.20 80.67 88.48 89.29 92.11

1.0

26.70 26.71 27.36 26.49 26.92

2.10 2.10 2.09 2.04 2.02

49.15 49.70 52.52 53.00 54.85

1.95

V -

4 rnirl - f,

r,l-Tri/7f~~1rortrerli~lrIitrohcrr=e~t~~

17.7 1 11.81 5.90 2.36 1.77

0.99 0.98 0.97 0.97 0.97

65 65 60 60 60

o-TrifY[tu~orrrrrIr~lr?ir~ohrtr=etle 16.99 1.02 70 11.33 1.02 70 5.66 1,oo 70 2.27 1.00 60 1.70 1.00 60

1.0

1.0 1.0 1.0

u All solutions 0.1 h1 in tetra-N-propylammonium h Scan rcvcrsal I20 mV past (E,),. r (i,,) measured by extrapolating current from

perchlorute. first wave.

This assertion is substantiated by the appearance of a reversible one-electron couple at potentials less cathodic than that representing the reduction of the parent compound (Fig. I). This couple is only present after voltage excursion past the second wave and is consistent with the production of the nitroso group, known to be more reducible than the parent nitro compound 6*lo. Data recorded for p-trifluoromethylnitrobenzene demonstrate similar electrochemical activity, with a weak nitroso couple apparent (Fig. 1, Table I). Chronoamperometric data taken under duplicate conditions at a p.p.d.e. confirms an tt (apparent) value of 1.8, 1.9 and 2.1 for the second reduction wave of o-, ??I- and p-trifluoromethylnitrobenzene, respectively.

In order to d’etermine the nature of reactions occurring in bulk solution and not totally evident from the rapid voltammetric experiments, the compounds o- and p-trifluoromethylnitrobenzene were mass electrolyzed at a platinum gauze cathode, at a constant potential above the second voltammetric wave of each. The progress of the electrolysis was monitored by current decay. The relative decay rates at an applied potential/of -2.0 V for electrolysis of 1 mM solutions of the ortho and fiara isomers are given in Fig. 2. The cell current *for electrolysis of the

TRIFLUOROMETHYLNITROSOBENZENE

Fig. 2. Decay of cell current p-trifluoromcthylnitrobenzenc

437

ISOMERS

in mass electrolysis at a platinum gauze

of 150 ml electrode.

of

1mM

solutions

of (a)

b- and

(b)

ortho isomer decayed exponentially to a residual current. Current decay for electrolysis of the pnra isomer was approximately exponential and decayed to a residual current slightly above that of the ortlzo isomer (Fig. 2). Aliquots of the catholyte were removed periodically and diluted with an appropriate amount of dry DMF, and the u.v.-visible spectrum recorded. The para isomer exhibits a U.V. absorption peak with a A,,,,, near 270 nm in DMF solvent (Fig. 3). A broad peak at 320 nm is observed after 15 min of electrolysis. It intensifies with continued electrolysis and predominates when a limiting cell current is established (Fig. 3). This broad absorption band is consistent with the azoxy coupling product. The broad visible absorption band near 425 nm is consistent with an azo coupling product. These results are analogous to those from electrolysis of o-trifluoromethylnitrobenzenex *. In summary, results of the voltammetric experiments demonstrate that each o.o-

3.00

350

400

WAVELENGTH

Fig. 3. U.v.-visible catholyte solution

spectra .of (a) after electrolysis

450

500

550

600

(nm)

DMI: solvent (b) p-trifluoromethylnitrobenzene of p-trilluoromethylnitrobenzene to a residual

in DMF cell current.

and

(c)

R. L. DICKERSON.

438

J. W. ROGERS

nitro isomer is initially reduced to the nitroso anion radical at its second current Some competing reactions are suggested. plateau by a common mechanism. Long-term experiments show that the products of exhaustive mass electrolyses of each nitro compound are primarily azoxy and azo compounds. Poterl tial step Comparison bf the cyclic voltammograms shown in Fig. 1 reveals that the follow-up wave, identified as the nitroso couple, produced by voltage excursion past the second wave of each parent nitro compound, is much less pronounced in the pnra isomer than in either the ortho or meta In general, under the same conditions of concentration, electrode size, potential sweep rate, and reversing potential, the order of peak current intensity of the nitroso couple is orrho > rmta > para. The relative intensities of these couples produced from each nitro isomer, i.e. the quantity of nitroso compound present after electrolysis at the second wave for a fixed time, was further investigated with potential step and hold experiments at a p_p.d.e. coupled with cyclic voltammetry following a predetermined holding time. Specifically, five experiments were conducted on each parent compound. In each experiment the potentiostat potential was stepped to a fixed potential on the diffusion plateau of the second wave and held for 15 s. The potential was then stepped to 0.0 V and triangularly swept at a predetermined sweep rate to a switching potential 100 mV cathodic of the nitroso-couple current maximum. Only the sweep rate of the cycle was varied in each experiment. Experiments conducted in such a manner, with following multi-cycles, demonstrated that diffusion of the nitroso product away

a0 E vs. SCE

(VI

Fig. 4. Cyclic voltammograms of tritluoromcthylnitrosobenzenc intermediates produced at a p.p.d.e. electrode by stepping potential of electrode to second reduction wave of the parent nitro compound. (a) Two separate voltammoprams of o-trifluoromcthylnitrosobenzene produced by potential step and hold to demonstrate reproducibility of tcchniquc, (b) o-Trilluoromethylnitrosobcncne recorded at varying potential sweep rates. (c) ltl-Trifluoromcthylnitrosobenzene recorded at varying, potential sweep rates.

TRIFLUOROMETHYLNlTROSOBENZENE

439

ISOMERS

from the stationary electrode is slow relative to the time of the experiment, and the voltammetric data proved to be reproducible, (Fig. 4). Such experiments not only signified the quantity of nitroso compound available at the electrode but allowed investigation of the electrochemistry of the intermediate at its lowest reduction potential. Potential step and hold produced a nitroso couple from o-trifluoromethylnitrobenzene with a cathodic peak current approximately twice that for the tneta isomer (Fig. 4). Experiments on p-trifluoromethylnitrobenzene, conducted under identical conditions, produced only a very weak nitroso couple. TABLE

II

CYCLIC VOLTAMMETRIC BENZENE INTERMEDIATES swc%Jp rot1 ( V ruin - ‘)

___----.

First

DATA FOR REDUCTION OF TRIFLUOROMETHYLNITROSOPRODUCED BY POTENTIAL JUMP“

wuvc

- f E,,)‘. (V)

o-T~.iJ’llroror,~erllplnirrnsohetl;erte 10.80 0.68 8.64 0.68 6.48 0.68 4.32 0.68 2.16 0.68 ~,r-Tri~llcoror~~eth~l~~itrosohetlzerle 10.56 0.72 8.45 0.72 6.34 0.72 4.22 0.72 2.11 0.7 I

--

-(i,),l~~~ (pA V-+

(-%),-(&A ( III v )

(ipM(ip)r

59 59 59 58 57

1.0 1.0 1.0 1.0 1.0

9.12 9.18 9.24 9.00 4.7G”

70 70

_c

-E

3.90 4.12 3.77 3.76 2.76h

67

-.c _c

65

-c

.67

rtiirl-+)

“ Potenti:tl jumped to value 200 mV cathodic of second wave current peak of parent nitro compound. ” Diffusion of intermediate away from working elcctrodc becomes significant ut cxtrcmcly slow sweep rates. c Relictble measurement of anodic current no possible due to close proximity of couple to Icnst negative couple of purcnt nitro compound.

The diagnostic criteria” taken from the cyclic voltammograms of the nitroso couple derived from the ortho and metcl isomers are given in Table II. The anodic to cathodic ,peak current ratio of 1.0 and the constant current function at all sweep rates is clearly consistent with the reversible production of a long-lived nitroso anion radical. Unsubstituted nitrosobenzene does not exhibit parallel behavior under similar condition@. DISCUSSION

The data presented herein suggest that the isomeric trifluoromethylnitrobenzenes are reduced by the same mechanism to the corresponding trifluoro-

440

R. L. DICKERSON.

J. W. ROGERS

methylnitrosobenzeneanion radical. However, cyclic voltammetric data demonstrate a significant difference in lifetimes of these radical species.’ The lifetime of the nitroso intermediate is determined by following reactions of a chemical and/or electrochemical nature”. The azoxy and azo products, found to predominate in bulk solution after electrolysis, are the result of coupling reactions between two nitroso groups, two hydroxylamine groups or nitroso with llydroxy1amin’e7-9. The abundance and stability of the o-trifluoromethylnitrosobenzene intermediate near the electrode surface suggests that the product found in long-term mass electrolysis of this isomer is largely a result of coupling between two of the nitroso inte!mediates. The lack of p-trifluoromethylnitrobenzene near the electrode after voltage excursion suggests that the electrochemical formation of reduced products, such as the hydroxylamine, competes favorably with coupling reactions at potentials of the second wave, or conversely that the coupling rate of p-trifluoromethylnitrosobenzene radicals is very rapid. The difference in the current-time curves in the long-term constant potential electrolysis experiments suggests that the route to the azoxy product of the pan isomer is via intermediates other than nitrosobenzene, and is consequently consistent with the former suggestion. The slightly larger chronoamperometric IZ (apparent) value of the second wave of the pura isomer is also consistent with these observations. Holtz14 has presented a discussion of the unique Hammett behavior of the trifluoromethyl substituent. An inductive model, called the n-inductive effect”, was employed to rationalize the greater than normal negative charge delocalization by the trifluoromethyl group on an aromatic ring. In this model, the strong electronwithdrawing effect of the trifluoromethyl group is explained by the existence of a dipole external to the benzene ring polarizing the n-system and enhancing negative charge delocalization as shown in structure 1. A model of this type appears to be consistent with the reactivity of the isomeric trifluoromethylnitrosobenzene radical intermediates as evidenced by the present data. The overall enhanced stability of these radical species is probably due to inductive charge delocalization from the nitroso group. The p-substituted intermediate would be more susceptible to further electron addition by structure 1, and visualization of a contributing structure such as II for the ortho isomer explains its great stability relative to further reduction and coupling reactions. Stability of the meta isomer should be intermediate between the ortho and pnru by this model.

The authors gratefully acknowledge the financial support of the Robert A. Welch Foundation (Grant No. AO-337). The authors are indebted to Dr. E. H. Sund for the synthesis and purilication of p-trifluoromethylnitrobenzene.

TRIFLUOROMETHYLNITROSOBENZENE

ISOMERS

441

SUMMARY

The relative lifetimes of the o-, VZ- and p-trifluoromethylnitrosobenzene anion radicals produced in the electroreduction of the corresponding nitro compounds in N,N-dimethylformamide are compared. Potential step experiments coupled with cyclic voltammetry facilitate the comparison. The fate of the nitroso species is discussed in relation to the results of experiments in which o-, I??- and p-trifluoromethylnitrobenzene were mass electrolyied and the products spectrophotometrically identified. REFERENCES 1 J. Pearson, 7hr1.x Fwutltrv Sec., 44 ( 1948) (583. 2 I. M. Kolthoff and J. J. L&.mc, Pofarogrq~h_~i Vol. 2, Interscicnce. New York, 2nd edn., 1952. 3 0. Paez, E. T. Seo and H. P. Silverman, Tdzrzicd Report ECOM-02464-2, U.S. Army Electronic Command. 4 A. H. Maki and D. H. Gcskc, J. ,4mev. Chew. Sot., 83 (1961) 1852. 5 P. H. Ricger and G. K. Fracnkcl, .I. Cheru. Phys.. 39 (1963) 609. 6 W. Kemula and R. Siodn. Bttll. Acad. PO/. Sci., Ser. Sci. Clzirzz.. 10 ( 1962) 507. 7 C. R. Noller, Clzerzzistry oj’ Orgattic Cornpo~rnds, Saunders, Philadelphia. 3rd edn. 1965. 8 E. J. Gccls, R. Konaka and G. A. Russell, Proc. Chem. Sm. Lozztlotz. (1965) 13. 9 G. A. Russell and E. J. Gcels, J. Aww. C/w,+ Sot!.. 87 (1965) 122. 10 R. D. Allcndoerfer and D. H. Rieger, ./. .4rtrer. C/IEIII. Sot., 88 (1966) 371 1. 11 W. N. Greig and J. W. Rogers. J. Anzer. Chezzz. SOC., 91 (1969) 5495. 12, R. S. Nicholson and 1. Shuin, AM/. Client., 36 (1964) 706. 13 R. S. Nicholson and I. Shain, Azzal. Chnz., 37 (1965) 178. 14 D. Holtz, Chenz. Rw, 71 (1971) 139. 15 M. J. S. Dewnr, HJ’pprcozljzrguliozz, Ronald Press, New York, 1962, pp. 155, 156. 16 J. W. Rogers and W. I-1. Watson. J. Ph~rs. Chetn., 72 (1968) 68. 17 E. B. Starkey, J. Anzer. C/rem Sm.. 59 (1937) 1479.