Indian Journal of Chemistry Vo l. 42A, April 2003, pp. 744-750

Electroreduction of isophthalaldehyde: An example of simultaneous reduction of two identical electroactive centers

W J Bover. D Johnson, M S Baymak & P ZUlllan *

Deparlment of Chemistry, C lark son University, Potsdam, NY 13699-58 10, USA

Received 14 Decelllber 2002

The diproton:Jted form of isophthalaldehyde is reduced by two electrons to a diradical which. reduced further by two electron s. reacts wi th two protons to form a diaicohol. The stabi lity of the diradieal is much lower than th at observed for o r/h o- and terephthalaldehydes. In acidic media isophthalaldehyde thus behaves like a simple benzaldehyde at tw ice the concentration. Thi s course of the electrode process, based on polarographic reduction, has been confirmed by cyc li c voil -ammetry and contro lled potential electro lys is usi ng a dropping mercury electrode. The diprotonated molecules of isophtha-la ldehyde are oriented during the reducti on process in such a way that probability of the electron transfer is the same for both groups. In the single prolOnated form of isophthalaldehyde, the group protonated at the electrode is reduced in a two-electron process to alcohol. T he reduct ion of the second carbony l group occurs at more negati ve potential s. A t the time of reduction the molecule is oriented with one CH=OH+ group towards the surface. Investigation of the reduction of the un-protonated form is limited by the reacti on of the diearboxaldehyde with hydrox ide ions. Additi ons of hydroxide ions to car-bony l groups result in geminal diol anions which are ox idi zed in alkaline so lutions in two consecuti ve two-electron steps. One of the CH(OH)O' groups is then oriented preferentiall y at the electrode surface. In isophthalaldehyde dioxime both pro tonated ox ime groupings arc reduced simultaneously in a single, eight -electron step.

The reducti on of organic compounds bearing two identical electJ'oactive groupings can occur either in two consecuti ve steps or simultaneous ly , in a single step. Consecutive reduct ions are observed for straight-chain compounds, in wh ich two electroactive groups are separated by several sp' carbons. An example of such a process is the reducti on of ,N-polymethy-lene-bi s-sydnones I, where two reduci ble rings are separated by two or four methy lene groups. Similar con secu tive processes have been reported" for reduc-ti ons of some aromati c compounds, like polyio-dophthalates . For consecuti ve reductions it is neces-sary to assume that the inves ti gated compound is ori-ented at the electrode surface in such a way that only one of the electroactive groups is in a sufficient di s-tance from the electrode surface for the electron trans-fer to occur. Alternatively, such consecutive processes arc observed for processes where an irreversible elec-tron transfer is preceded by a chemical reaction (e.g. protonation) where such heterogeneous chemical re-action is faci litated by the orientat ion of the reactants at the electrode surface. For the first step in consecu-ti ve reductions the second group acts as a substituent. As reducible groups are usuall y electron-withdrawing, the presence of the second group results in a shift of the first reduction process to more pos itive potential s.

Facilitation of the reduction o f the f irst group was observed, for example, for some din i trobenzenes3.

When two identical electroactive groups are either in a vicinal position in aliphat ic or alicycli c com-pounds or in orll/O- or para- position on an aromatic ring, mutual interaction of the two groups can be very strong. The reduction paltern in such compounds is then different from that observed fo analogous mole-cule with a single electroactive group. For example the electrochemical behavior of p-d iacety lbenzene differs principally from that of acetophenone4 and terephthalaldehyde from that of benzaldehyde5.

Observation of a simultaneous reduction o f two electroacti ve centers in a single step indicates that the probability of both the electron and proton transfers is the same for both elec troact ive group~ . That is possi-ble when the compound is oriented at the elec trode surface so that both electroacti ve groups are ina si mi lar distance from the electrode surface.

The number of examples of simultaneous reducti on of two identical groups reported in the literature is relatively limited. Such behavior has been observed, for example, for the reduction of some polynitroben-zenes in acidic mediae,. The reduction o f isophthalal-dehyde in acidic media is another example of such system, in which not only electron transfers, but also

BOVER ef 01. : ELECTROREDUCTION OF ISOPHTHALALDEHYDE 745

the chemical reaction accompany ing such trans fer fo r the compound bearing two fo rmy l groups is simil ar to benzaldehydes, bearing onl y a sing le formyl group.

Electrochemical reductio n o f isophthalaldehyde has so far received onl y limited attention. Its reductio n in DMSO was briefl y mentioned7. In aqueous solutio ns it has been observed that current-vo ltage curves o f thi s 1,3-dicarboxaldehyde differ princ ipa lly from those o f the 1,2- and the 1,4-dicarboxaldehydes8. The di fference has been attributed4 .5 to the lack o f stability of the biradical fo rmed and the absence of hydratio n in the 1,3- isomer.

Materials and Methods

In st rUlllentat ion Current-voltage curves were recorded for d c and

di ffe renti al pulse po larographi c ex periments using PAR Mode l 172 Po larographi c Analyzer (Princeto n Applied Research, Princeton, NJ) in connectio n with a Hewlett-Packard 7004B X- Y recorder. The capillary e lec trode fo r po larographic ex periments had the fo l-low ing characteri sti cs : /"II = 2.5 mgs· l , t = 3.2 s at Ii = 60 cm at 0.0 V (SCE).

Cyc li c vo lta mmetry was carri ed out us ing an in house built instrument. Contro ll ed potenti a l e lectro ly-sis with a dropping mercury e lectrode was performed using the Cambridge Po larographic Analyzer 82 P (Cambridge, U.K.) and preparative e lectrolys is with a mercury pool e lec trode was carried out using a Bipo-lar Potentiostat Mode l 100-A (M I Assoc iates , Cam-bridge, MA).

A Corn ing Digita l 11 2 Research and Sargent Model DR p H-meters w ith a glass e lec trode were em-ployed for pH-measure ments of buffers.

For recording of cun'ent- vo ltage curves an e lectro-lyti c ce ll with calo me l refe rence e lectrode separated by liquid j unctio n (Kalousek cell ) was used. For con-tro ll ed potenti a l e lectro lys is with DME a micro H-type cell was used in which I ml of the e lectro lysed solu tion was placed .

A Unicam SP-800-A (Pye-Unicam, Cambridge, U.K.) with temperature contro l of quartz cells was used fo r record ing of UV-vis spectra.

A vari ous Aerograph seri es 1200 Gas C hromato-graph with a Ilame ionizatio n detector was used for the analys is of puri ty of commercial a ldehyde samples and product identification .

Chelll icals and solutions Isophthalaldehyde, benzaldehyde and tolua ldehyde

were suppli ed by Aldrich Company (Mil wankee, WI ). Purity was checked by m.p., b.p . and g lc o f an ether solution of the a ldehydes. Che micals fo r preparation o f buffers were o f ana lytica l quality.

Stock solution of aldehydes (0.0 1M) were prepared in acetonitrile or in absolute ethano l and stored in

dark at 4°C. For initi a l studies Brilto n-Robinson buff-e rs and for definite pH va lues simple phosphate. acetate, B1C INE and borate bu ffers were used .

Procedure For e lectrochemical studies 9.8 ml o f the buffer

containing 10% aceto nitrile were deaerated by a stream of nitrogen. After additio n of 0 .2 ml of the 0 .0 I M stock solutio n o f the a ldehyde in aceto ni tri le and brief introductio n of nitrogen, the current- vo ltage

curves were recorded at 25 °C. Hence the fi na l con-centrations were 2 x 10-4M aldehyde . A lte rnative ly. in initia l ex periments buffers conta ining 30% acetoni -tril e or 2% or 52% ethano l were used.

Fo r spec trophoto metri c measurements 0 . 1 ml of the 0.0) M stock solution of the a ldehyde was added to 9 .9 ml o f the buffe r soluti on. Spectra were than recorded in the presence of air oxygen in solutio ns conta in ing I x lOAM aldehyde and I % e thano l.

Results and Discussion

Polarof5raphic reduction -'niti a l experiments were carri ed o ut in buffered

so luti ons containing 2% eth ano l and LX 10-4M isoph-thai a ldehyde (I ). Under such cond iti o ns po larographic i-E curves were strong ly affec ted by adsorp ti on. The effect of the adsorp tio n was s till marked when in such medi a the concentrati on of ] was dec reased to 5 x 1O-5M. Increase in content of organic co-solvent to 52% (v/v) ethano l or 30% (v/v) acetonitrile e liminated complications due to adsorpti on, but even after cor-rection fo r the change in viscos ity the current-vol tage curves mani fes ted co mplicating sol ute-solvent inter-act io ns. T hese were attributed to changes in solvation o f the e lectroactive species and to the effects o n rates accompanying the e lectron transfer. Hence the most straigh t forward informatio n about the cour e of the reducti on was obtained in bu ffe red solu tio ns contain-ing 10% (v/v ) acetoni tril e .l n such so lven t mixtures water was sti ll the predominating solvating agent. All results described be low were obta ined in such so lvent mixture. As boric aci d in te racts with the electroacti ve species. data reported for p H 8 to 10 were than o b-tained in a BtC rNE buffer.

746 INDI AN J CHEM. SEC A. APRIL 2003

• • 2: j

1.8 ---~ .. ~ ... }- .'~"""'~ '" 1.6 "

1.4 ct 2- 1.2

1 -

0.8 ,

0.6

0.4

02 j o I

0 2 4

•••

6 8 pli

10 12 14

Fig. I - The dcpendcnce o f lim iling currenlS of 2 x J(r.1 M so lu -lions or isophlhalakk hydc on p H in lhe presence of 10% accloni -lrik. i l ,\ ( .& ), ie,' ( . ). i2ll (+). i, (e ).

T he reduction of the dialdchyde I takes place in fi ve waves, denoted here as i I A. i lB. icA , il B, and i.l .

In ac idic media the reduction resembles closely that o f ary l mOllocarbony l compounds') at twice as high concentration than that o f the dialdehyde I. At p H4 thi s wave i 1,\ is accompan ied by another two-electron wave i l ll the hal f-wave potenti al of which is ()H-independent. At pH I O the heights of both of these waves decrease with in-crease in pH . Both plots of i2A and i2B as a function of p H have a shape of a decreasing dissoc iation curve w ith an inflection point (p K ') at a p H about 10.6 for wave i2A and about 11 .5 fo r wave i2IJ (Fig. I ). The slopes of EIi2-pH plots for wave i2A were 0.06SV/pH at p H 6 to 7 and 0.045V/pH at pH 7 to 10. At pH> I 0 the half-wave potential of wave i2A becomes pH-independen t. For the wave i2IJ the slopes dEI I2/dpH were O.OSOV/pH between pH 6 and 10 and the haif-wave potential of wave i211 becomes pH independent ;) t p H> IO.

1.ti

1.4

~ 1.2

0.8

0.6 +-----r-----------o 5 10 15

p H

Fig. 2- The dc:pcndcnce or hal f-wave po;enlia ls or 2 x to·.1 M so tuli ons of isophlhala ldchyde on pH in lhe presence of IO')f acelonilrite. i l/l (+), i !1l ( .& ), i211 C. ), i1n (e), il ( 0 ) ,

A t pl-l> I 0.6 the hal f-wave potential of wave i2B be-comes pH-independent (Fig. 2) , as expected for a re-duction preceded by a protonation.

Finally, at pH between II and 12 the wave i3 at more negati ve potent ials was observ d. Its height al so decreased with increase in p H (Fig. j).

The dependences of limiting "urrents and ha lr-wave potentials on pH can be interpreted by Scheme I .

For waves i lA and i lG the experimental ev idence in-dicates th" presence of processes ( I )-(4):

A r(C HOh + 2H+ .... surface .. A r(C HOWh ( I )

A r(CHOH+h + 2e- .. Ar(C OHh (2)

A r(CHOHh + 2e- .. A r(CHOH)2 (3)

A r(CHOH)" + 2H+ .... .. A r(CH20H )~ (4)

Scheme I

Ratio of limiting currents i2A i2B is close to 1.0. This indicates that di rneri zation of the dirad ica l formed in reaction (2) does not play an important ro ie under conditions used. The li fe-t ime of the diradica l for I is, nevertheless, much shorter than those ob-served for benzene 1,2- and I A-dicarboxaldehydes-l·5, as rnani fested by the absence o f anodic processes on curves obtained by cyc l ic vo l tammetry.

In the med ium p H-range, between pH 5.7 and 10 the reducti on of the dialdehyde I does not correspond

110VER e! (/1.: ELECTROREDUCTIO OF ISOPHTHALALDEHY DE 7..'t7

to a simultaneous reduction of both aldehydic groups. For simple benza ldehydes or acetophenones') in thi s pH range a single two-electron wave is observed, the hal f-wave potentia! of which is in most cases p H-independent. It is attributed, based on behav ior of higher p H value" to a sequence e', H+, e', H+. In first two steps of thi s sequence a radi cal anion formed is rapidl y protonated. The resulting radi cal is rapidl y further reduced to alcohol. Hence a single two-electron wave is observed as long as the rate of proto-nati on is sufficient to convert all the radica l ani on into an eas ily reducib le rad ical.

Compari son of the half-wave potenti al of the sec-ond wave of I (i:!n) with hal f-wave potenti als of 3-to lualdehyde ind icated that in thi s range in waves i lr\ and i113 a consecuti ve reduction of the two aldehydic groups occurs. The presence of the second formy l group, acting as a substituent, not only fac ilitates the reducti on and results in a shift of wave i IA to a more pos iti ve potenti al. but also fac ilitates the surface pro-tonati on of the formy l groups. Thi s is refl ected by pH -dependence or the half-wave potential of wave i1A. Hence the process in wave i1A can be depicted as given in Eqs (5) and (6).

, urfacc Ar(C HOh + W ... ~ Ar(CHOH+)C HO ... (5)

Ar(CHOW)C HO + 2e' + W ---.~ Ar(C H10H)C HO .. . (6)

In wave i11l, thus, the substitu ted benza ldehyde Ar(C H10H)C HO is reduced. At pH> I 0 even the sur-race protonat ion of Ar(CHOh in (5) is not rapid enough to convert all the di aldehyde into a monopro-tonated species according to (5). Thi s results at pH above 10 in a decrease of ilA and il B w ith increase in p H (Fig. I ). These waves are gradually replaced by wave iJ at more negati ve potenti als (Fig. 2) . The negati ve va lue of the hal f-wave potential of wave i} as well as its dependence of pH (Fig. 2) indicate a re-ducti on of all unprotonated spec ies (7):

Ar(CHO)1 + e' ---.~ Ar(CHO)(C HO ') (7)

The fate of the radi ca l anion and di stincti on be-tween a mono and dirad ica l is prevented by a bulk chemical reacti on. At p H> I I addition of hydroxide ions to the aldehyde I takes place in reacti ons (8a and b):

Ar(CHOh + OH' ... ~ A r(C HO)C H(OH )O' .. . (8a)

A r(CHO)C H(OH )O'+OW ... ~ A rrC H (OH )O' l c ... (8b)

The resulting geminal diol anion is electroinacti ve and its formation results in a decrease of wave i} with increasing ac ti vity of hydrox ide ions.

Limiting currents i lA and i lB , simil arl y as currents ilA and ilB in the p H region where they are pH-independent, are diffusion contro lled a. shown by the linear dependence on the square root of mercury pres-sure. Whereas the reducti ons of benzene 1,2- and 1.'1.-dicarboxa ldehydes in the presence or 52% eth anol indicate nucleophil ic additi on of the alcohol to ronn y I groups, no such effect o f ethanol has been observed for the studied I ,3-dialdehyde I.

Spectroscopic data indicate that nucleophili c addi-tion of hydrox ide ions occur in consecuti ve rather than simultaneous processes (8a, 8b). A ttack of one hydrox ide ion on the dicarboxa ldehyde I at a time is in agreement with the pattern of electroox idati on (. ce below), where the ox idati on of two formy l groups occurs in consecuti ve processes, both in vo lving addi-ti on of OH' groups prior to electron transi'ers.

Cyclic voltaill/llet rv (CV) The absence of anodic peaks on cyc l ic vol tam-

mograms at pH between 2 and 12 at scan rates up to 500 mys,l indicated irreversibility o f processes in waves ipl and ipl ' Otherwise CY did not offer addi-ti onal informati on about the processes in vo lved than DC polarography. The pH-dependence of peak cur-rents of ipl and ip1 as well as those of peak potentials resembled th at obtained with a higher accuracy in DC polarography. In particul ar, peaks ipl A and iplB practi -ca lly overlap, similarl y as peaks i2A and i21l at pH>8.

COllt rolled potelltia l electrolysis(CPE) To obtain informati on comparab le to that obtained

by DC polarography, the CPE was carri ed OLlt in 1 ml or a I x 10'-IM solution of isophthalaldehycle (1) con -taining 10% v/v eth anol to increase the so lubility.The electrolys is was carri ed OUl in a semimicrocelL where the so lution was stirred by the fa lling off mercury drops, using the dropping mercury elec trode. Typica l ex periments were carri ed out in the absence of oxy -gen for a peri od of about 7 to 8 hours, which resulted in a 70 to 80 % conversion.

The electro lys is at pH 2 at the l imiting current of wave i IA y ielded a straight plot of logi IA = f(t) with a

748 INDIAN J CHEM. SEC A, APRIL 2003

slope corresponding to a two-electron process. Wave i IB is at thi s p H overl apped by the current of hydrogen evo lution. T he time dependence of wave i lA resem-bled closely the decrease in absorbance at 230 nm in the UV spectrum. The product was identified as a pinacol using gas ch romatography, a dimer formed by interaction of two electrogenerated rad ica ls, involving some hydrogen abstraction from the solvent.

Electrol ys is in a phosphate buffer of pH . 6.8 con-tai ning 10% vlv ethanol at a potential corresponding to the limiting current of wave i2B resulted in a linear plot of log(i 2A+i2B ) = f(/) with a slope corresponding to a transfer of four elec tron s. No new wave appeared during the elec trolysis. In the course of electrolys is the absorbance at 230 nm decreased w ith time simi-larl y as the decrease in (i 2A+i:m). The decrease was accompanied by an increase in a low- intensity band at 250 nm. The electrolysis product was identified as dialcohol. Ar(C H20H):: using gas chromatography.

The electrolys is at pH 10 at the limiting current of wave i2A was carried out in a borate buffer which could have resulted in some of the observed compli-cations. A cle~u'e r picture has been observed at 14 h. In the course of electrolysis the II ---> r I * band at 230 nm decreased ar.d the ti me de-pendence was analogous to the decrease of i2A . The weaker benzenoid band at 250 nm also somewhat de-creased, This behavior is consistent with the above in terpretation that in i2A protonated form of one of the aldehyd ic groups is reduced.

At pH> II investigation of the electrolysis was compl iC;Jted by homogeneous reaction, i nvol vi ng hy-drox ide ions, observed also in the absence of appli ed potential.

AI/odic oxidatiol/ Similarl y as for the other substituted benzalde-

hydes I() isophthalaldehyde (I ) yields in al kal ine solu-tions containing 2% vlv ethanol anodic polarographic waves corresponding to the ox idation of gemi nal diol anions (grouping -CH(OH)U). In 1.0 M and more concentrated solutions of sodium hydroxide, the oxi-dation took place in two anodic wa\es (i A and ill) o f equal heigh l. separated by about 0.1 V . At lower pH-v,l iues than about 12 these waves were gradually su-j1erimposed by the current of anodic dissolution of

2

200 600 1000

t (mi n)

Fig, 3·- The dependence of limiting current s or reduction waves i I. i1. and i, of 2x lOAM isophthalaldehyde in borate butTer pH 10 on time.

Tab le I - Comparison of ~.nod i c limiting cclrrents of substituted benza ldehydes wi th thc tOlal limiting anodic current (i/\+ i ll) or isophlhalaldehyde. So lutions contai n 2% ethanol. 2x 10 -i M alde-hyde in 1.0M NaOH, (11=2.95s at h=IOOcm)

Substituent of benzaldehyde

/II ·CN

p-CN

p-CI

H

III-CH,

p-CH,

3.4,5-(OCH1h 1,3-(CHOh

1.25

1.29

1.29

1.24

1.25

1.29

1.25

2,50

mercury. Compari son of the total anod ic current ( iA + iB) with anod ic limiting currents of same monosub-stituted benzaldehydes (Table I ) indicated that the current (i A + ill) for the dialdehyde was twice that observed for the two-electron ox id' tion of monocar-boxa lclehyde. The half-wave potentials of both anodic waves (i A' is) of the dialdehyde I '",'ere , hiftecl with increase in pH to more negati ve val ues by about O.I3Vlp H.

As the DC polarographic curves Wf're difficult to measure, the differential pul se polarography was ap-pli ed. Peak currents irA and ipB corrected for changes in viscosi ty were independent or basicity from pH 13 to 14,5. In olut ions in O.5M to 0.005M NaOH at con-stant ion ic strength (~l=O . 5) th e anodic peak currents

BOVER el at.: ELECTROREDUCTIO OF ISOPHTI-IALA LDEHYDE 749

3 b (}f)- {} f)

(lJO ~ 2 0 o .

....... :::i! IA

750 INDIAN .I CHEM. SEC A. APRI L 2003

current-voltage cu rves. The l imiting current of the product increased w ith Innease in concentrat ion of hyd roxy iamine. A plot of the limiting current as a funclion of concentration of hydroxy lam ine shows two linear segments (Fig . 5). Their intersection, above wh ich the curren t does not increase wi th increase in concentrati on of hydroxylamine, occurs at a concen-Irat ion of hydroxylamine that is twice the initial con-cen trat ion of the dialdehyde I. This indicates forma-tion of a dioxime of the isophthalaldehyde, which, in thi s pH-range. is reduced in a single eight-electron slep.

The slope of the plot o f i = f( H:!OH) indicates that the eq uilibrium ( 13) between the dicarboxaldehyde and hydroxy lami ne is practi ca ll y comp leted in favor of the dioxime,which prevents determination of the eq uilibrium constan t of reaction ( 13):

A r(C HOh + 2 HcOH ....... I-----I~.Ar(CH=NOH )c +2HcO ( 13)

A single eight-electron reduct ion of the diox ime of isophthalaldehyde indicates simullaneous reducti on of both oxime groupings. Thus the orientation of the di-ox i me at the electrode surface must allow a compara-ble prohabililY of elec tron and proton transfers to both oX ime groups.

Conclusions Electroanaly ti ca l studies, in particular of variations

of li mi l ing currents and hal f-wave potentials with p H, oller use ful in format ion about the ori entation of or-ganic molecules w ith two identical electroacti ve

groups at the electrode surface duri ng the reduction or ox idation. In investi gated reducti ons of isophthalalde-hyde the rate of surface protonation enables either a si multaneous reduction of both elec troact i ve groups (in diprotonated species) or a consecu tive one (in a monoprotonoted isophthalaldehyde). In th e digem inal diol anion ic form, resulting in addi tion of hydrox ide ion in alkaline solution, the oxidation occurs as a con-secutive process. Thi s study also indicates the impor-tance o f choice of so l vent and of kind of buffer to obtain optimum (and simplest) conditi ons for a mechanistic study.

Acknowledgment M S 8aymak expresses gratitude for financial sup-

port to the Marmara University , Istanbul , Turkey.

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8 Halleck L & Marsen H. Zelektrochem , 57 ( 1953) 944.

9 Zuman P. Barnes D & Ry volova- Kej harova. A . Disc Fara -day Soc. 45 ( 1968) 202.

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