Electrochemical Nitration of Naphthalene in Micellar Systems

Foi es tu da da a ni tra ção do Naf ta le no (NapH) em um ele tro do de Pt em so lu ções aquo sas de NaNO2, na au sên cia e na pre sen ça de Brij 35, um sur fac tan te não-iônico. O com por ta men to ele tro quí mi co dos re a gen tes NapH e NaNO2 e de suas mis tu ras foi in ves ti ga do por vol ta me tria cí cli ca para de ter mi nar as con di ções de ele tró li se. A cor ren te de pico do NapH de cres ce mo no to na men te com a con cen tra ção de NaNO2, in di can do que o pro du to da oxi da ção do íon NO2 in te ra ge com o NapH. Fo ram re a li za das ele tró li ses a po ten ci al con tro la do e os pro du tos fo ram ana li sa dos por HPCL. Na so lu ção mi ce lar fo ram ob ti dos 1-nitronaftaleno, 2-nitronaftaleno, 1,2-naftoquinona, 1,4-naftoquinona e 1,1’-Binaftilo (Bi napH) en quan to que, na fal ta de Brij 35, so men te fo ram de tec ta dos naf to qui no nas e Bi napH. A ma i or se le ti vi da de para 2-nitronaftaleno em com pa ra ção com me i os ho mo gê ne os não-aquosos pode ter sido in du zi da pelo mi cro am bi en te nas mi ce las.


Intro duc ti on
Elec tro che mistry on mi cel les has mostly de ve lo ped in the last twenty ye ars 1 .Mo no me ric mo le cu les of amp hip hilic com pounds (ten so ac ti ves) pos sess re gi ons of hydrop hilic and hydrop ho bic cha rac ter.In aque ous so lu ti ons, ag gre ga tes of the se mo no me ric mo le cu les (or ga ni zed struc tu res or mi cel les) can be re a ched when the mo no me ric con cen tra ti on ex ce eds a cri ti cal va lue or cri ti cal mi cel le con cen tra ti on (cmc) 2 .
An im por tant fe a tu re of the se so-called mi cro he te ro gene ous systems is that by ca re ful elec ti on of the amp hip hi lic mo le cu le and ma king use of the mode of dis tri bu ti on and lo ca ti on of so lu tes in the ag gre ga tes, it is pos si ble to or ga nize re ac tants at a mi cros co pic (mo le cu lar) le vel 3 .Pro bably, the most im por tant pro perty of mi cel lar so lu ti ons is the ir ca pa city to so lu bi li ze wa ter-insoluble com pounds, a process des cri bed by the dyna mic equi li bri um mo nomer-micelle in the ac ti on mass mo del 4 .The so lu bi li za ti on phe no me non is very im por tant in che mistry, in dustry and bi o logy 5 .
The na tu re of the mi cel le-substrate in te rac ti on de ter mines the subs tra te elec tro che mi cal be ha vi or 6 .Mo re o ver, micel lar ef fects have been stu di ed from va ri ous pers pec ti ves, such as cmc and dif fu si on co ef fi ci ents de ter mi na ti ons 5,7 , Ar ti cle elec tro de ki ne tic and ther mody na mic pa ra me ters 8 , so lu bili za ti on 9 , ca taly sis 1,10 , elec trosynthe sis 11 , and elec tro de adsorp ti on 12 .
In a pre vi ous ar ti cle we re por ted a me cha nis tic study of the ni tra ti on of naph tha le ne (NapH) by its elec tro che mi cal oxi da ti on in the pre sen ce of ni tri te ion in ace to ni tri le so lu tion 13 .In the pre sent com mu ni ca ti on, si mi lar ex pe ri ments are re por ted in aque ous so lu ti on whe re the non-ionic surfac tant, Brij 35, was used to so lu bi li ze naph tha le ne in water.The elec trosynthe sis of ni tro de ri va ti ves was per for med by con trol led po ten ti al elec troly sis (CPE) on Pt elec tro des.The pro duct dis tri bu ti on was analy zed by high per for man ce li quid chro ma to graphy (HPLC).
The stan dard so lu ti ons of NapH, ni tro de ri va ti ves and ot her pro ducts were pre pa red by we ig hing.The exact concen tra ti on of NapH was de ter mi ned by UV spec tros copy (λ max =276 nm, log ε =3.75) in a Hew lett-Packard HP 8452 spec trop ho to me ter.
For cyclic vol tam metry (CV) and CPE me a su re ments, a po ten ti os tat-galvanostat EG&G PAR mo del 273 was emplo yed.The cur rent and po ten ti als were re gis te red eit her on an X-Y EG&G PAR mo del RE 0150, x-t REC 61 Ser vograph Ra di o me ter re cor der or with a Ke ith ley 194A high spe ed volt me ter con nec ted to a PC com pu ter.Con ven ti o nal three-compartment Pyrex cells were used for CV and CPE ex pe ri ments.The cell for CPE was ef fi ci ently agi ta ted by ver ti cal hydrody na mic mo ve ments of the elec tro de sup port.The wor king elec tro des were Pt disks of va ri a ble are as for CV, and Pt elec tro des of lar ger are as, ca. 4 cm 2 , for CPE ex pe ri ments.The coun ter elec tro de was a sta in less ste el foil with a lar ge area.All the po ten ti als were re fer red to a sa tu ra ted ca lo mel elec tro de (SCE) and were cor rec ted for iR drop by po si ti ve fe ed back tech ni ques.The ex pe riments were per for med at 20 °C.

Con trol ex pe ri ments in the ab sen ce of Brij 35
The so lu bi lity of NapH in aque ous so lu ti ons is low (i.e., 1.58 x 10 -4 M at 20 °C), as de ter mi ned by UV spec tros copy.In the pre sen ce of NO2 -ion, no chan ges in the aque ous solu bi lity were de tec ted.Cyclic vol tam mo grams sho wed only the oxi da ti on peak of ni tri te, but the NapH oxi da ti on oc cur red at 1.4 V be ca u se a sa tu ra ted so lu ti on of NapH without ni tri te de cre a sed the UV 276 nm band af ter EPC.When a sa tu ra ted so lu ti on of NapH + 1 x 10 -2 M NaNO2 was elec troly zed at 1.4 V, no ni tra ti on pro ducts were de tected.At this po ten ti al va lue, NO2 -ion oxi da ti on is very impor tant, but the re ac ti on bet we en NapH •+ and NO2 is very slow.The fas ter re ac ti on oc curs bet we en NapH •+ and H2O, to give only qui no nic pro ducts (1,2-NaphQ, 1,4-NaphQ), as were de tec ted by HPLC.

So lu bi lity of Naph tha le ne in mi cel lar so lu ti ons
In or der to ob ta in evi den ce of NapH dis tri bu ti on bet ween wa ter and Brij 35 mi cel les, the par ti ti on pa ra me ter (K p ) was eva lu a ted from the so lu bi lity me a su re ments, fol lowing a pre vi ously des cri bed pro ce du re 15,16 .
K p is de fi ned as: whe re [NapH] WS and [NapH] MS re pre sent the so lu bi li ti es of NapH in wa ter /0.4 M NaClO 4 and in the mi cel lar so lu tion, wa ter /0.4 M NaClO 4 /Brij 35 (2.5 x 10 -2 M), res pec tively.
[NapH] M re pre sents the NapH bound to the mi cel le.This met hod gi ves the va lue of K p at NapH sa tu ra ti on concen tra ti on and does not per mit stu di es at dif fe rent NapH con cen tra ti ons be low this so lu bi lity va lue.The NapH water so lu bi lity is [NapH] WS = 1.58 x 10 -4 M and in the mi cellar me di um [NapH] MS = 6.17 x 10 -3 M gi ving a K p = 0.026.This me ans that NapH is al most com ple tely so lu bi li zed in the mi cel le subp ha se at sa tu ra ti on.Thus, it is ex pec ted that un der the elec troly sis con di ti ons, the NapH so lu bi li za ti on is even more fa vo ra ble be ca u se the to tal subs tra te con centra ti on is 2 x 10 -3 M.

Cyclic Vol tam metry
NaNO 2 The elec tro che mi cal oxi da ti on of the ni tri te ion in aqueous so lu ti on 17 is well es ta blis hed.Fi gu re 1 shows the cyclic vol tam mo grams ob ta i ned in aque ous so lu ti on (a) and with Brij 35 (b); in both ca ses only one peak at E p,a = 0.800 V is de tec ted in the ano dic scan.No cat ho dic cur rent is ob ser ved when the scan is re ver sed.This fact in di ca tes a che mi cal com pli ca ti on cou pled to a first char ge trans fer.In aque ous and mi cel lar me dia the CV pat terns are si mi lar, in di ca ting no in te rac ti on of NO 2 -ion with Brij 35 mi cel les, as ex pected for an io nic subs tra te.This im pli es that NO 2 -ion is not par ti ti o ned into the mi cel les.In both ca ses, the peak cur rent va ri es li ne arly on v 1/2 and on the NO 2 -con cen tra ti on, but in the mi cel lar me di um the cur rent is only about 80% of the va lue in the aque ous system.This fact can be ex pla i ned by an in cre a se of the vis co sity of the mi cel lar system 18 .
The elec tro che mi cal re ac ti on in aque ous so lu ti on has been in ter pre ted in terms of a com plex me cha nism 17 (Eqs.2-4): The prin ci pal pro duct for med in the first char ge trans fer re ac ti on is NO 2 .Sub se quently, NO 2 suf fers dis pro por ti ona ti on to give NO - 3 and NO - 2 ions (Eq.3), in an ir re ver si ble way.The rate cons tant va lue ob ta i ned by elec tro che mi cal me a su re ment 17 is k = 4 x 10 6 l mol -1 s -1 .Hen ce, the NO 2 con cen tra ti on is only ap pre ci a ble in a re ac ti on la yer near the elec tro de sur fa ce.This la yer can be es ti ma ted to be less than the thick ness of the dif fu si on la yer of NO - 2 ion 19 .Howe ver, the NO 2 con cen tra ti on pro fi le is enough to re act with the NapH ra di cal ca ti on, NapH •+ (see be low).The refo re, un der vol tam me tric con di ti ons the ove rall elec tro de pro cess is gi ven by Eq. 4. In CPE ex pe ri ments in a un buf fered me di um the re ac ti on of the NO - 2 ion with H 3 O + gi ves: The pK a va lue 20 of this re ac ti on is 3.25, but this fact is not im por tant for CPE con di ti ons sin ce the pro ton trans fer ki ne tic in wa ter is fast.In or der to con firm this hypot he sis, the CPE of NO - 2 so lu ti on with Brij 35 was per for med at 1.45 V.The cur rent de ca yed ex po nen ti ally with time at a va lue near the blank cur rent, and the pH of the so lu ti on decre a sed from 6.31 to around 1.1.The ap pa rent elec tron num ber n app  1.8 Fa ra day per mole of NO - 2 ion is in agre ement with Eq. 4. The se facts show that the HNO 2 con centra ti on is ne gli gi ble at the end of the elec troly sis, sup por ting our hypot he sis.

Naph tha le ne
A typi cal cyclic vol tam mo gram of NapH in Brij 35 solu ti on is il lus tra ted in Fig. 2.An oxi da ti on wave (E = 1.40 V) due to the NapH oxi da ti on to give NapH •+ is ob ser ved on the first swe ep.This wave is not de fi ned as a peak pro bably due to the O 2 evo lu ti on.It is highly ir re ver sible and no com ple men tary cat ho dic peak is ob ser ved even at 50 V s -1 .This is an in di ca ti on of a very short life time for NapH •+ and/or a fast fol low-up re ac ti on cou pled to the char ge trans fer, as ob ser ved for the NapH •+ sta bi lity in anot her or ga nic me di um 13 .Assu ming that NapH is only in the mi cel lar subp ha se, its mi cro en vi ron ment would be more like an or ga nic sol vent and a si mi lar be ha vi or could be ex pec ted.
The cur rent li mit i LI shows a li ne ar de pen den ce on v 1/2 in the swe ep ra tes ran ge from 0.01 to 0.3 V s -1 .This ob serva ti on re in for ces the idea of a fast re ac ti on cou pled to the char ge trans fer.Typi cal re ac ti ons of the aro ma tic ra di cal ca ti ons are the di me ri za ti on and even furt her po li me ri za tion re ac ti ons 21 .The more pro ba ble di me ri za ti on pro duct of this ra di cal ca ti on is 1,1'-Binaphthyl (Bi napH) 13 .Anot her pos si bi lity is that NapH •+ re acts with a nu cle op hi le, wa ter for exam ple, to give oxy ge na te subs ti tu ti on pro ducts such as qui no nes 22 .
In the ne ga ti ve swe ep, two pe aks are ob ser ved.The peak at 0.15 V is as sig ned to the PtO re duc ti on in this medi um, as con fir med by blank ex pe ri ments (wit hout NapH).The ot her peak at -0.40 V is as sig ned to the solva ted pro tons re le a sed by the di me ri za ti on re ac ti on of NapH •+ , as con fir med by the ad di ti on of per chlo ric acid to the so lu ti on.No new peak was de tec ted by cycling bet we en -1.10 and 1.75 V.The re fo re, the elec tro o xi da ti on of NapH in mi cel lar me di um can be in ter pre ted by the fol lo wing global pro cess:

NaNO 2 + NapH so lu ti on
A typi cal cyclic vol tam mo gram ob ta i ned when both subs tra tes are in so lu ti on is il lus tra ted in Fig. 3. Two well-defined pe aks are ob ser ved which cor res pond to the NO - 2 ion oxi da ti on (peak I) and NapH oxi da ti on (peak II).The peak po ten ti als are the same as tho se of the se pa ra te subs tra tes (Fig. 1b and Fig. 2).The peak cur rent i pI does not chan ge with res pect to its va lue in Fi gu re 1b, ho we ver, the peak cur rent for the NapH elec tro o xi da ti on (peak II) changes when NO 2 -ion is pre sent.Thus, to de ter mi ne this depen den ce, syste ma tic stu di es were car ri ed out ke e ping NapH cons tant and chan ging the NO 2 -ion con cen tra ti on.In all ca ses, i pI chan ges li ne arly with the con cen tra ti on, but i pII de cre a ses when NO 2 -in cre a ses (Fig. 4).This sug gests that eit her the NO 2 -ion or its oxi da ti on pro duct in te racts with NapH •+ , chan ging its rou te of de gra da ti on, in agre e ment with si mi lar re sults ob ta i ned in a non-aqueous system 13 .

Con trol led-Potential Elec troly sis (CPE)
In view of the CV re sults re por ted abo ve, CPE was perfor med for two dif fe rent mi cel lar so lu ti ons: (a) only NapH pre sent, and (b) both subs tra tes NapH + NaNO 2 .

a) CPE of NapH
Fol lo wing the con di ti ons in Fig. 2, CPE of a 2 x 10 -3 M NapH so lu ti on with 2.5 x 10 -2 M Brij 35 and 0.4 M NaClO 4 as the sup por ting elec troly te was per for med at 1.45 V.The ini ti al co lor less so lu ti on tur ned to pale yel low at the end of the elec troly sis.The ex pe ri ment was con clu ded when NapH was no lon ger de tec ted by the HPLC tech ni que.Under the se con di ti ons, the de tec ted pro ducts are only 1,2and 1,4-NaphQ.Ho we ver, the mass ba lan ce with res pect to NapH sug gests anot her pro duct, pro bably the di mer Bi -napH or an in so lu ble oli go mer.In ad di ti on, the pH of the so lu ti on de cre a sed to pH  2, as ex pec ted when H + is re le ased in che mi cal re ac ti ons (Eqs.7 and 8).The elec troly sis = 5 x 10 -3 M. The ot her con di ti ons are the same as in Fig. 1 cur rent de ca yed ex po nen ti ally with time, and the n app ≅ 4.6 per mo le cu le of NapH was cal cu la ted at the end of the electroly sis.The n app va lue ob ser ved is con sis tent with the electro o xi da ti on of NapH un der the se con di ti ons to give qui no nes and bi naphthyl or an oli go me ric pro duct.
Typi cal re sult fors this CPE are shown in Ta ble 1.

b) CPE of NapH + NaNO 2
The pos si bi lity of the NapH ni tra ti on by the elec tro oxida ti on pro duct of the NO - 2 ion was ex plo red by CPE per formed at 1.2 V.At this po ten ti al, only the NO - 2 ion is elec troly zed.Under the se con di ti ons, no ni tra ti on nor quino nic pro ducts are de tec ted by HPLC.This pro vi des the evi den ce that NapH is not at tac ked by the in ter me di a te product of the NO - 2 ion elec tro oxi da ti on.The ab sen ce of the qui no nic and ni tra ted pro ducts would in di ca te that NapH •+ me di a tes in the ir for ma ti on.Thus, CPE was per for med at 1.45 V, and the pro ducts de tec ted by HPLC were 1-NO 2NapH, 2-NO 2NapH, 1,2-NaphQ, and 1,4-NaphQ.Also, the for ma ti on of Bi napH was as su med, and its yi eld was cal cu la ted from the mass ba lan ce.Typi cal re sults are shown in Ta ble 1.It should be no ted that the ra tio 1-NO 2 Naph/2-NO 2 NapH is lo wer than that ob ser ved in previ ous stu di es in ace to ni tri le me di um 13 , in di ca ting a mi cellar ef fect that may be due to pre fer red ori en ta ti ons of the subs tra te in the or ga ni zed me dia.

Con clu si ons
In aque ous systems in the pre ce of the NO - 2 ion , no ni tra ti on is ob ser ved.NapH •+ re acts fast with wa ter to form qui no nes.On the ot her hand, the elec tro ge ne ra ted NO 2 (Eq.2) de com po ses, as shown in Eq. 3.Both re ac ti ons are fas ter than tho se of ni tra ti on.
In mi cel lar systems, ni tra ted pro ducts are ob ta i ned.Pro bably, the mi cel lar mi cros truc tu re in cre a ses the NO 2 so lu bi li za ti on in a mi cel le subp ha se, fa vo ring its en coun ter with NapH •+ .The ra tio of 1-NO 2Naph/2-NO 2NapH is lower than in non-aqueous me dia, whe re the ra tio is al ways gre a ter than 50 13 .The hig her yi eld of 2-NO 2NapH may be ex pla i ned by in tra mo le cu lar se lec ti vity in du ced by the micel les mi cro en vi ron ment.

Fi gu re 1 .
Cyclic vol tam mo gram of the NaNO 2 oxi da ti on a) in aqueous so lu ti on; b) in mi cel lar so lu ti on, [Brij 35] = 0.025 M, [NaNO2] = Fi gu re 2. Cyclic vol tam mo gram for NapH oxi da ti on in mi cel lar so luti on.[NapH] = 5.5 x 10 -3 M, [Brij 35] = 0.025 M. The ot her con di ti ons

Fi gu re 4 . 1 Fi gu re 3 .
De pen den ce of i pII as a func ti on of the NO 2 -ion con cen tra tion.[NapH] = 5.5 x 10 -3 M, [Brij 35] = 0.025 M. The ot her con di ti ons are the same as in Fig. Cyclic vol tam mo gram for NapH and NaNO 2 oxi da ti on in mi cel lar so lu ti on.[Brij 35] = 0.025 M, [NapH] = 5 x 10 -3 M [NaNO 2] Ta ble 1. Yi eld of pro ducts for CPE of NapH in dif fe rent con di ti ons.