Organic Polarography Donald J. Pietrzyk, Department o f Chemistry, University o f Iowa, Iowa City, Iowa 52240
T
articles described in Chemical Abstracts, Analytical Abstracts, Electroanalytical Abstracts, Chemical Titles, and readily available journals ending November 30, 1969. I n the past reviews every attempt was made t o list all of the organic polarographic references. Such a n approach was not followed in this review. Generally, references dealing with detailed studies of the oxidation or reduction mechanism, studies of factors affecting half-wave potentials, correlation studies, and newer or unusual analytical procedures were selected. I n addition, only references from journals which were available to the author were used. HIS REVIEW SURVEYS
GENERAL REVIEWS
Several general review articles have appeared. Newer trends in polarography (63) and specific applications of organic polarography to organic systems (143, 547) were described. Other general polarographic reviews which include organic polarography were compiled (1, 10,449). General reviews on adsorption waves (201) and advances in alternating current polarography and tensammetry (72)are described. T h e Koryta adsorption equation was discussed and improved, which allows the estimation of maximum surface concentration of adsorbant (63). Electrochemical methods for the study of adsorption of organic molecules on platinum was reviewed (60).
Several reviews and general papers dealing with special electrodes have appeared. Voltammetry a t silicone rubber-based graphite electrode (418) and glassy carbon electrode (480) were investigated. Two interesting studies involving transparent electrodes were described. I n t h e first a n electrochemical study of eosin y and tris(4,7-dimethyl-1,lO-phenanthroline)iron(II) sulfate at thin film gold electrodes was carried out ( Q l 7 ) ,while in the latter tin oxide, gold, and platinum film optically transparent electrodes were evaluated by studying several oxidation and reduction systems in nonaqueous solvents (380). A general review of electrode oxidation reaction mechanisms in protophilic solvents and the effect of proton donors and acceptors (170) and in a variety of amide type solvents (420) appeared. Several reviews are concerned with biological and pharmaceutical applications. Polarographic behavior of nucleosides, nucleotides, purines, pyrimidines,
pyridines, and flavins (237) and purines and pyrimidines (142) was reported. Applications of polarography t o photobiochemistry (46) and biological problems (141) were described. The polarographic method was used in medicolegal practice for blood analysis on various articles b y observing the catalytic effect of blood on oxygen wave (482). Oscillopolarographic data for 160 different drugs ( l 3 4 ) , dc polarographic data for analysis of a variety of local anesthetics (79), and applications of modern electroanalytical techniques to pharmaceutical problems (2) were reported. Anodic organic reactions (103) and mechanisms (531), mechanisms for the polarographic behavior of aromatic hydrocarbons (437, 542), stereochemistry of electrode reactions (185), correlations of polarographic data to Hammett constants (473), and evaluation of polarographic maxima by ac polarography (544) were reviewed. Another article emphasized the methods and importance of isolating and identifying electrode reaction products before attempting to understand electrode mechanisms (549). A method was described for differentiating between a simple two-electron process and a fast first or second order ECE process (363). T h e effect of predischarge of metal ions on the polarographic behavior of organic compounds (318, 319) and the quantitative nature of hydrogen ion reduction in nonaqueous solvents (365)was studied. Methods for determining half-wave potentials of excited molecules (461),relating half-wave potentials of dyes to electron affinities and spectral properties (492), and insertion and polarographic investigation of positive carriers in single crystals (207) were described. Another interesting study has shown that anionic copolymerization of two monomers decreased with an increasing difference between their half-wave potentials (208). QUINONES
A cyclic voltammetric study of the hydroquinone-p-benzoquinonesystem in acetonitrile (138, 392) and of the reduction by de polarography of p-benzoquinone a t platinized P t and H g in dimethylformamide (109) was described. An anomalous behavior of hydroquinone a t p H 8.0 to 8.6 on P t was attributed to the presence of platinum oxide on the electrode (107). Polarographic data for 2,5-bis(trimethylsilyl)- and 2,j-tertbutyl-1,4-benzoquinone (64), for ubiquinone and 2,3-dimethoxy-j-methyI1,4-benzoquinone (375), 2,5-diethoxy-p-
benzoquinone and 2,5-bis-N-methylamino-p-benzoquinone (198), and a series of triptycene quinones (271) such as dihydrodioxotriptycene and dihydrodioxodibenzotriptycene (329) were reported. A half-wave potential-pH diagram for p-benzoquinone in diniet,hylformamide which also relates the electrochemical reactions a t the different pH values (28) was constructed. Voltammetric behavior of p-hydroquinone in the presence of sodium sulfite (yields hydroquinone sulfonates) was studied a t the Pt and stationary carbon paste electrode (438). Polarographic data for several quinones and their hydroxyl-substituted derivatives were used t,o determine the effect of intramolecular H-bonding on the ?r-electron energy levels and cornpared to calculated Huckel molecular orbital energies (135). Correlations between half-wave potentials for alkylated 0- and p-benzoquinones in acetonitrile were correlated to reduction potentials (4311. The effect of the addition of halide ion (487) and polycyclic aromatic hydrocarbons (488) on the polarographic maxima of methyl-p-benzoquinone in 2methoxyethanol was reported. Strong and weak acids were determined by observing the proton effect on the reduction of methyl-p-benzoquinone in 2methoxyethanol as solvent (486). Tocopherolinfoods, feedstuffs, and pharmaceut'ical preparations was determined polarographically after oxidation of the tocopherol to the corresponding quinone with Ce(1V) (532). Polarographic data for 2-hydroxy9,lO-anthraquinone and 2,9-dihydroxy10-anthrone in tautomerism st,udies ( 7 4 , the effect of the addition of protondonor solvents to nonprotogenic solvents on the waves for a variety of anthraquinone derivatives (162), and the effect of moisture on the reduction of anthraquinone derivatives in diniethylformamide (534) were reported. Adsorption of anthraquinone monosulfate on Hg in the presence of several different electrolytes was studied (11). Data for the reduction of 1,4-dimethylanthrasemiquinone and 1,4,5,8-tetramethylanthrasemiquinone in acetonitrile in ESR studies were described (444). Half-wave potentials for one-electron processes for a-@-substituted anthraquinones, phenazines, phenazine N,N'-dioxides, p-nitrodiphenyls, and p-nitrostilbenes in aprotic media were shown to be linearly correlated t o energies of the lowest vacant orbital (76). h detailed study of the effect of anions on the mechanism of the reduc-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
139R
tion of anthraquinone derivatives allowed several conclusions, such as shift in half-wave potentials, effect of dissociation of supporting electrolyte, effect of different anions, and relationship of reactivity to degree of dissociation of the supporting electrolyte to be made (533). Polarographic data for haloanthraquinones were correlated with infrared and ESR spectra (52) and for dichloroanthraquinone derivatives correlations were to first vacant molecular orbital energies (254). Haloanthroquinones substituted in the a- or P-position b y F, C1, Br, or I were studied in buffered methanol-aqueous mixtures (51), in methanol and dimethylforniamide solvents (5.9, and in acid media (54). Half-wave potential data for 17 (136) and 40 (137) derivatives of I, where reduction waves are due to the quinone
ALDEHYDES
Polarographic data for polycylic aromatic aldehydes (12), vanillin in ”&I, acidic, and basic media (306), aromatic aldehydes and ketones in acidic-aqueous solution (523), aldehydes of the type CH,(CH=CH),CHO, where n= 1,2,3 (326), and for a-hydroxyisobutyraldehyde at p H 10 to 12 (610) were reported. Half-wave potentials of substituted benzaldehydes were correlated to their spectral properties (156). The mechanism for the reduction of cinnamaldehyde was shown to involve a two-electron step to 3-phenylpropionaldehyde, which is further reduced; the path of the reduction depends on the p H (33). Details of the mechanism a t high p H (effect of cations) and a t low p H (dehydration) were also reported (54). The anion radicals produced in the 0 cathodic reduction of benzaldehyde and benzophenone in alkaline aqueous solution formed a t a rotating ring-disk electrode were studied in detail and observed by EPR (361). Effects of p H and the mechanism for the reduction of I propargylaldehyde which involves two and heteroring portion of the molecule. one-electron waves was reported (324). Polarographic data for 2,7-dimethyl-, The cyano group in 4-cyanobenzalde4,5-dimethyl-, 2,4,5,7,tetramethyl-, and hyde and 4-cyanoacetophenone is re3,4,5,6-tstramethyl-9,lO-phenanthrene- duced in acidic media in a four-electron quinone were measured in 75% dioxane step followed by a one-electron reducin order to measure steric effects of tion of the protonated carbonyl group methyl groups (249). (552)* ALCOHOLS, GLYCOLS, AND PHENOLS
KETONES
Oxidation data for 2-alkoxy-4,6-ditert-butylphenoxyl dimers (430),for 30 phenols substituted in 2,4, and 6 positions by various alkyl groups on graphite electrode (524), and for 2,4,6-tri-tertbutylphenol in acetonitrile (523). Anodic cyclic polarographic behavior of pentamethylanisole was described; t h e mechanism proposed involved a oneelectron oxidation to a cation radical, followed by irreversible proton loss and oxidation of benzyl radical (394). T h e mechanism for the anodic oxidation of a-tocopherol model compounds, reversible two-electron oxidation in aprotic media, was reported (391). Polarographic data for 12 different phenols a t p H 2 to 12 were correlated to their ultraviolet spectra (467). T h e sensitivity of the determination of poly(ethylene glycol) of molecular weight 1000 by a vareity of polarographic techniques was compared (240). Evidence for anodic hydroxylation of aromatic compounds, as found during oxidation of 2-substituted hydroquinone (hydroxy group goes on 3-position), under voltammetric conditions was reported (589). The oxidation of benzpinacol to benzophenone a t the DME in alkaline solution which was earlier suggested not to occur was recently confirmed (252,850).
Polarographic data are reported for 1,3-diphenyl-1,3-propanedione and other similar diones in dimethyl sulfoxide (77), 2-acetylfuran in acidic media (96), aromatic ketones and aldehydes (323), dehydrodianthrone (184), and for a homologous series of alkylphenones, CsH&OR, where R is CZ to C16 alkyl groups (407). Details of the effect of p H on the reduction of biacetyl and acetoin (149), deoxybenzoins (556), a,P-unsaturated ketones (337), isotinketyl radical (253), 2-aryl-1,3-indandiones (476), of benz[cd]indol-2(lN)one(naphthostyri1) (SQjY), and of propiophenone and o-hydroxypropiophenone (367) are reported. The effect of p H and phosphate concentration on the reduction of D-glucuronolactone (496),of acid, neutral, alkaline media, and cations on the reduction of ketones and aldehydes (648),and of p H on acetophenone, a-tetralone, and a-indanone reduction (305) was reported. Other studies listed the data and reduction mechanism for a series of 2-arylindandiones (512), and for 2-acetyl-, 2-propionyl-, and 2-benzoyl derivatives of 1,a-indandiones (556). Diene-like conipounds were suggested as products in the reduction of 2-phenyl-l,3-indandione and substituted derivatives a t p H 2.2 to 4 (511).
140 R
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
Reduction behavior of compounds of the type RC(O)P(O) R’R” in dimethylformamide and acetonitrile (439) and of the type R”(RO)P(0)C(O)CH3 in buffer solutions and dimethylformamide (440)were reported. A two one-electron process was observed in the polarographic investigation of 1-hydroxy-2-, 1-hydroxy-4-, and 2-hydroxy-1-acetonapthone (463). 0-Ketosulfides of the type CsHd2OCH&H(COHS)SR were found to be reduced a t the protonized carbonyl group in two one-electron steps, while the unprotonized form reduced in one two-electron process (452). Eleven unsymmetrical benzils were studied in 50% ethanol at p H 1, 5 , and 13 (466), while in another study the mechanism of benzil reduction in alkaline methanol-water was described (471). It was suggested that the benzil underwent a reversible overall reduction process of two electrons, which involves protons, to the product stilbenediolate ion; this undergoes further first-order arrangement to give the benzion anion. The effect of protonation, nietal cations, and substituents on the polarographic activity of aryl-alkyl ketones was reported (660). I n other studies the influence of proton donors on the mechanism of reduction for acetophenone in dimethylformamide (phenol proton donor) (462) and on ninhydrin in methanol (453) were studied. Several aryl-alkyl ketones were examined, in order to test an equation which considers that the electroactive protonated form is generated a t the electrode surface b y not only reaction with hydrogen ion but also with other proton donors (555). Polarographic data are reported in a study of the cyclization of o-benzylbenzophenones to 9-phenylanthracenes (468) and for the reaction of a,p-unsaturated ketones with phenylhydrazine (56). I n alkaline solution the reduction of w-benzyl- and w-phenylacetophenones was shown to be complicated by adsorption (554). An observed change in the drop time and ac and dc polarograms of benzophenone and methyl vinyl ketone was suggested to be the result of Hgalkyl compound formation (209). The catalytic wave of several keto acids was shown to be affected by the presence of thiamine and quaternary ammonium salts (360). Changes in the slope of Hammetthalf-wave potential plots for ring-substituted acetophenones were part of a study which also included a n attempt to put the data on a more absolute basis by considering the structure of the electrode double layer (243). Polarographic data for 39 m- and p-substituted benzophenones (551) and for 21 unsaturated ketones containing 2-furyl, 2-thienyl aryl groups (40) were correlated to Hammett values. The data were also used to interpret the electrode reac-
results obtained for the polarographic activity of trichloroacetate ion was explained b y the specific adsorption of anions and cations (605). R‘ “ 0 0 Kinetic waves for fumaric and maleic acids were studied (414). Pulse polar11 ographywas used to establish kinetic and equilibrium constants for pyruvic and culated energy for the lowest vacant glyoxylic acid (163), while potentiostatic molecular orbital (346). investigations were used in the measureIn pyridine, benzophenone was elecment of the rate constant for the dissocitrochemically reduced a t the D N E in ation of acetic acid in aqueous solutions two successive one-electron steps, with (445). The kinetics and effect of the the first producing a free radical anion presence of alcohols in the reduction of which proceeds to the benzhydrol phenylglyoxylic and maleic acids in bufdianion; additional mechanistic paths fered solution were reported (3, 4). are possible if proton donors are added Details of the polarographic behavior (349). Benxanthrone in 70% methanol of 4-cyanocinnamic acid as a function of gave a one-electron reversible wave t o a p H (YO)and for phthaloylglycine and its free radical which dimerizes. If the methyl ester which probably involves methanol is increased to SO%, the wave radical anion formation in dimethylis irreversible, while if the methanol is formamide (90)were described. Oxaluric 98%, a n additional reaction occurs acid was shown to be polarographically (258). Perinaphthenone a t p H 3 12 is reduced in a two-electron process apreduced in two one-electron steps with a parently to glyoxylic acid monourexide radical anion as intermediate (92), while (132). Cyclic voltammetry was used to benzene derivatives with two carbonyl study oxalic acid and oxalate anion (229) groups para to each other are reduced in and formate anion (228) a t a Au and Pt acid media in one two-electron reversible electrode in dimethyl sulfoxide. Br$nstep to form a biradical or quinoid-type sted acids in pyridine produced a wellproduct (553). defined one-electron diffusion-controlled The nature of the five waves found polarographic wave per acidic function for cyclohex-2-en-I-one in the p H and the half-wave potentials could b e range of 0 to 14 was suggested to be the correlated to potentiometric data (506). result of inhibition of the hydrogen Polarographic data for esters, amides, wave, one-electron diffusion-controlled and anhydride derivatives of carboxylic processes, wave combinations, and adacids in dimethylformamide (224), glysorption properties (112). Detailed col esters of phthalic acids (230), and studies of the reduction mechanism for the diethyl ester of pyrocarbonic acid chalcone and dihydrochalcone and effect (257) were reported. Several acrylate of p H (432) and for the reduction of the and methacrylate derivatives were studconjugated ketone in position 1 of ied in dimethylformamide in the preschromamycin and its derivatives (23) ence of proton donors and water (58). were reported. The availability of the electroactive Details of the reduction of phenylform of 4-hydroxycoumarin and chrofluorenone which involves a n irreversible mone was suggested to be controlled by two-electron process (9) and in the the extent of protonation (35~7). presence of proton donors (120) were reA study of nine alkyl benzoates and ported. The reduction and oxidation 17 0-, m-, and p- substituted methyl of fluorenone were studied a t a Au diskbenzoates in dimethylformamide by de ring electrode in NaOH solutions (278). and ac polarography was made in which The polarographic behavior of ninhydrin the data for a one-electron wave was (247) and 2-amino-2-phenylindonecorrelated to Taft’s polar substitutents dione-l,3 (519) was described. Periconstants (16). The electrochemical naphthenoiie, which was reduced in two reduction of a variety of esters of benzoic one-electron steps, was studied as a funcacid and isomers of phthalic acid which tion of pH (93). produced anion radicals was done by dc, ac, and oscillographic polarography as CARBOXYLIC ACIDS, ESTERS, AND NITRILES well as by other instrumental methods Polarographic data for ethylene (222, 228). glycol-bis-paminoethyl ether -NILVJPolarographic behavior of six N h”,N’-tetraacetic acid (157),ethylenedibenzoyl lactains was studied as a funcaminetetraacetic acid (EDTA) , and tion of p H (504). Adsorption and kiseven other EDTA-like compounds netics were considered in a study of N (477) oxalic acid in sulfurlc acid on P t in (2-methoxybenzoyl) caprolactam (505). N-Benzoyl lactam derivatives yielded the presence of benzene, hexane, or cyclohexane (276), and for several alitwo waves in which proton donors play phatic carboxylic acids as a function of an important role (145). environmental conditions (144) were Polarographic data for ICH2CH2Cn reported. Adsorption properties of and NCCH2CH2HgI in aqueous media (500), for acrylonitrile in aqueous dimaleic acid and its reduction properties N ere studied (413). The experimental methylformamide (22),for cis and trans tion mechanism, Half-wave potentials for derivatives of I1 were related to cal-
isomers of CH&H==CHCN and NCCH=CHCH~CHZCNand other isomers (465),and for aromatic nitriles (4.22)were reported. Benzonitriles with -COO-, -COOR, or -CY groups in m- or p-position were shown to be reduced at p H >8 in one two-electron step in which C N was a leaving group; half-wave potentials were correlated to Hammett values (332). I n another study the polarographic data for p-substituted benzonitriles m-ere correlated to simple molecular orbital calculations (95). ORGANIC HALOGEN COMPOUNDS
Polarographic data for the reduction of methyl, ethyl, propyl, and butyl iodide at a dropping Ge electrode (29), for alicyclic chlorinated compounds (99), isomeric bromocitraconic and bromoesaconic acids (94), for the effect of alcohol on the reduction of CHJ and CaHgBr (332),disodium salt of 2’,4’,5’,7’-tetraiodofluorescein a t p H 4.7 (62), alicyclic-halide derivatives of the type C6H6CH=CHCH2X and (CEHS)&= CHCHlX (338),a series of homologous esters of the type RCHBrC02CHs and BrCHzC02R (406),and for tris(difldoramino)fluoromethane, tetrafluoroformamidine, and pentafluoroguanidine (419) were reported. Cis and trans diethyl esters of dihalofumaric and dihalonialeic acids were studied polarographically (161, 336). Monosubstituted halopyridines (147), cis-P-4-methoxybenzoyl-P - bromoacrylic acid and its 8-chloro analog and related derivatives (246), and m-, p-methyl, 2,3-, and 3,4-dimethyl derivatives of diand trichloroacetanilides (483) were studied as a function of pH. Seven pairs of epimeric a-bromo-17- and -16ketosteroids in isopropanol-acetate buffer solution (183) and mono- and dichloroacetones in acetate buffer-acetonitrile-water systems (194) were reduced polarographically. 1,4-Dichloro-lJ3butadiene was identified as the product in the reduction of hexachlorobutadiene (15.3). Reduction data were reported a’,a’,a’-triflu~r~acetonephenone for (472)and for octachloro- and hexachlorocyclopentadiene (152). The polarographic behavior of benzoyl chloride (67) and of iodobenzene, bromobenzene, and p-chlorobenzaldehyde (50) in nonaqueous and mixed solvents was described. Half-wave potentials for some iodobenzenes were correlated to their respective nuclear quadruple resonance frequencies (86, 87). It was concluded from correlations between half-wave potentials of chlorobenzenes, bromobenzenes, iodobenzenes, and benzyl bromides and Hammett substituent constants in dimethylforniamide and acetonitrile that the reduction mechanism is not an SZ1 type reaction (447). Hammett-half-wave potential correlations
ANALYTICAL CHEMISTRY, VOL. 42,
NO. 5 , A P R I L 1970
141 R
were also reported for a series of benzyl bromides (187). An optically active (inverted) product was identified in the polarographic reduction of 2-phenyl-2-chloropropionic acid (106). Several 1,4-dibromoalkanes were studied and the following mechanism was suggested (127):
+
B P ( C H ~ ) ~ - B ~2e
-+
Br-(CH2)4:Br-(CH2)4:-
+ H+
-+
+ Br-
Br-(CH&H
(The same route as above to yield H(CH&-H finally.) The details of the reduction mechanism for a,@'-dibromo-p-xylene (105) and for triphenylbromoethylene and other vinyl bromides (564) were described. The compound FZBrCCFClBr was shown to be polarographically reduced in two waves to F&=CHF (syn form with respect to F atoms) or in one wave resulting in F,C=CFCI(anti form) (150). I n another interesting study simple alkyl halides were reduced a t a stationary lead electrode; iodides and bromides undergo an irreversible one-electron reduction and the bromide results are compatible with backside attack by the electrode, while the iodides have a different orientation (514). I n electrochemical studies of several halonitrobenzenes in nonaqueous solvents it was reported that the initial reduction is a one-electron process which yields the corresponding anion radical, followed by rapid dissociation to neutral nitrophenol radical and halide ion (298). NITROGEN COMPOUNDS
Nitro Compounds. Polarographic data for m-nitrophenol (73), nitropyrimidines (23'5), aliphatic nitroalcohol derivatives (328), nitrourea (400), eevera1 N'-substituted-2-methyl-4(and-5)nitroimidazoles (464),and 1-substituted 2,4-dinitrobenzenes in H2S04-EtOHHzO solutions (490) were reported. Several aliphat'ic tertiary nit'ro compounds were studied in alkaline media and acetonitrile (442,448). cis- and trans- CH,C(NO?)=CN02CH3 (19), CsH6CHBrCHBrN02 (102), 5-nitrobarbituric acid (251), 5-nitrouracil (252), and nitrofurazone (251) were studied in detail as a funct'ion of pH. Empirical equations which describe electron change as a function of p H were described in detailed electrochemical studies of 1,8-dinitronaphthalene and related dinitro compounds (508). The effect of proton donors on p-dinitrobenzene reduction (214) and the ability of pyridine and diethylamine to catalyze the proton transition between acids and nitro or carbonyl compounds ($21) were discussed. Half-wave POtentials for a series of 4- and 5-substituted 2-nitroghenols were correlated to electronic spectra and Hammet't substit142 R
uent constants (426). The pK, value for 1-nitro-2-chloroethane was related to its polarographic properties (491). 2-Nit'roacet'aldehyde oxime was studied cathodically as well as anodically i l l y ) , while o-substituted p-nitroacetophenone oximes were examined in order to gain insight in ortho effects (65). The polarographic behavior of p-nitrophenol (86) and nitrobenzene (179) which was adsorbed on activated carbon and coal, respectively, was described in studies of the adsorption process. An anomalous dip was observed in the polarographic wave for nitro derivatives of imidazole a t p H 6 to 10 (397). I n another study the steepness of the reduction wave for l-(p-bromophenyl)-2-nitroethyl alcohol and other aromatic nitro alcohols was suggested to be a measure of the ext'ent' of adsorption (3'00). The various important experimental variables were emphasized in a study in which p-dinitrobenzene served as the model compound (211). Cyclic chronovoltammetry was used to establish the reduction mechanism for nitrofuran in which very unstable products are formed and rapidly isomerize (266). Linear volt'age scanning was used to investigate intermediates in the reduction of 0-, m-, and p-nitrophenol in NaOH-KC1 a t pH 12.5 (148). .4 detailed electrochemical study including ac and dc polarography of nitrobenzene reduction was also reported (522). Effect of structure and p H on the reduction of 2-substituted-5-nitrofuran derivabives in water-ethanol was described (475). Reduction and oxidation studies of several nitro paraffins and their Na and K salts, respectively, were discussed in detail (478). Polarograph data for 24 nitro compounds were determined in 63: 27: 10 ethanol-pyridine-water and used in studies on the pyrochlorophyll-sensitized photo-reduction of nitro compounds (448). h fully instrumentally automated polarographic method with alarm system was described for continuous monitoring of nitroglycerin plant effluent (208). The mechanism for the kinetic wave of o-nitrophenglarsonic acid-Fe(II1) mixtures was reported (516). o- and mtrifluoromethylnit,robenzene in dimethylforniamide were shown to be reduced via two polarographic steps; the first is a one:electron diffusion-controlled wave producing a stable anion radical, while the second involves an E C E process which produces a trifluoromethyl nitrosobenzene anion radical (186). Oneelectron transfer was observed in reduction studies of anions of the type {RC(XO2)z)- (3'98). Nitroso Compounds. Nitroso and nitro derivatives of guanidine and urea were examined polarographically (296). The effect of proton donors on the reduction of p-nitrosodimethylaniline
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
to N,N-dimethyl-p-phenylenediamine in methanol was reported (239). Cupferron was studied as a funct'ion of p H by oscillopolarography (283). Effects of p H were also considered in the study of 2-isonitroso-l-indanone, 2-isonitroso1,3-indandione and other isonitroso derivatives (540). A variety of aromatic isocyanates were studied in attempts at, electrochemical preparation of stereoregular polyphenyl isocyanates (466, 456). Aliphatic-Aromatic Amines. Polarographic d a t a are reported for 14 polyamino- and aminohydroxypyrimidines at a rotating P t electrode (100) and for several amino acids in unbuffered 80% dioxane in which the data are related to pK,'s (577). Two catalytic waves were observed for =t-Iupanine (55). A four-electron process accompanied by catalytic effects was found for bis(ethy1eneimino)mannitol derivatives ($34). Half-wave potentials for 15 enamines were correlated to L.C .r1.0.--;11.O . calculations (164). The details of the oxidation of 17 aliphatic amines by cyclic voltammetry a t carbon electrode in aqueous alkaline solut'ion were reported where p-amines gave no waves, sec-amines one wave, and tert-amines gave three n.aves. X linear relationship was also found between peak potentials for the second wave of tert-amines and their pK, values (545). I n studies with aromatic amines oxidation of benzidine and alizarine S a t a rotating P t electrode (421), osidat,ion pathways for substituted triphenylamines (364), and oxidation dat,a and mechanisms for a variety of aromatic amines (515, S62) were described. Correlations of polarographic data and Taft-Hammett constants were used t'o show that electrochemical oxidation of amines and chemical Oxidation by chlorine dioxide are similar (220). Oxidation half-wave potentials for substituted anilines were correlated to calculated energy levels and Hammett values i294). Anodic oxidation of a series of p-substituted anilines in aqueous media revealed a common pattern of p-group elimination and head to tail coupling t,o give 4/-substituted 4-aminodiphenylamines (27). Optically transparent electrodes were used to study thekinetics and mechanism of o-tolidine oxidation in which the intermediate was ident)ified as a dimeric species (291). The details of the mechanism as a function of p H for the oxidation of p-dimethylaminophenol were reported (355). Tri-panisylamine, tri-p-tolylamine, and 9,lOdiphenylanthracene which provide stable cation radicals as a result of oxidation were stu(lied in presence of cyanide ion (588). Azo Compounds. Polarographic data were reported forsixderivativesof
p-(dimethy1amino)azobenzene (156), several azo dyes (221), hydrazobenzol (213), azo compounds of t'he type p (CH3)2NC6H4N=NC6HS as a function of p H (345), diazo compounds of the type CH3COCHN2, CH2ClCOCHN2, and CClCOCHN2 in 3070 ethanol as a function of p H (COS), aromatic azo compounds (43S), a'nd for p-bisazobenzene in acidic, neutral, basic and buffered solution (454). Half-wave potent'ials for substituted benzenediazonium tetrafluoroborates (139) and for diphenlldiazoniethane subst
