Pyridine polarographic potentials

Like benzenoid hydrocarbons, pyridine-like heterocycles give well-developed two-electron waves on reduction at the dropping mercury electrode. The latter are polarographically much more reducible than the former. This can be explained easily in terms of the HMO theory It is assumed (cf. ref. 3) that the value of the half-wave potential is determined essentially by the energy of the lowest free 7r-molecular orbital (LFMO) of the compound to be reduced, and for models of hetero analogues this quantity is always lower than that for the parent hydrocarbons. Introduction of an additional heteroatom into the molecule leads to a further enhancement of the ease of polarographic reducibility.95 On the other hand, anodic oxidation of the heterocyclic compounds is so much more difficult in comparison with benzenoid hydrocarbons that they are not oxidizable under the usual polarographic conditions. An explanation in terms of the HMO theory is obvious. 

Pyridine is not polarographically reducible in aqueous solvents but in aprotic media, such as acetonitrile,211 DMF,212 or liquid ammonia,213 it is reduced at rather negative potentials to the anion-radical, which then dimerizes. Some electron-attracting substituents, notably carboxyl derivatives, render the nucleus reducible even in aqueous solvents. Quaternary derivatives are generally reducible. JV-Alkylpyridinium ions thus give a free radical, which dimerizes rapidly the radical was trapped by a-phenyl-Af-tert-butylnitrone.214 1,3-Dimethylpyridinium ion is reduced in buffered aqueous medium to a 4,4 -dimer, which undergoes further chemical reaction, possibly an addition of water to one of the double bonds of the 1,4-dihydropyridine rings.215... 

Electron-attracting substituents facilitate electrochemical reduction. The reduction potentials for the polarographic reduction of quinoline and isoquinoline derivatives are much less negative than those for the pyridine analogues. Diazines are reduced electrochemically stepwise, usually as far as tetrahydro derivatives (70AHC(12)262). 

The oxidation-reduction potentials of metal ions differ in different solvents due chiefly to differences in the strength of coordination of the solvents to the metal ions. Thus, Schaap and coworkers,33 who measured reduction potentials polarographically in anhydrous ethylenediamine, found the order of half-wave potentials to be Cd2+ > Pb2+ > Cu2+ - Cu+ > Ti+, whereas, in aqueous solution, the order is Cd2+ > Ti+ > Pb2+ > Cu2+ -> Cu+. Oxidation—reduction potentials have been measured in a great variety of non-aqueous solvents, both protonic and non-protonic. Among the former are liquid ammonia and concentrated sulfuric acid.34 Among the latter are acetonitrile, cyanopropane, cyanobenzene, dimethyl sulfoxide, methylene chloride, acetone, tet-rahydrofuran, dimethylformamide and pyridine.34... 

Polarographic studies on pyrazine and methylpyrazines indicate that 1 4-dihydropyrazines are the products of reduction. The reduction of pyrazine itself at the dropping mercury electrode proceeds reversibly. The substitution of methyl groups makes the reduction more difficult with an increased number of methyl groups an increased tendency toward irreversible reduction is noted.102-104 The half-wave reduction potentials for pyrazine, methylpyrazine, 2,6-dimethyl-pyrazine, and tetramethylpyrazine are 2.17, 2.23, 2.28, and 2.50 eV, respectively. Pyrazine is thus more easily reduced than pyridine which has a half-wave potential of 2.76 eV, and less easily reduced than quinoxaline which has a half-wave potential of 1.80 eV.105... 

Polarographic studies have been made on pyrazine and methylpyrazines they indicate that 1,4-dihydropyrazines are produced, and that substitution (by methyl groups) makes the reduction more difficult. The reduction of the parent pyrazine proceeds reversibly (125, 586-588). The experimental half-wave reduction potentials [pyrazine (2.17 eV), methylpyrazine (2.23) 2,6-dimethylpyrazine (2.28), tetramethylpyrazine (2.50), pyridine (2.76), and quinoxaline (1.80)] also revealed that pyrazine was more easily reduced than pyridine but less easily reduced than quanoxaline (589). 

Quinoline and isoquinoline derivatives are polarographically reducible in a way similar to the pyridine compounds, but the reduction potentials required are generally less negative. 

A polarographic study of a pyridine solution of the quadrivalent (LXI), using 0.05 M lithium bromide as a carrier electrolyte, showed two reduction steps with half-wave potentials at —0.76 and —0.94 volt, ascribed to the... 

The polarographic reduction of quinoxaline and its derivatives has been studied by a number of workers. Half-wave electrode potentials are pH dependent and over the pH range of 1 to 10, Ei has been reported to vary from —0.254 to -0.863 The half-wave electrode potential in anhydrous dimethylformamide is — l.bOV for quinoxaline, —1.06 V for cinnoline, —1.41V for phthalazine, and —1.22 V for quinazoline. The benzodiazines are more easily reduced than the corresponding diazines, thus pyrazine has a half-wave electrode potential of -1.57V. Pyridine with a half-wave potential of —2.15 V is still more difficult to reduce. The reversible reduction potential of quinoxaline as determined by cyclic voltammetry is -1.097 V. The observed energy differences between azine and radical anion are well correlated with the results of CNDO and SCF iT-electron calculations. 

Isonicotinoyl amide in 0.8 N HGl electrolyzed at 5° and a cathode potential of —0.80 V vs. S.C.E. until 2 electrons per molecule are consumed 4-pyridine-aldehyde. Y 87% by polarographic determination. F. e., also reduction of subst. amides, s. H. Lund, Acta Ghem. Scand. 17, 2325 (1963). 

The reduction of papaverine methiodide has been facilitated by the use of new reagents. Its reduction (or of a similar salt) with sodium borohydride in aqueous methanol is now the best method for the production of racemic laudanosine (167) (Section III, F). If pyridine is employed as a solvent 1,2-dihydro-iV-methylpapaverine also is obtained in good yields (166). The 1,2-dihydro derivative is the main product when lithium aluminum hydride is employed although the tetrahydro compound is probably formed in small amounts (169). Polarographic reduction of papaverine and 3,4-dihydropapaverine in aqueous tetra-methylammonium hydroxide gave half-wave potentials of — 1.92 V and — 1.62 V, respectively (170). 

Since the latter are useful sources of pyridine derivatives, methods for their deoxygenation have been much studied. These compounds lack the oxidizing properties found with some amine oxides. Polarographic studies show that in acid media pyridine 1-oxide has a much lower reduction potential than oxides of tertiary amines with exocyclic nitrogen atoms -. The half-wave potentials are not useful guides to the abilities of the N-O bonds in pyridine oxides to take part in oxygen transfer reactions, and in 4-nitropyridine 1-oxide the nitro group is reduced first . 

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