RESEARCHES ON THE ELECTROLYTIC REDUCTION POTENTIALS. (PART VI.)

ON THE REDUCTION POTENTIALS OF NITPOPHENOLS

Abstract

(1) The electrolytic reduction potentials of ortho-, meta-, and para-nitro-phenol were measured with the dropping mercury cathode and the polaro-graph.
(2) Analogous in the case of nitrobenzene_??_Shikata: Trans. Faraday Soc., No. 61 XXI 42 (1925)_??_the first stage of reduction was thought to be the system nitrophenol-nitrosophenol.
(3) Two waves of reductions were found in slightly alkaline solution, in which the easily reducible wave was proved to be due to the reduction of the undissociated nitrophenol and the second wave to be that of the ionised nitro-phenol. It should be noticed that negatively charged nitrophenol ion was more difficultly reducible than the undissociated molecule, and this was quite contrary to the case of pyridine, that pyridine ion (cathion) was more easily reducible than the undissociated pyridine molecule at the dropping mercury cathode.
(4) Reduction potentials of these compounds in the solutions of various hydrogen ion concentrations were compared with the calculated values from the following formula at 25°C, using the 1 n calomel electrode as the standard electrode.
π=-0.05911/2 log k'/_??_H_??_²_??_Reducible compound_??_
(5) For o-nitrophenol, taking the reduction potential (0.072 V) of 10^-3 molar solution in the 0.1n HCl containing 3% ethanol as the standard,
we found log k'=-2.57
and for ionised o-nitrophenol,
log k'=0.02.
For m-nitrophenol, by taking the reduction potential (0.043 V) of 10-² molar solution in 0.1n HCI as the standard, we found
log k'=-2.55
and for ionised m-nitrophenol,
log k'=3.46.
For p-nitrophenol, i.e. 0.090V under the same conditions as the former standard, we found
log k'=-0.95
for the reduction of free molecule and
log k'=5.51
for that of p-nitrophenol ion.
(6) Concordance of the observed values with the calculated values was tolerably good in the slightly acidic solutions but the former was much more positive than the calculated value in the alkaline solution for all nitrophenols.
(7) The following series may be given for the reducibilities of isomeric nitrophenols and nitrobenzene.
o-nitrophenol>m-nitrophenol>nitrobenzene>p-nitrophenol.
_??_College of Agriculture, Kyoto Imperial University ??