P. Zuman

Quantitative Treatment of Substituent Effects in Polarography. V. Polarographic Reduction of Some Semicarbazones

Polarographic reduction of semicarbazones of aliphatic and aromatic aldehydes and ketones in the protonized form follows predominantly a four-electron course to form a primary amine and urea. Other acids in addition to the hydroxonium ion can participate in the protonation preceding the electrode process proper. Half-wave potentials of benzaldehyde semicarbazones are relatively little sensitive to the substituent effects (Q π = 0·10 V), and remain practically unchanged by m- and p-CH3, C2H5, i-C3H7 and t-C3H7 and m-CF3. Substitution by i-C3H7 and t-C4H9 in ortho-position results in a steric hindrance of coplanarity.

Polarographic Reduction of Aldehydes and Ketones. IV. Linear Free Energy Treatment of Substituted Benzophenones

Reduction of thirty nine meta- and para-substituted benzophenones at the dropping mercury electrode was investigated. Substituent effects on half-wave potentials were determined for the one-electron wave of the protonized form in acidic media, for the two-electron wave at pH 9·3 and for the wave in sodium hydroxide solutions that corresponds predominantly to a one-electron reduction. For correlations the Hammett equation and an equation relating meta with corresponding para values derived earlier were used. The deviations from the latter equation as well as the use of constants (sigma _{p - x}^ - ) in the Hammett plot has enabled some conclusions to be drawn about the electronic structure of the radicals involved. Even when the correlations of the half-wave potentials with Hammett substituent constants show only a limited precision, they allow the conclusion that half-wave potentials are controlled by substituent I and M effects transmitted through the benzene nucleus.

Fission of Activated Carbon—Nitrogen and Carbon—Sulphur Bonds. XI. Polarographic Reduction of Substituted Benzenesulphonamides

Substituted benzenesulphonamides bearing strongly electron-attracting substituents can be reduced at the dropping mercury electrode to ammonia, sulphur dioxide and monosubstituted benzene. The products have been identified in a controlled potential electrolysis using a mercury pool electrode. A pH-independent, two-electron wave is observed at pH 7 to 12. Fission of the C—S bond occurs before that of the S—N bond. The reaction represents a kind of aromatic SN2 substitution with the —SO2NH2 grouping as a leaving group. The substituent effects on half-wave potentials at pH 9·3 for derivatives I–XIX were examined. The reduction is strongly facilitated by conjugation with acceptor groups in the para-position; this effect is even stronger than would be expressed by (sigma {}_p^ - - x) constants.

The Use of Simple Molecular Orbital Theory to Elucidate the Polarographic Behaviour of Some para-Substituted Benzonitriles

The nature of the polarographic reduction of the para-substituted benzonitriles I–VI depends strongly on the para substituent. The first reduction step could involve cleavage of the C—C bond, which leads to CN− formation, or reduction of the cyano group to a−CH=NH function, or reduction of the para-substituted group. The experimental results obtained with compounds I–VI are compared with the squares of the expansion coefficients of the lowest free molecular orbital and with their bond orders before and after an uptake of two electrons.

Fission of activated carbon-nitrogen and carbon-sulphur bonds. XII. Polarographic reduction of substituted benzonitriles bearing in para -position a carbonyl group in acidic media

4-Cyanoacetophenone and 4-cyanobenzaldehyde are reduced in acid media in a four-electron step followed by a one-electrone reduction of the protonated carbonyl group. In the four-electron step the cyano group is reduced in a protonated species to methylenamino grouping. 4-Cyanobenzophenone is reduced in acid media in two two-electron steps. In the first step which is split into two one-electron waves, the carbonyl group is reduced. In the consecutive two-electron step 4-cyanobenzhydrol is further reduced.

Polarographic Reduction of Aldehydes and Ketones. V. Reduction Course of Some β-Ketosulphides and Effects of Alkyl Groups on Sulphur

β-Ketosulphides of the type C6H5COCH2CH(C6H5)SR are reduced at the carbonyl group. The protonized carbonyl group is reduced in two one-electron steps, the unprotonized form in one two-electron process, which differs from the reduction of corresponding β-aminoketones. The effects of alkyl groups at sulphur and nitrogen on the reduction of the carbonyl group are compared.

Polarographic Reduction of Aldehydes and Ketones. III. Effects of Alkyl Groups in the Side Chain on the Half-Wave Potentials of Deoxybenzoins and Some Other Alkyl Aryl Ketones

The polarographic waves of deoxybenzoins are in principle similar to the reduction waves of acetophenone. In acidic media the protonised form is reduced and the rate of the protonation (in which also acids other than hydroxonium ion participate) limits the wave-height. Even when this reaction is a “surface reaction”, the wave height is governed primarily by changes in chemical reactivity in the protonation reaction rather than by structural changes in adsorptivity. Alkyl groups R in deoxybenzoins of the type C6H5COCHRC6H5 exert predominantly a polar effect on half-wave potentials.

Polarographic Reduction of Aldehydes and Ketones. I. General Reduction Scheme

Reduction scheme of polarographic reduction of aryl alkyl ketones is discussed. Schemes are summarised for the reduction of the protonized form in acid media, for the two-electron reduction in medium pH-range and for the one-electron steps in alkaline solutions. Kinetics currents resulting in protonations, antecedent the electrode process or interposed between two electrode processes, which play a role in the transition regions of pH, are discussed. The effect of cations on the reduction waves is interpreted. Studied examples of reductions of various types of carbonyl compounds are summarised.

Fission of Activated Carbon-Nitrogen and Carbon-Sulphur Bonds. X. Polarographic Reduction of Substituted Methyl Phenyl Sulphones

Reduction of twenty-six meta- and para-substituted methyl phenyl sulphones at the dropping mercury electrode and at a mercury pool electrode was investigated and by identification of reduction products was decided which of the two functional groupings attached to the benzene ring is reduced. With most derivatives bearing an electron attracting group as a substituent, the methyl sulphonyl group leaves as methane sulphinate anion. A pH-independent, two-electron wave is observed between pH 7 and 14. The reaction represents a kind of aromatic SN2 substitution and is strongly facilitated by conjugation in the transition state with an acceptor group in the para- position. The effect of para-substituents is even greater than expressed by the values of (sigma {}_p^ - - x) substituent constants.

Polarographic reduction of aldehydes and ketones. II. pH-Dependence of polarographic limiting currents of aryl alkyl ketones governed by the rate of simultaneous reactions with several acids producing the electroactive species

For reactions in which the electroactive protonated form is generated at the electrode surface not only by reaction with hydrogen ions, but also with other proton-donors, the pH-dependence of limiting currents studied in buffers with a constant concentration of the acid buffer component gives a distorted dissociation curve. The dependence has the shape of a theoretical dissociation curve only, when the concentration of the base buffer component is kept constant. The inflexion point of such curves (pK″) depends on the concentration of the base buffer component [S−] and the curve is shifted towards lower pH-values with decreasing base component concentration. From dependence of 1/(K″)2 on [S−], values of rate constants with the acid buffer component (k SH) and with hydrogen ions (k H3O+) can evaluated. Derived equations are used for the study of the one-electron wave of the protonated form of aryl alkyl ketones, in particular some deoxybenzoins.