The irradiation of benzene/CCl4 solutions of meso-tetraphenyl porphyrin (TPP) leads to an electron transfer from an electronic excited state of the porphyrin to CCl4 giving an intermediate which is likely to be the encounter pair, (TTP+.. CCl4-.). This is completely reconverted to the original species by a fast back electron transfer except in the presence of ethanol where the reduction products of CCl4, Cl- and CHCl3, are formed with catalytic yield. A mechanism is proposed where ethanol plays the twofold role of favouring the dissociation of the encounter pair by increasing the polarity of the reaction environment, and of restoring TPP by reducing the TPP+. radical cation. The oxidation of ethanol produces H+ ions which give rise to the protonation of the pyrrolic nitrogens to give the TPPH22+ dication, which, however, fails to form in KOH containing solutions.
Catalytic reduction of CCl4 by a meso-tetraphenyl porphyrin excited state in benzene/ethanol mixed solvent.
VARANI, Graziano;MALDOTTI, Andrea;BARTOCCI, Carlo
1992
Abstract
The irradiation of benzene/CCl4 solutions of meso-tetraphenyl porphyrin (TPP) leads to an electron transfer from an electronic excited state of the porphyrin to CCl4 giving an intermediate which is likely to be the encounter pair, (TTP+.. CCl4-.). This is completely reconverted to the original species by a fast back electron transfer except in the presence of ethanol where the reduction products of CCl4, Cl- and CHCl3, are formed with catalytic yield. A mechanism is proposed where ethanol plays the twofold role of favouring the dissociation of the encounter pair by increasing the polarity of the reaction environment, and of restoring TPP by reducing the TPP+. radical cation. The oxidation of ethanol produces H+ ions which give rise to the protonation of the pyrrolic nitrogens to give the TPPH22+ dication, which, however, fails to form in KOH containing solutions.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
