Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrahalo-p-benzoquinones. Part V. The disubstitution reaction of iodanil

Iodanil reacts with an excess of triphenylphosphoniocyclopentadienide to yield exclusively the 2,5-disubstituted quinone through a second-order reaction. Kinetic data suggest that the reaction goes in two steps, involving pre-equilibrium formation of a betaine followed by an E1 type elimination of HI. The empirical rate law is derived from a multi-response non-linear least-squares analysis and the 2,6-/2,5-isomer ratio for tetrahalo-p-benzoquinone substitution is discussed for all the halogens. Finally, the synthesis and characterisation of 2-(triphenylphosphonio-3'-cyclopentadienyl)-3,5,6-trifluorocyclohexa-2,5-diene-1,4-dione (5, X = F) are also reported.