Kinetic and Mechanisms of the Homogeneous, Unimolecular Elimination of Phenyl Chloroformate andp‐Tolyl Chloroformate in the Gas Phase

Abstract

The gas‐phase elimination of phenyl chloroformate gives chlorobenzene, 2‐chlorophenol, CO2, and CO, whereasp‐tolyl chloroformate produces p‐chlorotoluene and 2‐chloro‐4‐methylphenol CO2 and CO. The kinetic determination of phenyl chloroformate (440–480oC, 60–110 Torr) and p‐tolyl chloroformate (430–480°C, 60–137 Torr) carried out in a deactivated static vessel, with the free radical inhibitor toluene always present, is homogeneous, unimolecular and follows a first‐order rate law. The rate coefficient is expressed by the following Arrhenius equations: Phenyl chloroformate: Formation of chlorobenzene, log kI = (14.85 ± 0.38) – (260.4 ± 5.4) kJ mol−1 (2.303RT)−1; r = 0.9993 Formation of 2‐chlorophenol, log kII = (12.76 ± 0.40) – (237.4 ± 5.6) kJ mol−1(2.303RT)−1; r = 0.9993 p‐Tolyl chloroformate: Formation of p‐chlorotoluene: log kI = (14.35 ± 0.28) – (252.0 ± 1.5) kJ mol–1 (2.303RT)−1; r = 0.9993 Formation of 2‐chloro‐4‐methylphenol, log kII = (12.81 ± 0.16) – (222.2 ± 0.9) kJ mol−1(2.303RT)–1; r = 0.9995 The estimation of the kI values, which is the decarboxylation process in both substrates, suggests a mechanism involving an intramolecular nucleophilic displacement of the chlorine atom through a semipolar, concerted four‐membered cyclic transition state structure; whereas the kII values, the decarbonylation in both substrates, imply an unusual migration of the chlorine atom to the aromatic ring through a semipolar, concerted five‐membered cyclic transition state type of mechanism. The bond polarization of the C–Cl, in the sense Cδ+…Clδ−, appears to be the rate‐determining step of these elimination reactions.

What a difference a chlorine makes: The remarkable unimolecular ion chemistry of phenyl formate and phenyl chloroformate

“…MIKES experiments determined that the primary channels for dissociation are loss of CO, Cl and CO 2 , and sequential loss from all of these contributes to the respective ions m/z 98 and m/z 77. The minimum energy reaction pathway confirmed these findings and showed that the CO loss is least energetically favoured, which is opposite to what is seen in the neutral chemistry of PCF, where H‐transfer to form chlorophenol is favoured 7 . Cl loss was shown to also dissociate in a non‐statistical process from its second excited state, in which the chlorine atom is weakly bound to the ring.…”

“…This fragment ion at m / z 121 can dissociate sequentially by two competing channels, by loss of CO or CO 2 , with a difference in energy of 0.51 eV favouring CO 2 . As a neutral, PCF undergoes a rearrangement process to transfer the Cl to the ring to form chlorophenol 7 . However, this process does not occur in the ion as the more energetically favourable process involves the ClCO moiety pulling away from the phenoxy group after which the Cl atom hops back to the O‐atom resulting in CO loss (green).…”

“…25 The observed pattern is similar to that observed in CH 3 Cl + where an excited state is responsible for a portion of the Cl loss pathway. 26 Indeed, OVGF energies for the ion states show that there is an excited state at 11.5 eV and three in close proximity between 11.9 and 7 Cl loss was shown to also dissociate in a non-statistical process from its second excited state, in which the chlorine atom is weakly bound to the ring. The MIKES of PCF +• showed that loss of CO 2 occurs with a large reverse energy barrier, consistent with the computationally derived minimum energy reaction pathway from B3LYP/6-311++G(d,p) calculations.…”

“…In contrast with PF, however, CO 2 loss dominates in PCF, because it has a larger activation entropy and lower overall activation free energy than CO loss. 7 The ion chemistry of PCF has not yet been reported in the literature.…”