Selectivity and reactivity of hot homolytic aromatic substitution by recoil chlorine atoms

Abstract

emission intensities were measured at 450 and 480 mm with readings being taken directly off the microphotometer. Tubes were then opened and aerated; the residual signal was measured and subtracted from that of the unopened tube. Emission spectra recorded on a Houston X-Y recorder were identical with those reported for benzophenone in other solvent^.^' EPR spectra of the biphenyls were obtained in both ethanol and methylcyclohexane glasses at 77 O K on a Varian E-4 EPR spectrometer. The light from a 1 kW high-pressure mercury-xenon arc, filtered only through water, was focused directly onto the cavity.Phosphorescence spectra of the biphenyls were obtained at 77 O K on the Aminco with the phosphoroscope attachment. Excitation was at 254 mm. Biphenyl concentrations were approximately IO-2 M. Absorption spectra were obtained on a Cary 14.Abstract: Hot homolytic aromatic chlorination has been studied in simple monosubstituted benzenes CsHsX (X = F, C1, Br, 1, NH2, OCH3, NOn) using recoil chlorine atoms produced via the 37Cl(n,y)38CI reaction. In the gas phase, CI-for-H substitution is a minor process, proceeding exclusively via hot direct replacement with little selectivity. In the liquid phase, dilution experiments reveal that two processes contribute to hydrogen substitution: a one-step reaction being first order with respect to the concentration of the aromatic substrate and a multistep process involving at least two substrate molecules. In either case the CI atoms exhibit slight electrophilic features. Application of the Hammett correlation reveals a higher selectivity (pf = -1.43) for the multistep than for the one step process (p+ = -0.56). Both chlorination reactions seem to proceed via complex formation. In the case of CI-for-X substitution, thermoneutral or exothermic processes (X = CI, Br, I, NOl) can be attributed to thermal a-complex formation, while endothermic processes (X = F, OCH3, NH2) are typically low yield, direct hot replacement reactions being first order with respect to the substrate concentration.

Hot Atom Chemistry of 18 F in Liquid Fluorobenzenes

“…On the basis of the above considerations we conclude to a hot process for the 18 Ffor-F-replacement in the compounds studied. This conclusion is in agreement with that of others for similar systems [7,11,16]. F-atom of 1.6% to 2.6% [17].…”

“…In all probability this equality is also pointing at a hot one-step mechanism for aromatic F-replacement. This conclusion is in agreement with that given by BEREI and STÖCKLIN [16] for the replacement by hot 38 Cl-atoms of halogen atoms in I 2 scavenged systems and with that of COENEN et al [11 ] for the replacement by hot 38 Cl-atoms of atoms and groups in endothermic reactions (-F, -NH 2 , -OCH 3 ), but not completely with that of BEREI and VASÁROS [7],…”

“…38 Cl-for-H-replacement has been explained by two processes: a one-step direct addition and a multistep process, for which at least one other substrate molecule is needed. The latter process is more selective with respect to the position of substitution than the former process [11].…”

“…In gaseous C 6 H 5 F or C 6 H S CF 3 thermalized 18 F-atoms do replace hydrogen atoms, but not fluorine or the CF 3 group as a whole [10]. Replacement of H and F by non-thermalized 38 CI in gaseous C 6 H S F has been explained by a hot "single" substitution mechanism [11]. Reactions in the liquid phase are often more complex.…”

“…Halogen or group replacement by 38 CI has been explained by a direct hot replacement in endothermic reactions, while for exothermic or thermoneutral reactions σ-complex formation has been invoked as the main pathway [11]. However, hydrogen abstraction (from -NH 2 , -OCH 3 ) and halogen abstraction (Br, I) can be competing reactions [11]. 38 Cl-for-H-replacement has been explained by two processes: a one-step direct addition and a multistep process, for which at least one other substrate molecule is needed.…”