Role of Hydrogen Abstraction Acetylene Addition Mechanisms in the Formation of Chlorinated Naphthalenes. 1. A Quantum Chemical Investigation

Abstract

The addition of chloroacetylene or tetrachlorovinylacetylene to 2,4,5-trichlorophenyl radicals, leading to the formation of tetra-, penta-, and hexachloronaphthalene congeners, has been explored at the M06-2X/6-311+G(3df,3p)//B3LYP/6-31G(d) level of theory. The accuracy of this method was justified by comparing the barriers of several pertinent reactions against energies from single point calculations at the B3LYP/cc-pVDZ, CCSD(T)/6-31G(d), and G2MS levels. Bittner-Howard and Frenklach hydrogen abstraction acetylene addition mechanisms were developed, as was a channel based on acetylene additions to chlorinated [4.2.0]octa-1,3,5-trien-7-yl congeners. While the latter channel exhibits relatively high C2HCl addition barriers and may be a minor growth channel at best, both the Bittner-Howard and Frenklach sequences appear facile. In all mechanisms, the additions of C2HCl leading to a β-chlorinated adduct is favored by ∼15 kJ mol(-1) relative to the α-chlorinated analogue, and the addition products typically access a variety of facile cyclization channels. The α-chlorinated product of C2HCl addition to 2,4,5-trichlorophenyl, however, undergoes a particularly rapid Cl-loss leading to 1-ethynyl-2,4,5-trichlorobenzene, effectively shutting down further growth. Generalization implies that α-chlorinated C6H5-CH═CH congeners do not participate in growth reactions. Addition of 2,4,5-trichlorophenyl to the C≡C bond of tetrachlorovinylacetylene and subsequent cyclization is found to be a facile route to hexachloronaphthalene formation and may be operative in fully chlorinated systems where the C6Cl5-CCl═CCl congeners cannot participate in the major growth processes.

Role of Hydrogen Abstraction Acetylene Addition Mechanisms in the Formation of Chlorinated Naphthalenes. 2. Kinetic Modeling and the Detailed Mechanism of Ring Closure

“…The chlorinated 2-vinylphenyl radicals to which the second acetylene adds are typically found to be higher in energy than the 2-phenylvinyl radicals which serve as the base in Bittner−Howard routes, but far less susceptible to the stabilizing H-or Cl-loss reactions that shut down naphthalene formation. Similarly, our previous results also indicate that Cl-loss at the ring closure site should exhibit appreciably lower dissociation energies than any 37 ). The prediction of 1,2,3,5,7,8-hexachloronaphthalene as the major product is, however, contrary to observation, which suggests that Frenklach-based cyclization channels are only minor contributors at best.…”

“…Early studies suggested dissociation barriers of 123.0 kJ mol −1 from C 8 Cl 7 , a downward revision of a mere ∼30 kJ mol −1 from analogous values utilized in nonchlorinated systems. 51,52 However, in part 1 37 we noted a downward revision of ∼70 kJ mol −1 was perhaps more realistic.…”

“…70 Rate constants have been estimated within the framework of transition-state theory. All energetic and structural data were obtained from M06-2X/6-311+G(3df,3p)//B3LYP/6-31G(d) level calculations (see part 1 37 ). The generalized transition-state theory rate constant is given by eq 1: 71…”

Quantum Chemical and Kinetic Study on Polychlorinated Naphthalene Formation from 3-Chlorophenol Precursor

“…For example, in Kim’s experiment, 2-CP produced mostly 1-MCN and 1,5/1,6/1,7-DCNs; 4-CP produced mostly 2-MCN and 2,6/2,7-DCNs; 3-CP produced nearely equivalent 1-MCN and 2-MCN, and nearly equal 1,5/1,6/1,7-DCNs and 2,6/2,7-DCNs [ 32 , 33 ]. In addition, this hypothesis was, again, denied by some experimental observations in thermal processes that the correlation between PCN and PCDF isomer distributions or mass concentrations is more closer than that between PCN and PCDD, which indicate a new PCN formation mechanism more similar with PCDF formation than PCDD [ 30 , 31 , 42 , 43 , 44 , 45 , 46 ]. In this situation, Kim proposed an alternative PCN formation mechanism based on experimental results [ 32 , 33 , 34 ]; in his scheme, PCNs are formed via carbon-carbon coupling at ortho -sites of CPR pairs, resulting in an intermediate chlorinated o,oʹ-dihydroxybiphenyl (chloro-DOHB) [ 32 , 33 , 34 ].…”

Homogeneous gas-phase formation of polychlorinated naphthalene from dimerization of 4-chlorophenoxy radicals and cross-condensation of phenoxy radical with 4-chlorophenoxy radical: Mechanism and kinetics study

“…The most direct route to the formation of PCNs is the gasphase reaction of chemical precursors, including unsubstituted naphthalene [12,13], polycyclic aromatic hydrocarbon (PAHs) [14,15], chlorophenyl radicals [16,17] and chlorophenols (CPs) [6][7][8][9][18][19][20]. Among the variety of precursors, CPs are the most direct precursors of PCNs along with PCDD/Fs and the most common aromatic compounds found in incinerator gas emissions [21][22][23].…”