Kinetics of the Reactions of OH‐Radicals with Benzene, Benzene‐d₆ and Naphthalene

Abstract

Absolute rate constants for the reactions of hydroxyl radicals with benzene (1), benzene‐d6 (2), and naphthalene (3) have been obtained using excimer laser photolysis to generate OH and time resolved resonance fluorescence as its monitor. Reaction (1) was studied at total pressures between 1.5 and 112 mbar at 298 K using Ar as a diluent gas. k1 is found to increase with pressure up to –40 mbar, but to be essentially pressure independent at still higher pressures, k 1∞ (298 K) = (7.0 ± 1.5) 1011 cm3/mol · s. The temperature variation of k1 as studied over the range 244 – 24 K. At temperatures below 330 K normal Arrhenius behaviour with k1 (T) = (3.8 ± 1.0) 1012 exp[‐(500 ± 50) K/T] cm3/mol · s is observed, corresponding to the addition reaction (1) OH + C6H6 → C6H6OH. Between 330 and 420 K a sharp decrease of k1 with increasing temperature is noted, which we attribute to reaction (1) becoming reversible. For the stabilization energy of C6H6OH we derive (79 ± 6) kJ/mol. Above 420 K a transition of k1 to a positive temperature dependence, corresponding to the abstraction reaction (1') OH + C6H6 → H2O + C6H5 is noted. Rate constants for the reaction of OH with benzene‐d6 (2) are essentially identical to those of reaction (1) except for temperatures above 420 K, where k2 falls considerably below k1. This isotope effect is a confirmation of the (1') abstraction route. Reaction (3) was studied at total pressures between 6 and 128 mbar (He) where it is found to be essentially pressure independent. Its temperature variation was studied between 337 and 525 K. Over this region the rate coefficient decreases from (8.8 ± 3.5) 1012 cm3/mol · s to (6.6 ± 2.6) 1011 cm3/mol · s, with the decrease with temperature being relatively weak up to 410 K, k1(T ≤ 410 K) = (2.2 ± 1.0) 1012 exp[(440 ± 50) K/T] cm3/mol · s, and stronger at still higher temperatures. This behaviour is interpreted as to correspond to the addition reaction (3) OH + C10H8 → C10H8OH and its equilibration. The stabilization energy of the OH‐naphthalene adduct is estimated to be (95 ± 6) kJ/mol.

Kinetic H/D isotope effects for gas phase hydroxylation of benzene and chlorobenzene between 520–1080 K. Hydroxyl radical versus O(3P) atom attack

“…In air (chloro)benzene, by itself, did not measurably react below 750 K, but in the presence of small proportions of .OH radical forming agents, including aliphatic hydrocarbons such as cyclohexane [6] or cyclohexadiene 171 undergoing slow combustion, phenolic products were obtained from a variety of benzene derivatives. Substitution patterns of ZC,H,OH from C&Z, as well as substrate selectivities were found to vary with temperature, clearly indicating a mechanistic change, under our conditions, around 650 K. A t increasing temperature, phenol formation appears to shift from a .OH addition pathway to one involving H-abstraction by .OH, a finding consonant with those based on -OH reaction rates with benzene [8,9]. Thermokinetic analysis [6] supported this interpretation and it was also shown that, at still higher temperatures, addition of O(3P) to arene could increase in importance as a pathway to phenol.…”

“…Due to its reversibility [8,9], addition (step 3) cannot effectively compete with reactions (1) and (21, (Scheme 1).…”

Reactions of naphthalene in N2O5NO3NO2 air mixtures

“…10. stant for the analogous decomposition of the OH-naphthalene adduct Alternatively, the rate constant k-3 can be estimated from the rate conand the thermochemistries of reactions (-3) and (6). Assuming that the resonance stabilization energy of the Nos-aromatic adduct is identical to that for the OH-aromatic adduct, the NO3-aromatic adduct is 10.9 kcal mol-' less stable towards thermal decomposition than is the OH-aromatic adduct [42] observed that the OH-naphthalene adduct was thermally more stable than the hydroxycyclohexadienyl radical by 3.8 kcal mo1-l (assuming a n identical pre-exponential factor), then the decomposition rate of the OH- The observation of Lorenz and Zellner [42] that the OH-naphthalene adduct is more stable towards thermal decomposition than the OH-benzene adduct by 3.8 kcal mol-' suggests that the NO3-naphthalene-adduct may also be correspondingly more stable towards thermal decomposition than the NO3-benzene adduct. If so, the rate constant for thermal decomposition of the NO3-benzene adduct is ca.…”

Fullerenes and Soot Formation— New Pathways to Large Particles in Flames

“…[37] The reactions of O and OH with benzene and substituted benzenes have also been studied. [38] The presence of partially oxidized higher aromatic species in fuelrich flames has also led to knowledge of the first intermediates of oxidative attack. [31] This leads to the following general picture, in particular with respect to the effects on the growth reactions.…”