Peroxyacetyl nitrate
Peroxyacetyl nitrate (PAN), chemically denoted as CH₃C(O)OONO₂ or C₂H₃NO₅, is a highly oxygenated, unstable organic nitrate compound that serves as a key intermediate in atmospheric photochemistry.[1][2] Formed primarily through the reaction of nitrogen dioxide (NO₂) with peroxyacetyl radicals derived from the oxidation of volatile organic compounds (VOCs) such as acetaldehyde, methylglyoxal, and products of isoprene and terpenes, PAN acts as a temporary reservoir for reactive nitrogen oxides (NOx), facilitating their long-range transport in the troposphere.[3][2] With a molar mass of 121.05 g/mol and CAS number 2278-22-0, PAN exists as a colorless gas or unstable liquid that decomposes thermally, especially in warmer conditions, releasing NOx and contributing to the formation of tropospheric ozone—a major air pollutant.[1][3] Its sources are predominantly anthropogenic in urban areas during non-growing seasons and biogenic from vegetation like isoprene emissions, with global production dominated by acetaldehyde (44%) and methylglyoxal (30%) precursors.[3] As a component of photochemical smog, PAN exhibits phytotoxic effects on plants, inhibiting photosynthesis and causing visible leaf damage, while posing risks to human health as an inhalation toxin and eye irritant, with concentrations typically ranging from 0.3 to 2 ppbv in polluted regions.[2][1] In colder environments, PAN's stability enhances its role in promoting ozone buildup by sequestering NOx until decomposition, whereas in warmer climates, it acts as a sink, modulating radical cycles and oxidant levels.[2][3]Chemical Properties
Molecular Structure
Peroxyacetyl nitrate (PAN) has the molecular formula C₂H₃NO₅ and is structurally represented as CH₃C(O)OONO₂.[4][5] This molecule consists of an acetyl group (CH₃CO–) linked to a peroxy bridge (–O–O–) that is bonded to a nitrate group (–ONO₂), forming the characteristic peroxycarboxyl nitrate functional group responsible for its role as a reactive nitrogen reservoir in the atmosphere.[6] A key feature of PAN's structure is the weak O–N bond in the peroxy nitrate linkage, with a bond dissociation enthalpy of approximately 35 kcal/mol, which contributes to the compound's thermal instability and susceptibility to unimolecular decomposition.[7] Although computational studies have identified up to 11 possible isomeric structures for PAN, including variations in the peroxy-nitrate connectivity, the dominant form observed in atmospheric conditions is the specific CH₃C(O)OONO₂ isomer.[6]Physical and Thermal Properties
Peroxyacetyl nitrate (PAN) appears as an unstable, colorless gas at room temperature, though it can be condensed to a liquid under certain conditions.[5] Its molecular formula is C₂H₃NO₅, corresponding to a molecular weight of 121.05 g/mol.[5] Due to its inherent thermal instability, PAN decomposes prior to boiling, with an extrapolated boiling point of approximately 106 °C based on vapor pressure measurements up to 291 K.[5] The vapor pressure at 298 K is 29.2 mm Hg, indicating relatively high volatility under ambient conditions.[5] PAN is moderately soluble in water, with a Henry's law constant of approximately 5 M/atm in acidic aqueous solutions at 298 K, and it also dissolves in organic solvents such as dichloromethane, pentane, and benzene.[8][9] The thermal decomposition of PAN proceeds via a first-order unimolecular reaction, primarily yielding nitrogen dioxide (NO₂) and the acetyl peroxy radical (CH₃C(O)OO•), with the rate constant expressed as k = 4.9 \times 10^{16} \exp\left(-\frac{27.5 \, \text{kcal mol}^{-1}}{RT}\right) s⁻¹ over the temperature range 283–313 K.[10] This process underscores PAN's sensitivity to temperature, limiting its stability in warmer environments. Spectroscopically, PAN exhibits characteristic infrared absorption bands useful for its detection and quantification, including a prominent peroxy nitrate (–OONO₂) stretch at 1842 cm⁻¹ with a peak cross-section of 0.74 × 10⁻¹⁸ cm² molecule⁻¹ at 295 K, alongside bands at 794 cm⁻¹ (C–O stretch), 1163 cm⁻¹ (C–O–O asymmetric stretch), 1302 cm⁻¹ (NO₂ symmetric stretch), and 1741 cm⁻¹ (C=O stretch).[11] In the ultraviolet region, PAN shows absorption cross sections that decrease with wavelength and temperature, enabling atmospheric monitoring via UV spectroscopy with significant bands between 200–300 nm.[12]Synthesis and Production
Laboratory Synthesis
Peroxyacetyl nitrate (PAN) was first prepared in the laboratory in 1956 by Edgar R. Stephens and colleagues through the photolysis of mixtures containing nitrogen oxides (NOx) and olefins in air, where PAN emerged as a key byproduct identified via infrared spectroscopy.[13] This method marked the initial isolation and characterization of PAN, enabling further studies on its role in photochemical reactions. A subsequent routine synthesis, developed in the early 1960s, involved the photolysis of ethyl nitrite vapor in dry oxygen within a Pyrex reactor under ultraviolet blacklight lamps, producing PAN through sequential radical reactions initiated by the nitrite decomposition.[14] This approach yielded approximately 30% PAN based on the ethyl nitrite consumed, with the product collected as a gas and stored under controlled conditions to prevent decomposition.[14] A standard laboratory method for PAN synthesis entails the reaction of acetaldehyde with nitrogen dioxide in the presence of oxygen, often conducted in the dark using dinitrogen pentoxide (N₂O₅) as a source of NO₃ radicals:\ce{CH3CHO + NO2 + O2 -> CH3C(O)OONO2}
This multi-step process generates the peroxyacetyl radical intermediate, which combines with NO₂ to form PAN, and has been detailed in mechanistic studies confirming high efficiency under controlled gas-phase conditions without photochemical initiation.[15] Alternative routes include the photochemical oxidation of acetone under ultraviolet light in the presence of NOx, such as the photolysis of acetone in synthetic air or mixtures with nitric oxide, which produces PAN via acetyl peroxy radical formation and subsequent nitration.[16] Another variant employs the nitration of peracetic acid (1.2 M) with NO₂, followed by extraction into n-heptane.[17] Purification of PAN typically involves low-temperature trapping in cold solvents or cryogenic distillation to isolate it from byproducts like acetaldehyde, acetic acid, and nitrogen oxides, leveraging its volatility and thermal lability.[14] Preparative gas chromatography with columns such as carbowax on firebrick is commonly used for final separation, eluting PAN after impurities under helium flow.[14] Normal-phase high-performance liquid chromatography provides an additional refinement step for solvent-extracted samples, yielding PAN free of acetyl nitrate contaminants.[17] Due to PAN's volatility and thermal instability, these techniques are performed at sub-zero temperatures to maintain stability.[13] Laboratory yields for PAN synthesis generally range from 20% to 50%, depending on the method and scale, though optimization is limited by side reactions and precursor purity.[14] Key challenges include PAN's inherent explosivity, with reports of detonations during condensation or mechanical shock, necessitating the use of explosion shields, remote handling, and avoidance of compression or heating during purification and storage.[13] These risks, combined with its sensitivity to trace impurities, demand rigorous safety protocols in controlled environments to ensure reproducible production for atmospheric and toxicity studies.[13]