BUTANE

Note: According to Chevron Phillips Company’s (2004) product literature, 99.5 wt % n-butane contains the following compounds: propane (0.1 wt %), 2-methylpentane (0.1 wt %), 2-methylbutane (0.2 wt %), and n-pentane (0.1 wt %). CASRN: 106-97-8; DOT: 1011; DOT label: Flammable gas; molecular formula: C4H10; FW: 58.12; RTECS: EJ4200000; Merck Index: 12, 1541 Physical state, color, and odor: Colorless, flammable gas with a faint, disagreeable, natural gas or gasoline-like odor. Odor threshold concentration in air is 1,200 ppmv (Nagata and Takeuchi, 1990). Detected in water at a concentration of 6.2 mg/L (Bingham et al., 2001). Melting point (°C): -135.0 (Stull, 1947) Boiling point (°C): -0.5 (Weast, 1986) Density (g/cm3): 0.6012 at 0 °C (Weast, 1986) 0.5790 at 20.00 °C (Dahloff et al., 2000) Diffusivity in water (x 10-5 cm2/sec): 0.89 at 20 °C (Witherspoon and Bonoli, 1969) 0.97 at 25 °C (quoted, Hayduk and Laudie, 1974) Dissociation constant, pKa: >14 (Schwarzenbach et al., 1993) Flash point (°C): -72 (Kuchta et al., 1968) -76 (NFPA, 1984) Lower explosive limit (%): 1.6 (NFPA, 1984) Upper explosive limit (%): 8.4 (NFPA, 1984) Heat of fusion (kcal/mol): 1.050 (Parks and Huffman, 1931) Henry’s law constant (atm⋅m3/mol): 0.356 at 5 °C, 0.454 at 10 °C, 0.568 at 15 °C, 0.695 at 20 °C, 0.835 at 25 °C (Ben-Naim et al.,

65 (estimated, CHRIS, 1984) Ionization potential (eV): 10.63 ± 0.03 (Franklin et al., 1969) Soil organic carbon/water partition coefficient, log Koc: Unavailable because experimental methods for estimation of this parameter for aliphatic hydrocarbons are lacking in the documented literature Octanol/water partition coefficient, log Kow: 2.89 (Hansch and Leo, 1979) Solubility in organics: At 17 °C (mL/L): chloroform (25,000), ether (30,000) (Windholz et al., 1983). At 10 °C (mole fraction): acetone (0.2276), aniline (0.04886), benzene (0.5904), 2-butanone (0.3885), cyclohexane (0.6712), ethanol (0.1647), methanol (0.04457), 1-propanol (0.2346), 1-butanol (0.2817). At 25 °C (mole fraction): acetone (0.1108), aniline (0.03241), benzene (0.2851), 2butanone (0.1824), cyclohexane (0.3962), ethanol (0.07825), methanol (0.03763), 1-propanol (0.1138), 1-butanol (0.1401) (Miyano and Hayduk, 1986). Mole fraction solubility in 1-butanol: 0.140, 0.0692, and 0.0397 at 25, 30, and 70 °C, respectively; in chlorobenzene: 0.274, 0.129, and 0.0800 at 25, 30, and 70 °C, respectively, and in octane: 0.423, 0.233, and 0.152 at 25, 30, and 70 °C, respectively (Hayduk et al., 1988). Mole fraction solubility in 1-butanol: 0.139 and 0.0725 at 25 and 70 °C, respectively; in chlorobenzene: 0.269 and 0.131 at 25 and 70 °C, respectively; and in carbon tetrachloride: 0.167 at 70 °C (Blais and Hayduk, 1983). Solubility in water: 61.4 mg/kg at 25 °C (shake flask-GC, McAuliffe, 1963, 1966) At 0 °C, 0.0327 and 0.0233 volume of gas dissolved in a unit volume of water at 19.8 and 29.8 °C,

respectively (Claussen and Polglase, 1952) 7 mM at 17 °C and 772 mmHg (Fischer and Ehrenberg, 1948) 1.09 mM at 25 °C (shake flask-GC, Barone et al., 1966) 3.21, 1.26, and 0.66 mM at 4, 25, and 50 °C, respectively (Kresheck et al., 1965) Vapor density: 2.38 g/L at 25 °C, 2.046 (air = 1) Vapor pressure (mmHg): 934.1 at 5.00 °C, 1,827.2 at 25.00 °C (equilibrium cell, Flebbe et al., 1982) 1,556 at 20.00 °C (Dahloff et al., 2000) Environmental fate: Biological. In the presence of methane, Pseudomonas methanica degraded butane to 1-butanol, methyl ethyl ether, butyric acid, and 2-butanone (Leadbetter and Foster, 1959). 2-Butanone was also reported as a degradation product of butane by the microorganism Mycobacterium smegmatis (Riser-Roberts, 1992). Butane may biodegrade in two ways. The first is the formation of butyl hydroperoxide which decomposes to 1-butanol followed by oxidation to butyric acid. The other pathway involves dehydrogenation yielding 1-butene, which may react with water forming 1butanol (Dugan, 1972). Microorganisms can oxidize alkanes under aerobic conditions (Singer and Finnerty, 1984). The most common degradative pathway involves the oxidation of the terminal

dehydrogenation steps forming butanal followed by oxidation forming butyric acid. The fatty acid may then be metabolized by β-oxidation to form the mineralization products, carbon dioxide, and water (Singer and Finnerty, 1984). Photolytic. Major products reported from the photooxidation of butane with nitrogen oxides under atmospheric conditions were acetaldehyde, formaldehyde, and 2-butanone. Minor products included peroxyacyl nitrates and methyl, ethyl and propyl nitrates, carbon monoxide, and carbon dioxide. Biacetyl, tert-butyl nitrate, ethanol, and acetone were reported as trace products (Altshuller, 1983; Bufalini et al., 1971). The amount of sec-butyl nitrate formed was about twice that of n-butyl nitrate. 2-Butanone was the major photooxidation product with a yield of 37% (Evmorfopoulos and Glavas, 1998). Irradiation of butane in the presence of chlorine yielded carbon monoxide, carbon dioxide, hydroperoxides, peroxyacid, and other carbonyl compounds (Hanst and Gay, 1983). Nitrous acid vapor and butane in a “smog chamber” were irradiated with UV light. Major oxidation products identified included 2-butanone, acetaldehyde, and butanal. Minor products included peroxyacetyl nitrate, methyl nitrate, and unidentified compounds (Cox et al., 1981). The rate constant for the reaction of butane and OH radicals in the atmosphere at 300 K is 1.6 x 10-12 cm3/molecule⋅sec (Hendry and Kenley, 1979). Based upon a photooxidation rate constant of 2.54 x 10-12 cm3/molecule⋅sec with OH radicals in summer daylight, the atmospheric lifetime is 54 h (Altshuller, 1991). At atmospheric pressure and 298 K, Darnall et al. (1978) reported a rate constant of 2.35-4.22 x 10-12 cm3/molecule⋅sec for the same reaction. A rate constant of 1.28 x 10-11 L/molecule⋅sec was reported for the reaction of butane with OH radicals in air at 298 K, respectively (Greiner, 1970). At 296 K, a rate constant of 6.5 x 10-17 cm3/molecule⋅sec was reported for the reaction of butane with NO3 (Atkinson, 1990). Chemical/Physical. Complete combustion in air produces carbon dioxide and water. Butane will not hydrolyze because it has no hydrolyzable functional group. Exposure limits: NIOSH REL: TWA 800 ppm (1,900 mg/m3); ACGIH TLV: TWA 800 ppm (adopted). Symptoms of exposure: High concentrations may cause narcosis (Patnaik, 1992). Toxicity: LC50 (inhalation) for mice 680 gm/m3/2-h, rats 658 gm/m3/4-h (quoted, RTECS, 1985). Source: Present in gasoline ranging from 4.31 to 5.02 vol % (quoted, Verschueren, 1983). Harley et al. (2000) analyzed the headspace vapors of three grades of unleaded gasoline where ethanol was added to replace methyl tert-butyl ether. The gasoline vapor concentrations of butane in the headspace were 7.4 wt % for regular grade, 6.9 wt % for mid-grade, and 6.3 wt % for premium grade. Schauer et al. (1999) reported butane in a diesel-powered medium-duty truck exhaust at an emission rate of 3,830 µg/km. Schauer et al. (2001) measured organic compound emission rates for volatile organic compounds, gas-phase semi-volatile organic compounds, and particle-phase organic compounds from the residential (fireplace) combustion of pine, oak, and eucalyptus. The gas-phase emission rate of butane was 25.9 mg/kg of pine burned. Emission rates of butane were not measured during the combustion of oak and eucalyptus. California Phase II reformulated gasoline contained butane at a concentration of 7,620 mg/kg. Gas-phase tailpipe emission rates from gasoline-powered automobiles with and without catalytic converters were 1,620 and 191,000 µg/km, respectively (Schauer et al., 2002). Reported as an impurity (0.4 wt %) in 99.4 wt % trans-2-butene (Chevron Phillips, 2004).