Atmospheric chemistry of alcohols

Journal of the Brazilian Chemical Society

DGA, USA¹  Grosjean, Daniel¹ 

关键词: alcohols;    atmospheric chemistry;    kinetics;    reaction mechanisms;   

DOI  :  10.1590/S0103-50531997000500002

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

It has become increasingly important to understand the atmospheric chemistry of alcohols in the context of urban and regional air quality. Alcohols, saturated and unsaturated, are emitted into the atmosphere by vegetation1-3. These biogenic emissions play an important role in the chemistry of the troposphere, especially in rainforest ecosystems such as the Amazon1. Saturated alcohols have long been used in large quantities as industrial solvents. Saturated alcohols have also been used as motor vehicle fuels and fuel additives. Ethanol, which has been used as a fuel since the invention of the internal combustion engine (N. A. Otto used ethanol in his classical combustion engine tests in 1897), is now a major component of light-duty vehicle fuels in many countries. In Brazil, which is the only country in the world where a nationwide, large-scale alcohol fuel program has been implemented, approximately 4 million automobiles run on neat ethanol and approximately 9 million automobiles run on an ethanol-gasoline mixture that contains a large fraction of ethanol, ca. 22 percent. In the United States, the 1990 Amendments to the Clean Air Act required that oxygenated compounds be added to gasoline in urban areas that did not meet national ambient air quality standards for carbon monoxide (CO) and for ozone (O3). This has generally been achieved by blending about 7 percent ethanol or about 15 percent of the ether methyl-t-butyl ether (MTBE) with gasoline. Other alcohols including methanol and t-butyl alcohol (TBA) and other ethers including ethyl-t-butyl ether (ETBE) have been used for many years or are under consideration as oxygenated fuels and fuel additives. Not surprisingly when considerable monetary investments and profits are at stake, scientific issues have not played a major role in the energy policy decisions that have been made regarding alcohols and other oxygenated fuels. Perceived benefits and possible drawbacks of oxygenated fuels will undoubtedly continue to be controversial topics in future policy debates4. It is not our intent here to discuss the merits and disadvantages of alcohol fuels. The purpose of this article is to review the atmospheric chemistry of alcohols with focus on the following objectives:  to compile available kinetic data that can be used to calculate the persistence of alcohols in the atmosphere.  to review data from laboratory studies of reaction products, and to outline the mechanisms of the photochemical oxidation of alcohols in the atmosphere.  to identify knowledge gaps and, when appropriate, to make suggestions for future research. In this article, we will not consider the atmospheric chemistry of unsaturated alcohols. These compounds, which are important biogenic emissions1-3, are hydroxy-substituted alkenes (e.g. allyl alcohol, CH2=CHCH2OH; cis-3-hexen-1-ol (leaf alcohol) CH3CH2CH=CHCH2 CH2OH). The atmospheric oxidation of unsaturated alcohols is similar to that of alkenes, i.e. the important reactions involve the unsaturated carbon-carbon bond and not the alcohol functional group5,6. This review will focus on the atmospheric chemistry of saturated alcohols (primary: RCH2OH; secondary: R1CHOHR2; tertiary: R1R2R3COH, where R = alkyl groups). The atmospheric chemistry of ethers, which along with alcohols constitute an important group of oxygenated fuels, has been reviewed elsewhere7.   Kinetic Data for the OH-Alcohol Reaction Examination of available kinetic data8 indicates that the following reactions of alcohols are slow and are of negligible importance in the atmosphere: photolysis, reaction with ozone, and reaction with the nitrate radical. The only chemical process by which alcohols are removed from the atmosphere is their reaction with the hydroxyl radical (OH).Reaction rate constantsRate constants for the gas phase reaction of alcohols with OH at ambient temperature are compiled in Table 1. Kinetic data published prior to ca. 1985 have been taken from the 1986 review of Atkinson9. Data published during the last decade (1986-1996) are from the original references10-18, which are listed in Table 1 for each alcohol studied. The kinetic data listed in Table 1 include OH reaction rate constants for thirty three alcohols, of which nineteen are C1-C8 monofunctional alcohols (primary, secondary and tertiary) and fourteen are difunctional alcohols including diols, hydroxy ethers, and hydroxycarbonyls.   For all but two of the alcohols listed, reaction with OH involves H-atom abstraction from a C-H bond (major) or from the O-H bond (minor and often negligible) as is discussed in more detail below. For allyl alcohol and hydroxyacetaldehyde, reaction with OH involves different mechanisms, i.e. OH addition at the C=C bond for allyl alcohol5,6 and mostly (ca. 78%) H-atom abstraction from the carbonyl carbon for hydroxyacetaldehyde15. For four of the alcohols listed in Table 1, the OH-alcohol reaction rate constant has been measured over a range of temperatures and the corresponding Arrhenius equations (k = Ae-E/RT) have been reported11,13. This information is summarized in Table 2 and indicates that, in the range of temperatures relevant to the atmosphere, temperature has only a modest effect on the OH-alcohol reaction rate constants.  Mechanistic implications of kinetic dataThe kinetic data listed in Table 1 indicate that the reaction of saturated alcohols with OH at ambient temperature involves H-atom abstraction from a C-H bond rather than from the O-H bond. This is consistent with theory since the O-H bond (bond strength = 104 kcal mol-1) is stronger than the C-H bond (bond strength = 94 kcal mol -1 in methanol). If the initial step in the OH-alcohol reaction involves H-atom abstraction from C-H bonds, then (a) OH reacts faster with methanol than with methanol-d3 (CD3OH), as observed10 (the C-D bond is stronger than the C-H bond), (b) the OH reaction rate constants for the series CH3(CH2)nOH increases from methanol (n = 0) to 1-octanol (n = 7), as observed,11,12,18 due to the increasing number of secondary C-H bonds, and (c) the reaction of OH with t-butyl alcohol, which contains only primary (and therefore stronger) C-H bonds, is slower, again as observed, than that of OH with 1-butanol and 2-butanol which contain weaker secondary and tertiary C-H bonds. Thus, the reaction of alcohols with OH can be written as follows:and the reaction pathway that involves H-atom abstraction from the O-H bond:is negligible under atmospheric conditions. This important mechanistic aspect of the alcohol-OH reaction is further supported by data for the reaction of alcohols with chlorine atoms. This reaction is expected to be similar to the alcohol-OH reaction since both OH and Cl are electrophiles. Indeed kinetic and product studies of the alcohol-Cl reaction19,20 have shown that H-atom abstraction involves C-H bonds and not the O-H bond:Accordingly, the initial step in the reaction of methanol with OH can be written as follows:where reaction (2) yields the simplest a-hydroxyalkyl radical, i.e. the hydroxymethyl radical CH2OH. For higher molecular weight alcohols, reaction with OH may involve H-atom abstraction from several C-H bonds. In this

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