Environ. Sci. Technol. 1986, 20, 992-997
Reaction Products and Rates of Disappearance of Simple Bromoalkanes, 1,2-Dlbromopropane, and 1,2-Dibromoethane in Water Timothy M. Vogei and Martin Reinhard” Environmental Engineering and Science, Civil Engineering Department, Stanford University, Stanford, California 94305
rn The reactivities of 1,2-dibromoethane (EDB), 1,2- and 1,3-dibromopropane (1,2-DBP and 1,3-DBP), l-bromoheptane (BH), and 1-bromo-3-phenylpropane (BPP) were studied in aqueous buffers in the pH range 7-11 and in the temperature range 45-90 “C. The half-lives at 25 OC were estimated by using the Arrhenius relationship: EDB (2.5 years); 1,2-DPB(320 days); BPP (290 days); 1,3-DBP (48 days); BH (=36 days). The dibromoalkanes with vicinal bromides, EDB and 1,2-DBP, eliminated HBr and yielded significant amounts of vinyl bromide and bromopropenes, respectively. The monobromo- and 1,3-dibromoalkanes (BH, BPP, and 1,3-DBP) hydrolyzed to alcohols with no elimination products detected. Reactivities of 16 bromoalkanes are correlated with Taft’s polar substitution constant (r = 0.88). Introduction Widespread contamination of surface water and groundwater with halogenated aliphatics has spurred investigations to identify chemical processes that remove these compounds from the environment (1-4). Bromoalkanes found in groundwaters include intermediates for chemical synthesis (4), pesticides such as EDB (5) and 1,2-dibromo-3-chloropropropane (DBCP) (6),and byproducts formed during water chlorination (7). This study reports on the reactivity of some mono- and dibromoalkanes in aqueous buffers. Hydrolysis and elimination reactions were considered. Reaction products and rates of compound disappearance were determined for EDB, 1,2-dibromopropane (1,2-DBP), 1,3-dibromopropane (1,3-DBP), and l-bromo-3-phenylpropane (BPP), and l-bromo-n-heptane (BH). The available data on the reactivity of alkyl bromides in water were analyzed with respect to Taft’s linear free energy relationship (LFER) (8-12).
Hydrolysis and Dehydrobromination Reactions of Brominated Aliphatics in Water. Alkyl halides may react with water (12). Mechanisms, rates, and product formation are strongly dependent on structural factors such as branching of the alkyl group and the type of halide substituent, on the polarity of the solvent, on the types of nucleophiles and bases present, and on temperature (1-4, 12). Several possible hydrolysis and dehydrobromination reactions of mono- and dibromoalkanes can occur (Figure 1). Hydrolysis of alkyl bromide (I) refers to the nucleophilic substitution of Br with a hydroxyl group in an aqueous medium to yield the corresponding alcohol (11). Dehydrobromination refers to the elimination of HBr to yield alkenes (3, 12). Elimination and hydrolysis reactions may compete. For example, a 1,2-dibromoalkane (111) may hydrolyze to monohydric (IV, V) and/or dihydric (VI) alcohols. Alternately, I11 may eliminate HBr to yield three different alkene isomers (VII, VIII, IX). Hydrolysis of 1,3-dibromoalkane (X) yields alcohols XI or XI1 when the 1-or the 3-bromine hydrolyzes, respectively, or the dihydric alcohol XI11 when both bromines hydrolyze. Eliminations tend to be favored over hydrolysis if there are electronwithdrawing groups in the P-position to the leaving group, 992
Environ. Sci. Technol., Vol. 20, No. 10, 1986
or if there is &branching (12). Thus, formation of 3bromo-l-propene from 1,2-DBP and dehydrobromination of X (not shown in Figure 1) are likely to be only minor reactions, because the 3- and the 2-carbons, respectively, do not carry electronegative substituents. Other factors that affect the elimination vs. hydrolysis rates are the type of leaving group, temperature, and solvent (12). Haloaliphatics in water exhibit a wide range of reactivities (13). For example, at 25 “C and pH 7 , estimated half-lives vary from 23 s for tert-butyl chloride to 1 year for fluorochloroiodomethane, 30 years for methyl fluoride, and to greater than 1000 years for tetrachloro- and trichloromethane (1). Rate Laws of Hydrolysis and Dehydrobromination Reactions. The reported observed rate law for hydrolysis (1) and/or elimination (3) reactions of halogenated aliphatics may have a term for the base-promoted reaction with the hydroxide ion, and one for the “neutral”reaction with water: -d[Cl/dt = k,bsd[C] = k,(OH-)[C]
+ k~[Cl
(1)
where kobsd is the observed first-order rate constant of the overall hydrolysis reaction, [C] is the molar concentration of the reactant, (OH-) is the activity of the hydroxide ion, kB is the second-order rate constant for the base-promoted reaction, and kN is the pseudo-first-order constant for the neutral process. Simple haloaliphatics (no vicinal halides) undergo hydrolysis independent of pH between pH 3 and pH 11(I). If hydrolysis and elimination reactions compete, or if two bromines hydrolyze, a rate law with multiple terms will apply (13). Temperature Dependence of Hydrolysis and Dehydrobromination Reaction. Hydrolysis and elimination rates are dependent upon temperature (1,3,12) and typically have activation energies (E,) in the range of 100 kJ/mol (1, 3). Rate constants that are slow at ambient temperatures are often measured at elevated temperatures and extrapolated to temperatures in the environmental range by using Arrhenius plots [In k vs. 1 / T (K)], which are generally linear over small temperature ranges, and therefore, constant activation energies are inferred (14). If k(T) is the rate of disappearance due to two competing processes with two different E, values, the product ratio changes as a function of temperature (14). Hughes, Ingold, and co-workers (15, cited in 12) report that elimination, in alcoholic solutions, is favored over substitution by increasing temperature. This suggests that activation energies of elimination reactions might be higher than those of hydrolysis reactions. Therefore, elimination and hydrolysis reactions would yield different relative amounts of products at different temperatures. Estimation af Reaction Rates from the Taft Correlation. Taft has developed an LFER for aliphatic compounds on the basis of the assumption that steric and electronic (inductive) effects can be separated: log (ki/kJ =
UI’
+ 6E,
(2)
where hi and k, are rate (or equilibrium) constants of compound i and a reference compound, respectively, UI is
0013-936X/86/0920-0992$01.50/0
0 1986 American Chemical Society
f - Brornoalkones 1
R-$H2-Br
R = ri,alhy, ll
k'
h_ R-FH,-OH
n!
1,2-Dibrornoalkanes
IE Br l
Br l
R-$H-FHz
-
m
