Alkyl bromides as mechanistic probes of reductive dehalogenation: Reactions of vicinal dibromide stereoisomers with zerovalent metals

Article abstract of DOI:10.1021/es0010195

Whether reductive dehalogenation proceeds via a one- or a two-electron mechanism has been suggested to affect product distributions, hence potentially influencing the success of engineered treatment systems. In this work, we explore vicinal dibromide stereoisomers as "probes" of the concertedness of electron transfer in reduction by aqueous suspensions of iron and zinc metal. Dibromides consisted of 2,3-dibromopentane (diBP) stereoisomers and (±)-1,2-dibromo-1,2-diphenylethane. All dibromides reacted with metals to give the same E:Z ratio of olefins observed during dehalogenation by iodide (a two-electron reductant). Reduction by Cr(II) (a one-electron reductant) yielded distinctly different proportions of E and Z olefins. Although this might be construed as evidence that metals function as two-electron reductants, high stereo-specificity was also obtained for reduction of diBPs by Fe(II) adsorbed to goethite, a presumed one-electron reductant; this can be explained by two single-electron transfers in rapid succession, facilitated by the locally elevated concentration of reducing equivalents at the oxide-water interface. The results suggest that reduction of alkyl halides by metals is not likely to produce free radicals that persist long enough to undergo radical-radical coupling or hydrogen-atom abstraction from minor dissolved constituents. Apparent free-radical coupling products are more likely to result from (possibly surface-bound) organometallic intermediates.

Full text of DOI:10.1021/es0010195

Environ. Sci. Technol. 2001, 35, 2268-2274
hydrogenation of multiple bonds, all involving a net transfer
Alkyl Bromides as Mechanistic
Probes of Reductive
Dehalogenation: Reactions of
Vicinal Dibromide Stereoisomers
with Zerovalent Metals
of two electrons. The principal goal of this research was to
explore one important aspect of the mechanism, namely the
concertedness of electron transfer during metal-promoted
reductive â-elimination.
Theoretically, reduction can proceed either via single
electron transfer (SET, which may occur via outer-sphere or
inner-sphere pathways) or via two-electron pathways, which
must necessarily be inner-sphere (10). Whether reactions
occur via outer- or inner-sphere mechanisms is of enormous
practical significance in designing catalysts or stoichiometric
reagents, as inner-sphere reductions occur much more
rapidly than outer-sphere reactions (11).
L I S A A . T O T T E N , † U R S J A N S , ‡ A N D
A . L YN N R O B E R T S *
Department of Geography and Environmental Engineering,
The Johns Hopkins University, 313 Ames Hall, 3400 North
Charles Street, Baltimore, Maryland 21218
The mechanism not only dictates reactivity; it also
influences selectivity (i.e., product distributions), potentially
affecting the success of treatment approaches. If reduction
occurs via SET, the resulting intermediates might persist long
enough to undergo characteristic free-radical reactions. Some
researchers (12, 13) have accordingly invoked coupling of
free radicals to account for products encountered in orga-
nohalide reactions with zerovalent metals. A better under-
standing of the mechanism through which zerovalent metals
reduce organohalides would explain why 1,1,1-trichloro-
ethane reacts with Fe(0) to generate significant C4 hydro-
carbons, but no such coupling products are observed in
reaction with Zn(0) (14). If free radicals were sufficiently
persistent, they might encounter H-atom donors, forming
hydrogenolysis products; Balko and Tratnyek (15) have, for
example, suggested that abstraction of H• from dissolved
Whether reductive dehalogenation proceeds via a one- or
a two-electron mechanism has been suggested to
affect product distributions, hence potentially influencing
the success of engineered treatment systems. In this work,
we explore vicinal dibromide stereoisomers as “probes”
of the concertedness of electron transfer in reduction by
aqueous suspensions of iron and zinc metal. Dibromides
consisted of 2,3-dibromopentane (diBP) stereoisomers and
(()-1,2-dibromo-1,2-diphenylethane. All dibromides
reacted with metals to give the same E:Z ratio of olefins
observed during dehalogenation by iodide (a two-electron
reductant). Reduction by Cr(II) (a one-electron reductant)
yielded distinctly different proportions of E and Z olefins.
Although this might be construed as evidence that
metals function as two-electron reductants, high stereo-
specificity was also obtained for reduction of diBPs by
Fe(II) adsorbed to goethite, a presumed one-electron
reductant; this can be explained by two single-electron
transfers in rapid succession, facilitated by the locally elevated
concentration of reducing equivalents at the oxide-water
interface. The results suggest that reduction of alkyl halides
by metals is not likely to produce free radicals that
persist long enough to undergo radical-radical coupling
or hydrogen-atom abstraction from minor dissolved
constituents. Apparent free-radical coupling products are
more likely to result from (possibly surface-bound)
organometallic intermediates.
•
organic matter by CCl3 could shift products toward the
relatively undesirable species CHCl3 during CCl4 reduction
by Fe(0). Other researchers have invoked one- versus two-
electron mechanisms to account for selectivity in reductive
dehalogenation. Criddle and McCarty (16) hypothesized
parallel one- and two-electron reactions during electrolytic
reduction of CCl4 to CHCl3 (via •CCl3) and formate (via
dichlorocarbene), respectively, while Buschmann et al. (17)
ascribed the increased yield of haloforms obtained in
reduction of tetrahalomethanes by cysteine to a two-electron
pathway rather than an SET pathway. To date, techniques
designed to confirm the existence of free radicals (and hence
SET pathways) have not been successfully applied to orga-
nohalide reactions with zerovalent metals in aqueous solu-
tion.
The phrase “two-electron reduction” is somewhat mis-
leading because electrons only can be transferred one at a
time (18). Even the classic SN2 reaction, which is not normally
perceived as involving ET, can be envisioned as taking place
via a process in which bond formation and “single electron
shift” (18, 19) or (equivalently) “inner sphere single electron
transfer” (20) are concerted events. Nonetheless, there is a
clear mechanistic distinction between SET processes resulting
in homolytic bond cleavage, and “two-electron” or “polar”
processes (such as SN2 reactions) that do not produce free-
radical intermediates.
To provide indirect evidence concerning the mechanism
through which zerovalent metals reduce alkyl halides, we
have examined the reduction of a series of stereochemical
probe compounds (Figure 1). These include (2R, 3S)-2,3-
dibromopentane (diBP) and its stereoisomeric enantiomer,
(2S, 3R)-2,3-dibromopentane (referred to as erythro-diBP),
as well as (2R, 3R)-2,3-dibromopentane and its stereoisomeric
enantiomer, (2S, 3S)-2,3-dibromopentane (threo-diBP). The
other probe employed was (1R, 2R)-1,2-dibromo-1,2-di-
phenylethane [and its enantiomeric isomer, abbreviated as
(()-SBr2].
Introduction
Zerovalent metals offer great promise in treating organohalide
contaminants through abiotic reductive dehalogenations
(1-9). Despite significant progress in recent years, many basic
questions about the mechanisms of these reactions remain
unanswered. Metal-promoted reductions include hydro-
genolysis (replacement of halogen by hydrogen), reductive
â-elimination (removal of two vicinal halogens, accompanied
by an increase in bond order), reductive R-elimination (e.g.,
reduction of carbon tetrachloride to dichlorocarbene), and
* Corresponding author phone: (410) 516-4387; fax: (410) 516-
8996; e-mail: lroberts@jhu.edu.
† Current affiliation: Department of Environmental Sciences,
Rutgers University, 14 College Farm Rd., New Brunswick, NJ 08901.
‡ Current affiliation: Department of Chemistry, City College of
CUNY, Convent Avenue at 138th Street, New York, NY 10031.
9
2 2 6 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 11, 2001
10.1021/es0010195 CCC: $20.00
2001 Am erican Chem ical Society
Published on Web 05/04/2001

“nucleophiles” (27, 28), though the appropriateness of
viewing a metal surface in this manner may be questioned.
Vicinal dibromides have been extensively used in the past
to investigate a variety of reduction reactions; Baciocchi (21)
provides a review. For example, Schubert et al. (27) invoked
differences in E:Z ratios as evidence that sodium metal in
NH3 reduced vicinal dibromoalkanes via a free-radical
mechanism, whereas magnesium (in tetrahydrofuran) and
zinc metal (in water) were hypothesized to react via a “two-
electron” mechanism. More recently, stereochemical evi-
dence for reactions of vicinal dihalides in homogeneous
solution has led to suggestions that polysulfide ions (29),
fluorenide ions (30), and diorganotellurides (31) can serve as
“two-electron” reductants. Studies have shown that reductive
â-elimination mechanisms are a function not only of the
reductant, but also of the structure of the dihalide probe, the
solvent, and the temperature. Thus, although some reactions
of zerovalent metals with vicinal dihalide stereoisomers have
previously been investigated (27, 28, 32, 33), experiments
typically were conducted in organic solvents or were carried
out at high temperatures, and the results may be of limited
applicability to environmental transformations.
The principal goals of this research were (1) to investigate
reactions of vicinal dibromide stereoisomers with a variety
of zerovalent metals under environmentally relevant condi-
tions (i.e., at room temperature, in water); and (2) to assess
the overall feasibility of vicinal dihalides as “probes” of
environmental reduction mechanisms. Reactions with model
reductants were therefore also investigated, including a one-
electron reductant [Cr(II) (aq)] (32, 34) and the nucleophilic
reductant I-. Some researchers have suggested that surface-
adsorbed Fe(II) species may represent important reductants
of organohalides in Fe(0) systems (4, 35, 36) as well as in
natural environments (37-39). Reductions promoted by
Fe(II) adsorbed to an Fe(III) oxide, goethite, were thus also
studied. Hydrolysis of the probe compounds, which competes
with reduction, was also examined.
FIGURE 1. Idealized reaction schemes for the model compounds
used in this study: (a) single electron reduction to free radical
intermediate, and (b) concerted two-electron reduction by electron
donor “D:” to olefin plus (potentially unstable) (DBr)+ complex. For
diBPs, R ) methyl, R′ ) ethyl (or vice versa). For (()-SBr2, R ) R′
) phenyl.
If these vicinal dihalides undergo reduction via SET, the
first one-electron reduction would yield a radical (Figure 1a)
that is partially stabilized by delocalization of the unpaired
electron over the remaining bromine. This “bridged” bromine
radical could experience rotation around the central C-C
bond before being further reduced, resulting in a mixture of
E and Z olefins. The E/Z ratio of the products is a function
of the rate at which the second electron is transferred relative
to the rate of C-C bond rotation, and also of the relative
stability of the E-like and Z-like radical conformers. If C-C
bond rotation is fast relative to the second electron transfer,
the E olefin may be favored slightly over the Z isomer (if the
R groups are small and steric interactions are minor), or the
E olefin may be formed exclusively (as in the case of bulky
R groups). The same E:Z ratio should, however, be attained,
regardless of whether the threo or the erythro isomer (or the
meso or the (()- pair) is reduced.
In contrast to the SET mechanism, if vicinal dihalides
were to undergo reduction via a “two-electron” pathway
(Figure 1b), the reaction typically would be highly stereospe-
cific, demonstrating a strong preference for anti elimination
(21): threo-diBP reacts to (Z)-2-pentene, and erythro-diBP
to form (E)-2-pentene. (Numerous exceptions to anti ste-
reospecificity have nonetheless been observed for (()-SBr2
(21), as discussed below.) This “two-electron” reaction may
be envisioned as involving transfer of halonium (X+) to the
electron donor D. Reductive elimination of vicinal dihalides
by I- has commonly been attributed to just such a “two-
electron” mechanism (22-24), leading to description of I-
as a “nucleophilic reductant”. Low-valent transition metals,
including Fe(I), Fe(“0”), and Co(I) porphyrins, also reduce
vicinal dibromides via an inner-sphere two-electron halo-
nium transfer mechanism (11). Several low-valent transition
metal complexes have been described as “nucleophiles” in
their reactions with alkyl halides (25, 26). Even zerovalent
metals have been referred to in the older literature as
Experimental Methods
Reagents. Threo- and erythro-diBPs and (()-SBr2 were
synthesized from (Z)- and (E)-2-pentene and (Z)-stilbene by
stereospecifically brominating the olefins with pyridinium
tribromide (40). The (()-SBr2 product was purified by
recrystallization in methanol, and was found by gas chro-
matographic/ mass spectrometric (GC/ MS) analysis to be free
of significant impurities. The threo- and erythro-2,3-diBPs
each contained less than 0.5% impurity of the undesired
isomer. The identities of the synthesis products were verified
by MS and proton NMR analysis (41).
Preparation of Metals. Each metal was acid-washed
before use (41). Fe(0) (Fisher, 100 mesh, 95.3%) was cleaned
with 1 N HCl. Zn(0) (Baker, 30 mesh, 100.5%), and Cu(0)
(Aldrich, 40 mesh, 99.5%) were cleaned with 0.4% sulfuric
acid; and Al(0) (Baker, 40 mesh, >99%) was cleaned with a
solution of chromic and sulfuric acids. All cleaning procedures
were conducted in an anaerobic glovebox (95% N2/ 5% H2).
The surface areas of the Cu(0) and Fe(0) were measured by
nitrogen BET and found to be 0.13 and 0.76 m2/ g, respectively.
The lower surface area of Zn(0) (0.035 m2/ g) required
measurement by krypton BET.
Goethite. Goethite slurry (44.2 g goethite/ L) was synthe-
sized by Barbara Coughlin (42) and was provided by Alan T.
Stone. The goethite had a specific surface area of 47.5 m2/ g
(42). For each experiment, 9.13 mM FeCl2‚4H2O was equili-
brated overnight with 2.95 g/ L of goethite (to allow sufficient
time for the Fe(II) to adsorb and to develop significant
reactivity; ref 37) before spiking with a vicinal dibromide.
The Fe(II) was present at 14 times molar excess of the
concentration of available sites (42). Experiments at pH 7.5
employed 5 mM tris(hydroxymethyl)aminomethane (Tris)
9
VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 2 6 9

buffer plus 0.1 M NaCl. Experiments at pH 10.5 used 5 mM
2-amino-2-methyl-1-propanol buffer plus 0.1 M NaCl. Above
pH 6, all surface sites on the goethite should be occupied by
Fe(II) (42).
Cr(II) Solutions. Cr(II) solutions were prepared in the
glovebox by the method of Castro (43). Cr(II) reactions were
carried out in 5 mM H2SO4 (pH 2.0-2.1) in a water bath at
25.0 (( 0.1) °C. Initial Cr(II) concentrations were determined
by oxidation with Fe(III), with quantitation of the resulting
Fe(II) by a bathophenanthroline disulfonic acid colorimetric
assay (44).
Iodide. Reactions with iodide were conducted in 1 M KI,
0.1 M NaCl, and 50 mM sodium tetraborate buffer (pH 9.3)
in a water bath at 25.0 (( 0.1) °C. This pH value was employed
to provide consistency with other experiments involving
strong nucleophiles (45).
Reactions of diBP Isom ers. Reactors were prepared in a
glovebox to minimize contamination with O2. Reactions with
metals were carried out in 1-L flasks. Reactions with goethite,
Cr(II), iodide, and hydrolysis experiments were carried out
in 125-mL flasks. Each flask was fitted with a ground-glass
stopcock adapter (Ace Glass) plugged with an NMR-type
septum (Aldrich catalog #Z10,070-6). The reaction solution
(typically unbuffered 0.1 M NaCl for reactions with metals;
50 mM Tris buffer/ 0.1 M NaCl for hydrolysis experiments at
pH 7.5; or 50 mM sodium tetraborate/ 0.1 M NaCl for
hydrolysis experiments at pH 9.3) was deoxygenated via
sparging with purified argon. The reductant was weighed or
pipetted into each flask, which was then filled completely
with aqueous solution. For reactions with zerovalent metals,
an aliquot was removed for pH measurement after equili-
brating with the metal overnight. The flask was then filled
completely with aqueous solution, leaving no headspace.
Flasks were spiked with either threo- or erythro-diBP in
methanol to achieve an initial dibromide concentration of
about 0.2 mM and a methanol concentration of 0.025 M.
Flasks were then removed from the glovebox and were placed
on a roller table at 40 rpm for mixing at room temperature.
Samples (1 mL) were taken without introducing headspace
by injecting an aliquot of deoxygenated solution through the
septum, forcing the liquid samples into a second syringe.
Aliquots (1 mL) were extracted into 1 mL hexane. Extracts
were analyzed on a Carlo-Erba Mega 2 GC equipped with a
flame-ionization detector (FID) and a 30 m × 0.32 mm × 5
µm Rtx-1 capillary column (Restek). The same analytical
column was used to confirm the identities of the reaction
products via GC/ MS.
Reactions of (()-SBr2. Experimental procedures were as
described for the diBPs except that all reactions were
conducted in 1-L flasks. Hydrolysis experiments were con-
ducted in 50 mM borate/ 0.1 M NaCl buffer at pH values
between 8.2 and 9.7. Each flask was spiked (via a methanol
carrier) to give an initial (()-SBr2 concentration of 4 µM
(methanol concentration 0.025 M). No attempt was made to
keep these systems headspace free, as the parent compound
and olefin products have relatively low Henry’s law constants.
Samples (10 mL) were taken from each flask, with the volume
being displaced by purified N2 or an N2/ H2 mixture. Samples
were extracted with 1 mL of hexane, and 1-µL hexane aliquots
were injected onto a 15 m ×0.25 mm ×0.25 µm EC-5 capillary
column (Alltech) for GC/ FID analysis. Selected samples were
also analyzed using a Hewlett-Packard (HP) 5890 GC
equipped with an HP 5970 mass selective detector.
FIGURE 2. E and Z olefins (as a percent of total olefin) formed from
the reaction of various reductants (top) with (()-SBr2 and (bottom)
with threo-diBP. Fe (neutral) refers to the reduction carried out in
0.1 M NaCl at pH 6.5-8.5. Fe (acidic) refers to the reduction carried
out in 0.1 M NaCl solution initially adjusted to pH 4 with HCl. Fe
(basic) refers to the reduction carried out in 0.1 M NaCl solution
initially adjusted to pH 10 with NaOH. Fe(II)/goethite experiments
were conducted at pH 10.5. See text for other reaction conditions.
Error bars represent 95% confidence limits.
extraction into hexane) as 2-bromo-3-pentanol and 3-bromo-
2-pentanol.
The rate constant for (()-SBr2 hydrolysis was 0.17 hr-1 at
pH 8.2-9.7. The major product was the epoxide, (E)-stilbene
oxide (95%). Traces of (Z)-stilbene oxide (less than 10% of
the total epoxide yield; confirmed by GC/ MS) were also
formed, along with minor amounts of (E)-stilbene. (Z)-
Stilbene was not detected as a hydrolysis product. Both (E)-
and (Z)-stilbene oxide were found to be stable in buffered
aqueous solutions at pH 9.2, and no isomerization of (Z)-
stilbene oxide to the E isomer was observed.
The distribution of epoxides obtained from (()-SBr2 and
the rapid nature of the hydrolysis together point to an SN1
mechanism to form a relatively stable benzyl carbocation
that may be in equilibrium with a bridged bromonium ion.
Reaction of the solvent or Br- with such a cationic inter-
mediate has been invoked to account for (E)-stilbene obtained
in the solvolysis of (()-SBr2 (46). Benzyl carbocations are
thought to be capable of extensive C-C bond rotation (21),
and the high E:Z ratio of the epoxides likely signifies that
C-C bond rotation approaches an equilibrium in which the
phenyl groups are largely trans to one another. This is an
important observation because nucleophilic reduction of (()-
SBr2 may also involve intermediate benzyl carbocations.
Results and Discussion
Hydrolysis of the Probe Com pounds. All of the dibromide
probe compounds hydrolyzed at a rate independent of pH
within the pH ranges tested. Hydrolysis rate constants for
the diBPs were 0.02 hr-1 (erythro) and 0.006 hr-1 (threo) at
pH 7.5. Products were tentatively identified by GC/ MS (after
Reductive Dehalogenation of the Probe Com pounds.
The stereospecificity of the reductive elimination reactions
is expressed as the percent E olefin present in the total olefin
yield. Results for (()-SBr2 and threo-diBP are summarized
in Figure 2. Note that the E and Z olefin products were stable
9
2 2 7 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 11, 2001

under the reaction conditions, neither undergoing reduction
nor isomerization.
Reduction by Cr(II). For (()-SBr2 and for both the diBPs,
the rate of reduction was fast relative to that of hydrolysis,
and disappearance exhibited pseudo first-order kinetics. (()-
SBr2 reacted with 0.06 mM Cr(II) to give (E)-stilbene as the
sole observed product (Figure 2), consistent with the results
of Mathai et al. (32). The second-order rate constant for this
reaction was 3.7 M-1s-1. The E:Z ratio results from the
eclipsing interactions in the intermediate radical, which drive
bond rotation (see Figure 1). These radicals apparently persist
long enough to attain a distribution in which essentially 100%
are in the E-like configuration. High E:Z ratios (up to 17.9:1)
have also been observed for reduction of (()-SBr2 by aromatic
radical anions (30), which are believed to serve as outer-
sphere SET reductants (11).
Second-order rate constants for reaction of 10 mM Cr(II)
with threo- and erythro-diBP were ∼0.012 M-1s-1 and 0.018
M-1s-1, respectively. Reduction of 2,3-diBPs by Cr(II) resulted
in 68% (E)-2-pentene from the threo isomer (Figure 2) and
73% from the erythro isomer. These results parallel those of
Kochi and Singleton (34) who observed a similar E:Z ratio in
2-butenes from the reduction of (()-2,3-dibromobutane by
68 mM Cr(II) in acetonitrile/ water (30:10). The formation of
both (Z)- and (E)-2-pentenes indicates that the free-radical
intermediate persists long enough to undergo C-C bond
rotation. Alkyl-substituted vicinal dibromides, unlike (()-
SBr2, are reduced to a mixture of E and Z olefins because the
radical rotamers are closer in energy and therefore significant
amounts of each exist at equilibrium.
Reduction by Iodide. (()-SBr2 was reduced by iodide to
form both (Z)-and (E)-stilbene (Figure 2; k ∼5.9 × 10-5
M-1s-1). Hydrolysis competed with reductive elimination.
Stilbenes comprised 55% of the total products, of which 70%
was (E)-stilbene. Mass balances were good (100-110%).
The observation of some (E)-stilbene from the reduction
of (()-SBr2 by I- might initially appear at odds with a
nucleophilic mechanism (Figure 1b), which should yield only
(Z)-stilbene. In contrast, a one-electron reductant such as
Cr(II) yields only the E isomer. Lee (24) notes, however, that
nucleophilic dehalogenation of (()-SBr2 in methanol most
likely occurs via a mechanism in which a bridged halonium
ion is in equilibrium with a benzyl carbocation. Because
rotation about the C-C bond of such an open carbocation
is hypothesized to occur freely, a halonium ion mechanism
would also account for the formation of (E)-stilbene during
the reduction of (()-SBr2 by I- in water.
Mathai et al. (32) have also observed (E)-stilbene in the
reduction of (()-SBr2 by iodide in organic solvents, ranging
from 4% to 35% of the total stilbene. Close scrutiny of the
data reveals that the % (E)-stilbene increases as the solvent
becomes more polar. This may reflect increasing stabilization
of charged intermediates in more polar solvents, favoring a
mechanism involving a cationic intermediate.
In contrast to the results obtained with (()-SBr2, threo-
diBP was reduced by iodide to form exclusively (Z)-2-pentene
(with a second-order rate constant kI- of approximately 6 ×
10-6 M-1s-1), and erythro-diBP was reduced by iodide to form
exclusively (E)-2-pentene (with kI- ∼ 1 × 10-5 M-1s-1). Mass
balances were essentially 100%. These results agree with
previous studies, which demonstrated that simple aliphatic
dibromides of the type RCHBrCHBrR′ (R, R′ ) alkyl) react
with iodide primarily via concerted nucleophilic reductive
elimination (Figure 1b) rather than via a halonium ion
mechanism (21). For example, reduction of (()-2,3-dibro-
mobutane by iodide yields 95-100% (Z)-2-butene in metha-
nol/ water (90:10, 59 °C) (47). The greater tendency of the
diBPs to react via a concerted mechanism probably reflects
lesser stabilization of a charged intermediate by alkyl versus
phenyl substituents.
FIGURE 3. Reaction of threo-diBP (b) (a) with 4.2 g/L iron (3.2 m2/L)
at pH 6.5-8.5; (b) with 0.83 g/L zinc (0.029 m2/L) at pH 6-7.5. All
experiments were conducted in 0.1 M NaCl. Products are (Z)-2-
pentene (9) and (E)-2-pentene (0). Solid lines represent fits to data
assuming pseudo first-order decay to observed products. Dashed
lines represent mass balances measured at each timepoint.
Reduction by Zerovalent Metals. Reduction of the diBPs
followed pseudo first-order kinetics with Fe, Zn, Cu, and Al.
Results for Fe and Zn are shown in Figure 3. Reaction rate
constants normalized to metal surface area (kSA) for threo-
diBP varied somewhat with metal loading and with mixing
speed, and ranged from 3 × 10-6 L‚s-1‚m-2 for Cu(0), to 0.7-1
× 10-4 L‚s-1‚m-2 for Fe(0), and 3-6 × 10-3 L‚s-1‚m-2 for
Zn(0). The surface area of the Al(0) could not be measured,
but the pseudo first-order rate constant for reaction of threo-
diBP with 4.2 g/ L of Al(0) was 0.05 hr-1 (after accounting for
hydrolysis). For Al(0) and Cu(0), throughout the reaction,
the pH remained relatively stable at 8.5 and 6.5, respectively.
For Fe(0), the pH at the beginning of the experiment was
about 6.5 and drifted to 8.5 by the end of the timecourse. For
Zn(0), the pH ranged from about 6 at the beginning of the
experiment to 7.5 by the end.
Each metal reduced both threo- and erythro-2,3-diBP in
a highly stereospecific manner, resulting in >95% of the anti
elimination product (Figure 2), similar to results obtained
for reaction with iodide and consistent with earlier studies
(27, 28, 32, 33). Mass balances were close to 100% for all
metals except Al(0). The low mass balance (about 65%)
observed for Al(0) was probably due to partitioning of the
volatile pentene products into the large amount of H2
headspace that formed. No hydrogenolysis products (2- or
3-bromopentanes) were detected. The absence of products
resulting from transfer of a proton from the solvent argues
against a mechanism involving a free carbanion intermediate.
9
VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 2 7 1

mixture was 71% for iron and 70% for zinc. The E:Z ratio
resulting from reduction of (()-SBr2 by Fe(0), Zn(0), and
iodide is the same (within experimental error; Figure 2).
Furthermore, reduction of (()-SBr2 in water by a variety of
strong nucleophiles (including HS- and polysulfides) results
in a similar percentage of (Z)-stilbene (45). This similarity in
product distributions may indicate that reactions with metals
and “nucleophilic” reductants all involve a common inter-
mediate. Alternatively, it is possible that the mechanism varies
but that exactly the same selectivity nevertheless results.
Reduction by Surface-Associated Fe(II). Reduction of
diBPs in the presence of goethite plus Fe(II) proved slow,
even though the surface area of the goethite suspension (140
m2/ L) was 40-200× greater than that of the Fe(0) particles.
At pH 7.5, the yield of pentenes was less than 10%, but only
the stereospecific isomer [(Z)-2-Pentene from threo-diBP and
(E)-2-pentene from erythro-diBP] was detected. Because
reduction was sluggish, hydrolysis of the probe compounds
consumed most of the spiked mass.
To speed up the reaction between the Fe(II)/ goethite and
the diBPs, some experiments were conducted at pH 10.5.
Modest (17%) conversion of threo-diBP to 2-pentenes was
achieved in 8 h. (E)-2-Pentene represented 3% of the
2-pentene products. The mass balance was 95% (consisting
mostly of unreacted threo-diBP). At this high pH, however,
Fe(II) is expected to precipitate as Fe(OH)2 (s). Control
experiments in which a solution of Fe(II) at pH 10.5 was
spiked with threo-diBP showed no production of pentenes
despite the formation of a cloudy gray-green precipitate
[presumably Fe(OH)2 (s)].
The rate of (()-SBr2 dehalogenation in goethite/ Fe(II)
systems could not be determined because of competition
from hydrolysis. Neither the diBPs nor (()-SBr2 underwent
reduction in goethite suspensions in the absence of Fe(II),
or in homogeneous Fe(II) solutions.
Mechanistic Interpretation. For both the diBPs and (()-
SBr2, the products observed from reduction by iron and zinc
have essentially the same E:Z ratio as those encountered
with the classic nucleophilic reductant, iodide. Although this
might indicate that reactions occur in each case via a “two-
electron” transfer of halonium (X+), the pronounced anti
stereospecificity could equally well reflect a situation in which
transfer of the second electron is fast compared to the rate
of C-C bond rotation in an intermediate radical.
FIGURE 4. Reaction of (()-SBr2 (O) (a) with 0.42 g/L iron (0.32 m2/L)
at pH 7.8-8.6; (b) with 0.83 g/L zinc (0.029 m2/L) at pH 7.8. All
experiments were conducted in 0.1 M NaCl. Products are (E)-stilbene
(1) and (Z)-stilbene ([). Solid lines represent fits to data assuming
pseudo first-order decay to observed products. Dashed lines
represent mass balances measured at each timepoint.
Because the adsorption and reactivity of surface-associ-
ated Fe(II) is highly pH-dependent (37, 42), as are the structure
and thickness of the passivated layer (48), differences in
reactivity and stereospecificity might be anticipated if
adsorbed Fe(II) represented the principal reactive species in
systems containing Fe(0). Hence, limited experiments were
conducted with diBPs in which initial solution pH was varied
by adding NaOH or HCl. Although the initial pH was not
maintained, the rate of reaction was affected to a modest
extent; the reaction was roughly 2 times faster at either high
or low pH than at neutral pH. The stereospecificity changed
at best only slightly with pH (Figure 2).
If we postulate that reactions in the heterogeneous systems
proceed via SET, estimates of the energy barriers for C-C
bond rotation in the intermediate radicals allow limits to be
placed on the pseudo first-order rate constants corresponding
to transfer of a second electron to the radical (k2). Lexa et al.
(11) used the E:Z ratios of 4-octenes obtained in the reduction
of D,L- and (()-4,5-dibromooctanes by radical anions to
estimate a rotational barrier of 19-29 kJ/ mol for the
4-bromooctyl radical. A similar rotational barrier (18-21 kJ/
mol) can be predicted (41) for the less-hindered 2- and
3-bromopentyl radicals using computational chemical tech-
niques employing density functional theory (DFT), corre-
sponding to a rate constant for C-C bond rotation of ∼109
s-1. To produce 70% (E)-stilbene from reduction of (()-SBr2
via SET, k2 would thus have to be of this magnitude. For the
bromostilbene radical, DFT calculations (41) yield an esti-
The disappearance of (()-SBr2 in the presence of iron
and zinc exhibited pseudo first-order behavior (Figure 4).
Rate constants kSA were 1 × 10-3 and 7 × 10-3 L‚s-1‚m-2 for
iron and zinc, respectively. The somewhat higher reactivity
of (()-SBr2 relative to the less sterically hindered diBPs is
counter to the trend that might be anticipated if reaction
were initiated by an SN2 attack on carbon by the metal to
form an organometallic intermediate, followed by a concerted
syn-elimination of a metal bromide. Hydrolysis did not
contribute significantly to the loss of (()-SBr2. Mass balances
were essentially 100%, and hydrogenolysis products that
might be indicative of a free carbanion intermediate were
not detected.
mated C-C bond rotation rate of 108 s-1
.
The reduction of the diBPs via stereospecific anti elimi-
nation by Fe(II) adsorbed to goethite might initially seem
somewhat surprising, because Fe(II) would presumably serve
as a one-electron reductant. Indeed, the Fe(II) organometallic
species C3H5Fe(CO)2P(C6H5)3 has been shown to reduce both
meso- and (()-3,4-dibromohexane to (E)-3-hexene in ho-
mogeneous organic solution (49). Amonette et al. (39) have
hypothesized that the rate-determining step of the reaction
of CCl4 with Fe(II) adsorbed to goethite involves simultaneous
(()-SBr2 underwent reduction to a mixture of (Z)- and
(E)-stilbenes. The percentage of (E)-stilbene in the olefin
9
2 2 7 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 11, 2001

transfer of two electrons within a termolecular complex
involving two adsorbed Fe(II) atoms; this suggestion was
based on the second-order nature of the reaction in adsorbed
Fe(II) concentration. We note that a second-order depen-
dence on the reductant concentration is characteristic of
noncomplementary redox reactions (50); an alternative
explanation could be simply that the reaction occurs via a
rapidly reversible transfer of a halogen atom, followed by
slower reaction of the resulting radical with a second adsorbed
Fe(II) atom.
This possibility will be explored further in a subsequent
publication.
Acknowledgments
We are grateful to Dr. Gary Posner and Ben Woodard for
their help in obtaining NMR spectra of synthesis products.
We are also indebted to Dr. Alan Stone and three anonymous
reviewers for their helpful insights and suggestions, which
greatly improved this work. This study was partially funded
through Contract No. F08637 95 C6037 from the Department
of the Air Force, as well as through an NSF Graduate Student
Fellowship to L. A. T. and an NSF Young Investigator Award
(Grant No. BES-9457260) to A. L. R.
For the Fe(II)/ goethite system, we can evaluate the
likelihood that transfer of a second electron could be faster
than C-C bond rotation. Given the Fe(II) binding-site density
on goethite (2.8 sites/ nm2 from ref. 42), a simple calculation
shows the sorbed Fe(II) atoms to be only about 0.44 nm
apart on average. For a bromopentyl radical, the calculated
characteristic diffusion time tD (computed as L2/ 2D, where
D is the aqueous diffusion coefficient estimated according
to ref 51) over such a distance L is 10-10 s. This is sufficiently
fast relative to C-C bond rotation to account for the high
stereospecificity observed in the Fe(II)/ goethite system.
Transfer of the second electron is even more likely to compete
with C-C bond rotation for reduction of (()-SBr2, and also
for reductions in the zerovalent metal systems, for which
reaction may occur with the naked metal (in corrosion pits).
The heterogeneous reduction of the vicinal dibromides
studied herein may be comparable to their reduction by glassy
carbon electrodes (11). Even though the latter reaction occurs
via outer-sphere SET, vicinal dibromides react completely
stereospecifically, in marked contrast to their outer-sphere
SET reduction by aromatic radical anions in solution. A
comparable situation may occur in the photoreductive
degradation of CCl4 on TiO2 surfaces (52), for which detailed
kinetic studies indicated that CCl4 reduction to dichloro-
carbene occurred via two single-electron transfers rather than
via a concerted two-electron-transfer step. In all of these
cases, reduction occurs at a surface containing an excess of
reducing equivalents, and transfer of the second electron is
fast relative to radical isomerization or diffusion away from
the surface.
Literature Cited
(1) Gillham, R. W.; O’Hannesin, S. F. Ground Water 1994, 32, 958-
967.
(2) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28,
2045-2053.
(3) Roberts, A. L.; Totten, L. A.; Arnold, W. A.; Burris, D. R.; Campbell,
T. J. Environ. Sci. Technol. 1996, 30, 2654-2659.
(4) Johnson, T. L.; Fish, W.; Gorby, Y. A.; Tratnyek, P. G. J. Contam.
Hydrol. 1998, 29, 379-398.
(5) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 1998, 32,
3017-3025.
(6) Scherer, M. M.; Balko, B. A.; Gallagher, D. A.; Tratnyek, P. G.
Environ. Sci. Technol. 1998, 32, 3026-3033.
(7) Arnold, W. A.; Ball, W. P.; Roberts, A. L. J. Contam. Hydrol. 1999,
40, 183-200.
(8) Farrell, J.; Kason, M.; Melitas, N.; Li, T. Environ. Sci. Technol.
2000, 34, 514-521.
(9) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 2000, 34,
1794-1805.
(10) Eberson, L. Electron-Transfer Reactions in Organic Chemistry;
Springer-Verlag: New York, 1987.
(11) Lexa, D.; Save´ant, J.-M.; Scha¨fer, H. J.; Su, K.-B.; Vering, B.; Wang,
D. L. J. Am. Chem. Soc. 1990, 112, 6162-6177.
(12) Lowry, G.; Roberts, P.; McNab, W.; Reinhard, M. Nat. Meet. -
Am. Chem. Soc., Div. Environ. Chem. 1997, 37 (1), 171-173.
(13) Liu, Z.; Betterton, E. A.; Arnold, R. G. Environ. Sci. Technol.
2000, 34, 804-811.
(14) Fennelly, J. P.; Roberts, A. L. Environ. Sci. Technol. 1998, 32,
1980-1988.
(15) Balko, B. A.; Tratnyek, P. G. J. Phys. Chem. B 1998, 102, 1459-
For these heterogeneous systems, the stereospecificity
therefore cannot be used to discriminate between a “two-
electron” versus an SET mechanism, nor can we conclude
from our results whether electron transfer is an inner- or an
outer-sphere process. Nonetheless, our studies indicate that
the lifetime of a putative radical intermediate formed by
reactions with zerovalent metals is so short as to preclude
characteristic free-radical reactions such as self-coupling and
hydrogen-atom abstraction from dissolved organic matter.
In order for other bimolecular reactions to compete with
transfer of the second electron, they would have to be
extremely fast, and one reagent would have to be present at
an abundance greater than typically observed in the dilute
aqueous solution that even contaminated groundwater
comprises. For example, a diffusion-limited reaction, such
as self-coupling of free radicals (which occurs with a second-
order rate constant on the order of 1010 M-1s-1) could begin
to compete with the second one-electron transfer only if the
local concentration of the radical species was greater than
∼10-2 M. Such a high concentration of reactive transients is
extremely unlikely in the presence of powerful reductants
such as zerovalent metals. Any intermediate radical that may
be formed will be short-lived and unlikely to escape from the
metal [or (hydr)oxide] surface before being reduced to a
relatively stable closed-shell species. This raises the possibility
that observed coupling products may result from a process
other than the collision of two free radicals, perhaps via an
intramolecular mechanism involving rearrangement of a
dialkyl organometallic intermediate, potentially one bonded
to the metal-water interface rather than free in solution.
1465.
(16) Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1991, 25,
973-978.
(17) Buschmann, J.; Angst, W.; Schwarzenbach, R. P. Environ. Sci.
Technol. 1999, 33, 1015-1020.
(18) Pross, A. Acc. Chem. Res. 1985, 18, 212-219.
(19) Pross, A.; Shaik, S. S. Acc. Chem. Res. 1983, 16, 363-370.
(20) Save´ant, J.-M. Adv. Phys. Org. Chem. 1990, 26, 1-130.
(21) Baciocchi, E. 1,2-Dehalogenations and Related Reactions. In
The Chemistry of Halides, Pseudo-halides and Azides; Patai, S.,
Rappoport, Z., Eds.; Wiley and Sons: New York, 1983; Vol. 1,
pp 161-201.
(22) Winstein, S.; Pressman, D.; Young, W. G. J. Am. Chem. Soc. 1939,
61, 1, 1645-1647.
(23) Lee, W. G.; Miller, S. I. J. Phys. Chem. 1962, 66, 655-658.
(24) Lee, C. S.; Mathai, I. M.; Miller, S. I. J. Am. Chem. Soc. 1970, 92,
4602-4609.
(25) Helvenston, M. C.; Castro, C. E. J. Am. Chem. Soc. 1992, 114,
8490-8496.
(26) Schrauzer, G. N.; Deutsch, E. J. Am. Chem. Soc. 1969, 91, 3341-
3350.
(27) Schubert, W. M.; Rabinovitch, B. S.; Larson, N. R.; Sims, V. A.
J. Am. Chem. Soc. 1952, 74, 4590-4592.
(28) House, H. O.; Ro, R. S. J. Am. Chem. Soc. 1958, 80, 182-187.
(29) Bosser, G.; Paris, J. J. Chem. Soc., Perkin Trans. 2 1992, 2057-
2063.
(30) Lund, T.; Bjorn, C.; Hansen, H. S.; Jensen, A. K.; Thorsen, T. K.
Acta Chem. Scand. 1993, 47, 877-884.
(31) Butcher, T. S.; Zhou, F.; Detty, M. R. J. Org. Chem. 1998, 63,
169-176.
(32) Mathai, I. M.; Schug, K.; Miller, S. I. J. Org. Chem. 1970, 35,
1733-1736.
(33) Vijayahree, N.; Samuelson, A. G. Tetrahedron Lett. 1992, 33,
559-560.
9
VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 2 7 3

(34) Kochi, J. K.; Singleton, D. M. J. Am. Chem. Soc. 1968, 90, 1582-
1589.
(35) Klausen, J.; Trober, S. P.; Haderlein, S. B.; Schwarzenbach, R.
P. Environ. Sci. Technol. 1995, 29, 2396-2404.
(36) Sivavec, T. M.; Horney, D. P. Natl. Meet. - Am. Chem. Soc., Div.
Environ. Chem. 1997, 37 (1), 115-117.
(37) Haderlein, S. B.; Pecher, K. Pollutant Reduction in Heterogeneous
Fe(II)-Fe(III) Systems. In Mineral-Water Interfacial Reac-
tions: Kinetics and Mechanism; Sparks, D. L., Grundl, T. J., Eds.;
ACS Symposium Series 715; American Chemical Society: Wash-
ington, DC, 1998; pp 342-357.
(38) Hofstetter, T. B. Reduction of Polynitroaromatic Compounds
by Reduced Iron Species - Coupling Biogeochemical Processes
with Pollutant Transformation. Ph.D. Dissertation, Swiss Federal
Institute of Technology, Zu¨ rich, 1999.
(39) Amonette, J. E.; Workman, D. J.; Kennedy, D. W.; Fruchter, J.
S.; Gorby, Y. A. Environ. Sci. Technol. 2000, 34, 4606-4613.
(40) Fieser, L. F.; Williamson, K. L. Organic Experiments, 4th ed.; DC
Heath and Co.: Lexington, MA, 1979.
(43) Castro, C. E. J. Am. Chem. Soc. 1961, 83, 3262-3264.
(44) Diehl, H. The Iron Reagents, 3rd ed.; G. Frederick Smith Chemical
Company: Columbus, OH, 1980.
(45) Jans, U.; Roberts, A. L. Natl. Meet. -Am. Chem. Soc., Div. Environ.
Chem. 1998, 38 (2), 123-124.
(46) Khurana, J. M.; Maikap, G. C. J. Org. Chem. 1991, 56, 2582-
2584.
(47) Schubert, W. M.; Steadly, H.; Rabinovitch, B. S. J. Am. Chem.
Soc. 1955, 77, 5755.
(48) Schultze, J. W. Passivity of Metals; Electrochemical Society:
Princeton, NJ, 1978.
(49) Wegner, P. A.; Delaney, M. S. Inorg. Chem. 1976, 15, 1918-
1921.
(50) Cannon, R. D. Electron-Transfer Reactions; Butterworth: Lon-
don, 1980.
(51) Hayduk, W.; Laudie, H. AIChE J. 1974, 20, 611-615.
(52) Choi, W.; Hoffmann, M. R. J. Phys. Chem. 1996, 100, 2161-
2169.
(41) Totten, L. A. The Use Of Model And Probe Compounds To
Investigate The Mechanisms Of Reductive Dehalogenation
Reactions. Ph.D. Dissertation, The Johns Hopkins University,
Baltimore, MD, 1999.
Received for review February 18, 2000. Revised manuscript
received February 22, 2001. Accepted March 5, 2001.
(42) Coughlin, B. R.; Stone, A. T. Environ. Sci. Technol. 1995, 29,
2445-2455.
ES0010195
9
2 2 7 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 11, 2001