Deuterium kinetic isotope effects in gas-phase SN2 and E2 reactions: Comparison of experiment and theory

Article abstract of DOI:10.1021/ja057491d

The competition between bimolecular nucleophilc substitution and base-induced elimination is investigated through kinetic isotope effect measurements for gas-phase reactions of RCl + ClO^- (R = methyl, ethyl, isopropyl, and tert-butyl) utilizing a FA-SIFT instrument. The overall reaction rate constants and the kinetic isotope effect for the reaction of C₂H₅Cl + ClO^- are compared to computational results. [Hu, W. P.; Truhlar, D. G. J. Am. Chem. Soc. 1996, 118, 860.] Experimental results show that as the degree of substitution in the neutral reactant increases the E2 channel becomes dominant. The systematic change in the overall kinetic isotope effects indicates that, for the reaction of ClO^- with C₂H₅Cl, both the S_N2 and E2 pathways do occur, as predicted by computation; however the experimental reaction rate constants and KIE deviate strongly from the computational result. Copyright

Full text of DOI:10.1021/ja057491d

Published on Web 12/22/2005
Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions:
Comparison of Experiment and Theory
Stephanie M. Villano, Shuji Kato, and Veronica M. Bierbaum*
UniVersity of Colorado, Department of Chemistry and Biochemistry, 215 UCB, Boulder, Colorado 80309-0215
Received November 2, 2005; E-mail: Veronica.Bierbaum@Colorado.edu
The competition between nucleophilic substitution and base-
induced elimination has been well-documented in the condensed
phase; however, few studies have addressed this competition in
the gas-phase. Studying SN2 and E2 reactions in the gas-phase
allows one to gain insight into intrinsic features that affect this
competition without interference from solvent effects. However,
these experiments are inherently difficult since these mechanisms
generate the same ionic product and detection of neutral products
presents serious analytical challenges.
Several groups have investigated SN2 and E2 reactions theoreti-
cally; however due to the lack of experimental data, less work has
addressed the competition between these two reactions. Hu and
Truhlar1 have evaluated the reaction rate constants and deuterium
kinetic isotope effects (KIE ) kH/kD) for both the SN2 and E2
pathways of the reaction of ClO- with C2H5Cl, shown in eq 1.
cies for the undeuterated species are lowered more substantially
q
than those for the deuterated species in the transition state, EH
<
EDq. An SN2 reaction proceeds through a trigonal planar transition
state where the CR-H bonds are tightened, thus raising these fre-
quencies relative to reactants. Since the frequencies for the undeu-
terated species are raised more substantially than those for the deu-
q
q
terated species in the transition state, EH > ED . When both reaction
pathways are viable an overall KIE is measured which provides
qualitative insight into the competition between these mechanisms.
Hu and Truhlar1 have quantitatively evaluated the competition
between SN2 and E2 pathways for ClO- with C2H5Cl and with
C2D5Cl using dual-level generalized transition state theory and
statistical calculations based on high-level, correlated electronic
structure calculations using extended basis sets (MP2/ADZP).1 Their
calculations predict an SN2 reaction efficiency of 2% with a
deuterium KIE of 0.60 and an E2 reaction efficiency of 26% with
a deuterium KIE of 3.1 at 300 K. They predict an overall reaction
efficiency of 28% and a KIE of 2.4.
We have measured the overall reaction rate constants and KIE
for the reactions of RCl + ClO- (R ) methyl, ethyl, isopropyl,
and tert-butyl) using a tandem flowing afterglow-selected ion flow
tube instrument, FA-SIFT.20,21 ClO- is an ideal choice for a
nucleophile because the rate constants for the entire neutral series,
methyl through tert-butyl, are within a measurable window of
10-9-10-13 cm3 molecule-1 s-1 and are all below the calculated
collision rate so that KIEs are evident. The reactant ion was formed
in the source flow tube from electron impact on N2O to produce
O- which was then allowed to react with CCl4 to form ClO-. ClO-
ions were mass-selected and injected into the reaction flow tube
where they were thermalized to 304((2) K by collisions with He
buffer gas (0.5 Torr, ∼104 cm s-1). Neutral reactant flow rates were
measured by monitoring the pressure change versus time in a
calibrated volume system. Reaction rate constants were determined
by introducing the neutral reagents at varying distances along the
reaction flow tube, thereby varying the reaction distance and time.
The 35ClO- intensity was monitored with a quadrupole mass filter
coupled to an electron multiplier. Rate constants for the perprotio
and perdeuterio reactions were measured sequentially on the same
day under the same conditions. The error reported for the reaction
rate constants is one standard deviation of three consecutive
measurements. The uncertainty due to systematic error is (20%;
however the systematic errors cancel in the rate ratio, and therefore,
the KIEs are more accurately determined. Helium buffer gas
(99.995%) was purified by passage through a molecular sieve trap
at 77 K. Neutral reagents were purchased from commercial sources
and used without further purification; however, it was experimen-
tally demonstrated that HCl, which would complicate the results,
was not a significant contaminant.22
Their work has provided us with the unique opportunity to compare
theory and experiment. In this account we report experimental,
overall reaction rate constants and deuterium KIEs for the reactions
of RCl + ClO- (R ) methyl, ethyl, isopropyl, and tert-butyl). This
systematic approach indicates that, for the reaction of ClO- with
C2H5Cl, both the SN2 and E2 channels occur, as predicted by Hu
and Truhlar; however the experimental KIE differs drastically from
the computational predictions.
There is extensive literature on gas-phase reactions that occur
exclusively by SN2 or by E2 mechanisms. (See refs 2-6 and
citations therein.) For reactions that can occur by both pathways,
Gronert7 has provided an excellent summary of experimental
approaches,8-13 including his novel utilization of dianionic nucleo-
philes that yield diagnostic product ions.14,15 Our research has
utilized reactivity trends16 as well as kinetic isotope effects17 to
explore these processes.
Kinetic isotope effects can be used to distinguish between SN2
and E2 mechanisms if the reaction proceeds at a rate slower than
the collision rate.18 For E2 reactions normal KIEs (kH/kD > 1) are
observed, while for SN2 reactions inverse KIEs (kH/kD < 1) are
observed. The origin of these effects is primarily a result of changes
in vibrational frequencies as the reaction proceeds from the reactants
to the transition state structure.19 Deuteration lowers the vibrational
frequencies in both the reactants and transition states relative to
the undeuterated compounds. An E2 reaction proceeds through a
transition state where the Câ-H bond is partially broken thus
lowering these frequencies relative to reactants. Since the frequen-
Measured reaction rate constants and deuterium KIEs are reported
in Table 1. The reaction of ClO- with methyl chloride can undergo
only an SN2 reaction, and an inverse KIE of 0.85((0.01) was
measured. This result is consistent with previous measurements of
9
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J. AM. CHEM. SOC. 2006, 128, 736-737
10.1021/ja057491d CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Experimental Reaction Rate Constants (10-10 cm3
molecule-1 s-1) and Deuterium Kinetic Isotope Effects for RCl +
ClO- f Products
mol-1. These small deviations in the activation enthalpy can have
a large effect on the branching ratios. Determining activation
enthalpies with an accuracy of 0.5 kcal mol-1 or better is a difficult
computational task.39
RCl
k
KIE
CH3Cl
CD3Cl
2.01 ( 0.01
2.36 ( 0.01
2.25 ( 0.01a
2.27 ( 0.01a
1.74 ( 0.03
1.01 ( 0.02
2.33 ( 0.03
1.01 ( 0.05
0.85 ( 0.01
These results illuminate the need for additional studies to describe
nucleophilic substitution and elimination reactions more accurately.
We hope that the discrepancy between experiment and theory will
spark interest for further exploration.
Acknowledgment. The authors thank Professors Charles DePuy
and Barry Carpenter for valuable discussions and Professor Gustavo
Davico for the initiation of the first experiment. This research was
supported by the National Science Foundation (Grant No. CHE-
0349937).
C2H5Cl
C2D5Cl
i-C3H7Cl
i-C3D7Cl
t-C4H9Cl
t-C4D9Cl
0.99 ( 0.01a
1.72 ( 0.05
2.31 ( 0.12
a Computational results predict k ) 6.7 × 10-10 and k ) 2.8 × 10-10
cm3 molecule-1 s-1 for ClO- with C2H5Cl and C2D5Cl, respectively, and
a KIE of 2.4.1
References
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by E2.16 A KIE of 2.31((0.12) was measured which is consistent
with previously measured normal KIEs for E2 reactions.5,25 As the
extent of substitution in the neutral reactants increases, the KIE
effects become increasingly more normal. These results indicate
that the E2 pathway becomes the dominant channel as the neutral
reagent becomes more sterically hindered.
The experimental reaction efficiencies (10% for ClO- with
C2H5Cl)26 and KIE reported here for the reaction of ClO- with
ethyl chloride differ from the theoretical values, suggesting that
the SN2 channel is more prominent than calculations predict. It is
difficult to account for the discrepancies between experiment and
theory; however nonstatistical dynamics27 or errors in the calculation
of the individual KIE or in the branching ratios of the two channels
could account for the discrepancy.
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the transition state. Nonstatistical effects have been observed for
SN2 reactions of monatomic nucleophiles with methyl halides, and
these effects have been extensively documented in the liter-
ature.23,24,28-32 However the use of statistical theories has been
successful for slightly larger systems.33-35 Furthermore, Hu and
Truhlar have previously calculated the KIE for the SN2 reaction of
solvated fluoride with methyl chloride using a statistical approach.36
Their results show excellent agreement with our published experi-
mental values.37 Thus the assumption that the ClO- + C2H5Cl
reaction behaves statistically is reasonable.
(18) Melander, L.; Saunders, J. H., Jr. Reaction Rates of Isotopic Molecules;
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i-C3D7Cl 99.4%, t-C4H9Cl 99%, t-C4D9Cl 99.1%). An HCl impurity would
complicate the rate measurements due to a rapid proton-transfer pathway,
which could not be distinguished from the SN2 and E2 channels. Therefore,
37Cl- was mass selected and allowed to react with the neutral reagents.
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contaminant.
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appear quite reasonable. It can be argued that the errors in the
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ratios were correct, the KIE for the E2 channel would need to be
1.0 or smaller or the KIE for the SN2 channel would need to be
0.13 or smaller to be consistent with the measured overall KIE.
These values are not reasonable based on previous results. On the
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SN2 and E2 channels to be 1.10 × 10-10 cm3 molecule-1 s-1 and
1.15 × 10-10 cm3 molecule-1 s-1, respectively, for ClO- + C2H5Cl.
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if the E2 barrier height was underestimated by only 1 kcal mol-1
while the SN2 barrier height was overestimated by only 0.6 kcal
,
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