Electron transfer reactions (ETR) of tert-butyl perbenzoates with dimethyl sulfide: The rates controlled by translational entropy

Article abstract of DOI:10.1002/poc.1099

Various tert-butyl perbenzoates were prepared according to the known methods. The mixtures of tert-butyl perbenzoate and dimethyl sulfide underwent thermolysis at several temperatures (T°C: 80, 90, 100, and 110). The rates of consumption of the peresters (k_obs) were obtained from In C ₀/C_t = k_obs × t where C_o and C_t are the concentration of the perester at time 0 and t, respectively. For example: The decrease of the perester satisfied pseudo first order kinetics, and k_obs were determined. k_Hom. and k _ET were obtained from k_obs = k_Hom. + k _ET[CH₃SCH₃] where [CH₃SCH ₃]/[perester] > 10. k_ET and its relative rates are produced from plot of k_obs against [CH₃SCH₃]. Hammett correlation are plotted against σ and σ+ in which σ is better correlated than σ+. Hammett ρ could be considered selectivity which stays constant irrespective of temperature. The phenomenon is not consistent with reactivity/selectivity principle. The differential activation entropies linearly increase with substituent constant σ to give slope of 2.63 while the similar slope for differential enthalpies shows much lower value of 0.892. The dominance of differential entropic terms clearly indicates the entropy control of the rates. Copyright

Full text of DOI:10.1002/poc.1099

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 625–629
Published online 6 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1099
Electron transfer reactions (ETR) of tert-butyl
perbenzoates with dimethyl sulfide: the rates
controlled by translational entropy
y
*
Sung Soo Kim and Sang Hak Lim
Department of Chemistry, Inha University, Incheon 402-751, South Korea
Received 9 January 2006; revised 4 May 2006; accepted 15 May 2006
ABSTRACT: Various tert-butyl perbenzoates were prepared according to the known methods. The mixtures of tert-
butyl perbenzoate and dimethyl sulfide underwent thermolysis at several temperatures (T 8C: 80, 90, 100, and 110).
The rates of consumption of the peresters (kobs) were obtained from ln C0/Ct ¼ kobs ꢀ t where C0 and Ct are the
concentration of the perester at time 0 and t, respectively. For example: The decrease of the perester satisfied pseudo
first order kinetics, and kobswere determined. kHom. and kET were obtained from kobs ¼ kHom. þ kET[CH3SCH3] where
[CH3SCH3]/[perester] > 10. kET and its relative rates are produced from plot of kobs against [CH3SCH3]. Hammett
correlation are plotted against s and sþ in which s is better correlated than sþ. Hammett r could be considered
selectivity which stays constant irrespective of temperature. The phenomenon is not consistent with reactivity/
selectivity principle. The differential activation entropies linearly increase with substituent constant s to give slope of
2.63 while the similar slope for differential enthalpies shows much lower value of 0.892. The dominance of differential
entropic terms clearly indicates the entropy control of the rates. Copyright # 2006 John Wiley & Sons, Ltd.
Supplementary electronic material for this paper is available in Wiley Interscience at http://www.interscience.
wiley.com/jpages/0894–3230/supplmat/
KEYWORDS: tert-butyl perbenzoates; dimethyl sulfide; entropy control of the rates
INTRODUCTION
shows dominance of entropic term over enthalpy. Triplet
d-hydrogen abstraction by o-tert-butyl benzophenones7
shows an entropy-controlled inductive effect. Recently
the rate of hydrogen abstractions by tert-butoxyl
radicals6–8 are suggested to involve entropy-controlled
process.9tert-Butyl peroxybenzoate (TBP) underwent
electron transfer reaction (ETR) with dimethyl sulfide
through reactions 1 ꢂ 4.10 The kinetic isotope effect
suggests that dimethyl sulfide-TBP system involves ET
while dimethylaniline-benzoylperoxide system proceeds
through SN2 mechanism.
The three intermediates occur in Eqn (1) which then
react via Eqn (2), Eqn (3) to give the intermediates I
which rearranges to give benzoyloxy dimethyl sulfide. A
similar ET mechanism between TBP and thiophenol has
also been reported by us.11 We would like to herein show
that the rate of ETR is controlled by entropy.
Hydrogen atom abstractions from toluenes1 and
cumenes2 by bromine atoms, b-scissions of the carbiny-
loxy radicals,3 and thermolysis of tert-butyl phenylper-
acetates4 proceed through polar transition state (TS) and
reveal entropy control of rates. The relative rates at
several temperatures provide differential activation terms
(DDHzYꢁH and DDSYz ꢁH) derived from Eyring equation.5
The sign and magnitude of differential activation
entropies (DDSzYꢁH) outweighs those of differential
enthalpies (DDHzYꢁH).1 ꢂ 4 When the substituent
approaches electron-donating character, thazt is, the value
of sþ maintains larger negative value, DDSYꢁH becomes
much bigger positive magnitudes than DDHzYꢁH. The
activation parameters for abstraction of phenolic hydro-
gen from p-methoxyphenol by tert-butoxyl radical6
(1)
*Correspondence to: S. S. Kim, Department of Chemistry, Inha
University, Incheon 402-751, South Korea.
E-mail: sungsoo@inha.ac.kr
yAekyung Petrochemical R&D Center, Daejeon, South Korea.
Contract/grant sponsor: Inha University Research.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 625–629

626
S. S. KIM AND S. H. LIM
(2)
(3)
(4)
RESULTS AND DISCUSSION
Table 1. Product analysis of thermolysis of tert-Butyl
p-nitroperbenzoate and dimethyl sulfide in absence and
presence of KF
Preparation and thermolysis of the peresters
and relevant kinetic data
Mole of Product/Mole of
Perester before Reaction
Various YC6H4CO2But [Y: p-OCH3, p-CH3, p-Cl, m-Cl,
and p-NO2] (TBP) were prepared according to the known
method.10 TBP (0.05 M), CH3SCH3 (0.5 ꢂ 2.0 M), a-
methoxycumene (0.02 M, internal standard) and trans-
stilbene (0.2 M)12were dissolved in CDCl3 and divided
into Pyrex ampoules. Potassium fluoride (KF) was added
to each ampoule which was then degassed and underwent
thermolysis at several temperatures. KF is to neutralize
HCl which is slightly contained in the solvent CDCl3. In
the presence of HCl reactions (5) and (6) occur to reduce
the yield of benzoyloxy dimethyl sulfide and tert-butyl
alcohol. Product analysis in absence and presence of KF
are shown in Table 1. That clearly shows that reaction (5)
is effectively inhibited by scavenging HCl with KF.
Products
Without KF
0.44
With KF
0.96
tert-BuOH
0.42
0.56
0.53
0.97
—
p-NO2C6H4COOH
–
–
(CH ) C CH
3 2
—
2
0.58
—
(5)
(6)
The rates of consumption of TBP (kobs) were obtained
by measuring the disappearing NMR peak of tert-butyl
group of TBP. Equation (7) has been utilized to
calculate kobs at various temperatures and with different
substituent (Y).
and by ET between the perester and dimethyl sulfide
to give
kobs ¼ kHom: þ kET½CH3SCH3ꢃ
(8)
kHom. and kET can be obtained from the linear relation
C0
between kobs and concentration of dimethyl sulfide. In
the case of a large excess of dimethyl sulfide (CMeSMe
ln
¼ kobs ꢀ t
(7)
/
Ct
CPerester > 10), Eqn (8) follows a pseudo-first order
reaction. kHom. corresponds to the intercept and kET
becomes the slope of the linear line. Utilizing this
method, kobs and kET are obtained for numerous TBP
(refer to Supporting Information).
where C0 is the initial concentration of the perester,
Ct is the concentration of the perester at time t. kobs
is obtained from Eqn (7) that could be dissected into
the rate constants caused by homolysis of the perester
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 625–629

THE RATES CONTROLLED BY TRANSLATIONAL ENTROPY
627
Absolute and relative rates of ETR at various
temperatures (T 8C: 80, 90, 100, and 110 8C) were
obtained. Hammett correlations at each temperature are
also studied (Table 2). Hammett correlations are positive
and better correlated with s rather than sþ. This indicates
that negative charge resides on benzoyloxy group without
conjugation with the phenyl ring (refer to II). This is well
compared with the previous entropy-controlled reac-
tions1–4 where the positive charge is delocalized into
phenyl ring for rþsþ correlations.
The relative rates of ETR, kYET/kEHT remains nearly
invariable irrespective of temperature. This means that
substituent effect is constant at various temperatures in
order to give the identical Hammett
r
value
(r ¼ 1.06 ꢄ 0.02) (refer to Table 2). The constant rþ
with temperature (not r in those cases) had already been
observed with photobromination of cumenes2 and
homolytic fragmentation of carbinyloxy radicals.3 The
Hammett reaction constant either r or rþ can be
considered selectivity and the invariance with tempera-
ture apparently inconsistent with reactivity/selectivity
principle (RSP).13 The activation parameters and their
differential values are tabulated in Table 3. The isokinetic
relationship is observed to give the straight line
indicating isokinetic temperature T ¼ ꢁ146 8C (Fig. 1).
The magnitudes of differential activation parameters also
suggest that cleavage of —O—O— in II is systematically
controlled by the substituents for the linearity.
Translational entropy responsible for the rates
Benzoyloxy group attains partial negative charge and tert-
butoxy becomes radical type in II. The extent of
heterolytic (into radical and anion) bond cleavage of
—O—O— is variable depending upon the substituent
(Y). The values of differential activation terms (Table 3)
indicate that electron-withdrawing groups (Y) boost more
bond cleavage than hydrogen H. The substituent effect
may weakly influence the magnitude zof DHzY to give
relatively smaller values of DDHYꢁH
. The plot
of DDHzYꢁH versus s is shown in Fig. 2 with slop of
0.891. Absolute rate theory 5 suggests that the vibration of
—O—O— bond in TBP should be replaced by translation
during the activation process. The transition from
vibration to translation should accompany dramatic
entropic increase since vibration is of one dimension
while translation occurs with three dimensions. There-
fore, the increase (decrease) in differential activation
entropy multiplied by the absolute temperature
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 625–629

628
S. S. KIM AND S. H. LIM
Table 3. Activation parameters and their differential values
p-OMe
p-Me
H
p-Cl
m-Cl
p-NO2
D HzY
a
18.9 ꢄ 0.1
ꢁ28.7 ꢄ 0.1
ꢁ0.1
18.9 ꢄ 0.4
ꢁ28.3 ꢄ 1.3
ꢁ0.1
19.0 ꢄ 0.1
19.4 ꢄ 0.2
ꢁ24.8 ꢄ 0.8
0.4
19.7 ꢄ 0.3
ꢁ23.6 ꢄ 0.8
0.7
19.7 ꢄ 0.2
ꢁ21.4 ꢄ 0.7
0.7
D SzY
b
ꢁ27.4 ꢄ 0.3
z
a,c
DD HYꢁH
0
0
0
z
b,d
DD SYꢁH
ꢁ1.3
ꢁ0.9
2.6
3.8
6.0
TDDSzYꢁH T ¼ 373 K
a
ꢁ0.47
ꢁ0.33
0.94
1.38
2.18
a unit: kcal/mol.
b unit: zcal/mol, K.
c DDHYꢁH ¼ DHYz -DHHz where the subscript Y and H indicate the substituents Y and H in the perester.
d DDSYz ꢁH ¼ DSzY-DSHz where the subscript Y and H indicate the substituents Y and H in the perester.
that of TS’s involved with the previous entropy
dominating reactions.1–4 The negative charge is localized
on benzoyloxy group (refer to II) to give rs correlation
while the positive charge is dispersed into the benzene
ring for rþsþ relationship.1–4
The selectivity S of a species with competing peresters
harboring substituents Y and H, respectively could zbe
defined
azs
S ¼ ln
kYET/kEHT ¼ ꢁ (DGYz ꢁ DGH)/
RT ¼ ꢁ DDGYꢁH with k and DGz being the corresponding
rate constaznt and free energy of activation of ETR.
S ¼ ꢁ DDGYꢁH/RT can be subdivizded into two parts:
ꢁDDGzYꢁH/RT ¼ DDSYz ꢁH/R ꢁ DDHYꢁH/RT. The present
ETR involve DDHzYꢁH/RT close to zero. Therefore, S is
mainly controlled by DDSzYꢁH/R which is temperature-
independent and takes almost constant figures with the
substituent (refer to values of kYET/kEHT of Table 2).
Figure 1. Plot of isokinetic relationship
CONCLUSION
(TDDSzYꢁH) should be much higher than that correspond-
ing to differential activationz enthalpy tzo control the
magnitude of DDGzYꢁH (TDDSYꢁH ꢅ DDHYꢁH). The plot
of TDDSzYꢁH versus s gives a straight line with
slope ¼ 2.63 (Fig. 3). The polarity of II is opposite to
The ETR proceeds through TS II in which —O—O—
bond is cleaved. Comparison of magnitude of DDHYz ꢁH
and DDSzYꢁH tells that the relative rates are controlled by
TDDSzYꢁH. The entropic dominance is derived from the
¼
¼
Figure 2. Plot of DDHYꢁH vs. s
Figure 3. Plot of TDDSYꢁH vs. s
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 625–629

THE RATES CONTROLLED BY TRANSLATIONAL ENTROPY
629
translational degree of freedom occurring in breakage of
—O—O— bond. Entropy-controlled rates may not be
uncommon for the ETR and the homolysis reactions take
place via polar TS.
Concentration of the perester at time t can be obtained
by measuring the magnitude of proton peaks of tert-butyl
group of perester and of methyl of a-methoxy cumene
utilizing Eqn (9).
Iper;t
9
6
Cper;t
Cst;t
¼
ꢀ
¼ aCper;t
(9)
Ist;t
EXPERIMENTAL SECTION
Materials and methods
ꢆ Iper,t: Magnitude of proton peak of tert-butyl group of
the perester at time t.
ꢆ Ist,t: Magnitude of proton peak of methyl group of a-
methoxy cumene.
Substituted benzoic acids, tert-butyl hydroperoxide and
other chemicals were purchased from the major suppliers.
Liquids were distilled with center-cut collection and
solids were purified according to standard procedures if
necessary.14 An NMR spectrometer was used for the
analysis of the peresters.
Accordingly kobs is obtained from Eqn (10).
kHom. and kET are thus acquired by plotting kobs against
concentration of dimethyl sulfide.
Iper;0=Ist;0
Iper;t=Ist;t
Cper;0
ln
¼ ln
¼ kobs
t
(10)
(11)
Cper;t
Preparation of various tert-butyl
perbenzoates
kobs ¼ kHom: þ kET½CH3SCH3ꢃ
All the peresters were prepared according to the known
method.15 IR C O stretchings are shown between
—
Acknowledgements
—
1747 ꢂ 1757 cmꢁ1. p-CH3O, p-CH3 and H peresters are
liquid while p-Cl (48 8C), m-Cl (46 8C), and p-NO2
(79 8C) peresters are solid with melting point indicated in
the parenthesis. The melting points are exactly match the
reported values.151H NMR spectra various YC6H4CO2-
But are as follows: p-OCH3: 6.8 ꢂ 8.0 (m, 4H), 3.86 (s,
3H), 1.40 (s, 9H); p-CH3: 7.2 ꢂ 7.8 (m, 4H), 2.41 (s, 3H),
1.41 (s, 9H); H: 7.2 ꢂ 7.9 (m, 5H), 1.42 (s, 9H); p-Cl:
7.4 ꢂ 7.9 (m, 4H), 1.40 (s, 9H); m-Cl: 7.3 ꢂ 7.8 (m, 4H),
1.38 (s, 9H); p-NO2: 8.1 ꢂ 8.3 (m, 4H), 1.42 (s, 9H).
This work was supported by Inha University Research
Grant of 2006.
REFERENCES
1. Kim SS, Choi SY, Kang CH. J. Am. Chem. Soc. 1985; 107: 4234.
2. Kim SS, Kim CS. J. Org. Chem. 1999; 64: 9261. DOI: 10.1021/
jo9905468.
3. Kim SS, Kim HR, Kim HB, Youn SJ, Kim CJ. J. Am. Chem. Soc.
1994; 116: 2754.
4. Kim SS, Baek IS, Tuchkin A, Go KM. J. Org. Chem. 2001; 66:
4006. DOI: 10.1021/jo10143j.
5. Eyring H. J. Chem. Phys. 1935; 3: 107.
6. Das PK, Encinas MV, Steenken S, Scaiano JC. J. Am. Chem. Soc.
1981; 103: 4162.
ETR of tert-butyl perbenzoates with dimethyl
sulfide
7. Wagner PJ, Cao Q, Pabon R. J. Am. Chem. Soc. 1992; 114: 346.
8. (a) Kim SS, Sohn SC. Tetrahedron Lett. 1982; 23: 3703. (b) Kim
SS, Koo HM, Choi SY. Tetrahedron Lett. 1985; 26: 891.
9. Finn M, Friedline R, Suleman NK, Wohl CJ, Tanko JM. J. Am.
Chem. Soc. 2004; 126: 7578. DOI: 10.1021/ja0493493.
10. Pryor WA, Jr. Hendrickson WH. J. Am. Chem. Soc. 1983; 105:
7114.
tert-Butyl perbenzoate (0.05 M), dimethyl sulfide
(0.5 ꢂ 2.0 M), a-methoxy cumene (0.02 M: internal
standard), trans-stilbene (0.2 M: inhibitor) and KF
(30 mg) were dissolved in CDCl3 (0.4 mL) in a tube.
This mixture was freezed, degassed three times and
sealed. The ampoule (lid. 4 mm, length 3 cm, 2/3 full) was
immersed in a constant temperature bath for more than
200 sec for the thermolysis. Less than 20 sec were
required for complete thermal equilibration, which was
measured by a Copper-Constantan thermo couple. At
various intervals, the tubes were removed from the bath,
quenched in ice water and opened for NMR analysis.
11. Kim SS, Tuchkin A, Kim CS. J. Org. Chem. 2001; 66: 7738. DOI:
10.1022/jo0106290.
12. (a) Blomquist AT, Berstein IA. J. Am. Chem. Soc. 1951; 73: 3412.
(b) Wallling C, Azar JC. J. Org. Chem. 1968; 33: 3888.
(c) Bentrude WG, Martin JC. J. Am. Chem. Soc. 1962; 84: 1561.
13. Leffler JE, Grunwald E. Rates and Equilibria of Organic Reac-
tions. Wiley: New York, 1963.
14. Perrin DD, Armarego WLF, Perrin DF. Purification of Laboratory
Chemicals, 2nd ed., Pergamon press: Oxford, UK, 1980.
15. Blomquist AT, Berstein IA. J. Am. Chem. Soc. 1951; 73: 5546.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 625–629