Transformation of OH-adduct of 1-chloro-4-iodobutane into intra-molecular radical cation in neutral aqueous solution

Article abstract of DOI:10.1016/S0009-2614(98)01370-0

The iodine centered OH-adduct formed on reaction of OH radicals with 1-chloro-4-iodobutane in neutral aqueous solution transforms (k=5.4×10⁵ s^-1) to an intra-molecular radical cation (). The unfavorable structural conformation of solute radical cation generated on reaction of OH radicals with 1-chloro-5-iodopentane does not allow the transformation of OH-adduct into an intra-molecular radical and instead a dimer radical cation () is formed.

Full text of DOI:10.1016/S0009-2614(98)01370-0

5
February 1999
Chemical Physics Letters 300 Ž1999. 493–498
Transformation of OH-adduct of 1-chloro-4-iodobutane into
intra-molecular radical cation in neutral aqueous solution
a
a
b
a,),1
Hari Mohan , D.K. Maity , S. Chattopadhyay , J.P. Mittal
a
Chemistry DiÕision, Bhabha Atomic Research Centre, Mumbai 400 085, India
Bio-organic DiÕision, Bhabha Atomic Research Centre, Mumbai 400 085, India
b
Received 28 September 1998; in final form 16 November 1998
Abstract
Ø
The iodine centered OH-adduct formed on reaction of OH radicals with 1-chloro-4-iodobutane in neutral aqueous
5
y1
solution transforms Žks5.4=10
s
. to an intra-molecular radical cation Ž
.. The unfavorable structural
Ø
conformation of solute radical cation generated on reaction of OH radicals with 1-chloro-5-iodopentane does not allow the
transformation of OH-adduct into an intra-molecular radical and instead a dimer radical cation Ž
q 1999 Elsevier Science B.V. All rights reserved.
. is formed.
1
. Introduction
many experimental and theoretical investigations w5–
x. These bonds can be formed on interaction of an
9
Hydroxyl radicals are known to react with alkyl
oxidized hetero atom such as S, N, P, I, Br with the
free electron pair of a second hetero atom both by
iodides in neutral aqueous solution and form a 2-
center 3-electron bonded OH-adduct which has ab-
sorption bands at 310 and 350 nm w1–3x. Dimer
radical cations formed in acidic solutions have ab-
sorption bands in the 410–450 nm region. On the
other hand, the yield and the life-time of the intra-
molecular radical cation generated from 1,n-di-
iodoalkanes are known to depend strongly on the
chain length between two iodine atoms w4x. In the
case of alkyl chlorides, neither OH-adduct nor OH
radical induced oxidation is observed. Radical cations
derived from organic compounds containing
2
)
inter- and intra-molecular association. 2sr1s
bonding between two dissimilar hetero atoms is pos-
sible if the difference in the electronegativity is small
and stabilization is provided by a favorable structural
conformation for sufficient p-orbital overlap w10–12x.
Generally, the formation of these radical cations is
very fast and is complete within a few hundreds of
nanoseconds. The observation of a 3-electron bonded
OH-adduct and its transformation into a solute radi-
cal cation in neutral solutions has not been reported
in the literature so far. The transformation of OH-ad-
duct does not take place in neutral solutions while it
is too fast to be detected in acidic solutions. The only
example of OH-adduct transforming into a solute
radical cation has been reported at pHs3.7 w13x.
Ø
)
sr1s three-electrons have been the subject of
)
Corresponding author. Fax: q91 22 550 5151
Also affiliated as Honorary Professor with the Jawaharlal
1
This conversion was fast and was complete within
Nehru Centre for Advanced Scientific Research, Bangalore, India.
600 ns w13x.
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž98.01370-0

4
94
H. Mohan et al.rChemical Physics Letters 300 (1999) 493–498
If the difference in the electronegativity of two
the mixture was heated at 808C for 2 h. It was
poured in benzene and the benzene layer washed
carefully with water, 10% aqueous Na CO , water
and brine. After drying, the residue was distilled to
get 5-chloropentyl acetate Ž33.5 g, b.p. 95–988Cr9
mm.. It Ž33.5 g, 0.2 mol. was then mixed with 6%
aqueous methanolic KOH and warmed to 608C. Af-
ter 2 h, the mixture was concentrated and the residue
in ether layer was washed with water and brine and
dried. Removal of the solvent gave 5-chloropentanol
Ž13.5 g, b.p. 80–818Cr8 mm. w15x.
hetero atoms is comparatively large and stabilization
of the solute radical cation is provided by a favorable
structural conformation, the transformation of OH-
adduct to a solute radical cation could be a too slow
process to be observed in microsecond time scale.
Given the large difference in the electronegativities
of iodine and chlorine Ž0.62 eV. and the possibility
of formation of a stable six-membered ring configu-
ration for the solute radical cation of 1-chloro-4-
iodobutane, the transformation of OH-adduct to so-
lute radical cation in neutral solutions might be slow
enough and detectable in microsecond time scale.
With this objective, 1-chloro-n-iodoalkanes were
synthesized and the results of pulse radiolysis studies
on their aqueous solutions are reported here.
2
3
2
.4. Preparation of 1-chloro-n-iodoalkanes
2
. Experimental
To a cooled Ž08C. and stirred mixture of the
2
.1. Synthesis of 1-chloro-n-iodoalkanes
chloroalkanol Ž0.01 mol. and NaI Ž0.011 mol. in
acetonitrile Ž20 ml., under Ar, was slowly injected
TMSCl Ž0.011 mol.. The mixture was brought to
room temperature and stirred for 12 h. It was poured
in ice–water, extracted with pentane. The pentane
layer was washed with aqueous Na S O , water and
1
-chloro-4-iodobutane ŽCIB. and 1-chloro-5-
iodopentane ŽCIP. were synthesized and purified in
the laboratory just before use and the detailed proce-
dure is mentioned below.
2
2
3
brine and dried. The concentration of the extract
followed by repeated Ž4–6 times. column chro-
matography of the residue over silica gel Žpentane as
eluent. gave the respective pure 1-chloro-n-iodoal-
2
.2. Preparation of 4-chlorobutanol
HCl Žgas. was bubbled slowly through refluxing
tetra hydrofuran Ž50.7 g, 0.79 mol.. The temperature
kanes.
of the reaction mixture rose gradually during 4 h. It
was heated for further 2 h when the temperature
became steady. The resultant brown liquid was evac-
uated at 50 mm pressure. The resultant mixture was
then distilled to get 4-chlorobutanol Ž45 g, b.p. 75–
The purity of the products was checked with
NMR Žin CDCl , Bruker AC-200, 200 MHz. and IR
3
ŽPerkin-Elmer IR spectrophotometer model 837..
7
88Cr6 mm. w14x.
2
.3. Preparation of 5-chloropentanol
2
.5. Pulse radiolysis studies
Dry tetrahydropyran Ž30.1 g, 0.35 mol. was added
to a mixture of acetyl chloride Ž23.6 g, 3 mol. and
The pulse radiolysis studies were performed with
dry ZnCl Ž5 g.. After the initial reaction subsided,
high-energy Ž7 MeV. electron pulses Ž50 ns. ob-
2

H. Mohan et al.rChemical Physics Letters 300 (1999) 493–498
495
tained from a linear electron accelerator w16x. The
dose delivered per pulse was determined by using an
aerated aqueous solution of KSCN and it was close
the transient absorption at 315 nm, was found to
increase linearly with solute concentration, Ž0.3–1.6.
y3
y3
=10
mol dm . The bimolecular rate constant,
y1
to 15 Gy Ž1 Gys1 J kg .. The solutions were
determined from the linear plot of k
vs. solute
obs
9
3
y1
prepared in deionized ‘nano-pure’ water containing
concentration gave a value of 4.7=10 dm mol
y3
y3
y1
1
=10
mol dm
phosphate buffer. All other
s
. The initial portion of the transient absorption at
details about sample preparation and kinetic analysis
are available elsewhere w11,12x.
315 and 360 nm decayed by first-order kinetics with
5
y1
ks5.2=10 s Žinset of Fig. 1.. Simultaneously,
the transient absorption at 420 nm was observed to
5
y1
grow by first-order kinetics with ks5.4=10 s
3
. Results and discussion
Žinset of Fig. 1., matching with the decay at 315 nm.
Time-resolved studies showed the formation of
The transient optical absorption spectrum ob-
transient absorption band at 420 nm ŽFig. 1b.. The
tained on pulse radiolysis of N O-saturated neutral
latter portion of the transient absorption at 315 and
2
aqueous solution of CIB exhibited bands with lmax
s315 and 360 nm ŽFig. 1a.. These bands were not
observed in the presence of t-butanol Ž0.3 mol
420 nm decayed by first-order kinetics with ksŽ3
4
y1
"1.=10 s . These studies also revealed that the
transient absorption bands at 315 and 360 nm ŽFig.
1a. were due to a transient species which on decay
y3
Ø
Ø
dm ., an efficient OH radical and weak H atom
scavenger, indicating that the contribution of reaction
of H atom with the solute was negligible. Thus, the
gave another transient species absorbing at 420 nm
ŽFig. 1b.. The transient absorption at 315 nm re-
mained independent of solute concentration Ž0.4–2.
Ø
absorption spectrum ŽFig. 1a. was due to the reaction
Ø
y3
y3
of OH radicals with CIB. The pseudo-first-order
=10 mol dm , suggesting that the entire amount
Ø
Ø
rate constant Žkobs. for the reaction of OH radicals
of OH radicals have reacted with the solute and the
with CIB, determined on monitoring the growth of
transient species was not a dimer. Under these condi-
tions, the concentration of the transient species could
be taken equal to the concentration of OH radicals
Ø
and the molar absorptivity at 315 nm was determined
3
3
y1
y1
to be 1.9=10 dm mol cm
.
Pulse radiolysis of N O-saturated aqueous solu-
2
tion of 1-iodobutane showed the formation of tran-
sient absorption bands at 315 and 360 nm without
any transformation into any other transient species.
The absorption bands were assigned to the OH-ad-
duct. Pulse radiolysis of N O-saturated aqueous solu-
2
tion of 1-chlorobutane did not show these transient
absorption bands. Therefore, the absorption bands
observed on pulse radiolysis of N O-saturated aque-
2
ous solution of CIB are assigned to iodine centered
OH-adduct wScheme 1, Ža.x. It is proposed that this
OH-adduct undergoes transformation to an intra-
molecular radical cation which is stabilized due to
the formation of a six-membered ring configuration
wScheme 1, Žb.x.
Fig. 1. Transient absorption spectra obtained on pulse radiolysis of
y3
y3
3.1. EÕidences in support of intra-molecular radical
cation
N O-saturated aqueous solution of CIB Ž1.6=10
mol dm
,
2
y3
y3
pHs6.5. in 1=10 mol dm phosphate buffer 1 Ža. and 6 ms
after a 50 ns electron pulse Žb.. Inset show absorption–time
1
7
Ža. If the proposed reaction mechanism is opera-
tive, then OH-adduct should not undergo any trans-
profiles at 315 Ža. and 420 nm Žb.. Dose per pulses1=10 eV
y1
g
.

4
96
H. Mohan et al.rChemical Physics Letters 300 (1999) 493–498
state absorption spectrum of CIB did not show any
variation with pH in this region. At pHs2, the
transient absorption spectrum showed a band at 420
nm ŽFig. 2b.. The absorbance of this band remained
independent of solute concentration, suggesting it to
be due to a monomeric species. Simple iodine cen-
tered monomer radical cations of alkyl iodides ab-
sorb at 310 nm w3x. Therefore, the transient absorp-
tion ŽFig. 2b. is not due to a simple iodine centered
Scheme 1.
radical cation. It could be due to a intra-molecular
radical cation formed on p-orbital overlap of oxi-
dized iodine with Cl ŽScheme 1.. Intra-molecular
formation at higher pH. In addition, at lower pH, the
transformation should be very fast and formation of
only intra-molecular radical cation absorbing at 420
nm is expected.
The transient absorption spectrum obtained on
pulse radiolysis N O-saturated aqueous solution of
radical cation formed on p-orbital overlap between
two halogens are reported to absorb in this region
w4,11,12,17x. Under these conditions, the molar ab-
sorptivity of the transient band at 420 nm was deter-
3
3
y1
y1
mined to be 5.5=10 dm mol
cm . The bi-
Ø
molecular rate constant for the reaction of OH with
2
CIB at pHs9.8 exhibited absorption bands at 310
and 340 nm ŽFig. 2a., without any growth at 420 nm
Žinset of Fig. 2, c.. The transient absorption spectrum
CIB at pHs2, determined by formation kinetic
9
3
studies at 420 nm, gave a value of 3.5=10 dm
y1 y1
mol
s
. The band at 420 nm decayed by first-
4
y1
at pHs9.8 is blue-shifted as compared to the spec-
trum obtained at neutral pH. Although the exact
reason for this shift is not known at present, it may
not be due to the hydrolysis of the solute as Iy
formed on hydrolysis would give an absorption band
at 385 nm. The nature of the transient species is not
expected to be different as the time-resolved studies
at neutral and acidic solutions have shown the tran-
sient absorption band at 420 nm. The absorbance and
growth rate of the transient absorption band at 420
nm increased at lower pH, indicating that the trans-
formation of OH-adduct at lower pH is very fast and
is in accordance with the proposed reaction mecha-
nism ŽScheme 1.. The pseudo-first-order rate con-
stant Žkobs. determined by monitoring the growth of
order kinetics with ks2=10 s ŽTable 1..
The comparison of interhalogen intra-molecular
Ø
radical cations revealed that: Ž1. the site of OH
the transient absorption at 420 nm as a function of
q
H
2
concentration gave a bimolecular rate constant of
8
3
y1 y1
.9=10 dm mol
s
. This value is two orders
of magnitude less than that observed for the reaction
Ø
of OH radicals with methyl thiomethylacetate w13x.
This explains the reason for observing the transfor-
mation of OH-adduct of CIB into solute radical
cation. Variation of normalized absorbance wD O.Dr
GŽOH.x at 420 nm gave an approximate value of
inflection point at pHf4 Žinset of Fig. 2.. This
Fig. 2. Transient absorption spectra obtained on pulse radiolysis of
y3
y3
y3
N O-saturated solution of CIB Ž1.6=10 mol dm . in 1=10
2
y3
mol dm
phosphate buffer at pHs9.8 Ža. and pHs2 Žb.. Inset
show absorption time profiles at 420 nm at pHs9.8 Žc. and
pHs2 Žd. and variation of normalized absorbance Ž420 nm. as a
function of pH Že..
should be due to the transformation of OH-adduct
into solute radical cation ŽScheme 1., as the ground

H. Mohan et al.rChemical Physics Letters 300 (1999) 493–498
497
Table 1
Ø
Kinetic parameters for the transient species formed on reaction of OH radicals with CIB and CIP
Solute
CIB
pH
lmax
kf
´
kd
3
y1
y1.
s
Ždm mol cmy1.
3
y1
Žsy1.
Ždm mol
4.7=109
7
315, 360
1.9=103 a
–
5.5=10
–
–
5.2=105
3.1=10
2.1=10
6.5=10
4
4
20
–
9
3 b
4
CIB
CIP
CIP
2
7
1
420
310, 350
430
3.5=10
3.8=10
2.1=10
9
5
9
4
6.3=10
a
3
15 nm.
b
4
20 nm.
radical attack is the halogen having lower elec-
at 310 and 350 nm ŽFig. 3a. without any transforma-
tronegativity and Ž2. the concentration of Hq re-
tion. The bands are slightly blue-shifted as compared
to those of OH-adduct of CIB at neutral pH. In
acidic solutions, the transient absorption spectrum
showed a band at 430 nm ŽFig. 3b.. The absorbance
at 430 nm was found to depend strongly on solute
concentration Žinset of Fig. 3., suggesting the 430
nm band to be due to a dimer radical cation. In
absence of a stable seven-membered ring configura-
tion in the case of CIP, inter-molecular dimer radical
cation was formed. The absence of transformation of
quired for acid-catalyzed dehydration was observed
to depend on the ionization potential of the alkyl
halide. Alkyl bromides having an ionization potential
y3
in the 10.1–10.3 eV region required G3 mol dm
HClO for the formation of radical cations as com-
4
y2
y3
pared to 10
mol dm
required in the case of
alkyl iodides whose ionization potential is in the
range of 9.1–9.4 eV. The stability of radical cation is
also observed to depend on the electronegativity
ŽE . of halogen. The iodine centered ŽE s2.21
N
N
eV. intra-molecular radical cation of 1-iodo-4-chlo-
robutane and 1-bromo-4-iodobutane has a half-life of
3
4 and 20 ms, respectively, whereas the bromine
centered ŽE s2.74 eV. intra-molecular radical
N
cation of 1-bromo-4-chlorobutane has a half-life of 9
ms. The presence of halogen having higher elec-
tronegativity has lowered the life time of the intra-
molecular radical cation. Chlorine ŽE s2.83 eV.
N
centered intra-molecular radical cations of alkyl
chlorides could not be experimentally observed. The
difference in the electronegativity of two halogens
along with the favorable structure of the interhalogen
intra-molecular radical cation seems to play an im-
portant role in the stabilization of the intra-molecular
radical cation.
Žb. The evidence for the assignment of the tran-
sient absorption spectrum obtained on pulse radioly-
sis of CIB to intra-molecular radical cation came
from the pulse radiolysis studies on CIP. The solute
radical cation of CIP would form a seven-membered
ring configuration which is expected to be very
Fig. 3. Transient absorption spectra obtained on pulse radiolysis of
y3
y3
N O-saturated aqueous solution of CIP Ž1.4=10 mol dm . at
2
y3
y3
pHs7 Ža. and pHs1 Žb. in 1=10
buffer. Inset shows variation of absorbance at 430 nm as a
function of solute concentration.
mol dm
phosphate
unstable. Pulse radiolysis of N O-saturated neutral
2
aqueous solution of CIP exhibited absorption bands

4
98
H. Mohan et al.rChemical Physics Letters 300 (1999) 493–498
indicating electron transfer from Iy to CIBØq wreac-
tion Ž2.x.
OH-adduct to solute radical cation at neutral solution
and formation of a dimer radical cation supported the
assignment of intra-molecular radical cation for CIB.
In the case of CIB, the formation of a stable six-
membered ring configuration has favored the trans-
formation of its OH-adduct to intra-molecular radical
cation. In contrast, with CIP, the unstable seven-
membered ring configuration did not favor the con-
version of OH-adduct into an intra-molecular radical
cation but led to a dimer radical cation in acidic
solutions.
CIBØqqI rBr ™ CIBqI rBr .
y
y
Ø
Ø
Ž .
2
The bimolecular rate constant was determined to be
9
3
y1
y1
4.8=10 dm mol
s
. Similarly, the electron
y
Øq
transfer was also observed from Br to CIB
wreaction Ž2.x with a bimolecular rate constant of
9
3
y1
y1
3=10 dm mol
s
. The electron transfer is
y
Øq
possible from Br to CIB , therefore, the oxidation
Øq
potential of CIBrCIB couple should be more than
Øy
y
o
Žc. The evidence in support of the assignment of
that of Br r2 Br couple ŽE s1.62 V..
2
the transient absorption band to radical cation was
also obtained from the electron transfer studies. Cl 2Ø y
is a strong one-electron oxidant with oxidation po-
All these studies suggested that the transient
Ø
species formed on reaction of OH radicals with CIB
is cationic in nature and the oxidation potential of
Øq
tentials2.06 V. The decay of the transient band of
CIBrCIB couple is between 1.6 and 2.1 V.
Øy
Cl , formed on pulse radiolysis of aerated acidic
2
y
y2
y3
aqueous solution of Cl Ž2=10
mol dm , pH
s1, ls345 nm., was observed to decay faster on
References
y4
addition of low concentration of CIB Ž1–3.=10
y3
mol dm
and the bimolecular rate constant was
w1x J.K. Thomas, J. Phys. Chem. 71 Ž1967. 1919.
8
3
y1 y1
2
w x
U. Bruhlmann, H. Buchler, F. Marchetti, R.E. Buhler, Chem.
Phys. Lett. 21 Ž1973. 412.
determined to be 1.6=10 dm mol
s
. Time-
resolved studies have also shown increased absorp-
w3x H. Mohan, K.-D. Asmus, J. Chem. Soc., Perkin Trans. II
tion in the 400–450 nm region. Thus it supported
Ž1987. 1795.
Øy
electron transfer from CIB to Cl2 wreaction Ž1.x and
4
w x
H. Mohan, K.-D. Asmus, J. Am. Chem. Soc. 109 1987
4745.
Ž
.
assignment of the transient absorption band to the
solute radical cation. Therefore, the oxidation poten-
w5x K.-D. Asmus, Acc. Chem. Res. 12 Ž1979. 436.
w6x K.-D. Asmus, in: C. Chatgilialoglu, K.-D. Asmus ŽEds..,
Øq
tial of CIBrCIB couple should be less than that of
Sulfur Centered Reactive Intermediates in Chemistry and
Biology, NATO Adv. Stud. Inst. Ser. A, vol. 197: Life
Sciences, Plenum, New York, 1990, p. 155.
w7x T. Clark, J. Comput. Chem. 4 Ž1983. 404.
w8x M.L. McKee, J. Phys. Chem. 97 Ž1993. 10971.
w x
Øy
y
o
the Cl r2Cl couple ŽE s2.06 V..
2
Cl qCIB ™ CIB q2Cly .
Øy
Øq
Ž .
1
2
However, earlier pulse radiolysis studies w11x had
Øy
9
S. Ekern, A. Illies, M.L. McKee, M. Peschke, J. Am. Chem.
shown that Cl2 is unreactive towards 1-bromo-2-
Soc. 115 Ž1993. 12510.
w10x E. Anklam, H. Mohan, K.-D. Asmus, J. Chem. Soc., Chem.
chloroethane, which may be due to the presence of
bromine having a higher electronegativity Ž2.74 eV.
as compared to iodine Ž2.21 eV.. The presence of
Commun. Ž1987. 629.
D.K. Maity, H. Mohan, S. Chattopadhyay, J.P. Mittal, J.
Phys. Chem. 99 Ž1995. 12195.
w11
x
halogen having higher electronegativity must have
increased the oxidation potential. Fluorine in alkyl
iodides has also increased the oxidation potential of
alkyl iodides and one-electron oxidation of fluori-
nated alkyl iodides becomes difficult as compared to
alkyl iodides w3,17x.
w12x D.K. Maity, H. Mohan, S. Chatopadhyay, J.P. Mittal, Radiat.
Phys. Chem. 49 Ž1997. 21.
w13x K. Bobrowski, C. Schoneich, J. Chem. Soc., Chem. Com-
mun. Ž1993. 795.
w14
x
D. Starr, R.H. Hixon, in: A.H. Blatt Ed. , Organic Synthesis,
vol. 2, Wiley, New York, 1963, p. 571.
Ž
.
w15x S. Voerman, Agric., Ecosyst. Environ. 21 Ž1988. 31.
w16x K.I. Priyadarsini, D.B. Naik, P.N. Moorthy, J.P. Mittal, 7th
Tihany Symposium on Radiation Chemistry, Hung. Chem.
Soc., Budapest, 1991, p. 105.
The decay of the transient absorption band of
Øq
CIB , formed on pulse radiolysis of N O-saturated
2
y3
y3
acidic aqueous solution of CIB Ž4=10 mol dm
,
pHs1, ls420 nm. was accelerated on addition of
w17
x
H. Mohan, D.K. Maity, J.P. Mittal, Chem. Phys. Lett. 220
y
y5
y3
low concentration of I Ž2.5=10
mol dm .,
Ž1994. 455.