Kinetics and mechanism of the quinolinium fluorochromate oxidation of some primary, secondary and unsaturated alcohols in acetonitrile

Article abstract of DOI:10.1135/cccc20010877

Quinolinium fluorochromate (QFC) in MeCN readily oxidises primary and secondary alcohols to the corresponding carbonyl compounds. Solvent effect on reactivity is quite significant. The reaction can be characterized by the rate equation v = k₃K₁K₂[Alc][TsOH][QFC]_t/ {1 + [TsOH](K₁ + K₁K₂[Alc])}. The (black star) value ca -1.6 is indicative of a partial bonding in the linear transition state envisaged. Large primary kinetic isotope effect (k^H/k^D > 10) in this study suggests a concerted, two-electron hydride transfer mechanism. Mechanistic studies on the oxidation of unsaturated alcohols with QFC in acetonitrile medium described by the rate equation k₀ = k₂K₁[TsOH]² + k₄K₃K1[Alc][TsOH] is verified. Product analysis studies rule out the possibility of attack on the double bond. Linear relation in the Exner plots implies the operation of a common mechanism in all the alcohols studied. The high reactivity observed in the cinnamyl alcohol oxidation can be ascribed to resonance interaction through conjugated double bonds.

Full text of DOI:10.1135/cccc20010877

Quinolinium FluorochromateOxidation
877
KINETICS AND MECHANISM OF THE QUINOLINIUM
FLUOROCHROMATE OXIDATION OF SOME PRIMARY,
SECONDARY AND UNSATURATED ALCOHOLS IN ACETONITRILE
1,
2
Subbiah MEENAKSHISUNDARAM * and Ramanathan SOCKALINGAM
Department of Chemistry, Annamalai University, Annamalainagar-608 002, India;
1
2
e-mail: meenas1957@rediffmail.com, dr_rmso@rediffmail.com
Received Jan uary 2, 2001
Accepted May 20, 2001
Quin olin ium fluoroch rom ate (QFC) in MeCN readily oxidises prim ary an d secon dary alco-
h ols to th e correspon din g carbon yl com poun ds. Solven t effect on reactivity is quite
sign ificant. The reaction can be characterized by the rate equation v = k3K1K2[Alc][TsOH][QFC]t/
{1 + [TsOH](K1 + K1K2[Alc])}. Th e ρ* value ca –1.6 is in dicative of a partial bon din g in th e
lin ear tran sition state en visaged. Large prim ary kin etic isotope effect (kH/kD > 10) in th is
study suggests a con certed, two-electron h ydride tran sfer m ech an ism . Mech an istic studies
on th e oxidation of un saturated alcoh ols with QFC in aceton itrile m edium described by th e
rate equation k0 = k2K1[TsOH]2 + k4K3K1[Alc][TsOH] is verified. Product an alysis studies rule
out th e possibility of attack on th e double bon d. Lin ear relation in th e Exn er plots im plies
th e operation of a com m on m ech an ism in all th e alcoh ols studied. Th e h igh reactivity ob-
served in th e cin n am yl alcoh ol oxidation can be ascribed to reson an ce in teraction th rough
con jugated double bon ds.
Keyw ord s: Alcoh ols; Quin olin ium fluoroch rom ate; Ch rom ates; Oxidation s; Aldeh ydes; Reac-
tion kin etics; Reaction m ech an ism s; Solven t effects.
Am on g th e selectively actin g oxo derivatives of variable valen ce m etals,
ch rom ium com poun ds play th e m ost im portan t role, because th e oxidative
reaction s based on th em can be readily carried out. Th e m ajor use of Cr(VI)
reagen ts in syn th etic ch em istry is in th e oxidation of alcoh ols to carbon yl
com poun ds. As a result, alcoh ol oxidation with th ese was exten sively in ves-
tigated. Th e n um ber of n ew Cr(VI) reagen ts togeth er with special reaction
con dition s h ave been developed. An h ydrous con dition s are m ore con du-
cive to com plexation of substrates with Cr(VI) species an d th erefore to m ild
oxidation . Th e m ost sign ifican t recen t Cr(VI) oxidan ts are quin olin ium
fluoroch rom ate1,2 (QFC), pyridin ium ch loroch rom ate3,4 (PCC), pyridin ium
dich rom ate5 (PDC), pyridin ium fluoroch rom ate6–13 (PFC) an d quin olin ium
ch loroch rom ate14 (QCC). QFC in dich lorom eth an e readily oxidises pri-
m ary, secon dary an d allylic alcoh ols to th e correspon din g carbon yl com -
Collect. Czech. Chem. Commun. (Vol. 66) (2001)
doi: 10.1135/cccc20010877

878
Meenakshisundaram, Sockalingam:
poun ds. Pan duran gan et al.15 reported th e kin etics an d m ech an ism of
substituted ben zyl alcoh ols with QFC in AcOH–H2O m edium . No kin etic
an d m ech an istic study of th e QFC oxidation of alcoh ols in an h ydrous con -
dition s h ave appeared. Here, we exam in e th e kin etics an d m ech an ism of
oxidation of twelve prim ary an d secon dary alcoh ols an d th ree un saturated
alcoh ols with QFC in aceton itrile m edium . A com parison is also m ade with
th e correspon din g reaction s of structurally sim ilar oxidan ts.
Coordin ation com plexes of ch rom ium can be used successfully for th e
con version of h ydroxy in to carbon yl groups in un saturated alcoh ols. Th e
oxidation of un saturated system s h avin g fun ction al groups as substituen ts
fin ds application s in th e syn th esis of h orm on es, ph erom on es,
prostaglan din s an d th eir syn th on es. Th e presen t study describes th e kin et-
ics of oxidation of th ree un saturated alcoh ols, viz. allyl alcoh ol,
but-2-en -1-ol an d cin n am yl alcoh ol in aceton itrile m edium .
EXPERIMENTAL
All th e ch em icals used were reagen t grade products. Th e alcoh ols were dried over an h ydrous
m agn esium sulfate an d fraction ally distilled before use. Th e reaction s were perform ed un der
pseudo-first-order con dition s by m ain tain in g large excess (1 : 100 or h igh er) of alcoh ols over
QFC. Th e reaction m ixture was h om ogen eous th rough out th e course of th e reaction . Th e re-
action s were followed by m on itorin g th e decrease in absorption of QFC at 360 n m em ploy-
in g
a JASCO 7200 UV-VIS spectroph otom eter with a variable-tem perature accessory.
Duplicate kin etic run s sh owed th at th e rate con stan ts obtain ed by titrim etric procedure
agreed with in ±3%. All th e rate con stan ts are average of two or m ore determ in ation s.
Th e stoich iom etry of th e reaction was determ in ed for a n um ber of reaction m ixtures con -
tain in g at least a th reefold excess of oxidan t over th e substrate. Th e estim ation in dicated
th at two m oles of th e oxidan t reacted with th ree m oles of th e substrate.
3 RCH2OH + 2 Cr(VI) → 3 RCHO + 6 H+ + 2 Cr(III)
Oxidan t (0.010 m ol), m eth an ol (0.015 m ol) an d 4-m eth ylben zen e-1-sulfon ic acid (TsOH;
0.015 m ol) were m ixed. After 24 h , th e solven t was evaporated. Th e residue was extracted
with eth er an d th e product obtain ed was iden tified by th e form ation of
2,4-din itroph en ylh ydrazon es. Th e keton es were also detected by ch aracteristic spot tests16
.
RESULTS AND DISCUSSION
Oxidation of Primary and Secondary Alcohols
Un der pseudo-first-order con dition s, it was observed th at first-order plots
sh owed curvature after 25–30% con version of QFC, i.e., th e reaction in -
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Quinolinium FluorochromateOxidation
879
volves two distin ct path ways – an in itial rapid step followed by a slow on e
due to depletion of QFC (Fig. 1). Th e depressed reactivity observed after
som e tim e m ay probably be due to th e in fluen ce of th e products on th e re-
action . Th is is rem in iscen t of th e effect n oted by Ram ach an dran et al. in
th e study of HSO –5 oxidation of am in o acids17. In Br(V) oxidation of
cin n am ic acids18 slow an d subsequen t fast reaction s are attributed to two
reactive species, Br(V) an d m olecular brom in e. In itial-rate m eth od was used
to determ in e th e order with respect to reactan ts in th e oxidation of alco-
h ols. A plot of k0–1 versus [Alc]–1 (Fig. 2) (r = 0.97; s = 55.5) sh ows an in ter-
cept on th e y-axis in dicatin g a Mich aelis–Men ten depen den ce on th e
con cen tration of alcoh ol. Sam e fraction al order depen den ce on th e con cen -
tration of alcoh ol is observed for th e oth er prim ary an d secon dary alcoh ols
studied. Th e kin etic order in TsOH used for catalysin g th e con version , ap-
pears to be fraction al, approach in g zero at h igh acid con cen tration s (Fig. 3).
In itial slope of ca 0.47 becom es virtually zero for acid con cen tration s
h igh er th an ca 2 · 10–2 m ol dm –3. Th e sam e tren d is observed in th e oxida-
tion of isopropyl alcoh ol with QFC. Th e kin etic results are sum m arised in
Table I.
Perm ittivity (D) values are approxim ately calculated from th e values for
pure solven ts. Th e ch an ge of perm ittivity of th e m edium was affected by
0.2
0.0
–0.2
3
2
–0.4
1
–0.6
0
250
500
750
1 000
time, s
FIG. 1
Tim e plots of logarith m s of absorban ce for th e oxidation of m eth an ol with quin olin ium
fluoroch rom ate (QFC) in aceton itrile m edium . 102 [MeOH] = 10.80 m ol dm –3, 103 [TsOH] =
7.53 m ol dm –3; 1 104 [QFC] = 4.76 m ol dm –3, 2 104 [QFC] = 6.98 m ol dm –3, 3 104 [QFC] =
9.52 m ol dm –3
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880
Meenakshisundaram, Sockalingam:
addition of dioxan e. A sm all ch an ge in th e D (37.50 → 33.99) of th e m e-
dium en h an ces th e rate appreciably (1.09 · 10–3 → 8.45 · 10–3 s–1) (Table II).
It can be seen th at th e solven t effect on reactivity is quite sign ifican t. A plot
of log k0 versus D–1 sh ows curvature. Such rate en h an cem en t in a less po-
lar solven t would be ascribed to th e facility of form ation of Cr(VI) esters19.
A com plex m ech an ism m ay also be a con tributin g factor to th is observa-
2 500
2 000
1 500
1 000
500
0
0
5
10
15
20
3
–1
1/[MeOH], dm mol
FIG. 2
Th e in verse–in verse plot for th e reaction between m eth an ol an d quin olin ium fluoro-
ch rom ate at 303 K. 103 [TsOH] = 7.53 m ol dm –3, 104 [QFC] = 4.76 m ol dm –3
1.6
1.2
0.8
0.4
0
1
2
3
4
5
3
–3
10 [TsOH], mol dm
FIG. 3
A plot of k0 versus [TsOH]. 104 [QFC] = 4.76 m ol dm –3, 102 [m eth an ol] = 10.80 m ol dm –3
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Quinolinium FluorochromateOxidation
881
tion . No polym erisation with acrylon itrile is observed. Furth er, th e rate of
con version is in varian t wh en acrylon itrile is added. Th ere was an appre-
ciable ch an ge in th e rate with th e ch an ge in ion ic stren gth of th e m edium
(Table II).
We h ave also exam in ed th e oxidation of CD3OD (m eth an ol-d4). Th e
pseudo-first-order rate con stan ts were derived from th e in itial-rate m eth od.
Kinetic behaviours are same as that of CH3OH. The rate increases with [alcohol]
with a fraction al slope an d th e fraction al order depen den ce on acidity ap-
proach es to zero at h igh acid con cen tration s. Th e kin etic data are sum m a-
rised in Tables III an d IV. Th e rate con stan ts were m easured at 293, 303 an d
313 K. Activation param eters evaluated from th e Eyrin g plots are listed in
Table V. Som e of th e reaction s were too fast to be followed spectroph oto-
TABLE I
Effect of reactan ts on th e rate of oxidation of m eth an ol in aceton itrile at 303 K
a
104 [QFC]
m ol dm –3
102 [MeOH]
m ol dm –3
103 [TsOH]
m ol dm –3
103 ko
s–1
2.54
4.76
6.98
9.52
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
10.80
10.80
10.80
10.80
4.31
7.53
7.53
1.02
1.09
1.05
1.02
0.45
0.71
1.09
1.38
1.79
0.54
0.74
1.09
1.26
1.50
1.46
1.47
7.53
7.53
7.53
7.53
7.53
10.80
16.14
21.52
10.80
10.80
10.80
10.80
10.80
10.80
10.80
7.53
7.53
7.53
2.15
4.30
7.53
10.80
21.50
32.30
43.00
a
k0 represen ts th e in itial rate con stan t.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

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Meenakshisundaram, Sockalingam:
m etrically at 313 K, wh ich led to a rough estim ate of rate con stan ts at h igh
tem peratures.
Correlation Analysis
Th e rate data fit reason ably well with Taft’s20 in ductive σ* values an d steric
substitution con stan ts, Es, separately.
At 303 K,
log k3 = –1.75σ* – 1.25
(1)
(r = 0.986; s = 0.076; n = 7) ,
wh ere k3 represen ts th e rate coefficien t of slow step.
TABLE II
Effect of acrylon itrile an d LiClO4 con cen tration an d solven t com position on th e reaction
rate at 303 K
[QFC] = 4.76 · 10–4 m ol dm –3
[MeOH] = 10.80 · 10–2 m ol dm –3
[TsOH] = 7.53 · 10–3 m ol dm –3
103 [AN], m ol dm –3
104 [LiClO4], m ol dm –3
D
103 ko, s–1
0.0
1.43
4.30
7.17
10.04
0.0
0.0
0.0
37.50
37.50
37.50
37.50
37.50
37.50
37.50
37.50
37.50
37.50
36.79
36.09
35.39
33.99
1.09
0.91
0.91
0.94
0.92
1.00
0.83
0.72
0.67
1.09
4.46
5.87
7.47
8.45
0.0
0.0
0.0
2.57
5.14
7.71
10.27
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
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Quinolinium FluorochromateOxidation
883
TABLE III
Effect of oxidan t, substrate an d acid con cen tration on th e reaction rate in aceton itrile at 303 K
104 [QFC]
m ol dm –3
102 [CD3OD]
m ol dm –3
103 [TsOH]
m ol dm –3
103 ko
s–1
1.90
3.17
4.76
6.98
9.52
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
4.76
10.80
10.80
10.80
10.80
10.80
6.90
7.53
7.53
7.53
7.53
7.53
7.53
7.53
7.53
7.53
7.53
2.15
4.30
7.53
12.90
21.50
32.30
1.09
1.07
1.08
1.08
1.06
0.68
1.08
1.56
2.16
2.46
0.72
1.16
1.56
1.91
1.86
1.88
10.80
20.70
31.10
41.40
20.70
20.70
20.70
20.70
20.70
20.70
TABLE IV
Effect of solven t com position on th e reaction rate
[QFC] = 4.76 · 10–4 m ol dm –3
[CD3OD] = 10.80 · 10–2 m ol dm –3
[TsOH] = 7.53 · 10–3 m ol dm –3
D
104 ko, s–1
37.50
37.15
36.79
36.44
36.09
34.68
1.56
28.40
43.40
51.00
58.80
71.00
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Meenakshisundaram, Sockalingam:
log k3 = –0.674Es – 1.25
(r = 0.977; s = 0.098; n = 7)
(2)
A good correlation is also obtain ed with Pavelich –Taft dual substituen t
param eter equation 21
.
log k3 = –1.67σ* – 0.032Es – 1.253
(3)
(100R = 98.6; SE = 0.085; n = 7)
Few alcoh ols are excluded from th e correlation sin ce th e correspon din g
substituen t con stan ts are n ot available. Th e n egative polar reaction con -
stan t in dicates an electron -deficien t tran sition state. Th e n egative steric re-
action con stan t sh ows steric acceleration . Steric con tribution s seem to be
m uch lower th an th e in ductive effect.
TABLE V
Rate con stan ts an d activation param eters for th e oxidation of alcoh ols with QFC in
aceton itrile
[QFC] = 4.76 · 10–4 m ol dm –3
[TsOH] = 7.53 · 10–3 m ol dm –3
103 k3, s–1
∆G#
∆H#
–∆S#
No.
Substrate
(303K)
r
s
kJ m ol–1
J K–1 m ol–1
kJ m ol–1
293 K
303 K
313 K
1
2
3
4
5
6
Meth an ol
Meth an ol-d4
Eth an ol
3.78
0.19
7.22
0.53
14.44
0.97
48.6
60.1
19.1
20.3
19.1
21.0
125.6
110.7
204.6
198.9
201.0
195.5
86.6
93.6
81.2
80.5
80.1
80.2
0.999 0.039
0.992 0.144
0.987 0.058
0.990 0.053
0.990 0.050
0.999 0.021
47.95
60.07
74.04
70.79
68.54
87.42
106.36
94.77
83.97
Propan -1-ol
Propan -2-ol
Butan -1-ol
109.67
130.27
130.86
2-Meth yl-
propan -1-ol
7
64.57
88.34
111.39
18.3
205.0
80.5
0.997 0.026
8
9
Butan -2-ol
Pen tan -1-ol
Pen tan -2-ol
Hexan -1-ol
Heptan -1-ol
111.79
73.71
119.74
69.27
74.50
131.13
99.87
154.97
127.22
162.05
125.36
128.41
9.9
18.3
8.8
229.3
204.0
232.6
197.9
203.8
79.4
80.1
79.3
80.2
80.1
0.998 0.012
0.999 0.017
0.999 0.001
0.998 0.021
0.995 0.034
10
11
12
140.60
96.56
20.3
18.4
102.98
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Quinolinium FluorochromateOxidation
885
MECHANISM
UV-VIS spectrum (Fig. 4) clearly con firm s th e com plex form ation between
oxidan t an d TsOH. Com plex form ation between im idazolium
dich rom ate–TsOH (ref.22) an d pyridin ium dich rom ate–TsOH (ref.23) h as al-
ready been reported. Also specific acid catalysis by TsOH gives supportin g
eviden ce for th e com plex form ation . QFC–TsOH com plex is m ore reactive
th an Cr(VI) species sin ce th e com plexation m akes th e oxidan t a better
electroph ile. It beh aves like a proton ated Cr(VI) species wh ich is a stron ger
oxidan t an d electroph ile. Form ation of a proton ated Cr(VI) species h as ear-
lier been postulated in th e reaction of QFC with α-h ydroxy acids in
perch loric acid m edium 24. Th e Mich aelis–Men ten depen den ce on [Alc] im -
plies th at com plex form ation 25 beween th e alcoh ol an d oxidan t/oxidan t–
TsOH com plex takes place in th e presen t experim en ts. Th e kin etic order in
th e acid, ch an gin g from fraction al to zero an d yieldin g a lim itin g k0m ax value
at h igh acidities, could be described by th e followin g sch em e.
(4)
K
2
C1 + RCH2OH
C2
(5)
(6)
(7)
(8)
(9)
k
3
C2
RCHO + Cr(IV) + TsOH
slow
Cr(IV) + Cr(VI) → 2 Cr(V)
2 Cr(V) + 2 RCH2OH → 2 RCHO + 4 H+ + 2 Cr(III)
(Rate)0 = k3[C2]
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886
Meenakshisundaram, Sockalingam:
[QFC]t = [QFC] + [C1] + [C2]
(10)
Th e steady-state treatm en t is applied an d th e [C1] an d [C2] values are substi-
tuted.
k3 K1 K2 [Alc][TsOH][QFC]t
(Rate)0 =
(11)
(12)
1 + K1 [TsOH] + K1 K2 [Alc][TsOH]
k3 K1 K2 [Alc][TsOH][QFC]t
(Rate)0 =
1 +[TsOH](K1 + K1 K2 [Alc])
1.25
1.00
0.75
0.50
0.25
1
2
300
350
400
450
Wavelength, nm
FIG. 4
UV-VIS spectra of QFC in aceton itrile. 1 QFC, 2 QFC with TsOH
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Quinolinium FluorochromateOxidation
887
Th e derived rate expression explain s th e reactivity depen den ce on acidity.
At h igh er [TsOH], th e factor of un ity in th e den om in ator can be n eglected
in com parison with th e oth er term .
k′[Alc][TsOH][QFC]t
(Rate)0 =
,
(13)
[TsOH](K1 + K1 K2 [Alc])
wh ere k′ = k3K1K2.
k′[Alc][QFC]t
(Rate)0 =
(14)
K1 + K1 K2 [Alc]
Th is rate law explain s th e zero-order depen den ce on acidity at h igh con cen -
tration s of TsOH. Th e rate equation (12) can be rearran ged.
1
1
1
1
=
+
+
(15)
(16)
k0
k3 K1 K2 [Alc][TsOH] k3 K2 [Alc] k3
1
1
1
1
1
=
+
+
k0 [Alc] k3 K1 K2 [TsOH] k3 K2
k3
From th e double reciprocal plot of 1/k0 versus 1/[Alc] at con stan t [TsOH]
(Fig. 2) (r = 0.997; s = 55.5), th e rate coefficien ts for th e slow step, k3, can be
determ in ed. Substrate effect studies were exten ded to all th e alcoh ols at dif-
feren t tem peratures an d k3 (s–1) values are listed in Table V.
Th e exact n ature of th e Cr(IV) species is n ot kn own . Form ation of
Cr(IV)–TsOH an d Cr(V)–TsOH com plexes in th e course of th e reaction is
n ot ruled out. In varian ce of rate with th e added acrylon itrile rules out a h y-
drogen abstraction m ech an ism . Furth er, ρ* values (ca –1.6) are n ot in agree-
m en t with a radical process. Gen erally, radical processes are ch aracterised
by sm all ρ values26,27. Studies on CD3OD oxidation give som e idea about
th e n ature of th e electron -deficien t tran sition state. Sign ifican t prim ary iso-
tope effects were foun d for CH3OH (kH / k0D > 10) upon deuterium substitu-
0
tion in all th e C–H bon ds. Isotope effects for som e reaction s th at proceed
by h ydride m ech an ism s are listed in Table VI.
Th e large prim ary kin etic isotope effect in th is study stron gly favours a
h ydride tran sfer m ech an ism . Th e ρ values h igh er th an –3 gen erally suggest
a fairly large degree of carbon ium ch aracter in th e tran sition state31. Th e ρ*
value of ca –1.6 in th e presen t study is in dicative of a partial bon din g. A lin -
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

888
Meenakshisundaram, Sockalingam:
ear tran sition state will certain ly exh ibits a larger isotope effect th an a cy-
clic in term ediate32. Hen ce th e probable structure of C2 will be
[TsOH····OCrO····H····CRHOH] .
We propose a con certed, two-electron h ydride tran sfer m ech an ism . Th is
m ech an ism is very sim ilar to th at proposed in th e oxidation of alcoh ols by
th e aquach rom ium (IV) ion 32
.
TsOH····OCrO + RCH2OH → [TsOH····OCrO····H····CRHOH]±
(C1) (C2)
↓ k3
products
(17)
Nearly con stan t ∆G# values im ply th at a com m on m ech an ism operates in
all th e alcoh ols listed. Th ere is a lin ear relation sh ip between ∆H# an d ∆S# (r =
0.995; s = 1.57). Th e excellen t lin earity observed in th e Exn er plot (Fig. 5) (r =
0.992; s = 0.050) furth er con firm s th e above view.
Comparison with Structurally Similar Oxidants
Quin olin ium fluoroch rom ate was foun d to un dergo a con certed n on -cyclic
two-electron h ydride tran sfer m ech an ism with alcoh ols in aceton itrile m e-
dium wh ereas th e available literature on pyridin ium fluoroch rom ate25 oxi-
dation of eth an ol in aceton itrile–n itroben zen e m edium sh ows th at th e
reaction follows a h ydride tran sfer m ech an ism via th e form ation of a
ch rom ate ester, sim ilar to oxidation s in volvin g ch rom ic acid33
.
TABLE VI
Isotope effects for alcoh ol oxidation
Oxidan t
Deuterium substitution
koH /koD
Referen ce
[RuO(bpy)(py)]2+
CH3OH/CD3OH
9
50
4.6
28, 29
28, 29
30
C6H5CH2OH/C6H5CD2OH
(CH3)2CHOH/(CH3)2CDOH
RuO4
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Quinolinium FluorochromateOxidation
889
Th e results of th e oxidation of ben zyl alcoh ols with QCC (ref.34) in a
DMSO–CH2Cl2 solution reveal th at h ydrogen tran sfer does n ot take place
by a n on -cyclic bim olecular process. A tran sition state h avin g a plan ar, cy-
clic sym m etrical structure is en visaged.
–0.75
10
12
8
11
5
6
9
7
4
3
–1.75
1
–2.75
2
–3.75
–3.75
–2.75
–1.75
–0.75
log k3293
FIG. 5
Th e Exn er plot (for n um berin g see Table V). 103 [TsOH] = 7.53 m ol dm –3, 104 [QFC] = 4.76
m ol dm –3
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

890
Meenakshisundaram, Sockalingam:
Detailed kin etic studies on th e oxidation of alcoh ols with QFC in dicate
th at th e oxidation with QFC an d PFC (ref.25) follow differen t m ech an istic
path ways.
Oxidation of Unsaturated Alcohols
Oxidation studies were carried out un der pseudo-first-order con dition s an d
th e first-order rate coefficien ts were determ in ed as explain ed before. Th ere
was n o sign ifican t oxidation in th e absen ce of TsOH. Th e reaction order de-
pen den t on acidity ran ges from 1 to 2 (r > 0.977; s < 0.052). A Mich aelis–
Men ten depen den ce on [Alc] is observed (r > 0.993; s < 15.34). Th e kin etic
data are sum m arised in Table VII.
TABLE VII
Pseudo-first-order rate con stan ts for variation of [TsOH], [QFC] an d [Alc] in aceton itrile at
303 K
103 ko, s–1
103 [TsOH]
m ol dm –3
104 [QFC]
m ol dm –3
103 [Alc]
m ol dm –3
cin n am yl
alcoh ol
allyl alcoh ol but-2-en -1-ol
2.80
2.80
2.80
2.80
2.80
2.80
2.80
2.80
1.87
2.24
2.80
3.70
4.48
4.16
5.56
8.34
5.56
5.56
5.56
5.56
5.56
5.56
5.56
5.56
5.56
5.56
16.95
16.95
16.95
5.65
5.38
1.84
1.16
0.77
1.58
1.84
2.26
3.05
1.16
1.44
1.84
2.96
4.07
5.04
4.12
1.74
2.16
3.28
4.12
5.01
5.31
2.40
3.30
4.12
6.46
–
6.29
3.41
1.87
1.85
2.86
3.41
3.92
4.18
1.71
2.82
3.41
5.11
–
11.30
16.95
22.60
28.25
16.95
16.95
16.95
16.95
16.95
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Quinolinium FluorochromateOxidation
891
Ion ic stren gth h as n o appreciable effect on th e reactivity (Table VIII). A
decrease in th e perm ittivity of th e m edium in creases th e oxidation rate
m argin ally. Added quin olin e depresses th e reactivity sign ifican tly as sh own
in Table VIII. Th e rate con stan ts were m easured at 283, 293, 303 an d 313 K;
th e activation param eters are listed in Table IX.
TABLE VIII
Depen den ce of th e rate of allyl alcoh ol oxidation on [AN], solven t com position , [LiClO4]
an d [quin olin e] at 303 K
[QFC] = 5.56 · 10–4 m ol dm –3
[allyl alcoh ol] = 16.95 · 10–3 m ol dm –3
[TsOH] = 2.80 · 10–3 m ol dm –3
103 [AN]
m ol dm –3
104 [LiClO4]
m ol dm –3
105 [Quin olin e]
103 ko, s–1
Da
m ol dm –3
0.0
0.63
1.26
1.89
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
37.50
37.50
37.50
37.50
37.50
35.74
33.97
32.21
30.44
37.50
37.50
37.50
37.50
37.50
37.50
37.50
37.50
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.73
1.45
2.18
2.90
0.0
0.0
0.0
0.0
0.0
0.0
1.84
2.23
2.11
2.17
1.84
2.15
2.16
1.96
2.08
1.78
1.72
1.68
1.58
1.78
1.65
1.28
1.21
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
3.49
6.98
10.46
17.44
a
Perm ittivity values are calculated from th e values for pure solven ts.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

892
Meenakshisundaram, Sockalingam:
Th e rate con stan ts decreased with in creasin g [QFC]. Th is observation is
very com m on in Cr(VI) oxidation s. Th e oxidan t m ust be in equilibrium
with a less reactive species. Th is beh aviour in aqueous solution is well ex-
plain ed by Wiberg35. Th e rate is depen den t on th e con cen tration of acid
ch rom ate ion rath er th an on total Cr(VI).
2 HCrO4–
Cr2 O 27 – + H2O
(18)
Th us, as th e con cen tration of ch rom ium (VI) is in creased, a progressively
decreasin g portion of th e total am oun t is in th e form of th e acid ch rom ate
ion an d th e rate con stan ts decrease with in creasin g ch rom ium (VI) con cen -
tration . In aceton itrile m edium , th e form ation of [QFC–aceton itrile] adduct
can be en visaged. Th is decreases th e effective con cen tration of QFC an d, as
a result, th e reactivity decreases. If th e form ation con stan t of th e adduct is
in sign ifican t, th en in verse depen den ce of th e rate on [oxidan t] will n ot be
observed. A sh ift in λm ax from 360 n m in aqueous m edium to 360–370 n m
in aceton itrile m edium is observed. A sim ilar sh ift in λm ax is m ore pro-
n oun ced in Cr(V) com plexes (510 n m in aqueous m edium to 540 n m in
aceton itrile m edium )36,37. Th e kin etic data can be described by a two m ech -
an ism s.
TABLE IX
Rate con stan ts an d activation param eters for th e oxidation of allyl alcoh ol, cin n am yl alco-
h ol an d but-2-en -1-ol with QFC in aceton itrile
[QFC] = 5.56 · 10–4 m ol dm –3
[TsOH] = 2.80 · 10–3 m ol dm –3
a
102
k
∆G#
∆H#
–∆S#
No Substrate
(303K)
r
s
kJ m ol–1 J K–1 m ol–1
kJ m ol–1
283 K 293 K 303 K
313 K
1
2
3
Allyl alcoh ol
2.18
4.88
1.78
3.25
9.58
2.56
4.43
14.33
4.03
7.57
26.44
6.59
27.2
37.7
29.5
180.5
136.2
174.3
81.9
79.0
82.3
0.990 0.083
0.996 0.076
0.994 0.070
Cin n am yl
alcoh ol
But-2-en -1-ol
a
k = k0 /[Alc]n, wh ere n is th e reaction order with respect to alcoh ols.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Quinolinium FluorochromateOxidation
893
K
1
OX + TsOH
C1
C2
(19)
(20)
K
2
C1 + TsOH
(th e forward reaction is rate-determ in in g)
C2 + [Alc]
CO + Cr(IV) + TsOH
(21)
fast
SCHEME 1
C2 is sim ilar to an activated com plex con tain in g two substrate m ole-
cules38–46
.
K
1
OX + TsOH
C1 + [Alc]
C1
(19)
(22)
(23)
K
3
C3
slow
k
4
C3
products
slow
SCHEME 2
Th e overall rate is
k K [TsOH]2 [QFC]t k K K [Alc][TsOH][QFC]t
2
1
4
1
3
(Rate)0 =
+
.
(24)
(25)
1 + K1 [TsOH]
1 + K1 [TsOH]
At low [TsOH],
k0 = k2K1[TsOH]2 + k4K1K3[Alc][TsOH]
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

894
Meenakshisundaram, Sockalingam:
k0
= k2K1[TsOH] + k4K1K3[Alc] .
(26)
[TsOH]
Th e plot of k0/[TsOH] versus [TsOH] is reason ably lin ear (Fig. 6) (r = 0.976; s =
0.031) with an in tercept th us provin g th e validity of th e assum ption s.
Product an alysis sh ows th at propen al, but-2-en al an d cin n am aldeh yde
are th e m ain products. Th is rules out th e possibility of attack of oxidisin g
species on th e double bon d of th ese alcoh ols. Nearly con stan t ∆G# values
im ply th e operation of a com m on m ech an ism with all th e alcoh ols. Th e
lin ear relation in th e Exn er plots (r > 0.992; s < 0.054) furth er con firm s th e
above view. Th e order of reactivity is, cin n am yl alcoh ol > allyl alcoh ol >
but-2-en -1-ol. A reverse order of reactivity is exh ibited in th e oxidation of
alcoh ols with selen ium dioxide47. Close observation of th e rate coefficien ts
of allyl alcoh ol an d but-2-en -1-ol in dicates th at th eir reactivities are alm ost
sam e. Th is is quite expected, sin ce th e m eth yl group is isolated from th e re-
action cen tre by th e C=C bon d. Th e +I effect of th e m eth yl group is n ot sig-
n ifican t in th is system . Th e h igh reactivity of cin n am yl alcoh ol can be
ascribed to reson an ce in teraction th rough th e con jugated double bon ds.
1.1
0.9
0.7
0.5
1.5
2.5
3.5
10 [TsOH], mol dm
4.5
3
–3
FIG. 6
A plot of k0/[TsOH] versus [TsOH]. 104 [QFC] = 5.56 m ol dm –3, 103 [allyl alcoh ol] = 16.95
m ol dm –3
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Quinolinium FluorochromateOxidation
895
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Collect. Czech. Chem. Commun. (Vol. 66) (2001)