# Polyethylene Glycols as Efficient Media for Decarboxylative Nitration ofα,β-Unsaturated Aromatic Carboxylic Acids by Ceric Ammonium Nitrate in Acetonitrile Medium: A Kinetic and Mechanistic Study

Authors: K. Ramesh; S. Shylaja; K. C. Rajanna; P. Giridhar Reddy; P. K. Saiprakash
Journal: Advances in Physical Chemistry
 (2013)

---

## Abstract

Polyethylene glycols (PEGs) were found to be efficient media for decarboxylative nitration ofα,β-unsaturated aromatic carboxylic acids by ceric ammonium nitrate (CAN) in acetonitrile to give β-nitrostyrene derivatives. Kinetics of the reaction exhibited second order kinetics with a first order dependence on [CAN] and [substrate]. Reactions were too sluggish to be studied in the absence of PEG; therefore detailed kinetics were not taken up. Reaction times were reduced from 24 hrs to few hours. The catalytic activity was found to be in the increasing order PEG-300 > PEG-400 > PEG-600 > PEG-200. Mechanism of PEG-mediated reactions was explained by Menger-Portnoy's scheme as applied in micellar kinetics.


---

## Body

## 1. Introduction

Cerium (IV) ammonium nitrate (CAN) is one of the most important reagents used for organic synthesis [1–4]. The formula of cerium (IV) ammonium nitrate, [(NH4)2[Ce(NO3)6], reflects that the cerium (IV) ion is surrounded by six nitrate groups and the ammonium ions are counterions to compensate for the negative charge of the hexanitratocerate (IV) coordinating unit. CAN is a one-electron oxidation reagent. Cerium (IV) ammonium nitrate can also be used as a nitrating agent [5, 6], initiator for radical polymerization reactions [7], and also as a reagent to remove protecting groups [8]. Even though cerium (IV) reagents are milder oxidation reagents than the other metal-based oxidation reagents such as Mn(VII) and Cr(VI) salts, they are relatively much less toxic. Because of their high molecular mass, large quantities of cerium (IV) salts are required for stoichiometric reactions. Therefore, indirect and catalytic reactions using Ce(IV) have been developed [9, 10]. The main advantage of CAN over other cerium (IV) reagents is its higher solubility in organic solvents. The most popular solvents are (in decreasing order of importance) water, acetonitrile, dichloromethane, THF, and methanol [11]. Often mixtures of these solvents are also used as reaction medium. Other solvents have found only marginal use for this type of reactions.Being good Michael acceptors,α,β-unsaturated nitroalkenes are widely applied in organic synthesis [12]. Among various methods reported for their preparation, a method involving the use of CAN provides a practical way to the synthesis of α,β-unsaturated “nitroalkenes” in good-to-excellent yields [13–23]. Recently polyethylene glycols (PEGs) have been used as catalysts and catalyst supports and also have been found to be an inexpensive, nontoxic, and environmentally friendly reaction medium, which avoid the use of acid or base catalysts. Moreover PEG can be recovered after completion of the reactions and recycled/reused [24–29] in another batch. Inspired by the striking features of PEG the author wants to use it as a catalyst by avoiding the use of acid in the present study; namely, ceric ammonium nitrate (CAN) triggered decarboxylative nitration of α,β-unsaturated aromatic carboxylic acids. Acetonitrile is used as solvent in order to facilitate kinetic studies.


## 2. Experimental Details

### 2.1. Materials

Cinnamic acid, ceric ammonium nitrate (CAN), and polyethylene glycols were obtained from SD Fine Chemicals or Loba. Substituted cinnamic acids were prepared by Perkins reaction as cited in the literature [30, 31].


### 2.2. General Procedure for PEG-Mediatedβ-Nitrostyrenes Synthesis

In a typical solid state synthesis, cinnamic acid (0.01 mol), PEG (0.02 mmol), and CAN (0.012 mmol) are placed in a clean two-necked R. B. flask containing acetonitrile (MeCN) and stirred for certain time. Progress of the reaction is periodically monitored by  thin layer chromatography (TLC). After completion, the reaction mixture is treated with 2% sodium carbonate solution, followed by the addition of “dichloromethane” (DCM) or “dichloroethane” (DCE). The organic layer was separated, dried over Na2SO4 and evaporated under vacuum, and purified with column chromatography using ethyl acetate : hexane (3 : 7) as eluent to get pure product. The products were characterized by IR, 1H-NMR, mass spectra, and physical data with authentic samples and found to agree well with earlier reports (Table 1). The yields of major products are compiled in Table 2.Table 1
Binding constants of[CAN-PEG] at 303°K using Benesi-Hildebrand method.
S. N
PEG
Benesi-Hildebrand equation
K
ε
-
Δ
G
dm3/mol
dm3/mol/cm
(kJ/mol)
1
PEG-200
y = 7E  − 05x + 0.052
743
19.23
16.7
2
PEG-300
y = 9E  − 05x + 0.055
611
18.18
16.2
3
PEG-400
y = 5E  − 05x + 0.071
1420
14.08
18.3Table 2
Activation parameters of cinnamic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −13.32x + 34.09
0.999
110
84.2
85.9
1.0
0.02
y = −9.181x + 20.97
0.999
76.3
83.3
−23.2
PEG-200
2.0
0.03
y = −8.399x + 18.75
0.995
70.0
95.5
−41.7
3.0
0.04
y = −8.625x + 19.84
0.998
91.8
81.8
−32.6
4.0
0.08
y = −7.047x + 15.06
0.999
58.6
55.6
−72.3
5.0
0.1
y = −8.112x + 19.01
0.995
67.5
39.7
39.5
0.5
0.01
y = −22.79x + 64.43
0.997
189
87.6
338
1.0
0.03
y = −4.655x + 6.308
0.996
38.7
82.3
−145
PEG-300
2.0
0.04
y = −5.385x + 8.930
0.999
44.8
81.8
−123
3.0
0.05
y = −2.684x + 0.252
0.998
22.4
81.0
−195
4.0
0.06
y = −8.743x + 20.63
0.999
72.7
80.5
−20.0
5.0
0.08
y = −7.750x + 17.60
0.999
64.5
80.0
−51.3
0.5
0.02
y = −6.979x + 13.65
0.999
58.0
83.3
−84.0
1.0
0.03
y = −6.106x + 11.05
0.997
50.8
82.5
−105
PEG-400
2.0
0.04
y = −9.224x + 21.84
0.999
76.7
81.5
−16.0
3.0
0.06
y = −8.718x + 20.54
0.999
72.5
38.0
−80.5
4.0
0.07
y = −7.054x + 15.38
0.999
58.7
80.1
−69.7
5.0
0.1
y = −7.687x + 17.63
0.999
64.0
79.3
−51.0
0.5
0.02
y = −5.421x + 8.550
0.998
45.0
83.0
−126
1.0
0.03
y = −4.644x + 6.274
0.999
38.6
82.2
−145
PEG-600
2.0
0.04
y = −3.131x + 1.542
0.999
26.0
81.4
−184
3.0
0.05
y = −5.555x + 9.813
0.999
46.2
81.0
−115
4.0
0.07
y = −3.622x + 3.753
0.998
30.1
80.0
−166
5.0
0.1
y = −4.247x + 5.956
0.999
35.3
80.0
−148


### 2.3. Kinetic Method of Following the Reaction

Thermostat was adjusted to desired reaction temperature. Flask containing known amount of ceric ammonium nitrate (CAN) in acetonitrile solvent and another flask containing the substrate (α,β-unsaturated acid) and suitable amount of PEG solutions were clamped in a thermostatic bath. Reaction was initiated by mixing requisite amount of CAN with the other contents of the reaction vessel. The entire reaction mixture was mixed thoroughly. Flask was coated black from the outside to prevent photochemical effects. Aliquots of the reaction mixture were withdrawn into a cuvette and placed in the cell compartment of the laboratory visible spectrophotometer. Cell compartment was provided with an inlet and an outlet for circulation of thermostatic liquid at a desired temperature. The CAN content could be estimated from the previously constructed calibration curve showing absorbance (A) versus [CAN]. Absorbance values were in agreement with each other with an accuracy of ±3% error.


### 2.4. [CAN-PEG] Binding Study

Spectrophotometric studies were performed in order to throw light on CAN binding with PEG (polyethylene glycol). UV-Visible spectrum of CAN in MeCN indicated a band at 459 nm. It underwent a hypsochromic shift (blue shift) of about 17 to 18 nm (band-shifted from 441 to 442 nm) in presence of PEG (Figure1), followed by hypochromic shift. This observation indicates the formation of [CAN-PEG] complex. The CAN-PEG binding constants (K) were evaluated by the method of Benesi-Hildebrand [32]. When the equilibrium for molecular complexation is taken into consideration,
(1)PEG+CAN⇌KC
The equilibrium constant K = [C]/[CAN][PEG], where [CAN], [PEG], and [C] are equilibrium concentrations of acceptor (CAN), donor (PEG), and complex, respectively. If [CAN]0 and [PEG]0 represent initial concentrations of CAN and PEG, respectively, then
(2)K=[C]([CAN]0-[C])([PEG]0-[C]).
Under condition that [PEG]0≫[CAN]0⇒[PEG]0≫[C] and ([PEG]0-[C])~[PEG]0(3)K=[C]([CAN]0-[C])[PEG]0⇒K=[C]([CAN]0[PEG]0-[C][PEG]0),[C](1+K[PEG]0)=K[CAN]0[PEG]0⇒[C]=K[CAN]0[PEG]0(1+K[PEG]0).
But according to Lambert-Beer’s law absorbance (A)=ϵcl.Figure 1
UV-Visible spectra of CAN in presence and absence of PEG in MeCN medium.In the previous equations,l is path length, A  is absorbance, ϵ is the molar extinction coefficient, and K is formation constant of the complex, respectively,(4)[C]=Aϵl=K[CAN]0[PEG]0(1+K[PEG]0).
According to this plot of absorbance (A) as a function of donor concentration ([PEG]0) the plot indicated a linear increase in the value of (A) with an increase in ([PEG]0) with a limiting value at its higher concentrations. Further, taking the reciprocals to the previous equation, it rearranges to
(5)([CAN]0Al)=(1K[PEG]0ϵ)+(1ϵ).
For one cm path length, the aforementioned equation can be written as(6)([CAN]0A)=(1K[PEG]0ϵ)+(1ϵ).
But the absorbance of CAN and [CAN-PEG] absorbs in the same region significantly; therefore the observed absorbance (A) could be written as(7)A=A(CAN)+A(Complex).
Now the true absorbance of [CAN-PEG] adduct is the difference between observed absorbance and absorbance of CAN:
(8)⇒A(Complex)=ΔA=A~A(CAN).
Therefore, a plot of ([CAN]0/ΔA) versus 1/[PEG]0 should give a straight line according to the previous equation. These plots (Figure 2) have been realized in the present study. Formation constant (K) has been calculated from the ratio of intercept to slope, while inverse of the intercept gave molar extinction coefficient (ϵ) and is represented in Table 1.Figure 2
Benesi-Hildebrand plot of PEG-300 interaction with CAN.


## 2.1. Materials

Cinnamic acid, ceric ammonium nitrate (CAN), and polyethylene glycols were obtained from SD Fine Chemicals or Loba. Substituted cinnamic acids were prepared by Perkins reaction as cited in the literature [30, 31].


## 2.2. General Procedure for PEG-Mediatedβ-Nitrostyrenes Synthesis

In a typical solid state synthesis, cinnamic acid (0.01 mol), PEG (0.02 mmol), and CAN (0.012 mmol) are placed in a clean two-necked R. B. flask containing acetonitrile (MeCN) and stirred for certain time. Progress of the reaction is periodically monitored by  thin layer chromatography (TLC). After completion, the reaction mixture is treated with 2% sodium carbonate solution, followed by the addition of “dichloromethane” (DCM) or “dichloroethane” (DCE). The organic layer was separated, dried over Na2SO4 and evaporated under vacuum, and purified with column chromatography using ethyl acetate : hexane (3 : 7) as eluent to get pure product. The products were characterized by IR, 1H-NMR, mass spectra, and physical data with authentic samples and found to agree well with earlier reports (Table 1). The yields of major products are compiled in Table 2.Table 1
Binding constants of[CAN-PEG] at 303°K using Benesi-Hildebrand method.
S. N
PEG
Benesi-Hildebrand equation
K
ε
-
Δ
G
dm3/mol
dm3/mol/cm
(kJ/mol)
1
PEG-200
y = 7E  − 05x + 0.052
743
19.23
16.7
2
PEG-300
y = 9E  − 05x + 0.055
611
18.18
16.2
3
PEG-400
y = 5E  − 05x + 0.071
1420
14.08
18.3Table 2
Activation parameters of cinnamic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −13.32x + 34.09
0.999
110
84.2
85.9
1.0
0.02
y = −9.181x + 20.97
0.999
76.3
83.3
−23.2
PEG-200
2.0
0.03
y = −8.399x + 18.75
0.995
70.0
95.5
−41.7
3.0
0.04
y = −8.625x + 19.84
0.998
91.8
81.8
−32.6
4.0
0.08
y = −7.047x + 15.06
0.999
58.6
55.6
−72.3
5.0
0.1
y = −8.112x + 19.01
0.995
67.5
39.7
39.5
0.5
0.01
y = −22.79x + 64.43
0.997
189
87.6
338
1.0
0.03
y = −4.655x + 6.308
0.996
38.7
82.3
−145
PEG-300
2.0
0.04
y = −5.385x + 8.930
0.999
44.8
81.8
−123
3.0
0.05
y = −2.684x + 0.252
0.998
22.4
81.0
−195
4.0
0.06
y = −8.743x + 20.63
0.999
72.7
80.5
−20.0
5.0
0.08
y = −7.750x + 17.60
0.999
64.5
80.0
−51.3
0.5
0.02
y = −6.979x + 13.65
0.999
58.0
83.3
−84.0
1.0
0.03
y = −6.106x + 11.05
0.997
50.8
82.5
−105
PEG-400
2.0
0.04
y = −9.224x + 21.84
0.999
76.7
81.5
−16.0
3.0
0.06
y = −8.718x + 20.54
0.999
72.5
38.0
−80.5
4.0
0.07
y = −7.054x + 15.38
0.999
58.7
80.1
−69.7
5.0
0.1
y = −7.687x + 17.63
0.999
64.0
79.3
−51.0
0.5
0.02
y = −5.421x + 8.550
0.998
45.0
83.0
−126
1.0
0.03
y = −4.644x + 6.274
0.999
38.6
82.2
−145
PEG-600
2.0
0.04
y = −3.131x + 1.542
0.999
26.0
81.4
−184
3.0
0.05
y = −5.555x + 9.813
0.999
46.2
81.0
−115
4.0
0.07
y = −3.622x + 3.753
0.998
30.1
80.0
−166
5.0
0.1
y = −4.247x + 5.956
0.999
35.3
80.0
−148


## 2.3. Kinetic Method of Following the Reaction

Thermostat was adjusted to desired reaction temperature. Flask containing known amount of ceric ammonium nitrate (CAN) in acetonitrile solvent and another flask containing the substrate (α,β-unsaturated acid) and suitable amount of PEG solutions were clamped in a thermostatic bath. Reaction was initiated by mixing requisite amount of CAN with the other contents of the reaction vessel. The entire reaction mixture was mixed thoroughly. Flask was coated black from the outside to prevent photochemical effects. Aliquots of the reaction mixture were withdrawn into a cuvette and placed in the cell compartment of the laboratory visible spectrophotometer. Cell compartment was provided with an inlet and an outlet for circulation of thermostatic liquid at a desired temperature. The CAN content could be estimated from the previously constructed calibration curve showing absorbance (A) versus [CAN]. Absorbance values were in agreement with each other with an accuracy of ±3% error.


## 2.4. [CAN-PEG] Binding Study

Spectrophotometric studies were performed in order to throw light on CAN binding with PEG (polyethylene glycol). UV-Visible spectrum of CAN in MeCN indicated a band at 459 nm. It underwent a hypsochromic shift (blue shift) of about 17 to 18 nm (band-shifted from 441 to 442 nm) in presence of PEG (Figure1), followed by hypochromic shift. This observation indicates the formation of [CAN-PEG] complex. The CAN-PEG binding constants (K) were evaluated by the method of Benesi-Hildebrand [32]. When the equilibrium for molecular complexation is taken into consideration,
(1)PEG+CAN⇌KC
The equilibrium constant K = [C]/[CAN][PEG], where [CAN], [PEG], and [C] are equilibrium concentrations of acceptor (CAN), donor (PEG), and complex, respectively. If [CAN]0 and [PEG]0 represent initial concentrations of CAN and PEG, respectively, then
(2)K=[C]([CAN]0-[C])([PEG]0-[C]).
Under condition that [PEG]0≫[CAN]0⇒[PEG]0≫[C] and ([PEG]0-[C])~[PEG]0(3)K=[C]([CAN]0-[C])[PEG]0⇒K=[C]([CAN]0[PEG]0-[C][PEG]0),[C](1+K[PEG]0)=K[CAN]0[PEG]0⇒[C]=K[CAN]0[PEG]0(1+K[PEG]0).
But according to Lambert-Beer’s law absorbance (A)=ϵcl.Figure 1
UV-Visible spectra of CAN in presence and absence of PEG in MeCN medium.In the previous equations,l is path length, A  is absorbance, ϵ is the molar extinction coefficient, and K is formation constant of the complex, respectively,(4)[C]=Aϵl=K[CAN]0[PEG]0(1+K[PEG]0).
According to this plot of absorbance (A) as a function of donor concentration ([PEG]0) the plot indicated a linear increase in the value of (A) with an increase in ([PEG]0) with a limiting value at its higher concentrations. Further, taking the reciprocals to the previous equation, it rearranges to
(5)([CAN]0Al)=(1K[PEG]0ϵ)+(1ϵ).
For one cm path length, the aforementioned equation can be written as(6)([CAN]0A)=(1K[PEG]0ϵ)+(1ϵ).
But the absorbance of CAN and [CAN-PEG] absorbs in the same region significantly; therefore the observed absorbance (A) could be written as(7)A=A(CAN)+A(Complex).
Now the true absorbance of [CAN-PEG] adduct is the difference between observed absorbance and absorbance of CAN:
(8)⇒A(Complex)=ΔA=A~A(CAN).
Therefore, a plot of ([CAN]0/ΔA) versus 1/[PEG]0 should give a straight line according to the previous equation. These plots (Figure 2) have been realized in the present study. Formation constant (K) has been calculated from the ratio of intercept to slope, while inverse of the intercept gave molar extinction coefficient (ϵ) and is represented in Table 1.Figure 2
Benesi-Hildebrand plot of PEG-300 interaction with CAN.


## 3. Results and Discussion

### 3.1. Determination of the Order of Reaction

General form of the rate law for a nitrodecarboxylation (nitro-Hunsdiecker reaction) reaction could be represented by considering the following general scheme:(9)CAN+S⟶Products
where CAN = ceric ammonium nitrate and S = α,β-unsaturated carboxylic acid:
(10)V=k1[CAN]x[CAN]y.Reactions were conducted under two different conditions. Under pseudo first order conditions[CAN]≫[CAN], plots of ln(Ao/At) versus time were straight lines with a negative slope indicating order (x) with respect to [CAN] to be unity. As typical examples pseudo first order plots are shown in Figures 3, 4, and 5. First order rate constant (k′) was obtained from the slopes of line plots cited (k′).Figure 3
Pseudo first order plot of cinnamic acid (ln(Ao/At) versus time) at 323 K. [CA] = 1.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−3 mol dm−3; [PEG-300] = 9.99 × 10−1 mol dm−3.Figure 4
Pseudo first order plot of acrylic acid (ln(Ao/At) versus time) at 315 K. [AA] = 1.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−3 mol dm−3; [PEG-300] = 13.32 × 10−2 mol dm−3.Figure 5
Pseudo first order plot of nitrocinnamic acid (ln(Ao/At) versus time) at 315 K. [NCA] = 1.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−3 mol dm−3; [PEG-400]= 1.25 × 10−1 mol dm−3.The reaction is also conducted under second order conditions with equal concentrations of [CAN]0 = [CAN]0. Kinetic plots of [1/(a−x)] or [1/(At)] versus time (Figures 6, 7, and 8) have been found to be linear with a positive gradient and definite intercept on ordinate (vertical axis), indicating overall second order kinetics.Figure 6
Second order kinetic plot of cinnamic acid (1/At versus Time) at 310 K. [CA] = 4.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−2 mol dm−3; [PEG-200] = 5.00 × 10−1 mol dm−3.Figure 7
Second order kinetic plot of acrylic acid (1/At versus Time) at 310 K. [AA] = 4.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−2 mol dm−3; [PEG-300] = 3.33 × 10−1 mol dm−3.Figure 8
Second order kinetic plot of nitrocinnamic acid (1/At versus time) at 310 K. [NCA] = 4.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−2 mol dm−3; [PEG-600] = 3.32 × 10−1 mol dm−3.In the present study, kinetic data have been collected at three to four different temperatures within the range of 300–315 K. Activation parameters such asΔH# and ΔS# have been evaluated from Eyring’s plots (Figures 9 and 10). Free energy of activation (ΔG#) is obtained from Gibbs-Helmholtz equation:
(11)ΔG#=ΔH#-TΔS#.Figure 9
Eyring’s plot of cinnamic acid (ln(k′′/T) versus 103/T). Kinetic study with [PEG-200] = 2.00 mol dm−3.Figure 10
Eyring’s plot of acrylic acid (ln(k′′/T) versus 103/T). Kinetic study [PEG-300] = 13.32 × 10−1 mol dm−3.


### 3.2. Mechanism of Nitrodecarboxylation of Unsaturated Acids by Ce(IV) Nitrate (CAN) in MeCN Medium

In order to gain an insight into the mechanistic aspects of CAN-CA reaction in MeCN medium, the knowledge of distribution of CAN species in HNO3 medium could be useful. In HNO3 Ce(IV) mainly exists as Ce(NO3)62-, Ce(NO3)5-, Ce(OH)(NO3)4-, Ce(NO3)4, which could arise from the following equilibria as cited in the literature reports [33–36]:
(12)Ce(NO3)4+NO3-⇌Ce(NO3)5-Ce(NO3)5-+NO3-⇌Ce(NO3)62-Ce(NO3)4+H2O⇌Ce(OH)(NO3)4-+H+
But the CAN species in the present study could be entirely different because the reactions are conducted in MeCN medium, which is large excess over [CAN]. MeCN may penetrate into the coordination spheres of Ce(IV) and form solvated CAN species according to the following equilibrium:
(13)Ce(NO3)4+2ACN⇌[Ce(NO3)4(ACN)2]
When unsaturated acid (USA) is added to the reaction mixture containing solvated CAN, it may form [Ce(IV) carboxylate] precursor which dissociates and releases HNO3  in situ. The HNO3 thus released in turn may combine with another solvated CAN to form nitronium ion (NO2+) which finally reacts with coordinated USA to afford β-nitrostyrene as shown in the following sequence of steps:
(14)RCH=CH–COOH+[Ce(NO3)4(CH3CHN)2]⇌[Ce(NO3)4(ACN)(RCH=CH–COOH)](Precursor)Precursor→(Slow)HNO3Precursor→(Slow)+[Ce(NO3)3(ACN)(RCH=CH–COOH)]HNO3+[Ce(NO3)4(ACN)2]⟶NO2++NO3-+[Ce(NO3)3(OH)(ACN)2]NO2++[Ce(NO3)3(ACN)(RCH=CH–COOH)]⟶RCH=CHNO2+CO2+[Ce(NO3)3(ACN)]+[Ce(NO3)3(ACN)]++NO3-+ACN⟶[Ce(NO3)4(ACN)2]


### 3.3. Mechanism of Nitrodecarboxylation in PEG Media

Progress of the reaction has been studied in the presence of a set of polyoxyethylene compounds (PEGs) with varied molecular weights ranging from 200 to 600 units, and it was found that the reaction is enhanced remarkably in all PEGs. Reaction times were reduced from 24 hrs to few hours. The catalytic activity was found to be in the increasing order PEG-300 > PEG-400 > PEG-600 > PEG-200. Further, it is also interesting to note that the absorbance of solvated Ce(IV) species is increased when PEG is added to [CAN] solution. This observation may indicate that solvated [CAN] species could bind with PEG to formPEG supported CAN species according to the equilibrium shown in Scheme 1.Scheme 1The plots ofkm (rate constant of PEG reaction) versus CPEG (concentration of PEG) indicated a rate maxima nearly in the vicinity of 1.50 mol dm−3 PEG-200, 0.99 mol dm−3 PEG-300, 0.99 mol dm−3 PEG-400, 0.500 mol dm−3 PEG-600. Efforts have been made to interpret PEG effect on the mechanism of CAN-USA reaction. Mechanism of PEG-mediated reactions was explained in the lines of micellar catalysis because PEG resembles the structure of Triton-X. The first and widely applied model is that of Menger and Portnoy [37] which closely resembles that of an enzymatic Catalysis. According to this model, formation of PEG bound reagent (PEG-CAN) could occur in the preequilibrium step due to the interaction of Ce(IV) with PEG. The complex thus formed may possess higher or lower reactivity to give products. A general mechanism is proposed by considering the bulk phase and micellar phase reactions as shown in Scheme 2, where km and k0 or (kb) represents rate constants for PEG and bulk phases, respectively, and K is the [PEG-CAN] binding constant. For the previous mechanism, rate law could be derived according to the following sequence of steps in the lines of micellar catalyzed reactions.Scheme 2Considering the total concentration of(Cs) as the algebraic sum of free species and PEG bound substrate complex (CAN-PEG) species,
(15)CS=[CAN]+[CAN-PEG].
From micelle-substrate binding equilibrium,
(16)CANK=[CAN-PEG][PEG][CAN],or[CAN]=[CAN-PEG]K[PEG].
Substitution of [CAN] in (15) gives
(17)CS=[CAN-PEG]K[PEG]+[CAN-PEG]=[CAN-PEG]+K[PEG][CAN-PEG]K[PEG]or[CAN-PEG]=K[PEG]CS1+K[PEG].
Similarly free substrate [CAN] is written as [CAN]=CS-[CAN-PEG](18)[CAN]=CS-K[PEG]CS1+K[PEG].
After simplification, the previous equation reduces to
(19)[CAN]=CS1+K[PEG].
Substitution of [CAN-PEG] and [CAN] in (15) gives
(20)k′=k0CS1+K[PEG]+kmK[PEG]CS1+K[PEG]orkφ=(k0+kmK[PEG])1+K[PEG]kφ=(k0+kmK[PEG])1+K[PEG],
where kφ=(k′/[CAN]), the second order rate constant in PEG media. Subtracting k0 from both sides of equation and rearranging
(21)kφ-k0=(km-kw)K[PEG]1+K[PEG].
But, since the reactions are too sluggish in the absence of [PEG], the rate constant (k0) would be much smaller than (kmK[PEG]); that is, (k0⋘kmK[PEG]). Therefore the (k0) term could be neglected in the previous equation. The rate law (21) could be then considered as
(22)kφ=kmK[PEG]1+K[PEG].This rate-law resembles Michaelis-Menten type rate law that is used for enzyme kinetics. Interestingly the plots of rate constant(kφ), that is, second order rate constant of PEG-mediated reaction versus [PEG], indicated either Hill type curve (i.e., a gradual increase with an increase in [PEG] passing through a maximum point in the profile). This observation could be explained due to the CAN-PEG binding interactions to give [CAN-PEG] species which is responsible for rate accelerations in PEG-mediated reactions. Formation of [CAN-PEG] species has also been established by UV-Visible spectroscopic studies, and binding constants are compiled in Table 1. These observations strengthened us to take up detailed reaction kinetics at various PEG concentrations in order to have an insight into the variation in the enthalpies and entropies of activation with [PEG].


### 3.4. Effect of Structure on the Reactivity, Enthalpy, and Entropy Changes

The enthalpy(ΔH#) and entropy of activation (ΔS#) are the two parameters typically obtained from the temperature dependence of a reaction rate, when these data are analyzed using Eyring’s equation. Both of these values are obtained from Eyring’s plots according to standard procedures [37–39]. Typical plots are shown in Figures 9 and 10. The entropy of activation (ΔS#) provides clues about the molecularity of the rate determining step in a reaction, that is, whether the reactants are bonded to each other or not. Even though structural change in the cinnamic acid exhibits slight variation on its reactivity, no quantitative conclusions could be obtained from rate constant data. However, the observed positive ΔS# values in lower PEG concentration (0.1 M PEG) decreased with an increase in [PEG] and are becoming more negative for all the unsaturated acids which probably indicates that PEG is tightly bound to CAN and lends support that PEG bound CAN is the active species in PEG-mediated reactions. The observed negative ΔS# values are in consonance with the concept that for associative mechanism, entropy decreases upon achieving the transition state, and values near zero are difficult to interpret [38–40]. By and large similar magnitude of observed free energy of activation (ΔG#) in this reaction series (Tables 2, 3, 4, 5, 6) indicates a similar type of mechanism which is operative in the present study. The observed negative magnitude of ΔG (Table 1) indicates the spontaneity of complexation of [CAN-PEG] due to PEG binding with CAN.Table 3
Activation parameters of crotonic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −9.907x + 22.7
0.999
82.4
85.1
−9.0
1.0
0.02
y = −6.742x + 12.86
0.999
56.0
83.3
−91.0
PEG-200
2.0
0.03
y = −5.069x + 7.688
0.999
42.1
82.3
−134
3.0
0.04
y = −6.020x + 11.13
0.999
50.0
81.5
105
4.0
0.05
y = −7.664x + 16.87
0.999
64.0
81.2
−57.3
5.0
0.06
y = −7.062x + 14.92
0.999
59.0
81.0
−73.5
0.5
0.01
y = −9.797x + 22.35
0.999
81.4
85.0
−12.0
1.0
0.02
y = −8.070x + 17.25
0.999
67.0
83.2
−54.2
PEG-300
2.0
0.04
y = −4.233x + 5.180
0.998
35.2
81.5
−154
3.0
0.05
y = −6.852x + 14.13
0.999
54.6
79.0
−80.0
4.0
0.06
y = −6.564x + 13.36
0.999
54.5
80.4
−86.5
5.0
0.08
y = −9.440x + 23.13
0.999
78.5
80.0
−5.3
0.5
0.01
y = −9.759x + 22.23
0.999
81.2
85.0
−13.0
1.0
0.02
y = −7.864x + 16.59
0.999
65.4
83.3
−57.0
PEG-400
2.0
0.04
y = −4.189x + 5.034
0.999
35.0
82.0
−155
3.0
0.05
y = −8.018x + 17.93
0.999
67.0
81.5
−48.5
4.0
0.06
y = −5.394x + 9.454
0.999
45.0
80.7
−119
5.0
0.08
y = −8.250x + 19.25
0.998
68.6
80.0
−37.5
0.5
0.01
y = −9.630x + 21.81
0.998
80.0
85.0
−16.3
1.0
0.02
y = −5.541x + 8.852
0.998
46.0
83.2
−123
PEG-600
2.0
0.03
y = −5.256x + 8.404
0.999
44.0
82.3
−128
3.0
0.04
y = −5.421x + 9.243
0.998
45.0
81.2
−120
4.0
0.06
y = −4.492x + 6.466
0.997
37.4
80.5
−144
5.0
0.09
y = −6.254x + 12.73
0.999
52.0
80.0
−91.8Table 4
Activation parameters of methoxy cinnamic acid in different PEG media. Units ofk′′ = dm3mol−1min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −12.85x + 32.55
0.996
107
85.0
73.0
1.0
0.02
y = −13.90x + 36.01
0.998
115
84.4
102
PEG-200
2.0
0.04
y = −6.038x + 11.21
0.999
50.2
81.50
−104
3.0
0.06
y = −5.027x + 8.448
0.999
42.0
80.2
−127
4.0
0.08
y = −3.844x + 4.748
0.999
32.0
80.0
−158
5.0
0.12
y = −4.403x + 6.945
0.999
37.0
79.0
−140
0.5
0.01
y = −10.36x + 14.11
0.997
86.0
35.3
110
1.0
0.03
y = −3.532x + 2.653
0.999
46.0
98.5
−175
PEG-300
2.0
0.04
y = −5.385x + 8.930
0.959
45.0
82.0
−123
3.0
0.05
y = −2.423x  − 0.582
0.999
20.2
78.0
−192
4.0
0.06
y = −8.743x + 20.63
0.999
73.0
81.0
−26.0
5.0
0.08
y = −7.965x + 18.28
0.999
66.2
80.0
−46.0
0.5
0.02
y = −7.638x + 15.83
0.999
63.5
83.3
−66.0
1.0
0.03
y = −10.01x + 24.07
0.999
83.2
77.0
21.3
PEG-400
2.0
0.04
y = −9.756x + 23.58
0.999
81.0
81.4
−2.0
3.0
0.06
y = −8.022x + 18.16
0.999
67.0
80.0
−41.0
4.0
0.08
y = −6.000x + 11.86
0.999
50.0
80.0
−99.0
5.0
0.10
y = −9.302x + 22.89
0.999
77.3
79.5
−7.3
0.5
0.02
y = −7.936x + 16.83
0.999
66.0
83.3
−58.0
1.0
0.03
y = −5.955x + 10.61
0.998
49.5
82.3
−110
PEG-600
2.0
0.05
y = −4.894x + 7.616
0.999
41.0
81.3
−134
3.0
0.06
y = −6.259x + 12.33
0.999
52.0
80.5
−95.0
4.0
0.08
y = −5.659x + 10.63
0.999
47.0
38.72
−110
5.0
0.1
y = −6.719x + 14.05
0.998
56.0
80.3
−81.0Table 5
Activation parameters of nitrocinnamic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −12.00x + 29.73
0.999
100
85.0
50.0
1.0
0.02
y = −10.87x + 26.63
0.999
90.3
83.1
24.0
PEG-200
2.0
0.04
y = −4.711x + 6.807
0.999
39.2
37.05
−141
3.0
0.06
y = −5.506x + 9.842
0.999
46.0
38.04
−116
4.0
0.07
y = −5.012x + 8.347
0.999
42.0
38.40
−128
5.0
0.09
y = −3.923x + 4.963
0.998
33.0
38.99
−156
0.5
0.01
y = −9.764x + 22.25
0.999
81.2
85.0
−13.0
1.0
0.02
y = −7.861x + 16.58
0.999
65.4
83.3
−60.0
PEG-300
2.0
0.04
y = −4.270x + 5.297
0.999
35.5
37.0
−153
3.0
0.05
y = −6.852x + 14.13
0.999
57.0
81.0
−80.0
4.0
0.06
y = −6.545x + 13.3
0.998
54.4
81.2
−89.5
5.0
0.08
y = −9.042x + 21.84
0.999
75.1
80.0
−16.0
0.5
0.02
y = −7.717x + 16.10
0.977
64.1
83.2
−64.0
1.0
0.03
y = −5.321x + 8.612
0.999
44.2
82.0
−126
PEG-400
2.0
0.04
y = −8.683x + 20.01
0.999
72.1
81.4
−31.2
3.0
0.06
y = −8.718x + 20.54
0.999
72.4
80.4
−27.0
4.0
0.07
y = −7.947x + 18.23
0.998
66.0
80.0
−46.0
5.0
0.10
y = −7.687x + 17.63
0.999
64.0
79.3
−51.0
0.5
0.01
y = −9.635x + 21.82
0.998
80.1
85.0
−16.2
1.0
0.02
y = −8.534x + 18.80
0.997
71.0
83.4
−41.3
PEG-600
2.0
0.04
y = −4.071x + 4.648
0.999
34.0
82.0
−159
3.0
0.06
y = −4.703x + 7.167
0.999
39.1
80.5
−138
4.0
0.08
y = −3.635x + 3.891
0.999
30.2
80.0
−165
5.0
0.10
y = −5.600x + 10.65
0.999
46.5
79.2
−109Table 6
Activation parameters of acrylic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −11.85x + 29.21
0.999
98.5
85.0
45.3
1.0
0.02
y = −8.384x + 18.31
0.998
70.0
84.0
−45.3
PEG-200
2.0
0.03
y = −7.860x + 16.98
0.998
65.3
82.2
−57.0
3.0
0.04
y = −8.164x + 18.27
0.997
68.0
82.0
−47.0
4.0
0.06
y = −6.581x + 13.42
0.999
55.0
81.0
−86.0
5.0
0.08
y = −6.061x + 11.96
0.998
51.0
80.5
−98.2
0.5
0.01
y = −11.85x + 29.21
0.999
98.5
85.0
46.0
1.0
0.02
y = −11.43x + 28.49
0.998
95.0
83.2
39.3
PEG-300
2.0
0.03
y = −10.67x + 26.25
0.997
89.0
83.0
21.0
3.0
0.04
y = −9.163x + 21.61
0.999
76.2
82.0
−18.0
4.0
0.05
y = −6.125x + 11.72
0.999
51.0
81.0
−100
5.0
0.07
y = −5.443x + 9.792
0.998
45.3
80.2
−116
0.5
0.02
y = −8.314x + 18.07
0.997
69.2
83.4
−47.4
1.0
0.03
y = −9.364x + 21.98
0.998
78.0
82.5
−15.0
PEG-400
2.0
0.04
y = −9.756x + 23.58
0.999
81.2
82.0
−1.6
3.0
0.06
y = −7.889x + 17.76
0.997
66.0
81.0
−50.0
4.0
0.08
y = −6.573x + 13.69
0.998
55.0
80.2
−84.0
5.0
0.10
y = −8.507x + 20.35
0.999
71.0
80.0
−28.4
0.5
0.01
y = −9.947x + 22.82
0.998
83.0
85.4
−8.0
1.0
0.02
y = −6.653x + 12.57
0.997
55.4
83.3
−93.0
PEG-600
2.0
0.03
y = −6.334x + 11.88
0.997
53.0
83.0
−99.0
3.0
0.05
y = −2.362x  − 0.780
0.998
20.0
77.4
−191
4.0
0.07
y = −3.078x + 1.893
0.998
26.0
80.5
−182
5.0
0.09
y = −4.666x + 7.455
0.998
39.0
80.0
−135


## 3.1. Determination of the Order of Reaction

General form of the rate law for a nitrodecarboxylation (nitro-Hunsdiecker reaction) reaction could be represented by considering the following general scheme:(9)CAN+S⟶Products
where CAN = ceric ammonium nitrate and S = α,β-unsaturated carboxylic acid:
(10)V=k1[CAN]x[CAN]y.Reactions were conducted under two different conditions. Under pseudo first order conditions[CAN]≫[CAN], plots of ln(Ao/At) versus time were straight lines with a negative slope indicating order (x) with respect to [CAN] to be unity. As typical examples pseudo first order plots are shown in Figures 3, 4, and 5. First order rate constant (k′) was obtained from the slopes of line plots cited (k′).Figure 3
Pseudo first order plot of cinnamic acid (ln(Ao/At) versus time) at 323 K. [CA] = 1.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−3 mol dm−3; [PEG-300] = 9.99 × 10−1 mol dm−3.Figure 4
Pseudo first order plot of acrylic acid (ln(Ao/At) versus time) at 315 K. [AA] = 1.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−3 mol dm−3; [PEG-300] = 13.32 × 10−2 mol dm−3.Figure 5
Pseudo first order plot of nitrocinnamic acid (ln(Ao/At) versus time) at 315 K. [NCA] = 1.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−3 mol dm−3; [PEG-400]= 1.25 × 10−1 mol dm−3.The reaction is also conducted under second order conditions with equal concentrations of [CAN]0 = [CAN]0. Kinetic plots of [1/(a−x)] or [1/(At)] versus time (Figures 6, 7, and 8) have been found to be linear with a positive gradient and definite intercept on ordinate (vertical axis), indicating overall second order kinetics.Figure 6
Second order kinetic plot of cinnamic acid (1/At versus Time) at 310 K. [CA] = 4.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−2 mol dm−3; [PEG-200] = 5.00 × 10−1 mol dm−3.Figure 7
Second order kinetic plot of acrylic acid (1/At versus Time) at 310 K. [AA] = 4.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−2 mol dm−3; [PEG-300] = 3.33 × 10−1 mol dm−3.Figure 8
Second order kinetic plot of nitrocinnamic acid (1/At versus time) at 310 K. [NCA] = 4.00 × 10−2 mol dm−3; [CAN] = 4.00 × 10−2 mol dm−3; [PEG-600] = 3.32 × 10−1 mol dm−3.In the present study, kinetic data have been collected at three to four different temperatures within the range of 300–315 K. Activation parameters such asΔH# and ΔS# have been evaluated from Eyring’s plots (Figures 9 and 10). Free energy of activation (ΔG#) is obtained from Gibbs-Helmholtz equation:
(11)ΔG#=ΔH#-TΔS#.Figure 9
Eyring’s plot of cinnamic acid (ln(k′′/T) versus 103/T). Kinetic study with [PEG-200] = 2.00 mol dm−3.Figure 10
Eyring’s plot of acrylic acid (ln(k′′/T) versus 103/T). Kinetic study [PEG-300] = 13.32 × 10−1 mol dm−3.


## 3.2. Mechanism of Nitrodecarboxylation of Unsaturated Acids by Ce(IV) Nitrate (CAN) in MeCN Medium

In order to gain an insight into the mechanistic aspects of CAN-CA reaction in MeCN medium, the knowledge of distribution of CAN species in HNO3 medium could be useful. In HNO3 Ce(IV) mainly exists as Ce(NO3)62-, Ce(NO3)5-, Ce(OH)(NO3)4-, Ce(NO3)4, which could arise from the following equilibria as cited in the literature reports [33–36]:
(12)Ce(NO3)4+NO3-⇌Ce(NO3)5-Ce(NO3)5-+NO3-⇌Ce(NO3)62-Ce(NO3)4+H2O⇌Ce(OH)(NO3)4-+H+
But the CAN species in the present study could be entirely different because the reactions are conducted in MeCN medium, which is large excess over [CAN]. MeCN may penetrate into the coordination spheres of Ce(IV) and form solvated CAN species according to the following equilibrium:
(13)Ce(NO3)4+2ACN⇌[Ce(NO3)4(ACN)2]
When unsaturated acid (USA) is added to the reaction mixture containing solvated CAN, it may form [Ce(IV) carboxylate] precursor which dissociates and releases HNO3  in situ. The HNO3 thus released in turn may combine with another solvated CAN to form nitronium ion (NO2+) which finally reacts with coordinated USA to afford β-nitrostyrene as shown in the following sequence of steps:
(14)RCH=CH–COOH+[Ce(NO3)4(CH3CHN)2]⇌[Ce(NO3)4(ACN)(RCH=CH–COOH)](Precursor)Precursor→(Slow)HNO3Precursor→(Slow)+[Ce(NO3)3(ACN)(RCH=CH–COOH)]HNO3+[Ce(NO3)4(ACN)2]⟶NO2++NO3-+[Ce(NO3)3(OH)(ACN)2]NO2++[Ce(NO3)3(ACN)(RCH=CH–COOH)]⟶RCH=CHNO2+CO2+[Ce(NO3)3(ACN)]+[Ce(NO3)3(ACN)]++NO3-+ACN⟶[Ce(NO3)4(ACN)2]


## 3.3. Mechanism of Nitrodecarboxylation in PEG Media

Progress of the reaction has been studied in the presence of a set of polyoxyethylene compounds (PEGs) with varied molecular weights ranging from 200 to 600 units, and it was found that the reaction is enhanced remarkably in all PEGs. Reaction times were reduced from 24 hrs to few hours. The catalytic activity was found to be in the increasing order PEG-300 > PEG-400 > PEG-600 > PEG-200. Further, it is also interesting to note that the absorbance of solvated Ce(IV) species is increased when PEG is added to [CAN] solution. This observation may indicate that solvated [CAN] species could bind with PEG to formPEG supported CAN species according to the equilibrium shown in Scheme 1.Scheme 1The plots ofkm (rate constant of PEG reaction) versus CPEG (concentration of PEG) indicated a rate maxima nearly in the vicinity of 1.50 mol dm−3 PEG-200, 0.99 mol dm−3 PEG-300, 0.99 mol dm−3 PEG-400, 0.500 mol dm−3 PEG-600. Efforts have been made to interpret PEG effect on the mechanism of CAN-USA reaction. Mechanism of PEG-mediated reactions was explained in the lines of micellar catalysis because PEG resembles the structure of Triton-X. The first and widely applied model is that of Menger and Portnoy [37] which closely resembles that of an enzymatic Catalysis. According to this model, formation of PEG bound reagent (PEG-CAN) could occur in the preequilibrium step due to the interaction of Ce(IV) with PEG. The complex thus formed may possess higher or lower reactivity to give products. A general mechanism is proposed by considering the bulk phase and micellar phase reactions as shown in Scheme 2, where km and k0 or (kb) represents rate constants for PEG and bulk phases, respectively, and K is the [PEG-CAN] binding constant. For the previous mechanism, rate law could be derived according to the following sequence of steps in the lines of micellar catalyzed reactions.Scheme 2Considering the total concentration of(Cs) as the algebraic sum of free species and PEG bound substrate complex (CAN-PEG) species,
(15)CS=[CAN]+[CAN-PEG].
From micelle-substrate binding equilibrium,
(16)CANK=[CAN-PEG][PEG][CAN],or[CAN]=[CAN-PEG]K[PEG].
Substitution of [CAN] in (15) gives
(17)CS=[CAN-PEG]K[PEG]+[CAN-PEG]=[CAN-PEG]+K[PEG][CAN-PEG]K[PEG]or[CAN-PEG]=K[PEG]CS1+K[PEG].
Similarly free substrate [CAN] is written as [CAN]=CS-[CAN-PEG](18)[CAN]=CS-K[PEG]CS1+K[PEG].
After simplification, the previous equation reduces to
(19)[CAN]=CS1+K[PEG].
Substitution of [CAN-PEG] and [CAN] in (15) gives
(20)k′=k0CS1+K[PEG]+kmK[PEG]CS1+K[PEG]orkφ=(k0+kmK[PEG])1+K[PEG]kφ=(k0+kmK[PEG])1+K[PEG],
where kφ=(k′/[CAN]), the second order rate constant in PEG media. Subtracting k0 from both sides of equation and rearranging
(21)kφ-k0=(km-kw)K[PEG]1+K[PEG].
But, since the reactions are too sluggish in the absence of [PEG], the rate constant (k0) would be much smaller than (kmK[PEG]); that is, (k0⋘kmK[PEG]). Therefore the (k0) term could be neglected in the previous equation. The rate law (21) could be then considered as
(22)kφ=kmK[PEG]1+K[PEG].This rate-law resembles Michaelis-Menten type rate law that is used for enzyme kinetics. Interestingly the plots of rate constant(kφ), that is, second order rate constant of PEG-mediated reaction versus [PEG], indicated either Hill type curve (i.e., a gradual increase with an increase in [PEG] passing through a maximum point in the profile). This observation could be explained due to the CAN-PEG binding interactions to give [CAN-PEG] species which is responsible for rate accelerations in PEG-mediated reactions. Formation of [CAN-PEG] species has also been established by UV-Visible spectroscopic studies, and binding constants are compiled in Table 1. These observations strengthened us to take up detailed reaction kinetics at various PEG concentrations in order to have an insight into the variation in the enthalpies and entropies of activation with [PEG].


## 3.4. Effect of Structure on the Reactivity, Enthalpy, and Entropy Changes

The enthalpy(ΔH#) and entropy of activation (ΔS#) are the two parameters typically obtained from the temperature dependence of a reaction rate, when these data are analyzed using Eyring’s equation. Both of these values are obtained from Eyring’s plots according to standard procedures [37–39]. Typical plots are shown in Figures 9 and 10. The entropy of activation (ΔS#) provides clues about the molecularity of the rate determining step in a reaction, that is, whether the reactants are bonded to each other or not. Even though structural change in the cinnamic acid exhibits slight variation on its reactivity, no quantitative conclusions could be obtained from rate constant data. However, the observed positive ΔS# values in lower PEG concentration (0.1 M PEG) decreased with an increase in [PEG] and are becoming more negative for all the unsaturated acids which probably indicates that PEG is tightly bound to CAN and lends support that PEG bound CAN is the active species in PEG-mediated reactions. The observed negative ΔS# values are in consonance with the concept that for associative mechanism, entropy decreases upon achieving the transition state, and values near zero are difficult to interpret [38–40]. By and large similar magnitude of observed free energy of activation (ΔG#) in this reaction series (Tables 2, 3, 4, 5, 6) indicates a similar type of mechanism which is operative in the present study. The observed negative magnitude of ΔG (Table 1) indicates the spontaneity of complexation of [CAN-PEG] due to PEG binding with CAN.Table 3
Activation parameters of crotonic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −9.907x + 22.7
0.999
82.4
85.1
−9.0
1.0
0.02
y = −6.742x + 12.86
0.999
56.0
83.3
−91.0
PEG-200
2.0
0.03
y = −5.069x + 7.688
0.999
42.1
82.3
−134
3.0
0.04
y = −6.020x + 11.13
0.999
50.0
81.5
105
4.0
0.05
y = −7.664x + 16.87
0.999
64.0
81.2
−57.3
5.0
0.06
y = −7.062x + 14.92
0.999
59.0
81.0
−73.5
0.5
0.01
y = −9.797x + 22.35
0.999
81.4
85.0
−12.0
1.0
0.02
y = −8.070x + 17.25
0.999
67.0
83.2
−54.2
PEG-300
2.0
0.04
y = −4.233x + 5.180
0.998
35.2
81.5
−154
3.0
0.05
y = −6.852x + 14.13
0.999
54.6
79.0
−80.0
4.0
0.06
y = −6.564x + 13.36
0.999
54.5
80.4
−86.5
5.0
0.08
y = −9.440x + 23.13
0.999
78.5
80.0
−5.3
0.5
0.01
y = −9.759x + 22.23
0.999
81.2
85.0
−13.0
1.0
0.02
y = −7.864x + 16.59
0.999
65.4
83.3
−57.0
PEG-400
2.0
0.04
y = −4.189x + 5.034
0.999
35.0
82.0
−155
3.0
0.05
y = −8.018x + 17.93
0.999
67.0
81.5
−48.5
4.0
0.06
y = −5.394x + 9.454
0.999
45.0
80.7
−119
5.0
0.08
y = −8.250x + 19.25
0.998
68.6
80.0
−37.5
0.5
0.01
y = −9.630x + 21.81
0.998
80.0
85.0
−16.3
1.0
0.02
y = −5.541x + 8.852
0.998
46.0
83.2
−123
PEG-600
2.0
0.03
y = −5.256x + 8.404
0.999
44.0
82.3
−128
3.0
0.04
y = −5.421x + 9.243
0.998
45.0
81.2
−120
4.0
0.06
y = −4.492x + 6.466
0.997
37.4
80.5
−144
5.0
0.09
y = −6.254x + 12.73
0.999
52.0
80.0
−91.8Table 4
Activation parameters of methoxy cinnamic acid in different PEG media. Units ofk′′ = dm3mol−1min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −12.85x + 32.55
0.996
107
85.0
73.0
1.0
0.02
y = −13.90x + 36.01
0.998
115
84.4
102
PEG-200
2.0
0.04
y = −6.038x + 11.21
0.999
50.2
81.50
−104
3.0
0.06
y = −5.027x + 8.448
0.999
42.0
80.2
−127
4.0
0.08
y = −3.844x + 4.748
0.999
32.0
80.0
−158
5.0
0.12
y = −4.403x + 6.945
0.999
37.0
79.0
−140
0.5
0.01
y = −10.36x + 14.11
0.997
86.0
35.3
110
1.0
0.03
y = −3.532x + 2.653
0.999
46.0
98.5
−175
PEG-300
2.0
0.04
y = −5.385x + 8.930
0.959
45.0
82.0
−123
3.0
0.05
y = −2.423x  − 0.582
0.999
20.2
78.0
−192
4.0
0.06
y = −8.743x + 20.63
0.999
73.0
81.0
−26.0
5.0
0.08
y = −7.965x + 18.28
0.999
66.2
80.0
−46.0
0.5
0.02
y = −7.638x + 15.83
0.999
63.5
83.3
−66.0
1.0
0.03
y = −10.01x + 24.07
0.999
83.2
77.0
21.3
PEG-400
2.0
0.04
y = −9.756x + 23.58
0.999
81.0
81.4
−2.0
3.0
0.06
y = −8.022x + 18.16
0.999
67.0
80.0
−41.0
4.0
0.08
y = −6.000x + 11.86
0.999
50.0
80.0
−99.0
5.0
0.10
y = −9.302x + 22.89
0.999
77.3
79.5
−7.3
0.5
0.02
y = −7.936x + 16.83
0.999
66.0
83.3
−58.0
1.0
0.03
y = −5.955x + 10.61
0.998
49.5
82.3
−110
PEG-600
2.0
0.05
y = −4.894x + 7.616
0.999
41.0
81.3
−134
3.0
0.06
y = −6.259x + 12.33
0.999
52.0
80.5
−95.0
4.0
0.08
y = −5.659x + 10.63
0.999
47.0
38.72
−110
5.0
0.1
y = −6.719x + 14.05
0.998
56.0
80.3
−81.0Table 5
Activation parameters of nitrocinnamic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −12.00x + 29.73
0.999
100
85.0
50.0
1.0
0.02
y = −10.87x + 26.63
0.999
90.3
83.1
24.0
PEG-200
2.0
0.04
y = −4.711x + 6.807
0.999
39.2
37.05
−141
3.0
0.06
y = −5.506x + 9.842
0.999
46.0
38.04
−116
4.0
0.07
y = −5.012x + 8.347
0.999
42.0
38.40
−128
5.0
0.09
y = −3.923x + 4.963
0.998
33.0
38.99
−156
0.5
0.01
y = −9.764x + 22.25
0.999
81.2
85.0
−13.0
1.0
0.02
y = −7.861x + 16.58
0.999
65.4
83.3
−60.0
PEG-300
2.0
0.04
y = −4.270x + 5.297
0.999
35.5
37.0
−153
3.0
0.05
y = −6.852x + 14.13
0.999
57.0
81.0
−80.0
4.0
0.06
y = −6.545x + 13.3
0.998
54.4
81.2
−89.5
5.0
0.08
y = −9.042x + 21.84
0.999
75.1
80.0
−16.0
0.5
0.02
y = −7.717x + 16.10
0.977
64.1
83.2
−64.0
1.0
0.03
y = −5.321x + 8.612
0.999
44.2
82.0
−126
PEG-400
2.0
0.04
y = −8.683x + 20.01
0.999
72.1
81.4
−31.2
3.0
0.06
y = −8.718x + 20.54
0.999
72.4
80.4
−27.0
4.0
0.07
y = −7.947x + 18.23
0.998
66.0
80.0
−46.0
5.0
0.10
y = −7.687x + 17.63
0.999
64.0
79.3
−51.0
0.5
0.01
y = −9.635x + 21.82
0.998
80.1
85.0
−16.2
1.0
0.02
y = −8.534x + 18.80
0.997
71.0
83.4
−41.3
PEG-600
2.0
0.04
y = −4.071x + 4.648
0.999
34.0
82.0
−159
3.0
0.06
y = −4.703x + 7.167
0.999
39.1
80.5
−138
4.0
0.08
y = −3.635x + 3.891
0.999
30.2
80.0
−165
5.0
0.10
y = −5.600x + 10.65
0.999
46.5
79.2
−109Table 6
Activation parameters of acrylic acid in different PEG media. Units ofk′′ = dm3 mol−1 min−1.
Type of PEG
PEG
k
′
′ at
Equation
R
2
Δ
H
#
Δ
G
#
Δ
S
#
% (V/V)
300 K
kJ/mol
J/K/mol
0.5
0.01
y = −11.85x + 29.21
0.999
98.5
85.0
45.3
1.0
0.02
y = −8.384x + 18.31
0.998
70.0
84.0
−45.3
PEG-200
2.0
0.03
y = −7.860x + 16.98
0.998
65.3
82.2
−57.0
3.0
0.04
y = −8.164x + 18.27
0.997
68.0
82.0
−47.0
4.0
0.06
y = −6.581x + 13.42
0.999
55.0
81.0
−86.0
5.0
0.08
y = −6.061x + 11.96
0.998
51.0
80.5
−98.2
0.5
0.01
y = −11.85x + 29.21
0.999
98.5
85.0
46.0
1.0
0.02
y = −11.43x + 28.49
0.998
95.0
83.2
39.3
PEG-300
2.0
0.03
y = −10.67x + 26.25
0.997
89.0
83.0
21.0
3.0
0.04
y = −9.163x + 21.61
0.999
76.2
82.0
−18.0
4.0
0.05
y = −6.125x + 11.72
0.999
51.0
81.0
−100
5.0
0.07
y = −5.443x + 9.792
0.998
45.3
80.2
−116
0.5
0.02
y = −8.314x + 18.07
0.997
69.2
83.4
−47.4
1.0
0.03
y = −9.364x + 21.98
0.998
78.0
82.5
−15.0
PEG-400
2.0
0.04
y = −9.756x + 23.58
0.999
81.2
82.0
−1.6
3.0
0.06
y = −7.889x + 17.76
0.997
66.0
81.0
−50.0
4.0
0.08
y = −6.573x + 13.69
0.998
55.0
80.2
−84.0
5.0
0.10
y = −8.507x + 20.35
0.999
71.0
80.0
−28.4
0.5
0.01
y = −9.947x + 22.82
0.998
83.0
85.4
−8.0
1.0
0.02
y = −6.653x + 12.57
0.997
55.4
83.3
−93.0
PEG-600
2.0
0.03
y = −6.334x + 11.88
0.997
53.0
83.0
−99.0
3.0
0.05
y = −2.362x  − 0.780
0.998
20.0
77.4
−191
4.0
0.07
y = −3.078x + 1.893
0.998
26.0
80.5
−182
5.0
0.09
y = −4.666x + 7.455
0.998
39.0
80.0
−135


## 4. Conclusions

Being a versatile chemical reagent CAN has been applied to organic reactions in catalytic or stoichiometric amounts. Our research group has succeeded in using CAN to perform nitrodecarboxylation ofα,β-unsaturated aromatic carboxylic acids to give β-nitrostyrene derivatives. Furthermore, use of PEG supported CAN could effectively initiate the decarboxylation of carboxyl group followed by nitration. Because the CAN reagent is cost effective, we believe that the present protocol offers a rapid and clean alternative and reduces reaction times.



---

*Source: 146585-2013-03-04.xml*