Kinetics of decarboxylation of aliphatic acids by cerium(IV) ammonium nitrate

0022~1902/8l/112899-06$02.0010 Pergamon Press Ltd.

1. inorg WC/. Chtm. Vol. 43, No. II. pp. 2899-2904, 1981 Printed in Great Britain.

KINETICSOF DECARBOXYLATIONOF ALIPHATICACIDSBY CERIUM(IV) AMMONIUMNITRATE RATAN RAJ NAGORI, MAHENDRA MEHTA and RAJ NARAIN MEHROTRA*? Department of Chemistry, University of Jodhpur, Jodhpur 342001,India. (Received 7 October

1980)

Abstract-The silver(I) catalysed decarboxylation of aliphatic acids by cerium(IV) ammonium nitrate in 2M nitric acid is a first order reaction in [Ce(IV)]and [Ag(I)]and an order < I is observed with respect to [RCOOH].Although there is a first order dependence in [Ce(IV)], the kobrdecreased with increasing [Ce(IV)] and there is a linear correlation between k;;d, and [Ce(IV),], where Ce(IV), is the monomeric cerium(IV) species. Hence monomeric cerium(IV) species are considered to be reactive. The initial presence of Ce(II1)retarded the rate and there is a linear correlation between k& and [Ce(III)]. The kobris in the increasing order: pivalic > isobutyric > butyric > propionic > acetic acid which is consistent with the relative stability of the respective alkyl radical. The observed rate constant kobsis given by the following equation: kobs =

2K(k+ k’[Ag(I)])[RCOOH],, 1+ K,[H’] + K1[Ce(III)])(l+2K&e(IV),I) + K[RCOOH],,

which under certain conditions and assumptions is reduced to

k = 2K(k+ k’LWMWXXG, Ohs 1t K,[H+]t K[RCOOH]e Monocarboxylic aliphatic acids are resistant to oxidative decarboxylation by cerium (IV) in dilute sulphuric acid even at refluxing temperatures[l, 21. The kinetics of decarboxylation of some aliphatic acids by cerium (IV) sulphate catalysed by Ag(I) has been reported[3]. The thermal decarboxylation of Ce(IV)-carboxylates in respective carboxylic acid medium is a slow process and the reduction of Ce(IV) is not complete even after prolonged heating[4]. An inner sphere mechanism is proposed for the oxidation of formic[S, 61 and acetic acid[7] by aquocerium(IV) ions. A similar mechanism is also proposed for the oxidation of substituted phenylacetic acids [8] in aqueous acetonitrile solution and isobutyric acid by cerium(IV) ammonium nitrate in aqueous solution. Unlike the oxidation of phenylacetic acids, the aliphatic acids are not readily oxidised and Ag(I) has been used as a catalyst[9]. This exhaustive study has brought out certain kinetic features of the reaction not yet reported. EXPERIMENTAL Reagents. The solution of cerium(IV) ammonium nitrate (G.F. Smith, primary standard) was always freshly prepared by the direct weighing of the sample in 2 mol drnm3nitric acid. Solutions of pivalic (Koch-Light, puriss), and isobutyric acid (B.D.H.) were prepared from respective samples without further purification. Solutions of acetic[lOa] (Basynth), propionic[lOb] (Koch-Light) and n-butyric[lOc] (Narden) acid were distilled before use. All solutions were standardised against a standard alkali. The silver(I) nitrate (Sarabhai-M, G.R.) solution was freshly prepared by the direct weighing of the sample. The stock nitric acid (Basynth’s, AnalaR) was heated to remove the dissolved oxides of nitrogen, cooled, diluted and standardised against a standard alkali. Sodium nitrate (B.D.H., AnalaR) was used to maintain the ionic strength. Stoichiometry. The nature of the oxidation product was much *Author to whom correspondence should be addressed. *Present address: Department of Chemistry, Kumaun University, Nainital263002,(U.P.), India.

dependent on the ratio in which the oxidant and substrates were mixed. Under the kinetic conditions where the [substrate]B [oxidant], it was assumed that the intermediate product(s) were not further oxidised because of the excess [substrate]. Thus for the characterisation of the first oxidation product, reaction mixtures were similarly prepared as for the kinetic runs. These were left in a thermostat at 50°C till all the cerium(IV) was reduced. The oxidation products were extracted several times with the solvent ether and the product was characterised to be an alcohol (RCOOH t 2Ce(IV)t Hz0 -+ROH t CO2t 2Ce(III)+ 2H’). The presence of methanol]1 la]. ethanol [ 1lb] and isopropanol[ 1Ic] in the oxidations of acetic, propionic and isobutyric acid respectively was confirmed by the spot tests. In another series of experiments in which [Ce(IV)]b [RCOOH] a number of oxidation products were noted at different stages of oxidation. Methanol, formaldehyde and formic acid were present in the oxidation of acetic acid; ethanol, acetaldehyde and acetic acid in the oxidation of propionic acid; similarly isopropanol, acetone and acetic acid were detected in the oxidation of isobutyric acid. However, in the oxidation of pivalic acid it was found that only two equivalents of Ce(IV) were used even after a long time. This could be attributed to the high resistance of the first molecular oxidation product, t-butanol, towards its further oxidation The results on the oxidation of methanol and ethanol[l2] indicated that these alcohols were oxidised slower than acetic and propionic acid. Rate measurements. The kinetics was studied in diffused room light under aerobic conditions. In all kinetic runs, [RCOOH]p [(Ce(IV)] and the ionic strength was kept constant at the desired level. The titrimetric method[l3] was used to follow the rate of the reaction. The pseudo first order rate constant kohr,with respect to Ce(IV), was calculated from the gradients of the linear plots between log(titre change) and time. These plots were linear beyond two half-lives and the knhr values were reoroducible to 2 3%. the average values from replicate runs are reported in the Tables. Testsfor free radicals. The existence of the free radical in the partially oxidised reaction mixtures was indicated by the polymerisation of acrylonitrile added to the reaction mixtures. Separate solutions of Ce(IV) and carboxylic acids did not induce the polymerisation of the monomer. The solutions were degassed with nitrogen

2899

before

the monomer

was added.

2!Joa

RATAN RAJ NAGORI RESULTS

Dependence on [Ce(IV)J. The disappearance of We(W)] over an eight fold variation was always first order since the usual first order plots were linear (r> 0.997) even beyond two half-lives of the reaction. However, the results in Table 1 indicated that kobs decreased with increasing [Ce(IV)] and the plot between ki& and [Ce(IV),l, Fig. 1, was linear where Ce(IV), denotes the monomeric cerium(IV) species. Dependence on [RCOOH]. The variation of kobs with [RCOOH], Table 2, was studied at four temperatures. The plot between k,;, and [RCOOH]-‘, Fig. 2, were linear at all temperatures. These plots indicated the formation of Ce(IV)-RCOOH complex prior to the rate limiting step. Dependence on [Ag(I)]. The effect of [Ag(I)], Table 3, on the observed rate was investigated in the oxidation of propionic acid. The linear plot between kabs and [Ag(I)], Fig. 3, having an intercept on the rate axis indicated that there was some uncatalysed oxidation of carboxylic acid. Dependence on [Ce(III)]. The effect of initial addition of Ce(III)nitrate was investigated at constant ionic strength. The kobsdecreased with increasing [Ce(III)] and the linear correlation between k;id, and [Ce(III)] is shown in Fig. 4. DISCUSSION

Ce(IV) Species. In aqueous nitric acid solution, a variety of Ce(IV)-nitrate complexes are known [ 14-171 and for some of these complexes the step wise formation constants have been reported[ 171.Ce(NOJ in the range [Ce(IV)] < 0.006 mol dm-’ is the most probable species and for [Ce(IV)] > 0.006 mol dmB3 a binuclear complex Ce2(N03)% is formed[ll]. The equilibrium constant for the equilibrium between monomeric Ce(NO& and dimeric Cez(NO&’ species is reported[ 181to have a value of 81 but other details were not given. The hydrolysis study[l9] in 3 moldm-’ nitrate solution indicated that Ce(IV) was unhydrolysed at [HNO,] > 0.5 moldme3 in [NO;] = 3 mol dmT3. Hence the existence of Ce(OH)3’ or the mixed hydroxy-nitrate complexes [ 15,161 was ruled out. The formation of dimeric cerium(IV) species in 5.5 mol dme3 nitric acid with Kd = 18? 2 at 30” was also reported but the specific composition of the dimeric species was not mentioned 1201. Hence it is seen that it is difficult to be specific about the reactive cerium(IV) species. This is perhaps one reason that Trahanovsky et al. [8,21] have consistently avoided to name the reactive cerium(IV) species in the oxidations of a variety of organic compounds in nitrate medium. Retarding efect of [Ce(IV)]. There are few studies reporting the dependence of the rate on the initial [Ce(IV)]. That the increasing [Ce(IV)] retarded the rate

0

0406

0012

0010

Fig. I. The linear plot between 10-’ k ii, and [Ce(IV),] obtained in the oxidation of propionic acid. The plot is based on the [Ce(IV),] calculated with Kd = 18mall’ dm3 (Ref.[20b]). Similar plots were also obtained when [Ce(IV),] was calculated with & = 81 mol-’ dmj (Retc;;llj (mE;;pe;mfntal conditions are de-

was reported in the oxidations of isobutyric acid[9] methanol [ 121and certain diols 1221in nitrate medium and in the oxidations of As(III)[23], and pentane- 1, 5diol[24] and butane - 1, 3 - dio1[25] in sulphate solution. It must be noted that the order with respect to [Ce(IV)] was always one in each case irrespective of the initial [Ce(IV)] used. In the oxidation of As(II1) it was explained that the rate was dependent on [Ce’“], Ce’” ion was the reactive species, whereas the method adopted to follow the rate determined the total concentrations of Ce’” and other Ce(SOJ-‘” species[23]. In the oxidation of pentane - 1, 5 - diol[24] the retardation was explained on the basis that monomeric cerium(IV) species were reactive and the rate law thus derived was consistent with the linearity of the plot between k,-d, and [Ce(IV)]. In the oxidation of butane

Table 1. The effect ot [Ce(IV),]e on kabs,the pseudo first order rate constant [CH,CHzCO>H]= 0.2, [Ce(IV)]o(mol dm-‘) [Ce(IV),* (mol dmm3) 104k,s,(s-l)

[H’] = 2.0, 0.005 0.004 4.00

[NO?] = 2.5, 0.01 0.008 2.73

002c

[AgNOJ = 0.005 and I = 2.5mol dm-’ at 40 0.03 0.04 0.015 0.02 0.018 0.022 0.011 0.013 I.31 I.11 2.03 1.82

*The [Ce(IV),] was obtained by solving the quadratic equation, given below, 2K&e(IV),]* + lCe(IV),] - [Ce(IV)la= 0 (Kd = I8 mall’ dm3)

Kinetics of decarboxylation Table 2. The effect of [RCOOH] . [RaxM](wl

on kob, at different temperatures. lCe(lV)lo and I = 2.5 mol dm-’

dii7

Temp. ( ‘C)

Propionic acid

Butyric acid

Isobutyric acid

Pivalic acid

of aliphatic acids by cerium(IV)

0.01

0.06

_ _ _ _ _--me

0.08 __

2901

ammonium nitrate

= 0.01, lHNO11= 2.0, lAgNO

0.40 , 0.10 0.20 10 k,,,,s- - - - - - - - - - -

K+

40

1.62

1.98

2.17

2.35

.2.70

2.98

42

45

3.40

3.81

4.10

4.24

4.62

4.90

81

50

5.02

5.78

6.20

6.46

7.28

7.72

55

7.76

a.80

9.36

9.72

10.4

66

11.4

80

40

2.12

2.42

2.63

2.78

3.11

3.32

75

45

3.61

4.18

4.50

4.75

5.29

5.62

77

50

6.02

a.48

106

55

a.96

40

2.94

3.20

3.33

3.40

3.62

3.72

45

4.62

5.04

5.24

5.42

5.74

5.90

77

50

6.98

7.55

7.86

8.05

8.52

8.80

167

6.74 10.2

7.18

8.14

7.70

10.8

12.3

11.2

96

13.0

55

9.S2

40

3.55

4.12

4.50

4.72

5.36

5.70

45

5.74

6.70

7.16

7.50

8.38

6.94

50

a.90

55

**

13.0

10:s

11.3

11.6

132 64 69

11.8

13.1

13.9

67

14.9

16.3

17.2

19.2

20.5

67

0.4

0.8

40

2.30

3.34

4.32

45 50

4.10 7.58

5.78 10.4

7.26 1%.6

16.0

19.4

12.0

13.0

11.0

0.2

55

80

10.3

0.1

LAcetic acid]

12.5

*

= 0.005

1.0

Kt

5.02

5.18

a

8.36 14.5

8.72 15.1

9 10

22.0

22.5

12

*K is the equilibrium constant for the reaction (5) and its values are reported in whole numbers. **[AgNOl] = 0.01 mol dm-‘.

0

Fig. 2. The linear plot between lO-‘k;& and [RCOOH]l’. The plot is consistent with the eqn (15). The plots are shown for 40” but such plots were linear at other temperatures too. The experimental conditions are described in Table 2. Propionic acid (-0-), Butyric acid (-Cl-), Isobutyric acid (4) and pivalic acid (-A-).

ool

&OS!

003

004

[AgN03](mol dm-))

Fig. 3. The linear plot between lo4 kobs and [AgNO?] in the oxidation of propionic acid. The plot is consistent with the eqn (15) and the experimental conditions are described in Table 3.

RATAN RAJ NAGORI

2902

Table 3. The effect of the initial [AgNO,] on the observed rate constant koba [Ce(IV)]o= 0.01, [AjNOj] (mol dm-‘) 10 kobr(s-l)

[HNOj] = 2.0 and I = 2.5 mol dm--’ at 40” 0.01 0.03 0.04 0.02 13.1 16.8 5.0 9.0

[CHXHZCO~H]= 0.2, 0.005 2.1

Retarding e#ect of [Ce(III)]. There were several possibilities that could explain the retarding effect of Ce(II1) on the observed rate. One of these was the reversible disproportionation of the Ce(IV)-RC02H complex into the products within the solvent cage. Since the backward rate of reaction (1) will be proportional to the mol fraction of Ce(II1) present within the cage, therefore k,-d, should not be proportional to [Ce(II1)][27]. In view of the linear correlation, Fig. 4, this possibility had to be ruled out. RCOIH-Ce(IV)zRCOO

. H’Ce(II1) +R’

2.0

I 0

I

0004

1

1

0012

0.006 [ce (III )]

( mot

dm-3

I

0016

)

Fig. 4. The linear plot between lo-’ kzs and [Ce(III)] in the oxidation of propionicacid is consistent with the eqn (14). The experimentalconditionsare describedin Table 4.

- 1,3 - dial [25] it was assumed that both monomeric and dimeric cerium(IV) species were reactive and the rate law obtained on this assumption was consistent with the observed linearity between kobsand [Ce(IV)]-‘. The existence of the dimeric cerium(IV) species in sulphuricacid

was first postulated in the oxidation of butane - 1, 3 - diol[25]whichwas later confirmedby the X-ray study of the crystal structure of the dimeric species Cez(OH)z(Hz0)4(S0,), obtained from sulphuric acid solution [26]. Since the present rate data on the oxidation of propionic acid gave a linear plot between k& and [Ce(IV),], as was obtained in the oxidation of pentane 1, 5 -dio1[24], the explanation that only monomeric Ce(IV) species were reactive may well explain the observed retardation in these oxidations too. The assumption that monomeric cerium(IV) species were reactive was made after considerations that excluded any possibility of a fractional or higher order in [Ce(IV)I because the usual first order plots were linear well beyond two half-lives of the reaction. These plots would have shown a deviation from the linearity if a fractional or higher order was involved.

t CO2t H’ t Ce(II1).

Another possibility was the formation of unreactive (IV)Ce-Ce(II1) dimeric species [20]. However, the dimerisation constant for the equilibrium (2) is small enough, Kd = 2.0t0.7 at 30” in 5.5 mol dmm3 nitric acid[20], to explain the observed effect. Ce(IV) t Ce(III)$(IV)Ce-Ce(II1)

(K:).

[Ce(III)] (mol dme3) IO4kotw(s-l)

[CH,CH?COzH]= 0.2, 0.002 2.42

(2)

Yet another possibility was the removal of free RCOzH as complex by Ce(II1). RCO*H-Ce(II1) complexes are known[28]. Since there wouldbe proportionalloss of [RC02H] with increasing [Ce(III)], some kind of linear correlation was expected between kobs and [Ce(III)]. The observed correlation is illustrated in Fig. 4. Formation of Free Radical. The formation of the free radical, indicated by the induced polymerisation of acrylonitrile, was consistent with the formation of alkyl radicals in the photo- and the thermal reduction of Ce(IV)-carboxylates in respective carboxylic acid medium. The reactive alkyl radicals were also characterised by esr spectroscopy during the photo-oxidation of acetic, propionic, isobutyric and pivalic acids by Ce(IV)[291. Need for protonation equilibrium. The K, values for all the aliphatic acids [30], except formic acid, is of the order of 10m5.On the other hand the protonation constant[31] for the equilibrium (4) is several times greater than the dissociation constant of the respective carboxylic acid. Thus there was a greater need to consider the protonation equilibrium (4) in any proposed mechanism than the consideration of the most unlikely dissociation equilibrium, considered in the oxidation of isobutyric acid[9], in a solution of 2 mol dm-3 nitric acid. Mechanism. In view of the results and the above

Table 4. The effect of the initial [Ce(lII)] on the observed rate constant k”t,, [Ce(IV)]0= 0.01

(1)

[AgNOs]= 0.005, 0.004 2.14

[HNO,] = 2.0 and I = 2.5 mol 0.008 1.71

0.012 I s3

0.016 1.32

Kinetics of decarboxylation of aliphatic acids by cerium(IV) ammonium nitrate discussion,

the most

probable

could be

mechanism

expressed in terms of the reactions (3)-(9) where reactions (7) and (8) are rate limiting and Ce’,” is the monomeric cerium(IV) reactive species. RCOzH t Ce(IIWRCO,H-Ce(II1)

(KJ

(3)

The rate law in eqn (14) was found to be consistent with the plots in Figs. l-4. Since 2KJCe’,vle 1 t K[RC02H] and [Ce(III)] = zero because it is initially absent, the eqn (14)is thus further simplifiedto (15). k Oh’

RCOzH t H+sRCO,H:

(K,)

RCOzH t Ce?SRC02H-CeF

(K)

R’ t CO2t Ce(III) t H’

Ag’ t RC02H-Cel,Vs[Transition R’ t CO*t Ag’ t Ce(II1) t H’ R . t Hz0 + Ce’” 2

(5) (6) (k)

(7)

state]*fast

(k’)

ROH t Ce(II1) t H’

-d[Ce(IV)l _- 2K(k t k’[Ag’l)[RCO,H][Ce’,“l. dt

The value of the equilibrium constant K for the reaction (5) was obtained by using eqn (16) where I and S are respectively the intercept and slope of the plots in Fig. 2 and is given in Table 2 in the last column. A perusal of these values indicated that these were constant within the experimental limits of variation except for one or two values which are perhaps in error. K=+K,[H+]).

(9)

(10)

It could also be shown that [RCOzHlo I t K,[H’] t K,[Ce(III)]

(11)

and

[CeZl =

(15)

(16)

(8)

Considering that two equivalents of Ce(IV) are used per mole of RCOlH oxidised to ROH, the first stable molecular product, which could be further oxidised to other products, the rate of disappearance of Ce(IV) in terms of the reactions (3)-(9) is expressed by the eqn (10).

[RCOzH] =

= 2K(k t k’[Ag+l)IRCOzHlo 1t K,[H+] t K[RC02H]o’

(4)

Ce? + CeLY$ (IV)Ce-Ce(IV) (Kd) (Dimeric species) RC02H-Cez-,

2903

[WV)10

1 t 2K,,ICe~l+ K[RCO2Hl

(12)

Since the p value in the plots between log K,and (T*, the sum of the Taft function[321, is same (-6.3) for the primary alcohols, ketones and carboxylic acids[31] therefore it was assumed that for carboxylic acids the variation of K, with temperature also exhibited a similar broad maximum as observed in the case of alcohols [33]. Thus the known value of K, could be used at other temperatures too. In the limiting case at high [RC02H],, when K[RC02Hlo P (1 t KJH’]), the eqn (15) suggested that koh5should become independent of [RC02Hlo and that it is so is indicated by the results reported in Table 5. The rate of oxidation of various aliphatic acids was in the order: pivalic > isobutyric > butyric > propionic > acetic acid. This could be linked to the ease of formation of the free radical which is in the order tertiary> secondary > primary radical. A similar conclusion was reached in the oxidation of substituted phenylacetic acids by cerium(IV) in nitric acid on the basis of a large negative p (2.91) value[8]. Thus the ease of the oxidation of aliphatic acids is directly linked with the stability of the alkyl radical formed in the decarboxylation process.

where [Ce(IV)lo and [RCO,Hlo express the initial analytical concentrations. The proper substitution of [RC02H] and [Ce’,‘] from the eqns (11)and (12)into eqn (10)gives the eqn (13) -d[Ce(IV)I = dt 2K(k t k’[Ag’l)[RC02Hl&e(IV)lo (1 t K,[H’l t K,~Ce(IIIl)(l t 2K, [Ce’,VI)+ KLRC02Hlo (13) The eqn (13) was further simplified on the assumption term the 2K&e~l(KJH‘l f IWe(III)I) @ that (1 t 2Kd[Ce?] t K,[H’l t KIICeW)I + K[RCOzHlo). 2K(k + k’[Ag+])[RCOzHla kobs= 1t KJH’] t K,[Ce(IIII) t2K&e?l+ URCOzHlo

(14)

Acknowledgements.-Thanks are due to C.S.I.R., New Delhi. for the financial help and the award of a J.R.F. to R.R.N. Similar thanks are due to U.G.C. for the award of a J.R.F. to Mahendra Mehta. Thanks are alzo due to Prof. R.C. Kapoor for the laboratory facilities

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Table 5. The invariance of the rate constant koh\at high [CHEH?C02H]

[Ce(IV)]0=0.01, [CHXH~COIH] (mol dmm3) 104k,&- ‘)

[AgNO,] = 0.005, 0.4 3.02

(HNO?]= 2.0 and I = 2.5 mol dm ’ at 40”. 2.0 0.8 I.0 I.5 0.6 3.18 3.15 3.12 3.21 3.14

2904

RATAN RAJ NACORI

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