INTRODUCTION:
Organophosphorus compounds play a key role1,2
in a number of biochemical i.e., enzymatically-catalysed processes. Commercial
applications of Organophosphorus compounds as floatation agents, stabilizers
and surfactants etc. have added much attraction to this area of study in the
modern era. Modern instrumental techniques3 like 31P NMR,
GC-MS etc. also allowed proper and suitable identification as well as
separation of the various components during synthesis.
EXPERIMENTAL STUDY:
Synthesis of Phosphoric bis-(o-methoxy
phenyl amide) chloride was achieved by thermocatalytic (with CTAB) treatment of
o-anisidine with phosphorus oxychloride4 (1:1), in benzene (dry),
while heating between 85-950C for 3.30 hours. A light-violet
coloured solid formed, gave m.p. 203-2050C. Washing with CHCl3
gave pure product with a sharp m.p. 2120C. Quantitative estimation
of phosphate by Allen’s modified method5 confirmed the structure as
Phosphoric bis-(o-methoxyphenyl amide) chloride. It was also characterised6
by elemental analysis, IR, 1H-NMR and 31P-NMR (
= 0.29 ppm)
spectral studies. All reagents used during the study were either of Aldrich or
Qualigens (Glaxo) brand.
RESULTS AND DISCUSSION:
Phosphoric bis-(o-methoxyphenyl amide)
chloride thus synthesized colud be kinetically studied in an aqueous medium
during hydrolysys in 0.05-7.0M HCl at 40(
0.5)0C. On hydrolysis,
this C-N-P diester liberates/generates both he parent amine, o-anisidine and
inorganic phosphate, but only the latter was estimated quantitatively by
Allen’s modified method5, with the help of Spectronic-20
spectrophotometer. The 2nd –order rate coefficients7 have
been determined in the acid range as above and presented in Table 1.
Neutral electrolyte effect study7
made at 1.0-3.0 
indicates as positive salt
effect. From the above data and the graphical representations (plot not shown),
an intercept kN0 = 1.60 x 10-2 L mol-1 min-1.
On applying the IInd empirical term of the Debye-Huckel equation, rates were
calculated Eq (i), and were taken to assign to the corresponding reactive
species.
ke = ko . e b …………….(i)
Further : ke = kN + kH+
CH+- ………..…. (ii)
Where, ke, kN, kH+,
cH+ and correspond to the observed rate
coefficients, via the Neutral species, specific acid-catalysed rate
coefficients, concentration of H+ -ions and ionic strength
respectively. The latter term kH+. CH+
is non-contributory due to the absence of the formation of the conjugate acid
species in the present bis-amide.
On further modification for each reactive
species. Eq. (ii) leads to the following :
KN = kNO ebN …………….
(iii)
or log kN = log kNO +
b’N
……………….(iv)
Equation (iv) has been used to determine
rates via the neutral species and since kN = kNO = 1.60 x
10-2 L mol-1 min-1, the second term, b’N, has no
significance. The observed rate coefficients resemble kN, suggesting
thereby that the Neutral form (or the Undissociated form) alone participates in
the entire acid range. Inspite of the increase in the acid molarity and the
appearance of a rate maximum at 4.0M-HCl, the conjugate acid species are not
found to be involved during hydrolysis in the entire acid range. This rate
maximum, in the present study is related to the maximum reactivity of the
Neutral species only, a totally different and unexpected behaviour of the
phosphoric diamide under observation.
Table-1 : OBSERVED AND CALCULATED
RATE-COEFFICIENTS FOR THE HYDROLYSIS OF PHOSPHORIC BIS-(o-MeO-PHENYL AMIDE)
CHLORIDE AT 40(0.5)0c
(KNO=1.60x10-2L mol-1min-1)
|
HCL
|
102
KNO
|
102ke(Calcd.)
|
102ke(Obsd.)
|
|
2.0
3.0
4.0
5.0
6.0
7.0
|
1.60
1.60
1.60
1.60
1.60
1.60
|
1.60
1.60
1.60
1.60
1.60
1.60
|
1.43
1.54
2.40
1.73
1.48
1.06
|
Note:- Between 0.1-1.0M HCl, ke
obsd. Varies between 0.77-0.74x10-2 L mol-1min-1
Arrhenius7,8 parameters (Table
2) suggest the involvement of the transition state9 and this data
was determined at 2.0M-HCl. The entropy of activation (
S#) favours
the bimolecular hydrolysis due to its –ve value of -14.98 e.u. Other related
parameters derived from temperature effect studies also support this.
Table-2: ARRHENIUS PARAMETERS FOR THE
HYDROLYSIS OF PHOSPHORIC BIS-(o-MeO-PHENYL AMIDE) CHLORIDE AT 2.0M-HCl
|
E
|
A
|
S#
|
H#
|
G#
|
|
52.27
KJ/mol
|
141x1010
sec-1
|
-14.98
e.u.
|
54.68
KJ/mol
|
74.27
KJ/mol
|
Solvent polarity changes (at 2.0M-HCl) were
made by increasing the percentage of acetic acid from 10.0 to nearly 90.0, and
the rates were observed to enhance by such changes in the % age. However, rates
were found to be independent of the varying %ages (10-50) of DMF as a reaction
medium. In an aqueous medium, the rates are slightly higher than 10% acetic
acid-water mixture, although the latter has a lower dielectric constant value
favoring protonation, while here only the Neutral species is the major reactive
form. This is attributed to its bimolecular hydrolysis, when water itself
becomes significant. The observed rate data with varied solvents’ compositions
is given as below :
ke (L mol-1 min-1)
: 1.43 > 1.37< 2.06 < 3.16 < 4.31 = 4.27
Medium (2.0M-HCl) Aq. AcOH AcOH AcOH DMF DMF
(% age) 10.0) (50.0) (90.0) (10.0) (50.0)
The varied AcOH-H2O compositions
were expected to promote the hydrolysis to a large extent as a result of protonation
of C-N-P nitrogen, but due to the absence of protonated species, the
protonation of o-methoxy subtituent present in each aryl matrix occure and the
rise in rates is not too large. In the presence of DMF, however, rates are
almost similar, whether the %age is as low as 10.0 or as high as 50.0.
Chart 1: Bimolecular Nucleophilic
Substitution SN2(P) of the Zwitter-ionic species with P-N bond
fission
Chart 2: Bimolecular Nucleophilic
Substitution SN2(P) of the Neutral species with P-N bond fission
In order to decide about the nature of bond
fission as P-N during acid hydrolysis, an azo dye test was conducted and was
positive, suggesting P-N bond rupture. Comparison of kinetic data of some
similarly-substituted esters when made gave similar results, again indicating
P-N bond fission in the present diamide.
On the basis of the above results and the
related discussion, phosphoric bis-(o-methoxyphenyl amide) chloride has been
observed to hydrolyse via the Nautral species (I) only in the higher acid region
(Chart 2), with one of its modified reactive forms, Zwitterionic species (II)
in the low acid region (Chart I). Both undergo bimolecular mechanism of
hydrolysis with P-N bond fission10 in the entire acid region.
REFERENCES:
1. Calvo, K.C., and Westheimer,
F.H.J. Am. Chem. Soc., 105, (1983).
2. Powers, V.M., ‘Biochemistry’, 30
(1991), pp. 9255.
3. M.M. Crutchfield, C.H., Dungan,
J.H. Letcher, Victor Mark and John R. Von Wazer, ‘31P Nuclear
Magnetic Resonance, Vol. 5, in Topics in Phosphorus Chemistry’, Interscience
Publ., (1967), pp. 169-175.
4. Jelly, Johan J., J. Chem. Eng.
Data, 1988, 33, 221.
5. Allen, R.J.L., Biochem. J., 34.
858 (1940).
6. Silverstein, R.M. Bassler, G.C.,
and Morrill, C. Terence, ‘Sepctrometric Identification of Organic Compounds’, 6th
ed., (2002), pp. 164, 222, 310.
7. Laidler, K.J., ‘Chemical
Kinetics’, IInd ed., Harper and Row Publ., INC, New York, 1987), pp. 384-393.
8. Arrhenius, S.Z., Physik Chem.,
4, 226 (1889).
9. Scientific American, Nobel
Prizes, (2000), 2-4.
10. Khorana, H.G., ‘Some Recent
Developments in the Chemistry of Phosphate Esters of Biological Interest’, John
Wiley and Sons, INC. New York, (1961).