The alkaline hydrolysis series of compounds derived from
O-alkyl-S-2-dimethylaminoethyl methylphosphonothiolate, O-alkyl-
S-2-trimethylammonioethyl methylphosphonothiolate methyl sulfato,
and 0,0-diethyl-S-2-dimethylaminoethyl phosphorothiolate
are studied in order to show substituent effects in these reactions.
The influence of structural change of alkyl radical in RO
group as well as electrophilic characteristic of phosphorus atom and
charge effects on nitrogen atom on the hydrolysis rate are discussed

D. Minić

D. Rakin

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CROATICA CHEMICA ACTA CCACAA 49 (4) 819-828 (1977) CCA-1055 YU ISSN 0011-1643 547.233 :547.241 :547.269 Original Scientific Paper Hydrolysis of Some Organophosphorus Compounds. Part I. Reactions in Alkaline Solution D. Minic and D. Rakin Military Technical Institute, 11000 Beograd, Serbia, Yugoslavia Received February 28, 1977 The alkaline hydrolysis series of compounds derived from O-alkyl-S-2-dimethylaminoethyl methylphosphonothiolate, O-alkyl--S-2-trimethylammonioethyl methylphosphonothiolate methyl sul-fato, and 0,0-diethyl-S-2-dimethylaminoethyl phosphorothiolate are studied in order to show substituent effects in these reactions. The influence of structural change of alkyl radical in RO group as well as electrophilic characteristic of phosphorus atom and charge effects on nitrogen atom on the hydrolysis rate are discussed. Tetrahedral organic phosphorus compounds undergo nucleophilic substitut-ion i - 3. Reactions in which the hydroxylic ion takes the part of nucleophil are the most interesting, since they are relatively simple and quick4 • In this work we examined the alkaline hydrolysis series of compounds derived from 0-alkyl-S--2-dimethylaminoethyl methylphosphonothiolates (a), O-alkyl-S-2-trimethyl-ammonioethyl methylphosphonothiolates methyl sulfato (b) and O,O-diethyl-S--2-dimethylaminoethyl phosphorothiolate (IX) (see Table I) , in order to show effects of alkyl in RO group as well as the effects of positive nitrogen in thio-alkyl radical in these reactions. The general structural formulas of these compounds are shown below. (a) (b) (IX) 820 D. MINIC AND D. RAKIN EXPERIMENTAL AND RESULTS Materials. - The preparation of the 0-alkyl-S-2-dimethylaminoethyl methyl-phosphonothiolates and their methyl sulfato derivatives has been described5• 0,0--diethyl-S-2-dimethylaminoethyl phosphorothiolate has been prepared by published methods~·7 • All compounds were purified by multiple (2-3 times) vacuum distillation and .their purity was determined by volumetric method8• O-alkyl-S-2-trimethylam-monioetpyl me,th,ylph0sphonothiolates methyl sulfato were purified by recristaly-za:tion 'fo constant ' melting point. Kinetics . - Hydrolysis of the 0-alkyl-S-2-dimethylaminoethyl rp.ethylphosphono-thiolates, 0-alkyl-S-2-trimethylammonioethyl methylphosph:onatiolates methyl sul-fato and 0,0-diethyl'-S-2-dimethylaminoethyl phosphorothi9late was followed by po-No 1. 2. 3. 4, 5. 6. 7. 8. 9. TABLE I Second-Order Rate: Constants of Alkaline Hydrolysis k 2 (in dm3/mol min) CHa(RO)P(O)SCH2CH2N(CH3)2 I 'O c 30 °c 40°c 50 °c ::s 0 R p. s 0 k2 k2' k2 k2' k2 k2' 8z I C2Hs I I 2.246 I 5.433' n-C4H9 II 0.748 0.768 1.754 ' 1.761 3.300 3.425 --n-CsH11 III 0.905 1.665 i-C3H1 IV 0.342 0.348 0.791 0.764 1.568 1.597 i-C4H9 v 0.510 0.511 1.018 1.016 1.933 1.936 neo-CsH11 VI 0.511 0.546 1.061 1.081 2.011 2.051 C6H13 (pinacolyl) VII 0.0628 0.0615 0.118 0,122 0.243 0.234 C6H11 (cyclohexyl) VIII 0.183 0.185 0.412 0.402 0.820 0.831 C2Hs* IX 1.726 1.712 3.538 3.594 7.2613 7.208 I I * CH, is substituted with C2H50 k2 - experimental values' of ·. rate constant k2' - estimated values of rate constant TABLE II Second-Order Rate Constants of Alkaline Hydrolysis k2 (in dm3/mol min) + CHa(RO)P(O)SCH2CH2N(CHa)a · CHaS04-'O c 20 °c 30 °c 40 °c ::s No 0 R p. s 0 k2 k2' k2 k' k2 k2' 8z 2 I I I 1. C2Hs x 8.0911' 16.201 2. n-CaH1 XI 3.805 7.706 3. n-C4H9 XII 6.183 11.757 4. n-CsHu XIII 3.891 8.095 5. i-C3H1 XIV 2.791 6. i-C4H9 xv 3.765 6.608 7. neo-CsH11 XVI 1.555 1.566 3.297 3.246 6.372 6.422 8. C6H13 (pinacolyl) XVII 0.409 0.424 1.070 0.990 2.100 2.187 9. C6Hu (cyclohexyl) XVIII 0.711 0.730 1.662 1.570 3.121 3.214 HYDROLYSIS OF ORGANOPHOSPHORUS COMPOUN DS 821 tentiomettic , titration using the »Radiomet~r« Titrator TTTl with glass electrode G 202B, and calomel electrode K 100. · · - · , The reaction mixture was prepared with 100 ml 0.01 mol/dm3 organophosphorus compounds and 100 ml of 0.025 mol/dm~ KOH. For titration 5 ml of this mixture was taken and the excess of alkali was titrated with 0.04 mol/dm3 hydrochloric acid. Results. - The second..:order rate constants for the hydrolysis of 0-alkil-S-2--dimethylaminoethyl methylphosphonothiolates, O-alkyl-S-2-trimethylammonioethyl methylphosphonothiolates methyl sulfato and O,O-diethyl-S-2-dimethylaminoethyl phosphorothiolate in alkaline solution are given in Tables I and II. Figures 1 and 2 show linear function lgb-x/a-x with . time and confirm that the reactions are second order. · · 200 400 t(min) Figure 1. The change of concentration O-alkyl-S-2-dimethylaminoethyl methyl-phosphonothio-lates with time. 100 · t (min) 200 Figure 2. The change of concentration o-alkyl-S-2-trimethylamonioethyl methyl-phosphonothio-lates methyl sulfate with time. 822 D. MINIC AND D. RAKIN Energy of activation, lg A, entropy of activation and energy activation (with standard error) are given in Tables III and IV. TABLE III Ac.tivation Parameters and lgA of the CHa(RO)P(O)SCH2CH2N(CH3) 2 'O i::: ;::s Ea± oE. ti G±ail.G No R 0 lgA ± algA ti S±ail.5 /e. u. p. s 0 kcal/mol kcal/mol 8z 1. C2H5 I 17.76 12.75 - 2.30 17.85 2. n-C4H9 II 14.55±0.92 10.38±0.64 -13.074±2.94 19.44±0.89 3. n-CsH11 III 14.55 8.25 -22.80 17.81 4. i-CaH1 ,IV 14.82±0.58 10.23±0.41 -13.75±1.86 19.35±0.56 5. i-C4H9 v 12.97±0.04 9.06±0.03 -19.11±0.14 18.23±0.04 6. neo-CsH11 VI 12.88±0.32 9.03±0.22 -19.25±0.01 17.59 ± 0.31 7. CaH1s (pinacolyl) VII 13.14±0.74 8.26±0.52 -22.75±2.38 18.20±0.72 8. C6H11 (cyclohexyl) VIII 14.59±0.42 9.79±0.29 -15. 76 ± 1.34 19.46 ±0.41 9. C2Hs"' IX 13.99±0.27 10.32±0.19 -13.33±0.86 16.98±0.26 • CHa is substituted with C2HsO e.u. = 4.184 JK-• mo1-1, cal = 4.184 J TABLE IV Activation Parameters and lgA of the CH3(RO)P(O)SCH2CH2N(CH8)s·CH3So4-'O i::: ;::s Ea± O Ea ti G±aAG 0 lgA ± algA ti S±aA5 /e. u. R p. so kcal/mo! kcal/mol .. 8z . . . . 1. C2Hs . . - . X .. - 12:26 10.05 -14.49 16.54 2. n-CaH1 XI 12.46 9.87 -15.31 16.37 3. n-C4H9 . XII 11.35 9.25 -18.15 16.08 4. n-CsHi1 · ' XIII 12.94 10.24 . -13.65 16.35 5. i-<:;4Ho XIV 12.87 7.98 -23~96 16.37 6. neo CsH11 XVI 12.87 ± 0.2.4 9.79±0.17 -15.69±0.80 17.20±0.23 7. CaH1a (pinacolyl) XVII 14.95± 1.23 10.78±0.89 -11.17±4.07 19.69±1.19 8. C6H11 (cyclohexyl) XVIII 13.51 ±0.90 9.94±0.65 -15.01±2.96 18.82±0.87 e.u. = 4.184 JK- t mo1-1, cal = 4.184 J DISCUSSION Fukuto and Stafford10, and Tammelin11 showed that O,O-diethyl-S-2-diethyl-aminoethyl phosphorothiolates, O-ethyl-S-2-dimethylaminoethyl methylphos-phonothiolates respectively, react with hydroxide ion, resulting exclusively in the cleveage of the P-S bond. Hydrolysis of O-ethyl-S-2-diethylaminoethyl methylphosphonothiolates and O-ethyl-S-2-diisopropilaminoethyl methylphos-phonothiolates, in strongly alkaline solution or in acidic medium, produces only ethyl methylphosphonic aeid and the corresponding dialkylaminoethyl mer-captan1fr-12. Our consideration based on this mechanism enabled us to follow all correlations as a result of the biomolecular reactions of nucleophilic sub-HYDROLYSIS OF ORGANOPHOSPHORUS COMPOUNDS 823 stitution. The second-order rate constant change of alkaline hydrolysis with temperature is in well agreement with Van't Hoffs and Arhenius laws. The temperature coefficient according to empirical Van't Hoff law for both series of compounds correspond to the most known reactions of nucleophilic sub-stitution (l.8-2.1). At the same time the change of the second-order constants of alkaline hydrolysis gives linear correlation with temperature as shown in Figures 3 and 4. Equations parameters are given in Tables V and VI. TABLE V Parameters of Linear Equation (1gk2 == B + A · 103/T) CHa(RO)P(O)SCH2CH2N(CHa)2 'tl ~ ;::s 0 No R 0. B A so 8z 1. n-C4H9 II 10.3792 -3.1814 2. i-CaH1 IV 10.2291. -3.2400 3. i-C4H9 v 9.0594 -2.8349 4. neo-CsH11 VI 9.0267 -2.8162 5. CsH1a VII 8.2624 -2.8719 . 6. CaH11 VIII 9.7911 -3.1901 7. C2Hs* IX 10.3216 -3. 0583 * CH3 is substituted with C2H , o lgk2 CH3(RO)P 0 SC2H4N(CH3)2 1 0 -1 3,1 3,2 3,3 Figure 3. Linear correlation second-order rate constant with temperature. 824 D . MIN IC AND D . R A KIN TABLE VI Parameters of L in ear Equation (1gk2 = B + A · 103/T) + CH3(RO)P (O)SCH2CH2N(CH3)3 · CH3S04-I 'O c ::l 0 No R 0. B A I s o o uz 1. neo-C5H11 XVI 9.7902 -2.8129 ' 2. C5H 13 XVII 10.7784 -3.2689 3. CsH11 XVIII 9.9396 - 2.9539 0 3,2 3,3 3 10 IT 3,4 Figure 4. Linear correlation second-order r ate const an t with t em perature . Our expedmental results show that the values of second-order rate con-stants depend on the structure of alkyl radical in the RO group. The constant of alkaline hydrolysis in O-alkyl-S-2-dimethylaminoethyl methylphosphonothiol-ates decreases from compound with ethoxy to compound with pinacolyloxy radical, i. e. the electrophilic characteristic of phosphorus atom decreases in the following order: C2H 50 > C4H90 - CsH110 > i -C4H90 > neo-C5H110 > i-C3H 10 > CsH110 > C5H130 . The slow est second-order rate constant of alkaline hydrolysis was found in O-pinacolyl-S-2-dimethylaminoethyl methylphosponothiolate (VII) (k240 oc = = 0.12 l/mol.min), and the highest one in O-ethyl-S-2-dimethylaminoethyl me-HYDROLYSIS OF ORGANOPHOSPHORUS COMPOUNDS 825 thylphosphonothiolate (I) (k240 oc = 2.25 l/mol.mih) (Table I) of phosphonothiola-tes series. As can be seen from Table I, 0,0-diethyl-S-2-dimethylaminoethyl phospho-rothiolate (IX) shows a higher rate of alkaline hydrolysis than -phosphono-thiolates. O-alkyl-S-2-trimethylammonioethyl methylphosphonothiolates methyl sul-fato has the same trend of reactivity (Table II). C2H50 > C4H90 > CsU110 - CaH70 > i-C4I:!90 > i-CaH10 > neo7CsH110 > CoH110 > CoH1aO In both series of compounds the second-order rate constant of alkaline hy-drolysis is changed gradually with the structural change of alkyl radical in the RO group. The relative rate constant (Table VII) confirms that the influence of the alkyl group on chemical reactivity is the most typical in the pinacolyloxy group. The rate constant of alkaline hydrolysis is about twenty times lower in O-pinacolyl-S-2-dimethylaminoethyl methylphosphonothiolate (VII) than in O-ethyl-S-2-dimethylaminoethyl methylphosphonothiolate (I). The rate diffe-rences observed on changing the substituent R with normal chain are very . small. The same relative reactivity correlation is noticed in the tetraalkylam-monio derivatives of methylphosphonothiolates. In this series of ~ompounds the TABLE VII Relq,tive Second-Order Rate Constant of Alkaline Hydrolysis of 0-alkyl-S-2--dimethylaminoethyl Methylphosphonothiolates CH3(RO)P(O)SCH2CH2N(CH3)2 No I R Compound Relative No k4o •c 2 1. CHaCH2- I 1 2. CHaCH2CH2CH2- II 0.78 3. CH3CH2CH2CH2CH2- III 0.74 CHa I 4. CHa-C-CH:r- VI 0.47 I CHa 5. CH3-CH-CH2- v 0.45 I _CHa 6. CHa-CH- IV 0.35 I CHa CHa I 7. CHs-C -CH- VII 0.05 I I CHs CHa 826 D. MINIC AND D. RAKIN phosphorus atom in O-pinacolyl-S-2-trimethylammonioethyl methylphospho-nathiolate methyl sulfato (XVII) was also of about twenty times lower electro-philic characteristics, in comparison with O-ethyl-S-2-trimethylammonioethyl methylphosphonothiolates methyl sulfato (X) (Table VIII) . TABLE VIII Relative Second-Order Rate Constant of O-aLkyL-S-2-trimethyLammonioethyL + . MethyLphosphonothioLates MethyL SuLfato CH8(RO)P(O)SCH2CH2N(CH3)8 • CH3So4-No R Compound Relative No k20 oc 2 1. CH3CH2- x 1 2. CHaCH2CH2CH2- XII 0.76 3. CHaCH2CH2CH2CH~ XIII 0.48 XI 0.47 5. . CHaCH · CH2- xv 0.46 I CH3 6. CHa-CH- XIV 0.34 1 CHs CHa I 7. CHs-C -CH2- XVI 0.19 J CHs CH a I CHs-C -CH- XVII 0.05 I I CHs CHs Our results suggest that a -substitution in the RO group has greater effect on the rate of alkaline hydrolysis than ·B-substitution. Thus, the isopropyl radical (compound IV), and especially pinacolyl radical (compound VII) in the RO group mostly decreases the rate of reaction to the greatest extent. The second-order rate constant of alkaline hydrolysis is more drastically changed in series of substituents C2HsO > i-CsH10 > CaH1aO than in series: C2HaO > C4H90 > CsHuO The second-order rate constant of alkaline hydrolysis was negligibly changed when the carbon in a-position contained two hydrogen atoms. HYDROLYSIS OF ORGANOPHOSPHORUS COMPOUNDS 827 Since the pinacolyl radical in the RO group has the greatest influence on the alkaline hydrolysis of the examined organophosphorus compounds of both series, one can say that steric effects are more likely to be responsible for the reactivity of the phosphorus atom13• Such, assumption can be confirmed by relatively change of lgA and entropy of activation (Table III). However, a more detailed analysis of the activation parameters (Tables III and IV) shows that the influence of the alkoxy group in nucleophilic substitution on the tetra-hedral phosphorus atom is very complicated. Qualitative evaluation of the effect of structural changes of the alkyl r~dicals in the RO group in the examined esters of methylphosphonothiolic acid must be taken with certain reserve, especially because space effects (among other things) can influence also the change of energy activation. It is even more complicated when one tries to explain the electrophilic characteristics of the phosphorus atom by hyper conjugation and other electronic 'effects of alkoxy groups. Positive Charge Effects of Nitrogen Atom on the Reactivity of Tetrahedral Phosphorus Atom The presence of a cationic group in molecules of O-alkyl-S-2-trimethyl- , ammonioethyl methylphosphonothiolates methyl sulfato gives to these com-pounds specific characteristics. The electron attracting properties of this group increase the partial positive charge on the phosphorus atom, and. consequently p" - ~ conjunction of the phosphoryl group is increased in a ground state. In this way, the increase in the electrophilic characteristics of the phosphorus atom can be explained in O-alkyl-S-2-trimethylammonioethyl methylphosphono-thiolates methyl sulfato in comparisor1 with O-alkyl-S-2-dimethylaminoethy] methylphosphonothiolates. Our experimental results suggest that the cationic group assists the rate of alkaline hydrolysis and confirms this hypothesis (Table IX). TABLE IX + Relativ e Second-Order Rate Constant of Alkaline Hidrolysis in Effects of Posi tiv e Charge of Nitrogen Atom in CHa(RO)P(O)SCH2CH2X 1. n-C4H9 2. n-CsH11 3. i -C4H9 4. neo-C4H11 5. C5H1a 6. CaHu • R elative K 2 = (k2 [X = N (CH,)3] (k2 [X = N (CHa),]) 15.40 8.96 12.92 7.69 15.96 8.49 Namely, the greater electrophilic characteristics of the phosphorus atom (10 to 15 times) in organophosphorus compounds with a cationic group in the thioalkyl radical seem to be due to the inductive effects of this radical only, since energy activation and free energy activation differ in comparison with these two series by about 1-3 kcal/mol. 828 D . MINIC AND D . RAKIN REFERENCES 1. R. 0. Br a in, Toksienie efiri kislot fosfora, »Mir« . Moskva, (1964), prilozenie Lo5adkina N. A., str. 457. 2. J. R. Cox Jr. and 0. B. Ramsey, Chem. Rev. 64 (1964) 317. 3. A. J. K ir by and S. G. Warren, The Organic Chemistry 6f Phosphorus, JW, Amsterdam, London, New York (1967) p . 274. 4. S. M. Markov, A. M. Po 1 eh in, N. A: Lo sad kin, G. A. Kostenko, Z. V. Moroz ova, and M. M. Jakubov i c, Zh. Obsch. Khim. 36 (1966) 1098. 5. D. Minic, Naucno-tehnicki pregled 26 (1976) 3. 6. L . E. Tamm e 1 in, Acta Chem. Scand. 11 (1957) 1340. 7. A. Calderbank and R. Ghosh, J. Chem. Soc. (1960) 637. 8. L. K. B each. and S. Sass, Anal. Chem. 33 (1961) 90L 9. L . E. Tamm e 1 in, Sv. Kem. Tidskr. 70 (1958) 157. 10. T . R. Fu k u to and E. M. St afford, J . Amer. Chem. Soc. 79 (1957) 6083. 11. L. E. Tamm e li n, Arkiv Chem. 12 (1958) 287. 12. J . Epstein, J. J. Ca 11 ah an, and V. E. Bauer, Phosphorus 4 (1974) 157. 13. R. F . Hudson and L. K ea y, J . Chem. Soc. (1956) 2463. SAZETAK Hidroliza nekih organofosfornih jedinjenja. Deo I. Reakcije u alkalnom rastvoru D. Minic i D. Rakin Izueavana je alkalna hidroliza 0-alkil-S-2-dimetilaminoetil-;metantiofosfonata i njihovih tetraalkilamonijum-derivata, kao i 0,0-dietil-S-2-dimetilaminoetil tiofosfata. Pokazano je na osnovu eksperirhentalnih rezultata da a-substitucija u alkil radikalu u RO grupi ima veci uticaj na brzinu alkaln~ hidrolize od ~-substitucije. Istovremeno, pozitivno naelektrisan atom azota u tioalkil-radikalu najverovatnije svojim induktiv-nim efektom u prelaznom stanju utice na brzinu ove reakcije. VOJNOTEHNICKI INSTITUT 11000 BEOGRAD JUGOSLA VIJA Prispjelo 28. veljace 1977. 

Hydrolysis of Some Organophosphorus Compounds. Part I. Reactions in Alkaline Solution

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