Kinetics of the Reaction Between Triphenylphosphine and Some Haloalkanes
Indian Journal of Chemistry Vol. 30A, November 1991, pp.929-935
Kinetics of the reaction between triphenylphosphine and some haloalkanes
DV S Jain* & R Chadha Department of Chemistry, Panjab University, Chandigarh 160014 Received 27 February 1991; accepted 8 July 1991
Kinetics of reactions RX + (C6Hs)3P- (C6Hs)3PR+X-(R = CCI3,CBr3,CH2Br, C4H9 and X = Br, Cl, Ii have been studied at different temperatures and in different dielectric media. The values of second or- der rate constants of the reactions with tetrahalomethanes are much higher than those with partially hal- ogenated alkanes. This suggests that the reaction may be of charge transfer type with tetrahalomethanes while with other haloalkanes it is simple nucleophilic substitution reaction of SN2 type.
A mixture of triphenylphosphine (TPP) and tetra- used as such. I-BB and l-lB were prepared in the halo methane (CX4) (especially X = CI or Br) has laboratory. Carbon tetrachloride was dried over been widely used as a versatile reagent for halogena- phosphorus pentoxide and the middle fraction, b.p. tion, dehydration, for creating P-N linkage and for 349 K was used. Acetonitrile was purified as de- replacing a hydroxyl group with halogen':" under scribed earlier". Tribromomethyltriphenylphos- mild conditions. lnspite of such wide applications of phonium bromide was prepared as described earli-
TPP/CX4 systems, there have been few detailed kin- er". Trichloromethyltriphenylphosphonium chlo- etic and mechanistic studies of the reactions between ride and trichloromethyltriphenylphosphonium TPP and CX4. We report herein the results of kinetic bromide were prepared by adding TCM (3.06 g) studies of the reactions of TPP with (CX4) such as and BTCM (1.98 ~ to TPP solution in acetonitrile CCI4, CBr4 and CBrCl3 in different solvents at dif- (5 mI) respectively. The first mixture was heated at ferent temperatures. For comparison we have stud- 323 K forS hr under nitrogen atmosphere while the ied the kinetics of the reaction of partially haloge- second mixture was stirred for 30 min at room tem- nated alkanes, viz. dibromomethane (DBM), perature. Precipitates started separating out in both l-bromobutane (I-BB) and l-iodobutane (l-lB) cases. The precipitates were filtered and recrystal- with TPP. lized from acetonitrile. These were analysed for The reactions between TPP and CX4 have been [(C6H5)3PCCl3]+Cl-, m.p. 448 K (lie m.p. 451 K) shown to lead to formation of various products de- and [(C6H5hPCCI3]+Br- m.p. 458 ± 2 K, respec- pending upon the ratioI.2,5.6in which the reactants tively. are mixed and to some extent on the manner? in Bromomethyltriphenylphosphonium bromide which the reactants are mixed. These complications was prepared by adding DBM (1.57 g) to a stirred arise from the consecutive reactions at higher molar solution of TPP (2.92 g) in dry toluene (10 mI) and ratios of (C6H5)3P/CX4.lt seems that trihalomethyl- mixture refluxed for 20 hr under nitrogen atmos- triphenylphosphonium halide is the initial product phere. The solid obtained was recrystallized from so long as the concentrations of both the reactants acetonitrile" m.p. 510 K (lit" m.p. 513 K).lt analysed are very low and the ratio (C6H5hP/CX4 is less than for [(C6H5)3PCH2Br]+Br-. l-Butyltriphenyl- unity. Therefore, in order to understand the com- phosphonium bromide and I-butyltriphenylphos- plex kinetics of the system it was decided to study ph onium iodide were prepared by refluxing 1-BB the kinetics of the initial reaction (1): (1.37 g) and l-lB (1.84 g) separately with TPP (2.62 g) in dry benzene (10 mI) at 353 K. The solids were CX (C6H5)3P--[(C6H5)3PCX,]+X- ... (1) 4 + recrystallized from acetonitrile. These were ana- The progress of the reaction was studied conduc- lysed for [(C6H5)3PC4H9]+Br-(m.p. 503 K) and tometrically as the product is ionic and the conduc- [(C6H5)3PC4H9]+I- (m.p. 493 K), respectively. tance increases with time. Kinetic study Materials and Methods The progress of the reaction was followed con- Triphenylphosphine (TPP)(Sisco Research Labo- ductometrically. In order to determine [phosphoni- ratories), TBM (BDH) and DBM (Fluka AG) were um salt] conductometrically the standard curves
929 INDIAN J CHEM, SEC.A, NOVEMBER 1991 were obtained between specific conductance and ious [reactants] and temperatures are given in Table corresponding [phosphonium salt]. The stock solu- 1. The reaction between TBM and TPP is extremely tions of desired [TPP] and [haloalkanes ] were pre- fast at room temperature therefore the kinetics of pared in dry acetonitrile (or any other solvent). Af- the reaction was studied in the temperature range of ter prethermostating the reactant solutions separ- 263 to 281 K. Activation energies of the reactions ately these were mixed in the reaction vessel and were determined from the Arrhenius plots and are conductance was measured at different intervals of given in Table 2 along with the frequency factors time. The conductance data were converted into the and entropies of activation. corresponding [phosphonium salt] with the help of The effect of dielectric constant of the medium on standard curves. the rate constants for the reactions of TPP with TBM and TCM was studied by carrying out the Results and Discussion reaction in mixtures of acetonitrile and l,4-dioxane The order of the reaction: (£ = 13.58 to 35.94). The dependence of rate con- (C6H:;hP+ RX -+ [(C6HshPR]+ X stant of a reaction on dielectric constant is generally
(R = CBr3, CCl3, CH2Br, C4H9) with respect to the given by Eq.(2). reactants was established by Ostwald's method. The lnk= In kg + 3/8kT(1- ell + e )(,u;/rl reaction was found to be first order in each reactant. 2 3 2/ 3) The values of the second order rate constants at var- + ,uB/rB -,u. r , ...(2)
Table 1- Values of second order rate constants of the reactions of TPP with halo alkanes
T{K) [TPPJ [TBMJ k T{K) [TPPJ [TBMJ k J 3 (mol dm-3) (mol dm-·l) (drrr' mol-I S-I) (mol dm- ) (moldm- ) (dm-'mol-Is-I)
TPP+ TBM; E = 19.58 E = 24.71 263 0.10 0.20 0.12 263 0.05 0.50 0.33 0.10 0.50 0.14 0.10 0.10 0.34 ~·O.lO 1.00 0.11 0.10 0.20 0.36 0;20, 0.50 0.14 0.10 0.50 0.37 0.40· 0.50 0.15 0.20 0.50 0.36 0.14 ± 0.02 0.35 ± 0.02 273 0.05 0.20 0.25 273 0.05 0.20 0.74 0.05 0.50 0.25 0.05 0.50 0.74 0.10 0.10 0.28 0.10 0.10 0.80 0.10 0.20 0.28 0.10 0.20 0.75 0.10 0.50 0.27 0.10 0.50 0.75 0.10 1.00 0.27 0.10 1.00 0.74 0.20 0.20 0.30 0.20 0.20 0.78 0.20 0.50 0.3\ 0.20 0.50 0.76 0.76 ± 0.Q2 0.28 ±0.02 281 0.05 0.10 0.48 281 0.05 0.10 1.28 0.48 0.05 0.20 0.05 0.20 1.23 0.49 0.05 0.50 0.05 0.50 1.02 0.10 0.10 0.50 0.10 0.10 1.26 0.10 0.20 0.49 0.10 0.20 1.21 0.10 0.50 0.49 0.10 0.50 1.23 1.00 0.10 0.50 0.10 1.00 1.14 0.20 0.20 0.52 0.20 0.20 1.23 0.20 0.50 0.50 0.20 0.50 1.17 0.50± 0.Q2 1.26 ± 0.07 Contd.
930 JAIN et al.: KINETICS OF TRIPHENYLPHOSPHINE-HALOALKANES REACTION
Table l=-Values of second order rate constants of the reactions of TPP with haloalkanes- Contd
T(K) J02 x [TPP] J02x [TBM] k T(K) [TPP][TeM] 10) k (mol drn t') (mol dm-3) (dm3mol-'s-') (mol dm-3) (mol dm-.l) (drn ' mol-' s-')
e = 35.95 313 0.02 0.20 0.57 263 0.02 0.10 1.41 0.05 0.05 0.68 0.05 0.40 1.47 0.05 0.1"0 0.66 0.10 0.10 l.38 0.05 0.20 0.61 0.10 0.20 1.47 0.05 0.50 0.47 0.10 0.40 1.40 0.10 0.20 0.57 0.10 0.80 1.41 0.20 0.20 0.55 0.20 0.20 l.34 0.59 ±0.06 1.41 ± 0.04 318 0.02 0.20 0.71 273 0.02 0.10 2.59 0.05 0.05 0.86 0.05 0.10 2.56 0.05 0.05 0.86 0.05 0.20 2.51 0.05 0.10 0.76 0.40 0.05 2.67 0.05 0.20 0.75 0.10 0.10 2.83 0.05 0.50 0.63 0.10 0.20 2.91 0.10 0.20 0.71 2.70 ± 0.17 0.20 0.20 0.72 281 0.01 0.20 3.87 0.74 ±0.06 0.02 0.10 3.67 0.05 0.10 4.31 e = 19.58 0.05 0.20 4.09 303 0.01 0.10 0.57 0.05 0.40 4.34 0.02 0.05 0.65 0.10 0.10 4.60 0.02 0.10 0.61 0.20 0.20 4.37 0.02 0.20 0.56 4.17 ± 0.26 0.02 0.50 0.61 0.05 0.10 0.61 T(K) [TPP] [TCM] 10-' k 0.10 0.10 0.56 (mol dm-3) (mol dm-J) (drrr' mol-' s-') 0.59 ± 0.04 TPP+TCM; f= 13.82 308 0.01 0.10 0.76 303 0.02 0.20 0~29 0.02 0.05 0.84 0.05 0.05 0.36 0.02 0.10 0.77 0.05 0.02 0.33 0.02 0.20 0.75 0.05 0.50 0.28 0.02 0.50 0.66 0.10 0.20 0.33 0.05 0.10 0.80 0.20 0.20 0.32 0.10 0.10 0.76 0.32 ± 0.Q3 0.76 ± 0.05 308 0.02 0.20 0.43 0.05 0.05 0.53 313 0.01 0.10 1.08 0.05 0.10 0.50 0.02 0.05 1.l6 0.05 0.20 0.45 0.02 0.10 1.l6 0.05 0.50 0.37 0.02 0.20 ).02 0.10 0.20 0.42 0.02 0.50 0.94 0.20 0.20 0.40 0.10 0.10 1.00 0.45 ± 0.04 1.06 ± 0.Q7 (Contd.i
931 INDIAN J CH~M, SEC.A. NOVEMBER 1991
Table 1- Values of second order rate constants of the reactions of TPP with ha1oalkanes- Contd
T(K) [TPPj [TCMj 103 k T(K) [TPPj [TCMj 103 k J 3 (mol dm- ) (mol dm--') (drn' mol-I S-I) (mol dm-J) (mol dm- ) (dmJmol-ls-l)
E = 35.94 318 0.01 0.10 1.36 303 0.01 0.10 0.88 0.05 0.10 1.40 0.02 0.05 0.95 0.10 0.10 1.37 0.10 0.93 1.37 ±0.02 0.02 0.02 0.20 0.92 0.02 0.50 0.90 E = 24.71 0.05 0.10 0.93 303 O.ot 0.10 0.62 0.10 0.10 0.93 0.02 0.05 0.64 0.94 ± 0.03 0.02 0.10 0.66 308 0.01 0.10 1.23 0.02 0.20 0.63 0.02 0.02 1.29 0.02 0.50 0.63 0.02 0.05 1.38 0.05 0.10 0.63 0.02 0.10 1.25 0.10 0.10 0.64 0.02 0.20 1.31 0.64 ±0.02 0.05 0.10 1.38 0.10 0.10 1.30 1.31 0.05 308 0.01 0.10 0.92 ± 0.01 1.56 0.02 0.02 0.94 313 0.10 1.66 0.02 0.05 0.95 0.02 0.02 1.56 0.02 0.10 0.91 0.02 0.05 1.60 0.02 0.20 0.90 0.02 0.10 1.62 0.02 0.50 0.95 0.02 0.20 1.58 0.05 0.10 0.88 0.05 0.10 0.10 0.10 0.90 0.10 0.10 1.68 0.92 ±0.02 1.60 ± 0.05 318 0.10 0.10 1.93 0.02 0.02 2.12 313 0.01 0.10 1.28 0.02 0.05 2.08 0.02 0.02 1.31 0.02 0.10 2.12 0.02 0.05 1.28 0.02 0.20 1.88 0.02 0.10 1.25 0.05 0.10 2.08 0.02 0.20 1.29 0.05 0.20 2.02 0.02 0.50 1.23 0.10 0.10 2.08 0.05 1.26 0.10 2.04 ± 0.08 0.10 0.10 1.27 1.27 ± 0.02 T(K) 1Q2 x [TPPj J02 x [BTCMj 102 k (mol dm+') (mol drn ":'] (dmJmol-1 S-I)
318 0.01 0.10 1.54 TPP+ BTCM; E = 35.94 0.02 0.02 1.64 303 0.10 1.00 LIS 0.02 0.05 1.65 0.20 0.50 1.15 0.02 0.10 1.58 0.20 1.00 1.23 0.02 0.20 1.68 0.20 2.00 1.20 0.05 0.10 1.55 0.40 0.50 1.25 0.10 0.10 1.62 0.40 1.00 1.21 1.61±0.05 1.20 ± 0.04 Contd.
932 JAIN et al.: KINETICS OF TRIPHENYLPHOSPHINE-HALOALKANESREACfION
Table 1- Values of second order rate constants of the reactions of TPP with haloalkanes- Contd 1Q2x[TPP] J02X[BTCM] T(K) 10' k T(K) [TEP] [I-BB] 10" k (mol dm ":'] (mQI drn"] (dmJmol-'s-l) (mol dm-.l) (mol dm-J) (dm ' mol-IS-I) 308 0.10 1.00 1.65 0.20 0.50 1.66 TPP+ 1-BB; e= 35.94 0.20 1.00 1.70 308 0.20 0.20 1.45 0.20 2.00 1.73 0.10 0.50 l.36 0.40 0.50 1.68 0.20 0.10 1.41 0.40 1.00 l.64 0.20 0.20 1.50 1.68 ± 0.03 0.20 0.50 l.43 313 0.10 1.00 2.12 0.20 1.00 1.45 0.20 0.50 2.12 0.40 0.05 l.33 0.20 1.00 2.16 0.40 0.20 l.35 0.20 2.00 2.10 0.40 0.40 l.30 0.40 0.50 2.16 1.40 ± 0.06 0.40 1.00 2.18 313 0.10 0.10 2.08 2.14±0.03 0.10 1.00 2.33 T(K) [TPPJ [OBM] 10' k 0.20 0.10 2.20 3 (moldm- ) (mol drn=') (dm+molr t s "") 0.20 0.20 2.29 TPP + OBM; e = 35.94 0.20 0.50 2.13 323 0.10 0.10 0.19 -D.40 0.05 2.16 0.10 1.00 0.19 0.40 0.10 2.16 0.20 0.10 0.19 0.40 0.40 2.14 0.20 0.50 0.19 2.20 ±0.08 0.20 1.00 0.19 318 0.10 0.10 3.50 0.40 0.40 0.18 0.10 0.20 3.30 0.19 ± 0.004 0.10 0.50 3.16 328 0.10 0.10 0.25 0.10 0.60 3.27 0.10 0.50 0.27 0.10 1.00 3.33 0.10 1.00 0.29 3.30 ± 0.11 0.20 0.20 0.26 323 0.10 0.20 5.20 0.30 0.30 0.28 If 0.10 0.57 5.18 0.27 ±0.Ql 0.10 1.00 4.75 333 0.10 0.10 0.45 0.20 0.10 5.25 0.10 0.50 0.46 0.2U 0.20 5.25 0.10 1.00 0.42 0.20 0.50 5.29 0.20 0.10 0.44 0.40 0.05 5.08 0.20 0.20 0.43 0.40 0.20 4.87 0.10 0.50 0.44 0.40 0.40 4.91 0.20 1.00 0.44 5.10 ± 0.18 0.33 0.30 0.42 0.44 ± 0.Ql 328 0.10 0.10 7.70 338 0.10 0.10 0.71 0.10 0.20 7.73 0.10 0.50 0.72 0.10 0.50 7.73 0.10 1.00 0.66 0.10 1.00 7.31 0.20 Q.lO 0.68 0.20 0.10 7.41 0.20 0.20 0.67 0.20 0.60 7.69 0.20 0.50 0.68 0.50 0.05 7.20 0.30 0.30 0.66 7.5 ± 0.26 0.68 ± 0.027 Contd.
933 INDIAN J CHEM, SEC.A, NOVEMBER 1991
Table I-Values of second order rate constants of the reactions of TPP with haloalkanes- Contd
T(K) [TPP] [I-IB] 105 k T(K) [TPP] [I-IB] 105 k J r ') 1 (mol drn (mol dm- ) (drrr' mol-I 5- ) (mol drn " ') (mol dm-J) (drn' mol-I S-I) TPP+ l-IB; E = 35.94 0.20 0.10 6.80 t - 0.20 6.50 313 0.10 0.10 2.80 0.50 0.40 6.40 0.10 0.20 2.80 0.05 0.40 6.41 0.10 0.50 2.80 0.10 0.40 0.20 0.10 2.80 0.20 6.87 0.20 0.50 2.75 6.69 ± 0.26 328 0,10 ·0.40 0.05 2.65 0.10 8.80 0.10 0.40 0.10 2.65 0.20 8.90 0.10 0.40 0.20 2.75 0.50 8.50 0.20 2.75 ±0.11 0.10 8.50 0.20 318 0.10 0.10 4.60 0.20 8.83 0.20 0.10 0.20 4.67 0.50 8.83 0.40 0.10 0.50 4.30 0.10 8.33 0.40 8.4l {l20 0.10 4.55 0.20 0.20 0.50 4.33 8.64 ± 0.21 9.40 0.05 4.L.0 333 0.10 0.10 12.80 0.40 0.10 4.45 0.10 0.20 13.30 0.40 0.20 4.52 0.10 0.50 13.60 4.45 + 0.16 0.20 0.10 13.30 323 0.10 0.10 6.70 0.20 0.20 13.33 0.10 0.20 7.20 0.40 0.10 12.28 0.10 0.50 6.60 13.10 ± 0.44
The dipole moments of TBM and TCM are zero, Table 2-Arrhenius parameters for the reactions of therefore Eq. (2) can be written as TPP with haloalkanes System s, (kJ) Log A ~S* lnk= lnkg + 3/8kT(1- ell + E )(,u1/rl- ,u;/r;) (JK-I mol-I) ... (3) TPP+TBM 19.58 40.2 7.1 -113 r The plot of In k versus (1 - e/ 1 + e ) is linear (Fig. 1). 24.71 39.2 7.4 - 107 For comparison the values of second order rate 35.94 36.4 7.4 - 107 constants in acetonitrile were reduced to a common TPP+TCM 13.82 47.0 4.5 -161 temperature (303K) using Arrhenius parameters. It 19.58 46.0 4.6 - 159 was found that k= 13.33, 0.94 x 10-3, 1.2 X 10-2, 24.71 46.0 4.7 - 158 1.19 X 10-7 and 12.02 x 10-6 for TBM, TCM, 35.94 46.0 4.7 - 155 BTCM, DBM, 1-BB and i-m respectively. TPP+BTCM 35.94 43.0 5.3 - 149 It seems that the mechanism of the reaction with TPP+DBM 35.94 76.6 6.6 -122 tetrahalomethanes is different from that of the par- TPP+ I-BB 35.94 73.0 6.5 - 125 tially halogenated alkanes. In the latter case pro- TPP+ l-IB 35.94 68.0 6.8 - 118 bably it is simple SN2 type of reaction, i.e.
<, <, ,-6+\/6+&- <, 1+_ x /: '. / -p +. -c-x -- -p·····c······x- [-P-C-I x -+ ././ ./ I/'I t(6HSJ) p\: tC6HS))P\ /. 0.) ill ex) In the first case the reaction takes place by electron IC6HSIJP.cX'-(CbHSIJP';;;x<- .: 1 transfer (see Scheme 1) leading to the formation of ion-pair (2) which rearranges immediately to 4 via [(C6HSJ)PX J+cx)-- (iC6HSI]PCx]1+X- (2) (4) the pentavalent phosphine stage (3) intermediate. Sc beroe 1
934 JAIN et al.: KINETICS OF TRIPHENYLPHOSPHINE-HALOALKANES REACTION
ition state. As a result the complex tends to bind sol- vent molecules much more strongly than do the 1.0 reactant molecules. This is also in agreement with the fact that the rate constant decreases as the die- 0.0 lectric constant of the medium decreases (Fig. 1). The lower entropy of activation in low dielectric media can be accounted for by the fact that the less -1.0 polar solvents have a greater loss in the degree of freedom during the transition from frozen state to -2.0 the ionic state as compared to the polar solvent. It --III- may be seen that the entropies of activation for the .~ -3.0 reactions follow the order: TBM > BTCM > TCM. ..• This can be accounted for by the smaller size of TCM compared to that of TBM leading to greater ordering of solvent due to solvation.
-5.0 Acknowledgement The authors are thankful to the CSIR (New Delhi) -6.0 for financial support.
References -7.0 1 Appel R, Angew Chern (int edn) 14 (1975) 801 and the ref- erences cited there. -8.0 2 Appel R, Knoll F, Michal W, Morbach W, Wihler H D & Veltman H, Chern Ber, 109 (1976) 58. -0.9' -0.92 -0.90 -0.88 -0.86 3 Appel R & Scholer H P, Chern Ber, 111 (1978) 2056. 4 Francisco G, Santiago V Ir, Patrie F & MC & Alberto M,] (1- ~ )/(1 +E) Chern Commun, (1985) 2196. 5 Ramirej F, Desai N B & Mckelvie N, J Arn chern Sac, 84 Fig. I-Plot of log k versus (1 - E )/( 1 + E) for the reactions of (O)TPP+TBM at 263 Kand (-)TPP+TCM at 303K (1962) 1745. 6 Wolkoff P, Can] Chern, 53 (1975) 1333. 7 Jain D V S & Chopra R, Indian] Chern, 21A {1982} 709.
As TBM is a better electron acceptor than TCM the 8 Driscoll J S, Grisley D W1 Pustinger J V, Harris JE & Ma- higher value of rate constant with TBM is under- thews C N,] org Chern, 2~ (196,4) 2424. standable. This is further supported by the complex 9 Bahnick D A, Bennet W E & Person W B, I phys Chern, 73 (1969) 2309. formation between tetrahalomethane (acceptor) and 10 Anderson R & Prausnitz J M,] chem Phys, 39 (1963) 1225. benzenes (donor)9,I0,16-20.Theformation constant of 11 Lautenberger W J, Jones E N & Miller J G,] Arn chern Sac, 1 : 1 benzene-TBM complex (Kf = 0.22 dm Vmol)? 90 (1968) 1110. is nearly 20 times higher than that i K, = 0.009 dm" I 12 Davis K M C & Farmer MjF,] chem Sac, (B)(1967) 28. mol)'? with TCM. Perhalomethanes are known to 13 Stevenson D P & Coppinger G M, J Arn chem Sac, 84 (1962) 149. accept electrons from amines'<" also. 14 Biaselle C J & MillerJG,] Arn chem Sac, 96 (1974) 3813. It can be seen from Table 2 that the activation ener- 15 Sharpe AN & Walker S,] chem Sac, {1962} 157. gies do not show much variation with the dielectric 16 North AM & Parker TG, Trans Faraday Sac, 67 {1971} constant of the medium. However, activation ener- 2234. gies for the reactions of three halomethanes follow the 17 North A M & Parker T G, ] chern .)oc Faraday II, (1972) 1094. order: TCM > BTCM > TBM: This is understand- 18 Gilson D FR & O'Konski C T, ] chem Phys, 48 (1968) able as C-Cl bond is stronger than C-Br bond. The 2467. large negative values for entropies of activation for 19 Hooper H 0,] chern Phys, 41 (1964) 599. all the reactions indicate polar nature of t}1etrans- 20 Weimer R F & Prausnitz J M,] chem Phys, 42 (1965) 36,43.
935