kinetics and mechanism of nucleophilic substitution of

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Indian Journal of Chemistry Vol. 32A, May 1993, pp. 406-408 Kinetics and mechanism of nucleophilic substitution of acridine by chlo ride ion in octahedral tin(IV) complexes K S Siddiqi", Fathi M A M Aqra, S A Shah & S A A Zaidi Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India Received 3 June 1992; revised 15 September 1992; accepted 27 January 1993 The nucleophilic substitution of acridine by chloride ion in SnCI4~ and RzSnClz~ (where R = Me, Bu, Ph, and L= acridine) carried out in acetonitrile at ambient temperature follows first or- der kinetics. A millimolar solution of the complexes in acetonitrile shows increase in conductance with time indicating solvation which is enhanced in the presence of SOCl 2 , C 6 H s COCI and CH 3 COCI suggesting the replacement of acridine by chloride ion. The rate constants k., k2 and the thermodynamic parameters (activation energy, entropy, and enthalpy) have been calculated. The nu- cleophilicity of these reagents has been found to be in the order :SOCl z > C 6 H s COCI > CH 3 COCl. While the spectrophotometric method may be available for the study of the substitution reaction of some group (IV) metal complexes 1 the kinetics of substitution of compounds yielding colourless solution may be conveniently followed by conduc- tometric method. In the present paper, we have selected SnCI4~ and R2SnCI2~ (where R = Bu, Me, Ph and L= acridine) in order to examine the influence of the alkyl groups on the rate of substitution. In these complexes, the ligand is replaced by chlo- ride ion liberated from different nucleophiles such as SOCI 2 C"H5COCI and CH,COCI. The con- ductometric measurements were carried out on the solutions of the complexes in acetonitrile. Since increase in temperature makes negligible change in conductance, the experiments were done only at ambient temperature. The rate con- stants k 1 and k2 for the successive replacement of acridine by CI- and the thermodynamic parame- ters (~G, ~H and ~S) have been calculated. Materials and Methods Millimolar solutions of the complexes were pre- pared in acetonitrile and their conductances were measured as a function of time. From the slope of the linear portion of a plot of molar conductance versus time, the rate constant (k) for solvation was found to be 7.6 x 10- 4 S-I. The solutions of the nucleophiles and the com- plexes were mixed separately in three different ra- tios. The conductance was measured immediately and after regular intervals over a period of one hr. The limiting conductance (Aa), was measured after 24 hr. Results and Discussion The molar conductance (Am) versus time curve for substitution was similar to that obtained for solvation (Fig. 1). However, the change in molar conductance in the former case was greater than in the latter. The probable mechanism for the sol- vation is outlined below: where R = Me, Bu, Ph; L= acridine; S = solvent (acetonitrile ) During solvation, either the chloride ion or the ligand may be replaced by the solvent+" (Eqs 1 and 2). The smaller increase in molar conduct- ance in the absence of the nucleophile may be ascribed only to the solvation of the complexes. The mechanism of solvation involving release of chloride ion and generating [R2SnSCI~l+ species would have caused substantial increase in the conductance and therefore, filled out. Hence, it is suggested that solvation is accompanied by the re- lease of the ligand as shown by Eq. 2. However, when the nucleophile is added the conductance increases due to the substitution of acridine by chloride ion, liberated from the nucleophile.

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Indian Journal of ChemistryVol. 32A, May 1993, pp. 406-408

Kinetics and mechanism of nucleophilic substitution of acridine bychlo ride ion in octahedral tin(IV) complexes

K S Siddiqi", Fathi M A M Aqra, S A Shah & S A A Zaidi

Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India

Received 3 June 1992; revised 15 September 1992; accepted 27 January 1993

The nucleophilic substitution of acridine by chloride ion in SnCI4~ and RzSnClz~ (whereR = Me, Bu, Ph, and L= acridine) carried out in acetonitrile at ambient temperature follows first or-der kinetics. A millimolar solution of the complexes in acetonitrile shows increase in conductancewith time indicating solvation which is enhanced in the presence of SOCl2, C6HsCOCI andCH3COCI suggesting the replacement of acridine by chloride ion. The rate constants k., k2 and thethermodynamic parameters (activation energy, entropy, and enthalpy) have been calculated. The nu-cleophilicity of these reagents has been found to be in the order :SOClz>C6HsCOCI >CH3COCl.

While the spectrophotometric method may beavailable for the study of the substitution reactionof some group (IV) metal complexes 1 the kineticsof substitution of compounds yielding colourlesssolution may be conveniently followed by conduc-tometric method.

In the present paper, we have selected SnCI4~and R2SnCI2~ (where R = Bu, Me, Ph andL= acridine) in order to examine the influence ofthe alkyl groups on the rate of substitution. Inthese complexes, the ligand is replaced by chlo-ride ion liberated from different nucleophiles suchas SOCI2• C"H5COCI and CH,COCI. The con-ductometric measurements were carried out onthe solutions of the complexes in acetonitrile.Since increase in temperature makes negligiblechange in conductance, the experiments weredone only at ambient temperature. The rate con-stants k 1 and k2 for the successive replacement ofacridine by CI- and the thermodynamic parame-ters (~G, ~H and ~S) have been calculated.

Materials and MethodsMillimolar solutions of the complexes were pre-

pared in acetonitrile and their conductances weremeasured as a function of time. From the slope ofthe linear portion of a plot of molar conductanceversus time, the rate constant (k) for solvationwas found to be 7.6 x 10-4 S-I.

The solutions of the nucleophiles and the com-plexes were mixed separately in three different ra-tios. The conductance was measured immediatelyand after regular intervals over a period of one

hr. The limiting conductance (Aa), was measuredafter 24 hr.

Results and DiscussionThe molar conductance (Am) versus time curve

for substitution was similar to that obtained forsolvation (Fig. 1). However, the change in molarconductance in the former case was greater thanin the latter. The probable mechanism for the sol-vation is outlined below:

where R = Me, Bu, Ph; L= acridine; S = solvent(acetonitrile )

During solvation, either the chloride ion or theligand may be replaced by the solvent+" (Eqs 1and 2). The smaller increase in molar conduct-ance in the absence of the nucleophile may beascribed only to the solvation of the complexes.The mechanism of solvation involving release ofchloride ion and generating [R2SnSCI~l+ specieswould have caused substantial increase in theconductance and therefore, filled out. Hence, it issuggested that solvation is accompanied by the re-lease of the ligand as shown by Eq. 2. However,when the nucleophile is added the conductanceincreases due to the substitution of acridine bychloride ion, liberated from the nucleophile.

SIDDIQI et al: KINETICS OF SUBSTITUTION OF ACRIDINE BY CI- IN Sn(IV) COMPLEXES 407

21

21

24

22

E 20<

• 11I!~

11

~ 140u 12

i~•

IV

10 20 30 40 50 60 70 eoTime ( min)

Fill. I-Molar conductance-time curve for (R =Me, Bu, Ph) (I, in'acetonitrile, (11)with CH3COCl'in acetonitrile,(III) with C6HsCOCI in acetonitrile, (IV) witlt SOCl2 in acetoni-

trile.

Table I-Rate constant for substitution of acridine by chlo-ride ion in the complex LzSnCl2R2 (R = Me.Bu, Ph)

NucleophiUic Ratio k, x 102 k2 X 102

reagent (S-I) (S-I)

SOCl2 1 : 1 3.10 7.41 : 2 3.10 7.4

1 : large 3.10 7.4

1 : 1 2.6 7.21 : 2 2.6 7.2

1 : large 2.6 7.2

1 : 1 1.8 5.91 : 2 1.8 5.9

1 : large 1.8 5.9

Plot of log [Aal Aa - Am] against time (Fig. 2)was linear suggesting first order kinetics in thepresence of nucleophiles. The values kl and k2for stepwise replacement of acridine by chlorideion were calculated from the slope of the first andsecond linear portions of the plots respectively,using the method of two mutually intersectinglines", Their values are listed in Table 1. The rateconstants were calculated by the equation:

The substitution may follow either SN1 or SN2mechanisms. In the present case, it has been

1.1III

E<IcI-<I

0~~ __ ~-4 __ ~~ __ ~~ __ ~

o 10 20 30 40 50 60 70 eoTlme( min)

Fig. 2- First order plot for various nucleophilic reagents.I. CH3COCl, II. C"HsCOCl, III. SOCI2, for 1 : 2 complex.

Table 2- Thermodynamic parameters for first and secondsubstitutions

For first substitution For second substitution

Nucleophi1lic AH* AS* AH* AS*reagent (kJ (JK-I (kJ (JK-I

mol-I) mol") mol--l) mol-I)

SOCl2 176.56 0.77 125.93 0.59C6HsCOCI 160.24 0.65 117.98 0.54CH3COCI 148.11 0.47 113.80 0.36

shown to follow SN1 reaction for which the fol-lowing mechanism in Scheme 1 has been pro-posed+':":

... (3)

... (4)

... (5)

... (6)

where R = Me, Bu, PhScheme 1

408 INDIAN J CHEM, SEe. A, MAY 1993

The rate of increase in molar conductance dur-ing nucleophilic substitution is as expected sincethe chloride ion is obviously a stronger nucleo-phi Ie than either acridine or the solvent? Thehigh nucleophilicity of the chloride ion is able tocompete with the greater concentration of the sol-vent in the substitution reaction. There is bondbreaking leading to the formation of five coordin-ate intermediate species" (steps 3 and 5) followedhy hond formation (steps 4 and 6). There areseveral sueh stahle five coordinate compounds ofR,SnCI, R2Sn(AC)2 known with quinoline deriva-rives". The mechanism proposed is also clearfrom Fig. 2 which consists of two intersectinglines indicating two different rate determiningsteps. Thus the dissociation of acridine by chlo-ride ion in reactions (3) and (5) is a slow processand henee rate determining step. Breaking one ofthe two metal ligand bonds (3) followed by the at-tack of chloride ion results in the formation of[R2SnCl}L]-. In the presence of high concentra-tion of chloride ion (high concentration of nucleo-philc l~ the second M - L bond breaks' (5) andM - CI hond is formed yielding [R2SnCI4r 2 as thefinal product (6). Consequently, the molar con-ductance as a function of time increases and at-tains the mixmum value after 1 hr. The A a wasobtained after 24 hr. The reaction (4) is faster thanreaction (6) due to the negative charge on the for-mer. It is interesting to note that all the complexesshow same results indicating no effect of sterichindrance of alkyl groups on the rate of substitu-tion reaction.

By substituting the values of kJ and k2 in Ar-rhenius equation, the energy of activation, entropyand enthalpy has been calculated (Table 2). Thesesupport the SNI mechanism and show that bondbreaking is a slow, and hence, rate determiningstep.

From all these observations, we conclude thatthe rate of substitution is faster than the rate ofsolvation. In the absence of the nucleophile, theinteraction with the solvent is greater. The substi-tution follows SNI pathway. On the basis of the!1S values the nucleophiles for substitution reac-tion may be arranged in the following descendingorder: SOCl2 > C6HsCOCI > CH3COCI.

AcknowledgementThe authors are thankful to Rasheed Jamhour

for his assistance in conductivity measurements.

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