Langmuir 1994,10, 662-669
662
Micellar Effects on the Nitrosation of Piperidines by 2-BromoethylNitrite and 1-Phenylethyl Nitrite in Basic Media Emilia Iglesias,'J J. Ram6n Leis,* and M. Elena Pefia* Departamento,de Qufmica Fundamental e Industrial, Facultad de Ciencias, Universidad de La C o r u h , La CoruRa, Spain, and Departamento de Qulmica Fkica, Facultad de Qufmica, Universidad de Santiago de Compostela, Santiago, Spain Received June 14,1993. In Final Form: November 19,199P The nitrosation of piperidine (P), 2-methylpiperidine (2-MP), and 4-methylpiperidine (4-MP) by 2-bromoethyl nitrite (BEN) and l-phenylethyl nitrite (PEN) is kinetically studied in basic media and in the preaenceoftetradecyltrimethylammoniumbromide (TTABr)and sodium dodecylsulfate (SDS)micelles. In the case of cationic micelles of TTABr and with BEN as the nitrosating agent, the results are explained assuming that only the alkyl nitrite is distributed between the aqueous and the micellar pseudophases, whereas the reaction takes place in water. When the nitrosating agent is l-phenylethyl nitrite, reaction in the micellar pseudophase is also observed. This result is evidence for the kinetic relevance of stoichiometricallynegligibleamountsof hydrophilicreagents in the micellesin the case of reactions involving highly hydrophobic substrates. The previous estimation, by indirect methods, of the binding constant of the unprotonated amine to the micelles allows a quantitative estimation of the rate constants in the micellar pseudophase, which could be compared with those in water. With anionic micelles of SDS, the results are analyzed by using the simple pseudophase ion-exchange (PPIE) model taking into account the partitioning of alkyl nitrites between the micellar and aqueous pseudophases and, simultaneously, the change in the basic ionization equilibria of the amine, because of the competition between the ammonium ion of the amine and Na+, for binding to the micelle. The electrostatic interaction of protonated amine with the micellar surface reduces the quantity of total unprotonated amine, the reactive form. Rate constants for the reaction in the micellar pseudophase and in water, obtained from the kinetic analysis, are reported and discussed. Binding constants of the unprotonated amine to the SDS micelles, as well as the ion exchange constants between the counterions of the micelle, Na+, and the ammonium ions of the corresponding amine, were previously determined from absorbance measurements.
Introduction The extent of micellar catalysis of bimolecular reactions is due, fundamentally, to the concentration of both reactants in the small volume of the micelle, promoted by hydrophobic and electrostatic effects between substrate and micelles.' The magnitude of this catalysis depends upon the incorporation of both reactants into the micelle and the rate constant in the micellar pseudophase.2 The dependence of the overall rate constant upon the surfactant concentration can, in principle, be treated quantitatively in terms of the distribution of the substrate between water and the micellar pseudophase. Micellar catalysis of reactions of substrates with hydrophobic reagents may be analyzed by calculating, directly or indirectly, the amount of each reactant in the micellar pseudophase, assuming that one reactant does not affect incorporation of the other.3 This method can be applied directly to reactions of nonionic reactants or to ionic reactants fully ionized under the reaction conditions. But the problem becomes more complicated when the reactants are involved in dissociation equilibria of weak acids, or bases, because the micelles almost certainly affect the acid+ Universidad de La C o d a .
Universidad de Santiago de Compostela. * Abstractpubliihedin Advance ACSAbstracts, January15,1994. (1) (a) Fendler, E. J.; Fendler, J. H., In Catalysis in Micellar and MacromolecuhrSystems;AcademicPreas: New York, 1975. (b)Bunton, C. A.; Savelli, G. Adu. Phys. Org. Chem. 1986,22,231. (2) (a) Bunton, C. A. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 19% Vol. 2, p 519. (b)Romsted, L. S. In Surfactantsin Solution; Mittal, K. L., Lindman,B., Eds.;Plenum Press: New York, 19% Vol. 2, p 1015. (3) (a) Sepcilveda, L.; Lissi, E.; Quina, F. Adv. Colloid Interface Sci. 1986,25, 1. (b) Ferreira, L. C. M.; Zucco, C.; Zanette, D.; Nome, F. J. phy8. Chem. 1992,96,9058.
base dissociation equilibria,' which makes it difficult to know the real concentrations of the basic and acidic forms of each conjugate pair. In this work we have kinetically studied the micellar effects upon a bimolecular reaction between (i) a nonionic reactant, alkyl nitrite (RONO), whose main driving force of interaction with micelles is a hydrophobic effect (alkyl nitrites are not very soluble in water), and (ii) amines, which are involved in ionization equilibria which may be altered by the micelles. The amines used in the present 2-methylpiperidine (2-MP),and work were piperidine (P), Cmethylpiperidine (4-MP),and the kinetic study is carried out in the absence of any buffer, contrary to other related systems found in the literature. We show that a combination of kinetic and spectroscopic measurements allows quantitative treatment of data in terms of a pseudophase model in the absence of buffers. This procedure avoids current problems found when working in the presence of added buffers, such as estimation of pH at the micellar surface or partitioning of the buffer forms between water and micelles.
Experimental Section Materials. Alkylnitrites were synthesized in aqueous sulfuric acid from sodium nitrite and the correaponding alcohol, following (4) (a)Bunton,C.A.InSurfactantsinSo~tion;Mittal,K.L.,Lindman, B., Eds.; Plenum Press: New York, 1984, Vol. 2, p 1093. (b)Chaimovich, H.;Bonilha,J.B. S.;Zane~,D.;Cuccovia,I.M.InS~actantsinSolution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 19% Vol. 2, p 1121. (c) Chaimovich, H.;Aleixo, R. M. V.; Cuccovia, 1. M.; Zanetta, D.; Quina, F. H. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. J., Ede.;Plenum Press: New York, 1982; Vol. 2, p 949. (d) Bunton, C. A.; Rometed, L. S.; Sepcilveda, L. J. Phys. Chem. 1980,84, 2611.
0743-7463/94/241Q-0662$Q4.5Q/Q 0 1994 American Chemical Society
Micellar Effects on the Nitrosation of Piperidines conventional methods? and were stored over molecular sieves at low temperature. The remaining reagentsused were suppliedby Merck or Aldrich of the highest commercially available purity. Rate Measurements. The kinetic experiments were performed with a great excess of the amine over RONO. Stock solutions of alkyl nitrites were prepared in dioxane to prevent their hydrolysis.6 The reactionwas initiated by additionto the rest of the reaction mixture, of 50 p L of a solution of alkyl nitrite in dioxane. The percentage of organic solvent in the final reaction mixture was 1.7% by volume. Reaction kinetics were studied by conventional spectrophotometry at 25 O C by following the increase in absorbance (generally in the range of 240-250 nm) corresponding to the formation of N-nitrosamine, using a Beckman DU-70 spectrophotometer. Absorbance-time data always fit the first-order integrated equation, and ko, the corresponding first-order pseudoconstant in s-l, waa reproducible to within 3 7%. Results and Discussion Reaction in Water. The nitrosation reaction of amines by alkyl nitrites in basic media is well The reaction takes place via the nucleophilic attack of the neutral form of the amine on RONO probably through a concerted mechanism. The second-order rate constants, k2, corresponding to the reaction between the amine and the alkyl nitrite obtained for the nitrosation of piperidine, 2-methylpip eridine, and 4-methylpiperidine by BEN and PEN in basic media ([OH-] = 0.1 mol~dm-~), are reported in Table 3, entry 3. Although, 2-MP has a similar basicity to the other two amines studied here, the low reactivity observed for this amine can be ascribed to steric hindrance by the methyl group in the ortho-position close to the center of the reaction. However, a carboxylate group in the same position does not reduce the nucleophilicity of the substrate? It is generally recognized that basicity is not a good parameter to correlate the reactivity of the different nucleophile^.^ Reaction in the Presence of Cationic Surfactant. (a) Reaction with 2-Bromoethyl Nitrite. The nitrosation reaction was studied in the presence of TTABr (0-0.25 mol-dm-s) for different concentrations of surfactant and a fixed concentration of amine and of BEN (1.5 X le mol*dm4) and in the absence of external buffers added to the medium. The addition of surfactant resulted in an inhibition of the reaction at concentrations greater than its critical micelle concentration, cmc. Figure 1shows typical results. The observed inhibition can be explained if one of the reactants is taken up by the micelles and the other remains mainly in water. Previous studieslo from our laboratory have shown that alkyl nitrites incorporate to TTABr micelles. The driving force of this interaction is the hydrophobic character of alkyl nitrites. On the other hand, the amines used in this (5) Noyes, N. A. Organic Synthesis; Wiley: New York, 1943;Collect. VOl. 2. (6) Garcla-Rio, L.; Iglesias, E.; Leis, J. R.; Pefia, M. E.; Williams, D. L. H. J. Chem. SOC., Perkin Tram. 2 1992, 1673. (7) (a) Casado, J.; Castro, A.; Lorenzo, F. M.; Meijide, F. Monatsch. Chem. 1986,117, 355. (b)Casado, J.; L6pez-Quintela, M. A.; Lorenzo, F. M. React. Kinet. Catal. Lett. 1986,32,413. (c) Calle, E.; Casado, J.; Cinos,J. L.;Garla-Mate-, F. J.;Tostado, M. J.Chem.Soc.,Perkin Tram. 2 1992,987. (d) Garcla-Rio,L.; Iglesias, E.;Leis, J. R.; PeAa, M. E.; Rios, A. J. Chem. Soc., Perkin Tram. 2 1993,29. (8) Bunton, C. A.; Huang, S. K. J. Am. Chem. SOC. 1974,96,515. (9) Richard, J. R.; Amyes, T. L.; Vontor, T. J . Am. Chem. SOC. 1992, 114.6626. and ref 7d. (10) Gkcla-Rio, L.; Iglesias, E.; Leis, J. R.; Pefla, M. E. Langmuir 1993,1263.
Langmuir, Vol. 10, No.3, 1994 663 0,
h
6
.. 'Y)
\
s
0
0 c
2
0
1
0.00
0.05
0.10
0.15
0.20
0.25
[TTA Br I/mol.dm" Figure 1. Influence of [TTABrl on the pseudo-firsborderrate constant of the nitrosation by 2-bromoethyl nitrite of ( 0 ) 4-methylpiperidine (3.5 X 1W mol-dm-8) at [NaOHl = 0.039 moladm" and of (A)piperidine (3.0X 1W molmdma),no added NaOH. Solid lines fit by eq 1. For parameters see Table 1.
study are very soluble in water, so that the incorporation to the TTABr micelles of the unprotonated amine, the reactive form, should be small. Bearing in mind these considerations, the overall reaction rate can be expressed as the rate in the aqueous phase, that is rate = k~[RONOl,[XPHl, [XPHI being the unprotonated amine concentration, assumed to be fully in the aqueous phase, k2 the bimolecular rate constant, and [RONO], the concentration of the alkyl nitrite in water. Using the pseudophase model put forward by Menger and Portnoyll and assuming that the presence of cationic micelles does not change the basic ionization equilibria of the amine in water, Scheme 1 can be proposed. In this Scheme 1 XPH + H,O RONO, RONO,
F! XPH;
+ D,
+ XPH
-
+ OH-
e RONO,
X-C,H,NNO
Kb
K :
+ ROH
k2
scheme, Kb represents the equilibrium constant corresponding to the basic ionization of the aminein the aqueous phase; KsNis the binding constant of the alkyl nitrite to micelles, defined in terms of the concentration of the micellized surfactant, [Dnl = [Dl - cmc (that is the total concentration of surfactant, [Dl, less the concentration of monomeric surfactant given by the critical micelle concentration, cmc). Scheme 1 leads to eq 1
ko = k,/(l + K:[D,l)
(1) where k, is the first-order rate constant determined in the absence of surfactant (see Table 1).Equation 1was found to satisfactorily fit the experimental data ko-[TI'ABr], which means that our previous hypothesis seems to be true. The theoretical fits-solid lines-to the experimental (11) Menger, F. M.; Portnoy, C. A. J.Am. Chem. SOC.1967,89,4968.
664 Langmuir, Vol. 10, No. 3, 1994
Iglesias et al.
Table 1. Experimental Conditions and Parameten Obtained in the Reaction of Piperidine (P) and 4-Methylpiperidine (4-MP) with 2-Bromoethyl Nitrite and 1-Phenylethyl Nitrite in the Presence of TTABr and POE Micelles
l@[amineI" P (5.0) P (3.0) 4MP (3.5) 4MP (3.5) no amine
P (3.0) P (3.0) no amine aP (3.0) no amine
[OH-Ia 6.7 X 10-9 0.039
l@k/s-'
(kPlV)& 2-BromoethylNitrite
* 0.5 * 0.3 0.3 * 0.2'
13.6 3.5 7.6 3.2
acid medium
0.027 acid medium 0.018 acid medium
0.39 0.71
1-PhenylethylNitrite 0.0121 0.0130
0.68
0.0043
d
10s[XPH]a
K.N
16.8 16.4 14.4 19*1 15.5
5.0 1.3 3.45 1.45
300 300 300* 2c 510 510 8d
1.7 3.0
~0.45
3.0
EO
EO
m0.57 =O
~0.58
=O
a All concentrations in mol.dm-8. b The estimated degree of ionization of amine at the working conditione. Determined in the acid hydrolysis of alkyl nitrites, ref 10. d Experiments carried out in the presence of POE. Unpublished results.
data are shown in Figure 1and lead, using experimental values of k,, to the values of KsNwhich are listed in Table
I+$ 4
2.0
1.
The ion-exchange equilibrium for OH-, released in the protonation of the amine, and Br,the micelle counterion, seems to have no effect on the basic ionization equilibrium of the amine: a perfect fit of the experimental data to eq 1 occurs not only in the case of experiments carried out in the presence of hydroxide ions but also for those performed in the absence of NaOH. This result is to be expected if the magnitude of the correspondingequilibrium constant is considered, aa KO* = 0.1-0.025.12 The binding constants, KaN,obtained in both experimental situations, are the same, within the possible experimental error, and they are also in perfect agreement with the value obtained for this alkyl nitrite, BEN, in a previous study of its acid hydrolysis and, therefore, in the absence of amine.10 ( b )Reaction with 1-Phenylethyl Nitrite. The results obtained in the nitrosation of piperidine by PEN in the presence of TTABr do not fit eq 1 (see Figure 2, dotted line). At high [TTABrl the values of ko calculated from eq 1are smaller than those obtained experimentally. This deviation suggests the presence of an additional reaction pathway in the micellar pseudophase. Since PEN is much more hydrophobic,l°KaN= 300 mol-l*dm3,than BEN, small amounts of the amine in the micellar pseudophase can be kinetically detected more easily. Addition of a further reaction pathway corresponding to reaction between micellar-bound alkyl nitrite and micellar-bound amine (whose concentration is related to the concentration of amine in water and to D, through a binding constant KA) leads to eq 2,where k p and V represent the bimolecular
rate constant in the micellar phase and the molar reaction volume, respectively, and [XPHlt was calculated as the kw/k2ratio. In the denominator of this equation the term which corresponds to the association constant of the unprotonated amine to the TTABr micelles, KA,does not appear because, as will be seen, the value which can be estimated for that equilibrium constant turns out to be very small. This means that the amount of micellar-bound amine is stoichiometrically negligible with respect to the total amine, although this small amount has kinetic relevance. (12) (a) Chaimovich, H.; Bonilha,J. B. S.;Politi, M.J.; Quina, F. H. J. Phys. Chem. 1979,83,1851. (b)Zanette, D.; Chaimovich, H. J. Phys. Org. Chem. 1991,4, 643.
.u
U
t:
,9'
1.0
,9' ,Q' 0'
1
0
I
I
k"
= I-
c
I
lb.
b
0.00
0.0
0.05
0.10
0.15
0.20
0.25
0.30
[T T A B r I/ mo I.dm" Figun, 2. Influence of [TTABrl on the pseudo-fiit-orderrate constant of the nitrosation by 1-phenylethylnitrite of piperidine (3.0 X 10" mol-dm-9, no added NaOH. Solid line fit by eq 2; dotted line, fit by eq 1,and dashed line, see text. For parametere see Table 1.
The plot in Figure 2 shows the excellent fit of the experimental data to eq 2. The distribution constant of PEN, K,N, between water and TTABr micelles10 as well as the values of k2 and Kb were determined previously. So, the plot in this figure-solid line-corresponds to the fit of experimental data to eq 2 by iteration varying exclusively the value of (kplV)K~.According to eq 2 a plot of k(l +KSN[DJ)us ED,] should yield a straight line, as it is also observed experimentdy-dashed line. From the experiments carried out in the presence and in the absence of a fiied quantity of NaOH,the values of the constants which appear in Table 1 are obtained. For an estimation of the binding constant of the amine, KA,the reaction was studied in dioxane-water mixtures. The bimolecular rate constants, kz, obtained in these mixtures of different dielectric constant appear in Table 4. For a value of the dielectric constant around 35 (similar to that estimated for the micellar Stern layer) the bimolecular rate constant for the reaction of PEN with the piperidine achieves a value close to 5 X 10-3 dm3*m01-~-s-~. If this value is adopted as the one that is to be expected for k p , assuming a similar reaction media in the micellar pseudophaseand taking V = 0.14 dms-mol-',
Micellar Effects on the Nitrosation of Piperidines
Langmuir, Vol. 10, No. 3, 1994 666
the molar reaction volume in the micellar pseudophase,ls
K A would be estimated as 0.3 dms*mol-l. It is obvious that this constant cannot be calculated by using kinetic methods (eq 2) when the maximum [TTABrl used is 0.25 mol-dmd (in fact K A [ T T A B ~E] ~0.07 is negligible with respect to 1). However, as nearly all the amount of 1-phenylethyl nitrite is in the micellar pseudophase, and kzmis not much lower than k2, it is possible to observe the reaction in the micelle. Similar studies when carried out in the presence of nonionic micelles of polyoxyethylene-20-cetylether (POE) show the same pattern of behavior and KA,the binding constant of the unprotonated piperidine to POE micelles, can be estimated as 0.4 mobdm4 if the molar reaction volume for the Stern layer of POE micelles is taken as 0.5 mol-dm-3roughly estimated followingref 14,and assuming kzm be equal to the value of k2 determined in a dioxanewater mixture of dielectric constant around 35. The polarity parameters show little dependence on the charge of the mi~el1es.l~ This small association of piperidine to TTABr or POE micelles is a consequence of the low hydrophobicity of this species, which is in accordance with the high solubility of this amine in water (Merck Index) and with the low polarity of this species, the other possible source of interaction with micellar surface. Reaction in the Presence of Anionic Surfactant.1. Binding Studies. Aqueous solutions of P, 2-MP,or 4 - W , in the absence of hydroxide ions, absorb at wavelengths lower than ca. 220 nm. This absorption corresponds to the unprotonated form of the amine, because if the corresponding spectrum is taken in acidic conditions where the total amine is protonated, the absorption band disappears completely. The same behavior is observed in the presence of SDS. Addition of SDS in concentration higher than its cmc to aqueous solutions of those amines produces a considerable decrease in the measured absorbance. This means that amines interact with SDS micelles giving rise to an increase in its protonation degree, which is favored by two factors, (1) the exclusion from the micellar surface of the hydroxide ion being formed and (2) the attraction for the micellar surface of the piperidinium ion that is formed. This effect has already been observed.l'jl* In this study, the magnitude of this interaction is quantified from absorbance measurements,A (carried out between 210 and 220 nm), of aqueous solutions of amine at variable [SDSI. (1a )Measurements at Fixed pH. These measurements were performed in the presence of a fixed quantity of NaOH, between 0.02 and 0.055 mol.dm-3, which was present both in the sample and in the reference, and always in excess over the [amineIbw (-3 X 103 mol.dm-3). The apparent base ionization constant, Kb(ap),of the amine in the presence of SDS micelles can be defined by eq 3, where [OH-], represents the intermicellar concen-
I
Q)
Q
I
*.sa
0.60
n L
2 a a
'k 1
0.45
0.30 0.15
I '
0.00 0.05 0.10
0.15 0.20 0.25
0.30
[SDS 1/ mo Ldm" Figure3. Variation of the absorbance of aqueous amine solutions with [SDS] for ( 0 )2-methylpiperidine (3.5 x 10-8 mol&+), [NaOHl = 0.035 moladm" at X 217 nm;(A)piperidine (3.0 X 10-8 mol-dm"), [NaOHl = 0.027 moldm" at 220 nm. Solid lines fit by eq 4. For parameters see Table 2. Insert figure corresponde to the variation of P K ~ , (of~ ,piperidine with [SDS] at [NaOH] = 0.054,0.027, and 0.013 mol-dm".
tration of hydroxide ions. The subscripts w, m, and t represent the free, bound, and total analytical concentrations of each species.
The molar absorptivities of the protonated amines, determined in 0.05 mol-dma HC1, were found to be near zero at 217-220 nm (the working wavelengths). The absorbance at these wavelengths of a solution of an amine can be described by A = e~[xPH]t,since EA, the molar absorptivity of the unprotonated amine, is insensitive to the presence of micelles. Equation 3 can be therefore written in the form of the variation of pKb(ap) (=pOH + log{A/(Ao- A)], being A0 the absorbance in the absence of surfactant) with [SDS]. The results are shown in the insert of Figure 3. The change in Kb upon addition of SDS can be related to the electrostatic attraction of XPH2+ by the surfactant head groups. As for other ions, competition between X-piperidiniumions and Na+can be put forward according to the equation XPH2+(w)+ Na+(m) XPH2+(m) Na+(w), governed by an equilibrium constant KI. The use of this ion-exchange constant, together with the binding (13)(a)Bunton,C.A.;Carresco,N.;Huang,S.K.;Paik,C.H.;Romated, constant KA,correspondingto partitioning of the neutral L. S. J. Am. Chem. Soc. 1978,100,5420. (b) Bunton, C. A.; Sepaveda, amine between water and micelles, and the definition of L. Isr. J. Chem. 1979,18,298. (14) F&msted,L. S.InSurfactantsinSolution;Mittal,K.L.,Lindman, &p) given in eq 3, allows eq 4 which relates the B., Eds.; Plenum Press: New York, 19W,Vol. 2,p 1015. experimental absorbanceto the surfactant concentration. (15)Mukerjee, P.;Cardinal, J. R. J. Phys. Chem. 1978,82,1620. (18)(a) Yamashita, T.;Yano, H.; Harada, S.; Yasunaga, T. J . Phys. A= Chem. 1984,88,287. (b) J. Phys. Chem. 1983,87,5482.(c) Harada, S.; Sanno, T.;Yamashita, T.;Yano, H.; Inone, T.; Yasunaga, T.Bull. Chem. Ao(1 KA[D,]) Soc. Jpn. 1988,61,1045.(d)Yamashita,T.;Sumino, M.;Yano,H.;Harada, S.; Yasunaga, T.Bull. Chem. SOC.Jpn. 1984,57,2352. (1+ KAID,l) + (K,,/[OH-l,)(l+ KIINa+l,/[Na+lw) (17)Bonilha, J. B. S.; Georgetto,R. M. Z.; Abuin, E.; Liasi, E.; Quina, F. J. Colloid Interface Sci. 1990,135,238. (18)Bunton, C. A.; Chan, 5.;Huang, S. K. J. Org. Chem. 1974,39, 1282.
-
+
(4)
The analytical concentration of Na+(,) and Na+(,, can
Iglesias et al.
666 Langmuir, Vol. 10, No. 3, 1994 Table 2. Experimental Conditione and Thermodynamic Parameters Obtained from the Measurements of the Absorbance of the Solutions of the Amines Varying the [SDSI lP[aminell mol.dm4 P (3.0) P (3.0) P (3.0) 4MP (3.5) 2MP (3.5) a
[NaOHl/ mol.dm3 . 0.054 0.035 0.027 0.035 0.035
pKaa 11.30b 11.30b 11.30b 11.06e ll.W
KA 3.8,E5Ad 3.5: 3 . 9 3.0: 3.3d 6.7,'7.0d 5.0,' 5.8d
KI 14,'14.9" 13.7: 12.8d 12.7: 13d 62,'69 33,' 32d
Values of PKb = 14 - PKa of the amine used in the f i b of the data
*
absorbance-[SDS] to eq 4. Determined from absorbance measurementa in this work. c Determined by measuring absorbancesat X 220 nm. Determinedby measuring absorbances by 217 nm. e Take from ref 21.
be described by eq 519 in terms of the degree of micellar charge neutralized, /3 (=0.B2O). Since the term [XPH2+lm [Na'l, [Na'l,
= /3[Dnl - [XPHCI, and = (1- /3)[D,l+ [XPH;],
+ tNaOHI, + cmc (5)
is negligible in our experimental conditions, [Na+], and [Na+lwcan be readily evaluated. The constants KIand K Awere calculated by fitting the data for the variation of A with [D,] to eq 4 (see Figure 3), using both constants as adjustable parameters and Ao, ID,], [OH-], [Na+lm, and [Na+lwas inputs. The results are given in Table 2, together with the experimental conditions. Probably, the formation of hydrogen bonds between the )N-H groups of X-piperidines and the sulfate head groups of the SDS micelles can explain the difference in the calculated values for K A in the case of SDS micelles and those estimated for the binding of the unprotonated piperidines to TTABr or POE micelles. However, the electrostatic interaction (involving the solubilization of the piperidinium ions in a polar "adsorbed state") associated to KI is much more important than hydrophobic interaction (nonpolar "dissolved state" for the solubilized neutral piperidine). It is worth mentioning also that KI increases with methylation of piperidine, as expected from the increased hydrophobicity of the cation. The fact that KI is sensibly lower for 2-MP ion than for 4-MP ion probably reflects the steric difficulty to association produced by a methyl group close to the cationic center. (lb) Measurements without Added NaOH. In these experimental conditions the real [OH-] ,increases with [SDS]. The formation of OH- was verified by the ratio A217/A220, which does not remain constant, especially at high [SDSI. (The extinction coefficients measured from aqueous-Na0H solutions were e217 = 10.1 f 0.1 and e220 = 3.8 f 0.1 mol-1-dm3.cm-l.) The absorbance measurements obtained at high [SDSI were corrected from the error introduced by the contribution to the absorbance measure due to OH-which is formed in the process and which was determined from pH measurements. This procedure was tested under the same experimental conditions of the binding and the kinetic measurements, by measuring the pK, of piperidine in water which resulted, 11.30 f 0.05, in perfect agreement with published values.21 (19)Quina, F. H.;Chaimovich, H. J. Phys. Chem. 1979,83,1844. (20) Bravo,C.;HervBs, P.; Leis, J. R.;Pefia, M. E. J.Phys. Chem. 1990, 94,8816, and references therein. (21) (a) Bunton, C. A.; Huang, S . K.J. Am. Chem. SOC.1974,96,515. (b)Jencks, W. P.; Gilchrist, M. J. Am. Chem. Soc. 1968,90, 2622. ( c ) Critical Stability Conatants; Plenum Press: New York Vols. 1, 2, and 6.
In the present experimental situation, eq 4 is very difficult to applied since A, ID,], and [OH-lw are all changing simultaneously and the [Na+l in the micellar and aqueous phases cannot be evaluated by eq 5. So the experimental data A-[SDSI were perfectly fitted to the empirical eq 6 (see insert of Figure 5). The parameters a
and b were obtained from the fitting procedure. From eq 6 and the values of molar absorptivity of the amine it is possible to calculated the unprotonated amine concentration as a function of [SDSI (eq 7). The values of molar absorptivity of the amines were determined in water in the presence of a constant quantity of hydroxide ion (-0.035 mol-dmq), which was put in the sample and in the reference, simultaneously. The [amine] used was between (0.3 and 3.3) X 103 mol-dmq. The results are listed in Table 5, in which the values correspondingto the parameters c, the concentration of [XPHI in the absence of SDS, d , and e are also reported. These empirical parameters relate the unprotonated amine concentration to the surfactant concentration through eq 7. [XPHI = (c + d[D,I)/(l+ e[D,l) (7) 2. Kinetic Studies. The reaction was studied in the presence of SDS carrying out different experimentsvarying (i) the SDS concentration in the presence and absence of a fixed quantity of hydroxide ions, (ii) the amine concentration, (iii) the concentration of hydroxide ions, and (iv) the counterion concentration. (i) Influence of [SDSI in thehesence of NaOH. These measurements were performed in the presence of a fixed quantity of NaOH, between 0.01 and 0.06 mol-dm3, keeping constant the [amine] and [RON01 and varying the [SDSI. The kinetic data for the nitrosation of amines by RONO were analyzed quantitatively using the ion-exchange model. This formalism takes into account (1)the partitioning of the alkyl nitrites, RONO, and the neutral form of the amines between the micellar and aqueous pseudophases, (2) the selective ion exchange of ammonium ions of the amines with sodium counterions, and (3) independent reactivities in the micellar and aqueous phases. Following the work of Chaimovich and Quina,l9 our system can be described by eq 8. k, = {{k2+ (k2m/V)K~K/[D,l)[aminelt]/{(l+ K~[D,l){(l+ KAIDnI)+ (K,,/[OH-l)(l+ KIINa+l,/[Na+lw)J) (8) The [OH-] is maintained constant by the use of a [NaOH] much higher than the [amine]. The distribution constants ofRONO were determined previously.1° The distribution constants of XPH and the values of KIand Kb were taken as the data determined by absorbance measurements and reported in Table 2. The data in Figure 4 were fitted to eq 8 by iteration varying excusively the value of k p / V . The correspondencebetween the data and the ko us [SDSI functions generated at several [OH-], by the ion-exchange model, and with the three amines studied, was excellent (Figure 4). Calculation of second-orderrate constants in the micellar pseudophase in conventional units is possible after selection of a volume element for the reaction. With the molar reaction volume as V = 0.14 dm3.mol-l, the kzm values, calculated from the respective best fit values of
6
Micellar Effects on the Nitrosation of Piperidines 9.0
7.5
Table 3. Experimental Conditions and Kinetic Parameters Obtained in the Nitrosation of X-Piperidines by Alkyl Nitrites in Aqueous Micellar Solutions of SDS
(v)y-axis=1.2.k0
(A)y-axis=k,/l.2
amine P (3.0 X 10-9) 4MP(3.5 X 10-9) 2MP(3.5 X 10-9)
6.0 c
Iv) \
Langmuir, Vol. 10, No. 3, 1994 667
4.5
*e t
P (3.0 X 10-9) P (3.0 X 10-9) P (3.0 X 10-9) 4MP(3.5 X 10-9) 2MP(3.5 X 10-9)
z 3.0
[NaOHI/ kaal kaml mol.dm-9 mol-l.dm*.s-l mol-1dmS.s-1 K , N b 2-BromoethylNitrite 0.035 2.70 0.02 0.16 5.5 0.035 2.20 h 0.01 0.098 6.0 0.035 0.444 h 0.004 0.017 5.5 1-PhenylethylNitrite 0.054 0.227 0.002 6.1 X 10-8 79 0.035 0.227 0.002 6.9 X 10-9 79 0.027 0.227 0.002 6.9 X 10-8 79 0.035 0.196 0.001 5.1 X 10-9 79 0.035 0.0430 0.0003 7.6 X l(r 79
*
** *
a Experimental conditions: aqueous solution with [NaOH] = 0.1 moledmg. b Values of used in the fits and taken from ref 10. Values in parentheses are amine concentrations in mol.dm-8. The values of KI and KA used to fit the data to eq 8 are given in Table 2. In all cases a value of 4 X 10-9 mol.dm-8 was used for the cmc of SDS. This value was taken as the minimum [SDS] necessary to observe a kinetic effect.
x,N
1.5
0.0 0.00
0.05
0.10
0.15
0.20
0.25
0.30
[SD S I/ mol-dm"
3.0
Figure 4. Influence of [SDS] on the pseudo-fit-order rate constant of nitrosation of piperidine (3.0 X 10-9 mol-dm4) by 1-phenylethylnitrite at [NaOH] = (V)0.054, ( 0 )0.036, and (A) 0.027 mol.dm-9. Solid limes fit eq 8. For parameters see Table 3.
2.5
2.0
4 c
Iv) \
1.5
N
0
3
7
1 .o
r
Iv) \
0
-
0 0.00
002
0.04
006
006
0 10
1 0.!2
$ 2 0
0.5
a.
1
0.0 0.0
0 0.0
0.1
0.2
0.3
[SD SI/m ~ l - d m - ~ Figure 5. Influence of [SDSI on the pseudo-fist-order rate constantof nitrosation of 4-methylpiperidine(3.5 X 10-9moEdm-8) by 2-bromoethyl nitrite, no added NaOH. Solid line fit eq 9. For parameters see Table 5. Insert figure corresponds to the variation of the absorbance of aqueous solutions of 2-methylpiperidine (3.5 X 10-9mol.dm-8) with [SDSI in the absenceof added NaOH. Solid line fits eq 6, being A0 = 0.547.
k2m/V, are reported in Table 3. These values exhibit a decrease with respect to kz, the corresponding values in water, and are in perfect agreement (see Table 4) with the values of the bimolecular rate constants obtained in a water-dioxane mixture corresponding to a dielectric constant of ca. 35,the value of the dielectrical constant estimated for the Stern layer of SDS micelles.22 This means a reduction in the polarity at the binding site of SDS micelles, and the results also seem to indicate t h a t the site for the reaction with PEN is of lower polarity than that for the reaction with BEN. I n fact, the k2/kzmratio (22) (a) Sarpal, R. S.; Belleate, M.; Durocher,G. J. Phys. Chem. 1993, 97,5007. (b) Cordee, E. H. Pure Appl. Chem. 1978,50,617,and ref 15.
0.5
1 .o
1.5
2.0
1 02[P ip er id ine I/ mo lad m-' Figure 6. Influence of [piperidine] on the nitrosation rate constant by 2-bromoethylnitrite (0)at [NaOHl = 0.1 moledm-8 (see Table 3 for the result of kz), (v)in aqueous solution, no added NaOH, and (A)in aqueous solution of [SDS] = 0.055 molsdm", no added NaOH. Solid lines fit by ko = ka piperidine]; dotted line, to guide the eye.
takes a value around 20 for the reaction with BEN and around 40 for the reaction with PEN. This result could be considered as evidence that the Stern layer of micelles is highly anisotropic,23 not being uniform in composition and properties, if we assume that the preferential location of PEN makes it reside on the average in a deeper zone of the Stern layer, in a less polar environment than that felt by BEN. Influence of [SDSI with No Added NaOH. Under these experimental conditions the &cap) is not constant and the expression corresponding to [Na+ld [Na+lwisnot simple, because the value of [XPH2+]m in eq 5 is not negligible. Taking into account that the reaction may take place in water and in the micellar pseudophase, between the unprotonated amine and the alkyl nitrite, the results can be analyzed quantitatively making use of eq 7,obtained (23) Bunton,C. A.; Nome, F.;Quina,F. H.; Brometed,L.S. Acc. Chem. Res. 1991,24,355. (24) Anderson, J. E. J. Phys. Chem. 1991,96,7062.
Iglesias et al.
668 Langmuir, Vol. 10, No. 3, 1994
Table 4. Variation of the Second-Order Rate Constant, kr, in mol-l.dm*-s-l, Corresponding to the Nitrosation of Piperidine by 2-Bromoethyl Nitrite (BEN) and 1-Phenylethyl Nitrite (PEN) with the Dielectric Constant, DC, of the Medium (Dioxane-Water Mixtures) Determined Following Ref 24.
6
5
DC-
4 7
77.3 69.1 57.5 48.5 39.8 33.6 30.8 23.0
v)
\
I
0
r
lo
r
*-
z
2
L
0
1.5
4.5
3.0
6.0
7.5
9.0
102*[ Na OH 1/ m o I-dm-' Figure 7. Influence of [NaoH]&,~don the pseudo-first-order rate constant of the nitrosation of piperidine by ( 0 )2-bromoethyl nitrite and (A) 1-phenylethyl nitrite. Solid lines fit eq 10. 2.5
1.5
In \
+
m
0 c-
1.0
0.5
V."
I
I
I
I
0.00
0.05
0.10
0.15
[MeC I]/ mo 1.d m-' Figure 8. Influence of [Cs+l (solid points) and [Na+l (open points) on the nitrosation of piperidine by 2-bromoethyl nitrite at [SDS] (A)0.033,( 0 )0.055, (V)0.1, and ( 0 )0.089 mol.dm-8: for solid points [PI= 3.3 x 1V moldm-9; for open points, [PI = 2.7 X 109 mol.dm-8. Solid lines to guide the eye.
empirically, which gives the variation of the [XPHlt with [SDS]. The resulting expression for ko is that corresponding to eq 9.
k, =
0.23 0.125 0.052 0.024 0.0098 0.0047 0.0040
[aminel/mol.dm-8 c/mol-dm-8 10% 1.33 X 10-8 2.8 P (3.0 X 1 p ) P(3.3X le) 1.80X 10-8 4.7 4.0 X l@ 4.1 P (6.7 X 109) 4hfP (3.5 X 10-8) 1.65 X 10-9 2.6 2MP (3.5 X 1W) 2.1 X 10-8 2.4
kam/
mol-1.dma.s-1 cz17 166 0.146 131 176 160 5.0X lVC 131 176 74 0.16b 131 176 180 O . l l b 179 245 95 0.0196 140 190 e
a The values of molar absorptivitycoefficienta,t/cm-1.mol-l.d"d, corresponding to the neutral amine are also given (see text). Nitrosation by 2-bromoethylnitrite. Nitrosationby 1-phenylethyl nitrite. The reported [amine] were used in absorbance and kinetic studies.
1
2.0
7
kz (PEN)
2.7 2.0 1.15 0.68 0.35 0.19 0.14 0.070
Table 6. Values of c, d, and e Obtained Experimentaly (Equation 7) and Used in Equation 9 to Fit Kinetic Experimental Data.
1
0.0
kz (BEN)
(C + 4D,l) k2 + (k2m/V)K~K:[D,l (1+ K,N[D,])(l + KA[D,]) (1+ e[D,l)
The presence of anionic micelles modifies the basic ionization equilibrium of the amine in such a way that the quantity of unprotonated amine-the reactive species-does not change proportionally to the totalamine until the micellar surface is saturated with ammonium ions. Once the saturation is reached, a linear relationship is restored. (iii) Influence of [NaOKl. At constant [piperidinel (=3 X 103,in the nitrosation by BEN, and 3.3 X 103,in the nitrosation by PEN) and [SDSI (=0.055), the concentration of NaOH was varied between 4 X 103and 8 X le2, all concentrations in mol-dmd. The addition of OHshould produce an increase in the reaction rate, as is observed experimentally, because of the displacement of the equilibrium represented byKb(ap,toward the formation of the uncharged amine. As can be seen in Figure 7, the experimental data fit perfectly to eq 10, which can be easily reached by taking into account the expression of Kb(ap). In this equation ko' represents the observed rate constant at [NaOHIaddd = 0. k,-k,'=
-
(k2+ (k2m/V)K~~~[D,l~[aminelt[OH-lad (1+ K,N[DJ)(1 + KAID,I)(Kb(ap) + [OH-])
(9)
Analyzing the experimental data in a manner similar to that followed in the previous section, the values obtained for k p are given in Table 5. As can be observed, they compare very well with those obtained in conditions of constant pH (see Table 3). (ii)Influence of [Amine]. The analysis of the influence of [amine] in the absence of surfactant leads to a linear relationship of ko with [amine]. The same plot in the presence of 0.055 mol.dm-9 of SDS does not show a linear relationship (see Figure 6).
The values obtained as 0.032, in the nitrosation of P by BEN, and as 0.029 moldma, in the nitrosation by PEN, agree well with that value obtained in the study of the influence of [SDSIat a fixed concentration of NaOH. At the same time the values of y = 8.1 X 103 s-l and y = 5.3 X 10-4 s-1 are in perfect agreement with the results expected in both cases. The y-values correspond to the observed rate constant when all [piperidine] is in its unprotonated form and the reaction takes place in aqueous and micellar pseudophases simultaneously. (iu) Influence of Counterions. At constant concentra-
Micellar Effects on the Nitrosation of Piperidines tions of SDS and amine, the concentration of NaCl(o-O.3 mobdm") and of CsCl(O-O.08 moldma) was varied. In Figure 8the experimental data of ko us [MClI, M = metal, are plotted. The pseudo-fmt-order rate constant increasea when the metal concentration increases, and the effect is greater in the presence of Cs+ than in the case of Na+ (or Li+, in which case there is practically no variation in the observed rate constant). These results are explained by the ion-exchangebetween piperidinium ion in the micellar surface and the metallic ions added to the medium.
Conclusion The results of these studies corroborate that micellar
Langmuir, Vol. 10, No. 3, 1994 669 systems can affect both reaction and equilibria processes. The simultaneous occurrence of both effecta can be quantitatively explained, in our case, by considering the distribution of all species in aqueous and micellar pseudophases and the change in the equilibrium constant for amine ionization produced in the presence of SDS micelles. The use of neutral ionizable nucleophiles ( e g . amines) in studies of reactivity at micellar surface has been very scarce in the past, in spite of the biological relevance of aminolysis-type reactions. A possible reason for such a lacuna is the use of buffers, which makes quantitative treatments difficult. The present work provides an alternative way of dealing with systems with ionizable reagents, in which the use of buffers is avoided.
