Kinetics of acid and nucleophile catalysis of the diazotization of 1-naphthylamine

JULIO CASADO,' ALBINO CASTRO, EMILIA IGLESIAS,' M. ELENA PERA, AND Jose VAZQUEZ TATO Departamento de Quimica Fisica, Facultad de Quimica, Universidad, E-Santiago de Compostela, Esparia

Received June 24, 1985

JULIO CASADO, ALBINO CASTRO, EMILIA IGLESIAS, M. ELENA PERA, and Jos6 V h z ~ u ~ z TATO. Can. J. Chem. 64, 133 (1986).

The diazotization of 1-naphthylamine has been studied at acidities ranging from to 1.5 M. At low acidities nitrosation takes place via the unprotonated form of the amine, and the rate-controlling step is either the interaction with the nitrosating agent or, if high concentrations of a nucleophile are present, the loss of a proton from the protonated nitrosamine (the nucleo- philes studied were C1-, Br-, SCN-, and thiourea). At high acidities the protonated and free amines also react, the former of these reactions involving proton transfer to the solvent. In each case the proposed mechanism was supported by studying the reaction in D20, which also allowed the isotopic effect of the solvent on the formation equilibria of the various nitrosating agents to be obtained.

JULIO CASADO, ALBINO CASTRO, EMILIA IGLESIAS, M. ELENA PERA, et J O S ~ V h z q u ~ z TATO. Can. J. Chem. 64, 133 (1986).

On a CtudiC la diazotation de la naphtylamine-1 dans un intervalle d3aciditCs allant de 10-'a 1,5 M. A faible aciditC, la nitrosa- tion se produit via la forme non protonee de l'amine; 1'Ctape dkterminante de la rtaction est soit l'interaction avec l'agent de nitrosation ou, s'il y a une forte concentration de nuclCophiles, la perte d'un proton a partir de la forme protonee de la nitrosamine (les nuclCophiles CtudiCs sont les ions C1-, Br-, SCN- et la thiourCe). A des acidites ClevCes, les amines protontes et libres rCagissent Cgalement. Dans le cas des amines protonCes, la reaction implique un transfert de proton au solvant. Dans chaque cas, le mCcanisme proposC repose sur une Ctude de la rCaction dans le D20, qui permet Cgalement d'obtenir l'effet isotopique du solvant sur la formation a 1'Cquilibre des divers agents de nitrosation.

[Traduit par le journal]

Introduction The rate equations of diazotization reactions are heavily

dependent on the experimental conditions, whose modification may change both the nitrosating agent (nitrosation being a preliminary step towards diazotization) and the slow step of the reaction, and at one time this behaviour gave rise to con- siderable controversy, the general validity of conflicting rate equations being claimed by their authors (I) . Most research in this area has involved aromatic amines such as anilines and pyridines, whose diazonium ions are relatively stable and for which the effect of substitution has been analysed in some detail, but rather less is known about compounds with multiple rings. These considerations have led us to carry out a detailed study of the diazotization of 1-naphthylamine (1-NA) in water and in D 2 0 , at acidities ranging from lo-' to 1.5 M and in the presence of various nucleophiles. I -NA was chosen for its simplicity and the value of its pK,, and because the only published study of its diazotization (2), in which ClNO was used as nitrosating agent, yielded a value of 1.9 x 10" M-' s- ' for the attack constant at O°C, which is surprisingly high compared with the values of (1 .O-5.9) X 10' M-' s-I obtained at 25°C for anilines and their derivatives (3), and which are widely accepted as evidence of these reactions being diffusion controlled. It was expected that varying the experimental conditions might result in I-NA exhibiting nucleophile catalysis, rate-limiting proton transfer to the solvent, and reactions via its free and protonate forms.

Experimental

tion, but fresh solutions were in any case prepared at short intervals. The stability of the diazonium salt produced by the reaction and that of the 1-NA were verified experimentally under the various working conditions used. The decomposition of nitrite was also observed to be negligible during the course of the reaction, even under the most unfavourable conditions.

Kinetic measurements were carried out in a UVIKON 820 spectro- photometer, the temperature of the cell carrier being kept constant by a HETO 03T623 thermostat. A Radiometer 82 pH-meter with a GK 2401C combined electrode was used to measure pH.

The formation of the diazonium salt was monitored spectrophoto- metrically at 362 nm, a wavelength at which this salt is the only component of the reaction mixture to exhibit any appreciable absorp- tion (E = 6400 M-' cm-'1. The integration method was used for kinetic analysis. When equal concentrations of nitrite and 1-NA were employed, the equation fitted to the experimental absorbance-time data was

1 - 1 A, - A, A, - A,, (k2W 1

A,, A,, and A. being the absorbances at times t, infinity, and 0 respec- tively. When the reaction was slow enough for more than a 20-fold excess of m i n e to be used, the method of Davies, Swann, and Campey (4, 5) was used to fit the equation

[2] In ( A , - A,) = In (A, - Ao) - k,t

In either case the corresponding graphs (respectively l/(A, - A,) against rand In (A, - A,) against t) were linear for at least 85% of the reaction (Fig. 1) with standard deviations of less than 2%. When eq. [I] was used the values of A, and A. were obtained experimentally. The values of kl and k2 were reproducible to within 23%. All experiments were carried out at 25OC.

All the reagents employed were Merck p.a. products and were used Results and discussion directly after drying. D20 was supplied by;he Spanish ~ u c l e a r ~ n e r g ~ ~t acidities in the range 0.085-1.62 M HC1O4 the diazotiza- Board (Junta de Energia Nuclear) and contained 99.77% D. 1-NA and tion of ~ - N A was found, using excess amine, to be offirst order its solutions were protected from light to prevent their photodecomposi-

with respect t o both reagents (Table 1). First-order behaviour with respect to HN02 was confirmed by the agreement between

'present address: Departamento de Quimica Fisica, Facultad de the experimental A,-f data and eq. [21. Thus, Quimica, Universidad, Salamanca, Espaiia. 2Author to whom correspondence may be addressed. [3] v = kl [HN02] = k211 -NA] [HN02]

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FIG. 1. (a) Typical pseudo-second-order plot of the diazotization of 1-NA at 25"C, I = 0.35 M, [HN02] = [I-NA] = 3.27 X M, [NaSCN] = 2 x M, [H+] = 0.1 M. (b)'Typicalpseudo-first-order plot of the diazotization of 1-NA at 25"C, I = 0.35 M, 11-NA] = 1.05 X M, [HN02] = 3.27 X M, [NaCI] = 0.0266 M, [H'] = 0.1 M.

Catalysis by nucleophiles The influence of the nucleophiles C1-, Br-, SCN-, and

(NH2)2CS was studied at 25°C in 0.1 M HC104 using constant concentrations of nitrous acid and 1 -NA (I = 0.35 M). C1- , Br-, and SCN- were used in the form of their sodium salts. The concentrations of NaCl and NaBr employed varied from 0.017 M to 0.25 M, those of NaSCN from 0.001 M to 0.015 M, and those of thiourea from 1.48 x M to 2.23 x M. The concentrations of HNO, and 1-NA were 3.3 X M except in the experiments with C1-, which were slow enough to allow the use of excess amine (1.06 X M).

Under the above conditions the halide ions and thiourea all catalysed the reaction. The graphs of k2 against [Nucleophile] are curves in which k2 tends with increasing [Nucleophile] to a limit that is the same for all nucleophiles employed (Fig. 2). This limiting value may be evaluated from the intercepts of the plots llk2 vs. l/[X-I (the average value is 4500 M-' s-I for all of them). This suggests the reaction mechanism to be of the kind put forward by Woppmann (2), in which the nitrosating agent is the nitrosyl salt of the nucleophile (Scheme 1). No evidence of nitrosation by any other agent (NO+(H2N02+) or N2O3) was observed.

Assuming the steady-state condition to be applicable to the intermediate I in Scheme 1,

I where KXNo = K1K2 is the equilibrium constant of the forma- tion of the nitrosyl salt of the nucleophile, and Ka is the dissociation constant of the protonated amine (the value of 3.96

\ obtained spectrophotometrically by the present authors for the pK, at 25°C and an ionic strength of 0.2 M agrees well with the

TABLE 1. Values of the first-order pseudo-constant k, and A, under various conditions

[HCIO,] (M) [I-NA] (M) [Nitrite] (M) A, kl (s-')

*Calculated from the pH.

FIG. 2. Influence of the concentration of SCN- (0) and Br- (0) on the second-order rate coefficient, kz (eq. [4]) at 25OC, I = 0.35 M.

value published by Hall and Sprinkle (6). Except for t h i ~ u r e a , ~ the values of k2 were unchanged on working at pH 1.5.

The values of k3 and k3/k4 obtained by plotting k2-I against [Nucleophile]-' are listed in Table 2. The values thus found for k3(CINO) and k3(BrNO) indicate diffusion-controlled pro- cesses and are compatible with those reported by Williams and co-workers (3) and Schmid and Fouad (1 1) (Table 2). The much lower values of k3 for SCNNO and nitrosothiourea are in keeping with the lower reactivity of these latter nitrosating agents, their greater catalytic effect being due to their large KxNo. Figure 3 shows that the values of log (k-,lk4) are satisfactorily correlated with Pearson's nucleophilicity param-

3 ~ n this case it is necessary to take into account that the initial stoichiometric concentration of nitrous acid is equal to [HNO,] + [Nitrosothiourea]. Thus, k2 = k3KxNoK,[TU]l{1 + (k-3/kd)[TUl} (1 + KXNO[H+I[TUI).

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1. HN02 + Hf NOf + H20 KI 2. N O + + X - XNO K2

y ~ 2 H,$NO

+ XNO 2 @ + X- 3. 0 0 k- 7

I k'Islow) HNNO

several & + -& H +

( X = CI-, Br-, SCN-, thiourea) SCHEME I

TABLE 2. Values of k3 and k-,lk, (eq. [4]) at 25OC and I = 0.35 M, together with some published values

! k3 x I O - ~ ( M - ' s-I) I k3 x *

X - KXNO ( M s ) k-3/k4* A p-CIA p-MeA

C1- 1.14 x 10-~(7)' 2.22 0.359 2.2" 1.8" 5.50 x 1 0 - ~ ( 7 ) ~ 2.5' 2.3'

I Br- 5 .10X10-~(8)" 3.70 5.80 1.7' 2.8' I I 3 . 2 3.2d i sCN- 32 (9)' 0.382 248

TU 5000 (10)" 0.00245 548 I I

*This paper; A = aniline; p-ClA = p-chloroaniline; p-MeA = p-methoxyaniline; TU = thiourea. "25°C (ref. in parentheses). *O°C. C200C. "~c id i ty 0.2 M, cf. ref. 3 . 'Acidity 1-5 M, cf. ref. 3. Qeference 11.

eter, n (12), the value of 1.1 for the slope of the line being similar Experiments carried out to determine the influence of to those usually found for typical S N 2 substitutions on the temperature on the diazotization of 1-NA by ClNO yielded the carbon atom. The present results are thus in keeping with the value of 79.67 kJ/mol for the Arrhenius activation energy and usual order of nucleophilicity, C1- < Br- < SCN-< (NH2),CS. the values AH* = 13.89 kJ/mol and AS+ = - 19.45 J/(mol K)

Support for the proposed k4 step was sought by repeating the for the k3 step.4 The enthalpy of activation is within the accepted Br- and SCN- experiments in heavy water. The constants limits for a diffusion-controlled step (10-20 kJ/mol), and thus obtained in water and D 2 0 are shown in Table 3. If, as is agrees with the value obtained for the constant k3. reasonable, the isotopic effect on the k3 process is assumed to be negligible, then a value of 3 .11 may be calculated for the Catalysis by HClO, and NaC10,

isotopic effect on the k4 step. Since this value is that of a primary The influence of ionic strength o n the diazotization of 1-NA

isotopic effect, the slow k4 step must involve proton transfer. was studied by varying the concentration of NaC104 in a series of experiments carried out at 25OC in 0.5 M HClO4, using

The D2° the effect On the K~ constant concentrations of nitrite and 1-NA of 2.75 x 10-5 M ecluilibrium to be calculated from the values of k?KxNnK,. If - - thk effects on the K2 equilibrium constant and the diffusion- controlled k3 step are assumed negligible, then when the 4 ~ h e s e quantities were calculated from the values at different tem- nucleophile employed is SCN- or Br- the values of p ~ , peratures of ~ ~ K c I N o K , (the slope of the graph of k2-' against [CI-I-')

obtained in water and D 2 0 and listed in Table 3 imply isotopic together with published values for KclNo at 0 and 2 5 " ~ (7) and the values of K, at 14 (5.55 x M, I = 0.2 M) and 25°C (Table 3). The

effects Of 2' l 9 and 2'967 On the formation Of latter were obtained spectmphotometrica1ly at 310 nm by the present These agree with each other and with those authors because the values found in the literature for K, at 15°C (14) by Challis e t a / . (13) for the nitrosation reaction and 25°C (6) afford a value of 136.6 kJ/mol for the enthalpy of ioniza-

I- + H~NO?' -+ I N 0 + HzO tion of the 1-naphthylammonium ion, which is enormously high com- pared with the values of 20-53 kJ/mol found for anilines (15) and

which supports the assumptions made in their derivation. would imply a negative enthalpy of activation.

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CAN. J. CHEM. VOL. 64. 1986

TABLE 3. Values of the kinetic and thermodynamic parameters of eq. [4] in water and D20, t = 25°C

H = H20; D = D20. "Determined by us.

FIG. 3. Variation of log (k-3/k4) with Pearson nucleophilicity, n .

and 1.32 X M, respectively. The observed reaction rates reveal increasing catalysis as the concentration of salt rises, the dependence of the logarithm of the second-order experimental rate constant k on the ionic strength of the medium being linear, with a slope of 0.485 ? 0.017 (Fig. 4). These findings are similar to those obtained for other substrates (15), and may be considered as evidence of a secondary saline effect acting to increase the equilibrium concentration of the nitrosating agent (NO' or H2N02').

The influence of acidity was studied by varying the concen- tration of HC104 from 0.15 to 1.5 M at 25OC and an ionic strength of 1.5 M controlled by NaC104, the concentrations of nitrous acid and 1-NA being those mentioned in the previous paragraph. In these conditions the rate equation is (16, 17)

Figure 5 shows that the slope of the graph of log k against -Ho varies with acidity, Ho being the Hammett acidity function, whose values for concentrations of HC1O4 corrected for an ionic strength of 1.5 M were taken from ref. 17. The experimental constant k itself, however, is a linear function of ho. These results are in keeping with there being interaction between the

FIG. 4. Influence of the ionic strength on the second-order rate constant, k , at 2S°C, [HC1O4] = 0.5 M, [NaN02] = 2.75 X M, [I-NA] = 1.32 X M.

nitrosating agent (NOf or H2N02') and the free and protonated forms of the amine, as in the diazotization of o-chloroaniline (17) and 2- and 4-aminopyridine- 1-oxides (1 6a), for then

which, since [I -NA] ho = [l-NAH'IK,, implies the linear relationship

between k and ho. If this hypothesis is accepted, comparison of eq. [7] with Fig. 5 implies values of 2.17 + 0.09 and 2.44 + 0.04 for k5Ka and kg, respectively, and the former of these in turn implies a value of 1.97 x lo4 for k5 = kattackKNO+, (kattack being the rate constant for the elementary process of reaction between NO' and the free amine), which is extremely high compared with the values of 2000-6000 found for the nitrosa- tion of other amines by NO', and accepted as indicating a diffusion-controlled process (18). One reason for the high value of k5 may lie in the exponential influence of ionic strength on the reaction (see above). If this effect is allowed for by using the data of Fig. 4 to obtain an equivalent value for zero ionic

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NO+

Products SCHEME 2

FIG. 5 . Dependence of k (eq. [ 7 ] ) on Ha (a) and ha (0) at 2S°C, I = 1.5 M , [NaN02] = 2.75 X M, [I-NA] = 1.32 X M .

strength according to the equation

PI log k5 = log kSO + 0.485 1

the result is a value of 3690 for k5 that is fully compatible with published values for other substrates.

For the k6 interaction between the nitrosating agent and the protonated form of the amine, a mechanism analogous to that shown in Scheme 2 for aniline is proposed, involving the association of the NO+ ion and the aromatic ring in a complex that, upon losing a proton, rearranges to yield the nitroso compound. Naphthylamine's possession of two condensed rings increases the number of possible positions for NO+ in the initial complex and hence the importance of nitrosation via the protonated form and, indeed, the value of k6 for 1 -NA is greater by a factor of 10 than for anilines and pyridines (16c), making this pathway significant at intermediate acidities.

Since the above scheme involves proton transfer to the solvent in the rate-controlling step of the reaction, a primary

isotopic effect ought to be detectable. Measurements of k6 in D 2 0 yielded the value of 1.59 for (k6)H,0/(k6)D20 and, since k6 = PK'KNO+, the value of (KNO+)H,d(KNO+)D20 obtained earlier implies an isotopic effect of 3.50 for K'k", which is indeed a primary isotopic effect and thus supports the validity of Scheme 2.

Acknowledgements The authors wish to thank the Spanish Comisi6n Asesora de

Investigaci6n Cientifica y TCcnica for subsidizing the research reported in this article as part of an investigation into the mechanisms of formation of nitroso compounds. E.I. likewise wishes to thank the Spanish Ministry of Education and Science for the grant (Formaci6n de Personal Investigador) allowing her to take part in this research.

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