Electrophilic Aromatic Substitution Re- Used After Purification by Standard Methods

Indian Journal of Chemistry Vol. 26A, April 1987, pp. 333-335

Electrophilic Aromatic Substitution Re- used after purification by standard methods. actions: Part III -Iodination of Anilines with Methanol, isopropanol and DMF (AR, BDH) were Iodine Monochloride in Presence & Absence used after distillation. All the reactions were carried out in organic solvent-water mixtures containing an of a Surfactant excess of H + and Cl - ions. The reactions were carried out under pseudo-first order conditions (jsubstrate], G V S SHASHIDHAR, N SATYANARA YANA & E V > > [ICl].) and were monitored by estimating the SUNDARAM· unreacted leI iodometrically at regular intervals upto Department of Chemistry, Kakatiya University, Warangal 506 009 two half-lives. The rate constants were reproducible Received 12 May 1986; revised 29 August 1986; accepted 22 within ± 2'/0' The products were isolated by the September 1986 method suggested by Berliner" and identified by TLe Rates of iodination of aniline and substituted anilines with iodine and other spot tests s. monochloride have been measured in 30':-,,(v/v)aqueous methanol, isopropanol and dimethylforrnamide. The effects ofperchloric acid, The first order dependence of rate each on [ICl] and sodium chloride and surfactant (NaLS) have been investigated. [substrate] is revealed by the constancy of kobs at Effect of dielectric constant on rates has also been tested. The varying [ICl] and the linearity (unit slopes) of plots of Hammell p + values have been measured in 30;,o(v/v)methanol-H ,0, log kobs against log [substrate]. This unit dependence isopropanol-H 10 and dirnethylformarnide-H ,0 mixtures in the of rate on [substrate] has also been observed in the presence and absence of sodium lauryl sulphate. The Hammell p + values range from - 6.82 to - 6.00. These results confirm Wheland presence of surfactant. Hence the rate law can be given type of transition state. The activation parameters are consistent by eq.t l) with the proposed mechanism.

-d[ICl]/dt = k2[Aniline] [ICl] ... (1) The mechanism of halogenation of aromatic amines depends on the nature of the halogenating agent 1.2. In where k2 is second order rate constant. the halogenation of phenol, aniline and anisole with Rate of the reaction is inversely dependent on [H +],

iodine monochloride the active halogenating species plot oflog k 1 versus log [HCl04] being linear with unit

may be H201 + or Hal. The title investigation is an negative slope. This indicates that the substrate species attempt to find the actual iodinating species and the involved in iodination is free aniline or substituted type of transition state. Thus the reactions of anilines anilines. Rate is also found to be inversely dependent with lCl have been carried out in aqueous methanol, on [Cl -] suggesting the active iodinating species as isopropanol and dimethylformamide (DMF) both in hypoiodous acidium ion, (H201 +) and not hypoiodous the presence and absence of surfactant, sodium lauryl acid (Hal). If Hal were to be the active species, the sulphate (NaLS). rate would have been inversely dependent on [H +]2. Standard iodine monochloride solution was Similar behaviour is observed in the iodination of prepared as described earlier:'. Aniline, (J- and p- phenols:'. Increase in the proportion of organic solvent toludines, o-chloroaniline, o-anisidine, m-amino- in the reaction medium has a pronounced retarding benzoic acid and p-nitroaniline (all AR, BDH) were effect on reaction rate (Table 1).

Table 1- EfTect of Varying Solvent Composition on Reaction Rate Constant ([Substrate] = 0.01 mol dm -3; (ICI] =0.001 mol dm -3; [NaCl] = 1.0 mol dm -3)

MeOH DMF MeOH DMF

4 I~(v/v) k 1 X 10 '';~(v/v) k 1 X 10· I~(VIv) (s -1) (s -I)

[HClO.] =0.2 mol dm -3; temp=25°C [HCIO.] =0.4 mol dm -3; temp=41°C Aniline o-Anisidine 20 35.20 10 28.37 20 66.37 10 77.38 30 27.48 20 11.76 30 37.30 20 32.58 40 12.73 30 4.43 40 21.54 30 11.60 50 5'79 40 1.37 50 10.15 40 3.67 -----_._---

333 INDIAN J CHEM .. VOL. 26A. APRIL 1987

Table 2- Effect of Temperature Variation on Reaction Rate Constant and Thermodynamic Parameters

([Substrate] =0.01 mol dm -3; [ICl] =0.001 mol dm -3; [HCI04] =0.4 mol dm -3; 1= 1.4 mol dm -3; [NaCl] = 1.0 mol dm -3; MeOH-H20 =30:70% (v/v); [NaLS] = 0.01 mol dm :")

3 Substrate ·k2 x 10 r E. sn; -t.st t.Gt log A dm+mol -IS -I kJ mol r ' kJ mot :' JK -'mol-I kJ mol :" Aniline 245.70 0.9995 61.62 59.10 61.6 77.76 10.01 (252.70)t (51.12) (48.60) (97.8) (18.24) (8.24) o-Toluidine 543.70 0.9922 57.93 55.41 67.2 75.76 9.75 (722.30) (44.10) (41.50) (110.4) (74.96) (7.46) o-Anisidine 173.50 0.9999 55.10 52.58 86.0 78.63 8.74 (191.40) (65.18) (62.66) (51.9) (78.39) (10.52) o-Chloroaniline 118.70 0.9996 55.52 53.00 87.8 79.59 8.65 (152.00) (53.40) (50.88) (92.7) (78.97) (8.39) p-Toluidine 40.00 0.9996 56.67 54.15 93.0 82.30 8.37 (43.00) p-Nitroaniline 4.43 0.99H9 59.93 57.41 100.6 87.88 7.98 (5.08) m-Aminobenzoic acid 1.17 0.9990 69.51 66.99 80.0 91.22 9.05 ( 1.95) (56.12) (53.66) (119.9) (89.94) (6.96) ·at 30°C tValues in the paranthesis refer to micelle catalysed reactions

The effect of added surfactant on the rate of meters are reported in Table 2. From the data in Table iodination of aniline at a given ionic strength has been 2, it is evident that electron donating. substituents studied. The catalytic effect of surfactant confirms the increase the rate while electron-withdrawing groups stabilization of positively charged transition state in decrease it. micellar phase. But the observed catalysis docs not Hence taking al1 factors into consideration the follow a regular trend. A sigmoid curve is obtained mechanism of the reaction can be envisaged as shown when pseudo-first order rate constant is plotted in Scheme 1. against [NaLS]. Two maxima are noted at 0.015 and 0.04 mol dm -3 of surfactant. After each maximum • I~H though the rate constant values decrease with increase x---y in [NaLS]. rate constants are higher than those in the NH2 absence of NaLS. In the initial stages the increase in rate of reaction with increase in [surfactant] may be due to the coloumbic attraction between anionic fast micelles, in the micro-environment of which the substrate is present, and H20l ". After reaching the first maximum value further increase in [NaLS] decreases the effective [substrate] because of Scheme 1 hydrophobic effect. With further increase in [NaLS]. the concentration of counterions (Na +) increases The rates are higher in methanol than those in which neutralise the negative charge on the anionic isopropanol and DMF (Table 3) implying that the micelles, thns facilitating the reaction and accounting sslvent plays an active role in stabilising the transition for second maximum. state by actually participating, to different degrees, in The effect of [surfactant] on the rate constant bond breaking and making processes and that MeOH- follows the mathematical model proposed by Bruice et H20 solvent mixture is able to do it much more at. b and the cooperativity index (n) is found to be 1.4. effectively than isopropanol-HyO or DMF-HzO This positive value implies stimulation of the mixture. However, in the case of m-aminobenzoic acid interaction of additional substrate molecules by the and p-nitroaniline, this trend is not consistent. The

interaction of first molecule with the micelles 7. rates increase with increase in dielectric constant of The reaction has been studied at different medium for a given aquo-organic mixture indicating temperatures both in the presence and absence of that the transition state is more polar and the surfactant and the second order rate constant, (k 2 = iodination of these substrates may be of an ion-dipole kobs/[substrate]) at 30e and thermodynamic para- type.

334 NOTES

---~~------... -~------p +-value in the presence of surfactant may be due to Table 3- Effect of Nature of Solvent on Reaction Rate stabilization of transition state in micellar phase. The Constant in Presence and Absence of Surfactant variation in p T -values in three solvents is in ([substrate] =0.01 mol dm ":': [1('1] =0.001 mol dm '; [NaCt] = 1.0 consonance with electronic effects operating in the mol dm '; [HCIO.] = OAlJlol dm -'; [surfactant] = 0.0 I mol dm - '; respective solvents. temp = 30 C: Organic solvent -H 0 = 30:70 (v/v)) 2 The magnitudes of !lHt and !lst (Table 2) are in

2 good agreement with those found previously in the Substrate k 2 X 10 (drn' mol ~'s ') in solvent electrophilic substitution reactions 1.3. Variation of MeOH Isopropanol DMF substituent has little effect on the values of !lHt, but Aniline 24.57(25.17)* 17.97(18.28) 7.36( 8.20) in general, values of !lSt increase as the electrqn- a-Toluidine 54.37(72.23) 26.97(28.30) 12.63(15.60) withdrawing ability of the substituent increases. This o-Anisidine 17.35(19.14) IDS( 13.82) 4.9S( 6.24) trend is consistent with an increasing need for o-Chloroaniline 11.87(15.20) 9.03( 9.47) 3A7( 3.80) stabilization of the transition state by solvation as a p-Toluidine 4.0~ 4.30) 3.27( 3.53) IA5( 1.83) result of increasing destabilization of the developing p-Nitroaniline OA4( 0.51) 1.03( 1.17) 0.35( 0.54) m-Aminobenzoic 0.12( 019) 3.19( 342) 0.29( 0.39) charge by substituent I 0.1 I acid The variation of !lHt values with the nature of ·values in the parenthesis refer to micelle catalysed reactions substituent is higher in the presence of NaLS than in its absence. This may be due to the increased ability of nuclear substituent to interact with the developing The structure-reactivity relationship has been tested positive charge in the transition state. In the presence by Hammett plots in the presence and absence of of surfactant the average value of !lst (-94.6 JK -I) is surfactant by the method described elsewhere:'. The more negative than that in its absence (!lst = - 76.5 order of reactivities of anilines is: o-toluidine > aniline JK -1). This is a-ttributed to the greater solvent > o-anisidine > o-chloroaniline > p-toluidine > p- participation in the transition state because of its nitroaniline > m-aminobenzoic acid. The same order stabilization in micellar phase. is observed in the presence of micelles also. The One of the authors (GVSS) is thankful to the CSIR, unusual low reactivity of p-toluidine and o-anisidine New Delhi, for the award of junior research fellowship. compared to aniline is due to reduced n-donation by the second substituent at ortho or para position", References In all these cases good correlations are obtained I Raghavan P S. Srinivasan (Sr) V S & Venkatasubramanian N. when Pia relation is employed. This shows that Indian J Chern. 19A (1980) 322. reaction centre is conjugated with aryl group and 2 Radhakrishna Murthy P S & Janardhana Ch, Indian J Chern. 19A (1980) 333. stabilised by electron-releasing substituents, and 3 Shashidhar G V S. Satyanarayana N & Sundaram E V.lndian J conjugation is less important in transition state. The Chern. 25A (1986) 289. p +-values for the iodination of anilines in MeOH, 4 Berliner E. J Am chem Soc. 72 (1950) 4003. isopropanol and D M F in the absence of surfactant are 5 Feigl F. Spot tests in organic analysis. (Elsevier. New York). 1975. - 6.82, - 6.25and - 6.52 and in presence of surfactant p.63. 6 Bruice R & Cordes E H. J Am chem Soc. 90 (1968) 4395. the values are -6.66, -6.00 and -6.25 respectively. 7 Piszkiewicz Dennis. J Am chem Soc. 99 (1977) 1550. These reaction constant values are comparable with 8 Reynolds W F. Mezey P G & Hamer G K. Can J Chern. 55 (1977) that of bromination of benzene'! (p + - 6.2), 522. indicating a similar type of transition state in both the 9 Richard A Jackson. Mechanism: An introduction to the study of cases. In the bromination there is considerable organic reactions. (Oxford Press). 1972. (a) ) 16;(b) 107. 10 Lucchini V. Modena G. Scorrano G & Tohellato U. J Am chem evidence that brominating agent is H 0Br+. These 2 Soc. 99 (1977) 3387. observations confirm the Wheland type intermediate II Modena G. Rivelti F. Scorrano G & Tohellato U. J Am chem formation during iodination. The small decrease in Soc. 99 (1977) 3392.

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