cthaiii~lby couliiig t 0 - - L5'. \VIicri ill-icd to coiistaiit weight a t 100' the crystals y~ilverized.3~ -1nnl. Calcd. for CsHsBr204SSa: C , 20.36; If, 0.85; 13r, 45.16. Foulid: C, 20.17; 13, 0.87; Br, 45.06. 2,4,6-Tribromophenol ( I h s t m i n Prxctical Grade) was recrystallized four times from etliaiiol, i n . ~94-95", ~. lit. m.p.
:u . 3 3
Kinetic Measurements of the Bromodesulfonation Reaction of I.-Medsurements were carried nut in distilled water a t 24.93 i 0 . 0 3 O . The conceiitratioiis of the reactants and i-atc roiistants are given in Tables I a r i d 11. The following rcagents were used : sodiuiii perchlorate, Fisher Purified Grade recrystallized from water; sridiuiii bromide, Mallinckrotlt Reagent Grade; perchloric acid, C . P . Grade; and the bromine wias a center fractioii, b . p . 5 8 O , of refractionatvti Merck Reagent Grade. Stock solutions of I 10.0200 'If),stantlardized perchloric x i ( ! and bromine water i r n . Z X 10 - 3 ) were used in preparirig thr reaction mixtures. S ~ ~ d i u iperclilorate ii and sodium bromide were weighed out sq):tr:itely for eacli euperiiiicnt. The reaction mixture was l)rcparetl, a t the reaction teinperature, b y mixing a solution ( a ) rontainirig sodium perchlorate and I with an equal volume 1250 or 500 1111.) of a wlution (h) containing bromine, ~ivrrliloricarid :iiid sodium bromide. The rate was followed 1)y n.itli~lrawirigaliquots (26.59 inl.), pipetting eacli aliquot i i i t o 1 0 n i l . of 0.8:"(aqueous potassium iodide, arid titrating t lie 1il)erated iodine immediately with ;I standardizetl sodium 0.0027 N) solution measured from a semitarch was used as the indicator. The normality o f tlic sotliuni thiosulfate solution was determined Iicritidicaliy by titrating against a standard potassium iodate wlutioii. Sixteen to tvveiity poiiits were taken in each exIicrimcnf . When I \vas iii excess, there was no problem witli bromine volatilitj-. However, when bromine was in i'uccss, the loss of broriiiie became important (as detcr( 3 2 ) Analysis was performed b y Schwarzkopf hlicroanal>-tical Laboratories, Woodside 77, N. Y . ( 3 3 ) I. Heilbron. e t n l . , "Dictionary of Organic Componnds." Vol. IV. Is;yre a n d Sjmttisw.rmcle 1,tii.. T . o n d o i ~ ,1053, 1). S:l
[ CONTKILIUTION FROM
milled by blaiik deterniinatioiis) alter iiiie-third to oiie-half of the solution had been removed. Therefore, the volume of the solutions (a) and (b) was increased t o 500 ml. and lrsi than one-half of the mixture was used. Reaction of 2,4,5-Tribromophenol with Bromine. -- Solutions of 2.00 X dd tribromophenol were prepared 11). dissolving 0.0662 g. of the phenol in hot distilled Tvater and then diluting t o one 1. upon cooling. The desired arnourit~ of sodium perchlorate and sodium bromide were dissolved in a measured amount of tribromophenol solution. Stmilardized perchloric acid was added and, Lifter bringing t o 24.93 zt 0.02", the solution was then niixed \vith lironiilii. water such that the total volume was one liter. Tlie initial concentration of tribromophenol was calculated from thc 34 solution used to prepare the r e x volume of 2.00 X tion mixture. The disappeararice of bromine together Xvitli tetrabroinocj-clohexadienone 111arid other coinpounds wliicli with iodide liberate iodine was followed. Aliquots (50 OT 26.59 nil.) were withdrawn, added t o 10 i d . of 0~85$potassium iodide solution, and the liberated iodine n-as titrat eil with staudardized sodium thiosulfate sulutioi: . The reaction was homogeneous throughout the course of tlle reartioil. The data are given i n Table 111. When trihrornoyhenol was in excess, tlie values of the first-order rate constant, k ~ , were t.aken as being equal t o the slope of a plot of In (B)i ' i . time, where B is the iodine titer in moles/]. \YitIi broiiiiiic in excess, kq !vas taken as being equal to the slope o f a plot of hi (A0 - BO B ) 11s.time. Ultraviolet spectra were takeii iii distilled w,iter 0 1 1 i' Car). recordiiig spectrophotometer, model 1I .
+
Acknowledgment.-The author wishes to express his gratitude to Dr. I;. H. Westheimer for his encouragement and helpful discussions throughout the course of this work. The author is also indebted to the National Science Foundation for financial support. CA!dIiR l I J ( ; l i , k I A S S A C l f 1 3 E T T S
THE >~.?LLINCKRODT I , A U O K A l O R I E S O F
HARVARD UNIVERSITY]
The Bromodesulfonation of Aromatic Sulfonate Salts. 11. The Effect of Amino, Methoxy, Methyl and Nitro Substituents' R Y LAWRENCE ( > ,C . L X N K L I , ~ 1i.iL.e been studied in cicluvotl$ t i i t . t l ~ i ~ l l l ' J ~ ~ S ~ ~ of f ~ ,i ~ series l i ~ ~ ti r~E ~p J- i~l i i l ~ s t i t x i ni >~!~i i ~ , i tli c ,,;ulfoii,itt. 'l'iir rtwi\ts give additioiial experinieilt:tl cul)lii)rt tu the theory that ?kctriJphilic aromatic substitutioii rc.:tcti,i!i\ I)roccwl h y !vLty ( J f quiiiotioid iiiterinediatc,.;. Tile following p-substituents were studied: amino (1111, metho\-! ( I V ) , inc.tliy1 ( \ ' J , ,ti111 liytir,Juyl, with vi-nitro groups (L.1'1, ,111 four conipounds gave secotid-order kinetics, but they varieti i i i rrgartl t o their ilclimilence of rate oii (Br-). The kitietic-; For V showed little or :lo dditioilal dependence on Br- aside froin tli,it required ,)L T3ri-- fiiriuation. However, Tvith 111, I\' and V I it wa.: found th3t decrezses in rate occurred which cairiot be accounted for h y the Bra- effect alone (rf. Table 11). The kinetics can be accounted for by a mechanism wherein bromine and the sulforiate aiiioii uridergo a reversible reaction to give Br- and a "steady state" concentration of a quinonoid
1lit,
kitit t i [ " .
. o ~ u t i ~ ~att i
0'
intermediate. To complete the reaction, the quinonoid intermediate decomposes in a unimolecular step (cf. eq. 1 and 2 ) . Structural features affecting the stability of quinonoid reaction intermediates are discussed.
In a previous paper the bromodesulfonation of sodium (I) was 3,5-dibromo-4-hydroxy reported.3 Spectral and benzenesulfonate kinetic data showed that the formation of a 3,5-cyclohexadienLone (11) as :F reaction intermediate took place ininiediately upon mixing the reactants, and that the rate-determining step was the first-order decomposition of the quinonoid I1 to give tribromoI 1) T h , s paper SIRS presented in part a t a symposium on aromatic culiatitutiun held h p t h e Division of Organic Chemistry a t the 130th k l e i , t i n c of the American C'iirmical Society, Atlantic City, N . J., ,'cpt. 2 0 , I!?&, ,E) SI1e11 I , c \ e l t , i j m e n t Cc,,, l C n ~ e r y \ i l l e , C.tlif A7,ilio~i,alSricilc.r I o t I n d : i t i , j r i l, C ~ L I L I ITHIS ~ I I , J O I J K N A L , 7 9 , 2972 (19.57).
0
y
y
r
k. I, Br '$0,- I I
phenol.4 Since the bronlodesulfonatiorl reaction aromatic Of is an ti0IL3 this work COnStitUteS a Clear example Of ekCtrophilic aromatic substitution proceeding by id a quinonoid intermediate. ii) I i i .irjueous sulutioii ic,iinil t o l i e l?G min.
.it
2 . ; ' t h e 1i;df.Iife of I h e ; n l ~ r ~ i i c d \i\ ~ : I \~ t ~ ~
BROMINATION OF AROMATIC SULFONATE SALTS
June 5, 1957
This paper reports the investigation of the kinetics and mechanism of the bromodesulfonation of the aromatic sulfonate salts
2933
Results
Reaction Kinetics.-The kinetics for the bromodesulfonation of III, IV, V and V I have been deNa termined in water a t -0.10 f 0.02'. The reaction I rates were followed by removing aliquots, quenchCHI 8 ing with an iodide solution and titrating the liberated iodine with a known sodium thiosulfate solution. Starch was used as the indicator. Tables I and 111 summarize the findings. The reactions were carried out essentially a t a constant acidity and bromide concentration, in Previous investigators have shown that the prod- most cases, by having an initial concentration of ucts obtained in aqueous solution are, respectively: sodium bromide and perchloric acid large enough 2,4,6-tribr0moaniline,~J4-brom0anisole,~ l-bromo- that the amounts of acid and bromide produced in 4-methylnaphthalenes and 4-bromo-2,G-dinitro- the reaction were small in comparison. The equilibrium constant for the formation of Br3- is phenol.$-l A study of the rates of reaction as a function of 19.6 a t 0.0' (equation 3)13; and, consequently, (Br-) shows that bromide ion retards the rate of very little of the bromide produced during a reacproduct formation with 111, IV and VI, but that tion was converted to Brs-. For example, a t 90% the extent of this participation by bromide ion is reaction in expt. 11 and 4 the ratios of Br3- to Br2 dependent on the structure of the sulfonate salt. are 0.01 and 0.005, respectively. Second-order Kinetics.-By varying the initial These results are explainable in terms of the usual mechanism for electrophilic substitution when ratio of the sulfonate salt to bromine by factors of there is a "steady state" concentration of an inter- 31, 13,24 and 34 for 111, IV, V and VI, respectively, mediate quinonoid, and they therefore constitute, the kinetics of these reactions were found to be together with the results of the previous paperJ3 second-order, that is, first order in each reactant. Reactions with the anisole I V were homogeneous strong experimental support for that theory. The and followed the second-order equation to points mechanism presented here is similar to that procompletion when the anisole was in a posed by Grovenstein and Henderson to account beyond fourfold excess but only to 60% completion when it for the decrease in the rate of bromodecarboxylation of 3,5-dibromo-4-hydroxybenzoicand 3,5- was in a twofold excess. With the bromine in exdibromo-2-hydroxybenzoic acids with added so- cess, the second-order expression was followed to only 35-40% completion. In all cases, the apdium bromide. l 2 For the dibromo-p-aminobenzenesulfonate I11 parent rate constant for bromination increased with time, and, therefore, it appeared that p-brothe reaction scheme is moanisole, the reaction product, was undergoing NH* fKH* further bromination. With V, 1-bromo-4-methyl,Br naphthalene began to separate as an oil from the solution after about 25% reaction. The rate folBr? kz -!- Br- ( 1 ) lowed the second-order expression out to 40% reaction and then the rate constant drifted to a value I A G to 10% lower presumably because of bromine adSO3 Br SO3sorbed on the product which thereby reduced the \*I1 amount of bromine in solution. Reactions with SI12 the amine I11 gave a precipitation of 2,4,6-tribromoaniline after about 30% reaction, and the kinetics followed a second-order plot out to 60-70% reaction and then the rate decreased by about 10% apparently because of adsorption of bromine on the A Br Br SOnsolid. Reactions with the dinitrophenol VI were VI1 homogeneous. Bromide Dependence.-The kinetics of the reThe SO3 is hydrated to give sulfuric acid. actions were studied as a function of bromide (5) J. J. Sudborough and J. V. Lakhumalani, J . Ckem. Soc., 111, concentration over a change in (Br-)o from zero 41 (1917). to 0.15 M . The ionic strength was maintained (6) 0. Heinichen, A n n . , 253, 268 (1889). (7) A. N. Meldrum and M. S. Shah, J. Chem. Soc., 133, 1982 (1923); constant by adjusting the amount of (NaC104)o. M. S. Shah, et al., J . Univ. Bombay, 3, 153 (1934), C. A , , 29, 4747 Upon increasing (Br-)o the rate constants decreased (1935). as shown in Tables I and 111. ( 8 ) L. F. Fieser and D. M. Bowen, TXISJOURNAL, 62, 2103 (1940). Ionic Strength.-Changes in the ionic strength (9) E. Sakellarios, B e y . , 65, 2846 (1922); cf. R. King, J . Chem. Soc., 119,2106 (1921). had little effect on the rates.I4 For example, a (10) M. M.Marqueyrol and P. C a r d , Bd1. soc. chim. France, [4] 27, change in ionic strength from 0.152 to 0.012 127 (1920). (expt. 6 and 10) changed the rate constant for the (11) Because 4-bromo-2,6-dinitrophenol reacts further with bromine
Br\b'B&r Br'o +
under certain conditions, the products from this reaction a r e discussed separately in a later section. (12) E.Grovenstein, Jr., and U. V. Henderson, Jr., THISJOURNAL, 78, 569 (1956); E. Grovenstein, Jr., and G. A. Ropp, ibid., 78, 2560
(1956).
(13) G.Jones and M . L. Hartmann, Trans. Am. Elcctro. Sac , SO,295 (1916). (14) The small salt effect is in line with the Bronsted equation whlch would predict no s a l t effect for the reaction of a neutral molecule, Br2, with an ion.
LAWRENCE G. CANNELL
2934
brornodesulfonation of sodium dibromo-p-aminobenzenesulfonate from 6.02 to 5.91 liters/mole-sec. Similarly, only a small effect was noted with the p-methoxybenzenesulfonate IV (expt. 20 and 23). The essential point of interest is that small changes in media due to the substitution of one monovalent ion for another, a t constant ionic strength, will not significantly alter the interpretation of the results which rely upon changes in rates by factors of 4 to over 1,000. Effect of Acidity.-Increasing the amount of (HC104)o a t constant ionic strength showed that the rates of bromodesulfonation of 111, IV and V are quite insensitive to acid. The amount of acid was increased by factors of 43 (expt. 2 and 7), 2 (expt. 11 and and 4 (expt. 29 and 33) with the sulfonate salts: 111, IV and V, respectively. With the dinitro-p-phenolsulfonate VI, however, an increase in acid by a factor of 5 decreased the rate by a factor of 4.4 (expt. 39 and 44). Bromination of o-Methoxybenzoic Acid.-The decrease in rate of bromination of o-methoxybenzoic acid accompanying a change in bromide concentration from 0.0025-0.15 M was found to be slightly greater than 4 (see Table 11). Calculation of Rate Constants.-The secondorder rate constants, kobs, were calculated by plotting log (A0 - Bo B ) / B us. time (see eq. 8 ) , where A O is the initial concentration of the aromatic sulfonate salt and B is the apparent bromine concentration (equal to the titer of iodine in mole/l.) a t a specified time. The graphical method was preferred to the direct calculation of kobs for each point because i t gives less weight to the value of Bo. The rate constants obtained by using experimental method 1, where checked, were reproducible to 1 to 3% of the average value. The wider variation
+
Vol. 79
occurred with those experiments which had shorter half-lives (cf. expt. 34 and 31 where the half-life was 80 sec.). The reproducibility of rate constants obtained with experimental method 2 , where checked, was found to be ca. f 1.5% of an average value (cf. expt. 11 and 12). The accuracy of the data is also indicated by its fit to a second-order equation as the ratio of the initial concentration of reactants is varied and, in the case of I11 and 117, by the agreement with eq. 13 (see Fig. 1). Discussion Kinetics of the Second-order Bromodesulfonation Reactions.-The second-order kinetics for the reaction of bromine with the dibromo-p-aminobenzenesulfonate (III),9-methoxybenzenesulfonate (IV) or 4-methylnaphthalenesulfonate (V), suggest a bimolecular mechanism between bromine and the aromatic sulfonate salt. Reactions with 111, I V and V were insensitive to changes in acidity; this is in agreement with the expectations that the sulfonic acids would be completely ionized and that the anion would be the reactant. Further the observed independence of rate on acidity shows that neither HOBr nor H20Br+ is the brominating agent. A decrease in the rate of bromination as the bromide ion concentration is increased would be expected because of the decrease in bromine concentration due to tribroinide ion formation Br-
+ Br2 Jr Bra-
(3)
for which the equilibrium constant is
The second-order rate equation has the form
_ _d B- _dt
- - dA =
dt
k(Brd(A)
where k is the second-order rate constant, anti A is the concentration of sulfonate salt, and B is the sum of bromine and tribroiuide ion (;.e., the iodine titer of the solution). For a given experiment (Br -) is essentially coiistaiit, and incorporating the bromine-trihromidc ion equilibrium into the equation gives ( 5 0
-
The equation shows that the reaction will follow a second-order rate law with an observed rate constant, kobs, which obeys the relationship 0.5
1 / I- /
k =
!1
0.0 0.05 0.10 0.15 (Br-), mole/l. Fig. 1.-Plot of rate constants, corrected for Bra- effect, for the bromodesulfonation of sodium 3,5-dibromo-4-aminobenzenesulfonate V S . (Br-).
0
(15) Also compare expt. 18 and 24 for a n increase in acid b y a factor of 5 b u t with a change of ionic strength.
kobs
[l
+ K(Br-)I
(7)
Integration gives the rate equation used in finding hobs
The correction for reaction 3 has been applied to the data for the bromination of 111, IV, V and 0methoxybenzoic acid and the corrected constants, kobs [l K(Br-)], are given in Table I. It was found that hobs [l K(Br-)] is nearly constant for the 4-methylnaphthalenesulfonate V and for o-methoxybenzoic acid. Table I1 summarizes the decrease in rate upon increasing (Br-)o from 0.0025 to 0.15 ill. JVith a value of K = 19.6,13ey. 7 pre-
+
+
2935
BROMINATION OF AROMATIC SULFONATE SALTS
June 5 , 1957
TABLE I SECOND-ORDER RATECONSTANTS FOR BROMODESULFONATION REACTIONS AT -0.10 f 0.02' Concn. given in moles/liter Expt.
(ArS0aXa)o
x
(Brdo x 104
104
(HClO4)o
(NaBr)a
(NaClO4)a
Sodium 3,5-dibromo-4-aminobenzenesulfonate
1 2 3 4 5 6 7 8 9 10
25.0 5.00 2.50 5.00 5.00 5.00 5.00 5.00 5.00 5.00
1.92 3.00 5.97 2.74 2.95 2.65 2.93 2.92 2.99 2.87
11 12 13 14 15 18 19 20 21 22 23 24 25 26
10.0 10.0 15.0 20.1 2.51 2.20 10.0 10.0 10.0 5.00 5.00 5.00 5.00 5.00 5.00 5.00
5.50 5.45 5.58 5.53 5.13 7.93 5.08 5.20 4.72 2.83 2.62 2.94 2.95 2.84 2.98 3.08
27 28 29 30 31 32 33
5.00 5.00 10.0 12.5 2.50 5.00 10.0
2.80 2.46 2.11 1.04 5.08 3.30 2.31
34 35 36 37 38
5.005 5.00
2.73 2.50 2.95
16
17
a
5.00 5.00 5.00
( C H ~ O C ~ H , C O ~x H )104. ~
2.82 2.55
0.100
0.0500
.loo
.0500
o-Metlioxybcnzoic acid 0,0025
0.0103 .0103 ,0103 .0103
0025 1500
.0206
0026
dicts a decrease by a factor of 3.76. For an increase from zero to 0.15 M (Br-)o the factor is 3.94. The observed decrease of 4.5 and 4.8 for the 4-methylnaphthalenesulfonate V and o-methoxybenzoic acid, respectively, are in reasonable agreement with the calculated factor of 3.76.16 However, for the $-methoxybenzenesulfonate I V and (16) It appears t h a t K would not be affected greatly by changes in ionic strength since this is t h e case in the closely analogous system for the formation of Ia- fr6m It and I-. For the latter system, t h e work of W. C. Bray and C. M. J. McKay (THIS JOURNAL, 82, 914 (1910)) and L. I. Katzin and E. Gebert ( { b i d . , 7'7, 5814 (1955)) indicates t h a t the maximum variation in K for changes in t h e concentrations of KI, NaClOd and N a I under 0.15 A4 would be less t h a n 12%. For a 12% increase in the value of K for the formation of Bra- from B r - and Brz, eq. 7 predicts a decrease of 4.2 in rate of bromodesulfonation. Experiments with P-methoxybenzenesulfonate I11 wherein (Br-) was varied from 0.05 t o 0.005 a t II 0.06 showed t h a t the large deviations from eq. 7 were not due to a salt effect on K (see expt. 12, 17 and 18).
-
0.734 .744 .692 153 26.4 6.02 0.707 .236 .116 5.91
0 0 .loo .0500 0 .0023 0 0.1500 .0023 .0025 .1475 .0023 .OlOO ,1400 .0023 .0500 .loo0 .0023 .loo0 .0500 ,0023 ,1500 0 .0023 .OlOO 0 Sodium p-methoxybenzenesulfonate 0.0200 0.0300 0.0100 .0200 .0300 .0100 .0300 .OlOO .0200 .0100 ,0200 .0300 .0200 .0300 .0100 .0300 .OlOO .0200 .045 .OlOO .0050 0 .OlOO .0500 .0200 .0200 * 0200 ,0100 0 ,0015 0 .1500 .0023 .0023 .0025 .1475 ,0100 .1400 .0023 .loo0 ,0023 ,0500 .0023 .loo0 .0500 0 .0023 .1500 Potassium 4-methylnaphthalenesulfonate 0.1475 0.0023 0.0025 ,0023 .0200 .1300 .0023 .0200 .1300 .0023 .0200 .1300 .0200 .1300 .0023 .0023 .1500 0 .lo23 ,0200 .0300
1500
153 27.7 7.20 1.40 0.699 0.457
0.184 .189 .178 .175 .177 .212 .322 .0964 .181 .234 ,434 .404 .286 .0986 .0453 .0254 0.134 .0857 .0985 .0878 .0992 .0274 .lo5
.1475 0 0 .1475
,354 ,191 .434 .424 .342 .195 ,134 * 100 0.141 .120 ,138 ,108
20.0 18.9
19.1 18.0 3.70 3.00 13.0
0 . 1475
0.264
14.6
15.4
dibromo-p-amiiiobenzenesulfonate 111 the decrease in rate is too great to be accounted for by this correction for Br3- formation. Therefore, the simple bimolecular mechanism scheme must be modified to account for this additional dependence on bromide ion. The proposed mechanism scheme is
A< r:
----t 3-
7'
ks
ArBr
+ SO,
(10)
Ar is an intermediate having a quinonoid structure \
S0,-
293G
LAWRENCE
G . CliNNELL
which is assumed to be in a “steady state” with respect to the reactants. This mechanism is basically the same as that proposed earlier for the bromodesulfonation of sodium 3,5-dibromo-4-hydroxybenzenesulfonate (I); however, the magnitude of the rate constants are such that in the absence of bromide ion the bromination step, eq. 9, is now rate-determining. If a “steady state” concentration is assumed for the quinonoid, Q,the reaction scheme given by eq. 9 and 10 predicts that
Equation 11 has then the same form as eq. 5, and by making a similar correction for the formation of Bra- (cf. eq. 6) the following expression is obtained
+
Equation 12 predicts that a plot of (Robs rl K(Br-)])-I against (Br-) will give a straight line. \Then the experimental data for the bromination of the dibromo-p-aminobenzenesulfonateI11 and p rnethoxybenesulfonate IV are plotted in this manner, straight line correlations are obtained (see Fig. 1). The agreement further verifies the correctness of the mechanism scheme given by eq. 9 and 10. The intercept at (Br-) = 0 becomes equal to l / k l ; this value can also be determined directly. The slope is equal to Kz/klka, and it is therefore possible to determine K*/k3. For the anisole IV &/ha is equal to 22 l./mole, and for the amine I1 it is 2,200 I./mole. Since the (rate forward/rate reverse) = kz(Br-)/k3, it follows that when (Br-)a is 0.10 AI the intermediate quinonoid VII, formed From the amine, returns to the initial reactants 220 times for every time it goes on to products. When kz/k3 is small, eq. 12 reduces to eq. 7 (;.e., the bromination step is rate-determining regardless of the bromide ion concentration). Dibromo-p-aminobenzenesulfonate 111. Effect of Acidity.-As noted above, the rate of bromodesulfonation of the amine 111 is unaffected by increasing (HCIOl)a from 0.002:J to 0.100 AI. This indicates that the concentration of the compound undergoing bromination is not significantly changed over this range of acidity. Allso, the ultraviolet spectrum of 111, which has maxima at 208 and 253 in,^^, does not change even though the composition of the solution is changed from 0.1 ;If HC101 to 0.1 Ad NaOH. Sodium p-aminoben~enesulfonate’~ undergoes a 19-fold decrease in absorption a t Xmax 247 mp in acid solution due to protonation of the amine group. Therefore, the data indicate that the amine 111 is so weak a base that it remains unprotonated even in 0.1 M HC104. This behavior is in line with the expected base strength of 111. Gillois and Rumpfls have shown that the 2,B-dibromoanilinium ion has a ~ K ofH 0.35 at 25’; and since the ~ K for H the K-protonated p-aminobenzenesulfonate ion is 1.35 log units less than that for the anilinium ion,lg the expected pK11 for the 9-pro(17) For previoiis work, see, for examgle, II. Bohrne and J. Wagner, ;li.ch. Phavm., 280, 255 (1942): C. ~ l .37, , 2517 (1913). (15) hl. Gillois and P. Rumpf, B d l . soc. chim. Fvance, 21, 112 (1954). (19) R . 0. RTacLaren and D. F Swinehart, THIS J O U R N A L , 73, l Y ? ? (1931).
Vul. 79
tonated 3,5-dibromo-4-aminobenzenesulfonate ion is about -1.0. Therefore, in 0.1 izI acid only about 1% of the amino groups would be protonated. TABLE I1 DECREASE IS RATEO F BROMODESULFONATION o s ISCREASING (Br-)o FROM 0.0025 TO 0,1500 Jf AT 0.0” Sodium 3,5-dibromo-i-atninobenzenesulfonate 230 (1300jO Disodium salt of 3,5-diriitro-~-hydroxybenzeriesulfonic acid 236 Sodium 4-methoxybe11zenesulfonate 16 Potassium 4-methylnapthalenesulfonate 1.5 o-Methoxybenzoic acid 4.8 3.76 Predicted decrease due to Bra- effect only a Experimentally determined for a change from (I3r-i” = 0 t o (Br-?a = 0.15dl. This value should b e compared to a predicted decrease for tlie effect of Rr3- formation of 3.94.
Destabilization of the Quinonoid State in Phenols by o-Nitro Groups.-The disodium salt of 4hydroxy-3,5-dinitrobenzenesulfonicacid (VI), in contrast to the salt of dibromo-p-phenolsulfonic acid 1, gave a very rapid reaction with bromine even a t 0”, and the kinetics showed that the decomposition of the quinonoid intermediate was not rate-determining a t low bromide concentrations. The data (see Table 111) can be summarized as follows: (1) the kinetics were second order. (2) An increase in (Br-) from 0.0025 to 0.1500 Ad decreased the reaction rate by a factor of 246; the rate constant is still decreased by bromide by a factor of 6.5 after taking into account the Br3- effect (cf. Table 11). The dependence of the rate on bromide ion was thus very similar to that found for the dibromo-p-atninobenzenes ulfonate (111): the bromination step is rate-deterinin ing at low (Br-10, but as (Br-)o is increased, the desulfonation reaction begins to become rate-determining. ( 3 ) The rate was inversely dependent on acid (an increase in acid by a factor of 5 resulted in a 4.4-fold decrease in rate), and this stands as e\-idence that it is the phenoxide anion which undergoes brotninatioii. The mechanism is comparable to that given in CY]. 1 and 2. The proposed explanation is given in the form of a potential energy curve, Fig. 3, which compares the bromodesulfonation of I and VI. I t is well known that the introduction of a nitro group o r t h o to an OH group greatly increases the acidity of the phenol. This is attributed largely to the resonance stabilization of the anion.?O A similar interaction would be expected in the ground state of the doubly charged anion of VI and, therefore, Fig. 2 shows a lower energy for the ground state of VI than for I. However, in the transition state, for the bromination step this interaction is partly lost because the phenoxide anion is being converted to a carbonyl group, and the C-C double bonds are becoming localized in positions unfavorable to resonance between the phenol oxygen and the nitro groups. Therefore, in the transition state, the energy difference between I and VI is considerably reduced. It has been shown that quinones become better ( 2 0 ) G. Mi. Wheland, “Resonance in Organic Chemistry,” John Wiley and Sons, Inc., N e w York, N. Y., 1955, p. 346.
June 5, 1957
BROMINATION OF AROMATIC SULFONATE
2935
SALTS
TABLE I11 &NETlC
DATAFOR
THE BROMORESULFONATION O F DISODIUM
SALT OF 3,5-DINITRO-4-HYDROXYBENZEXESULFOXIC I l C I n AT
-0.10 f 0.02" Concn. given in moles/liter Expt.
(NaOArS0aNa)o x 10'
39 40 41 42 4 :3 44
5.00 5.00 1.60 20.00 ;i, 00 5.00
(Rrdn
x
(HCIO~D
104
2.69 2.56 5.56 1.67 2.67 2.70
0.0200 ,0200 ,0200 .0200 ,0200 .lo0
(NaBr)o
(NaC10dq
0.0025 ,0100 ,0100 ,0100 ,0100 ,0025
0 1475
,1400 ,1400 ,1400 ,1500 ,0675
13.7 2 70
13.1 2.26 2.40 2.37
0.oFi32
0.210
3.00
oxidizing agents when substituted with electronegative groups and that the nitro group is particularly effective in this regard.21 Therefore, the potential energy of the tribromoquinonoid intermediate is represented in Fig. 2 as being less than that for the bromodinitroquinonoid intermediate VIII. 0
02x\A,so2 )I \ \
SO$-VI11 Finally, in the desulfonation step, as the G-S bond is broken the C-C double bonds again become delocalized and the resonance energy between the phenoxide anion and the nitro groups can again be important. Consequently, the activation energy for the desulfonation step for VI is much less than that for I. The over-all result is that in the absence of bromide ion the rate of desulfonation is increased to such a great extent that it is now no longer rate controlling. But the quinonoid has sufficient stability that with bromide ion the reverse reaction to give starting materials can take place and the desulfonation step then becomes important as the rate-determining step. It should be rioted that this change is brought about not by replacing one activating group by another but by modifying it through ortho substituents. The activation is clearly due to a combination of the phenoxide anion and the nitro groups and can be described as a resonance interaction. This is shown by the observation that potassium 4-methoxy -3-ni trobenzenesulfonat e gives a much slower reaction with bromine than does sodiuni 4-methoxybenzenesulfonate (IV). The deactivating effect that introducing a nitro group has on the rate of bromination of sodium P-methoxybenzenesulfonate IV thus illustrates the effect which is usually observed: the nitro group retards electrophilic aromatic substitution by an inductive effect. In the bromination of the dinitrophenol VI the nitro groups must also deactivate the ring by an inductive effect, but this is completely hidden by the resonance interaction. The difference in the kinetics of bromination of the dibromophenol I and the dinitrophenol VI demonstrates that the rate-determining step in aromatic electrophilic substitution can be changed by the modification of substituents. Dinitro-p-phenolsulfonate VI. Products.-Br
(?lJ
I-, I(. i:ieser,
Tiits J l ) r r n N A r . , 52, > 3 J 4
(l!):H)),
61, :1101 iI!l2!1).
\
Initial state Final state Reaction coordinate. Fig. 2.-Bromodesulfonation of OH Y I Y __ Y is Br, ( I ) \/\/ ___.__ Y is KO*, ( V I ) I 11
\/
SO3Na
Sakellarios reports that the reaction of V I with an equal molar amount of bromine water a t 24-30' gives a 93% yield of 4-bromo-2,6-dinitrophenol before recrystalli~ation.~Marqueyrol and Carre report that VI with an equal molar amount of bromine in water gives 3 crude product which contains 31.7y0 bromine (3 broniodinitrophenol has 30.47,) wherein the main product is 4-bromo-2,6dinitrophen01.~~With excess bromine at room temperature they reported that the crude product contained 45% bromine (547, required for a dibromonitrophenol) from which the chief product and a isolated was 2-brom0-4,B-dinitrophenol~~ smaller amount of 2,4-dibromo-B-nitrophenol. They further showed by titration data that VI reacted with more than a molar amount of bromine. The replacement of a nitro group on 4-bromo-2,6dinitrophenol by bromine is not surprising in view of other work which shows that picric acid will brominate in aqueous solution a t room temperature to give 2-brom0-4,6-dinitrophenol.~~ Dinitro-p-phenolsulnate VI. Side Reactions.The reaction rate measurements, reported in this (22) Marqueyrol a n d Carre suggest t h a t 4-bromo-2,F-dinitrophenol, t h e initial reaction product, isomerizes in the presence of excess bromine into 2-bromo-4.6-dinitrophenol (cf., El. E. Armstrong, J . C l i f i i ? . Soc., 13, 520 (1875); I
