Use of Indicators to follow Acid-base Reactions in Benzene

BY J. H. T. BROOK

‘‘ Shell ” Research Ltd., Thornton Research Centre, P.O. Box 1, Chester

Received 1st February 1967

To demonstrate the utility of the azoarylamine indicators for studying acid-base neutralization reactions in benzene solution, the reaction of trichloroacetic acid with aliphatic amines in benzene was studied by infra-red spectroscopy and indicator techniques. The amines (and the indicators) form complexes of stoichiometry B . HA, B .2HA and B . 3HA, and order-of-magnitude estimates for the formation constants of the B .2HA and B -3HA complexes are reported. It was confirmed that trichloroacetic acid is essentially monomeric in dilute benzene solution but, owing to the forma- tion of acid-salt complexes, the indicator bases react with the acid in accordance with the expression, log bH+]/p] QI n log [HA], where n is rarely integral. This, however, proves to be no severe limita- tion to their use. The hydrogen bond donors, N,N-dimethylacetamide and trioctylphosphine oxide, also were shown by indicator methods to give acid complexes with trichloroacetic acid.

The behaviour of acid-base reactions in acetonitrile, an aprotic solvent of moderate dielectric constant, is well understood, but less attention has been devoted to other aprotic solvents, such as the commercially important hydrocarbon oils. In aceto- nitrile the products of acid-base reactions are ion-pairs which dissociate to a small extent to give separated ions, and a characteristic feature of the reactions is the forma- tion of complex anions A . HA-, where HA is an acid and A- its conjugate base. In hydrocarbon and halohydrocarbon solution, a characteristic feature of acid-base reactions (for weak acids such as the phenols, at least) is the formation of hydrogen- bonded complexes as an alternative to the formation of ion-pairs, and it is not clear whether the formation of hydrogen-bonded complexes merges gradually into the formation of ion-pairs as the strengths of acids and bases increase, or whether acids can be regarded as “ neutralized ” by the formation of hydrogen-bonded complexes.

Electrometric methods cannot be used to follow such reactions, but spectroscopic methods may be employed in favourable cases : for instance, Bell and his collabor- ators 4-7 studied the reaction between nitrophenols and amines by spectroscopy in the visible region, and Barrow 8 9 9 used infra-red spectroscopy to study the reactions between haloacetic acids and amines. However, these methods are either not gener- ally applicable or not precise enough for quantitative work and Bruckenstein and Saito lo introduced a more general method based on vapour pressures (using the vapour-phase osmometer) to examine the haloacetic acid + amine reaction in benzene.

It should be possible to use indicator methods generally to follow acid-base reac- tions in hydrocarbons. Davies and her collaborators have already made exten- sive use of the phenolic indicator Bromophthalein Magenta E for studying the reaction of weak acids with bases, and we have now extended the work of La Mer and Downes l3 to show that the azoarylamine indicators can be used to follow the reaction of trichloroacetic acid with amine bases and hydrogen-bond donors.

EXPERIMENTAL Benzene (A.R.) was distilled through a column of 10 theoretical plates, washed with sul-

phuric acid and water, dried by distillation and stored over sodium. The halogenated 2034

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J . H . T . BROOK 2035

acetic acids were commercially available products shown to be pure by gas liquid chromato- graphic (GLC) analysis of their methyl esters ; solutions made up in benzene were dried by azeotropic distillation, and the strengths of the solutions measured by titration. p-Ditolyl phosphate, recrystallized from benzene, had the theoretical equivalent weight of 280, and melting point 77-79°C. Dibutyl phosphate, prepared by alkaline hydrolysis of tributyl phosphate had equivalent weight of 200 (theory 210). The indicators were recrystallized from ethanol, and the amines, purified by distillation through a 30-plate column, were shown by GLC to be 99 % pure. They were stored under nitrogen over potassium hydroxide, and distilled immediately before use. Dimethylacetamide (BDH) and trioctylphosphine oxide (Albright and Wilson), were used as received, solutions of the latter being dried by azeotropic distillation.

Spectra in the visible region were measured on a spectrophotometer (Unicam SP600) fitted with a water-jacketed cell holder. Infra-red spectra were recorded with a Grubb- Parsons Spectromaster instrument fitted with water-jacketed cell holders. Standard solu- tions of indicator (usually 2 x lov3 M), acid and base were prepared, with normal precautions to dry the glassware, and guard tubes fitted to the tops of the burettes, but no rigorous precautions were taken to exclude atmospheric moisture. There was no indication that such small amounts of moisture as might be absorbed from the air affected the results.

RESULTS A N D DISCUSSION

BEHAVIOUR OF THE AZOARYLAMINE INDICATORS

The spectra of solutions of the indicator bases, phenylazo-l -naphthylamine (PAlN) and p-aminoazobenzene (PAAB), with various concentrations of acid showed

-4 0 -3 0 - 2 0 ’ -I 0 0

log [HA1 FIG. 1 .-The equilibrium of haloacetic acids with phenylazo-1-naphthylamine in benzene.

absorption peaks corresponding to the presence of the unprotonated (B) and proton- ated (BH+) forms of the indicators l4 at 430 and 540 mpm for PAlN and at 375 and 505mpm for PAAB, with the unprotonated form absorbing at the shorter wave- length. Isosbestic points were found at 480mpm for PAlN and at 430mpm for

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2036 ACID-BASE REACTIONS I N BENZENE

PAAB. The indicator ratio [BH+]/[B] could be determined directly from the absorp- tions at the appropriate two wavelengths, and small corrections were applied for the overlapping of the absorption peaks. To obtain the absorption spectrum of the BH+ forms of the indicators, it was advisable to use ditolyl phosphate as the reference acid, since the wavelength of maximum absorption shifts to longer wavelength in the presence of high concentrations of trichloroacetic acid.

Plots of log [BH+]/[B] against log [HA] were linear over a satisfactory range of acid concentrations (fig. 1 and 2) for all the acids and indicators used. The slopes of the

- 4 0 -3.0 -2.0 -1.0 0

1% [HA1 FIG. 2.-The equilibrium of phosphorus monoacids with phenylazo-I-naphthylamine in benzene.

lines were generally non-integral, though for the reaction between P A l N and tri- fluoroacetic or trichloroacetic acid the slope was exactly 2, implying that the reaction is

B+2HA+B. ZHA, and for the reaction of P A l N with dibutyl or ditolyl phosphate, the slope of the line was 1.50, corresponding to a reaction

The different slopes of the various indicator plots show that the acid-base reactions are of different orders of reaction, and this prevents an exact comparison being made either of the relative strengths of the acids in benzene or of the relative strengths of the indicators, though, qualitatively, the relative strengths fall in the expected order.

2B+ 3(HA),+2B .3HA.

TEMPERATURE-DEPENDENCE OF EQUILIBRIUM CONSTANTS

Experiments with trifluoroacetic, trichloroacetic and dichloroacetic acids and PAlN at five temperatures between 13.5 and 36°C gave results in agreement with the van? Hoff isochore.

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J . H . T . BROOK 2037

The units of the equilibrium constants in table 1 are L2 and of the energy terms, kcal mole-l. The dimensions of K for dichloroacetic acid are more properly 1. mole-l raised to the power 2.16, and the value given is that of the hypothetical K having dimensions of L 2 taken from the experimental points where the indi- cator ratio is unity. The high values of -AH" show that the equilibrium constants

TABLE EQ EQUILIBRIUM PARAMETERS AT 2 5 ° C

loglo K - AFa -AH" -AS"

CF3COOH 5-76 8.0 20-2 41 CCISCOOH 5.54 7.7 22.8 51 CHClZCOOH 3 . 8 7 5-4 16.6 38

have a sharp temperature-dependence. AH is a composite of a heat of neutralization plus an appreciable heat of association of the anion with an extra molecule of acid,15 and the highly negative entropy of reaction is largely due to the association of three molecules to give a single species.

ASSOCIATION OF T H E O R G A N I C A C I D S I N BENZENE

The experiments with indicators are most easily interpreted in terms of the reaction of one molecule of indicator with two or three molecules of monomeric haloacetic acid, but there is no satisfactory agreement 1 o p 1 6 * 2 0 as to the state of aggregation of these acids in dilute solution in benzene. The older data l6 derived from freezing point depressions show that trichloroacetic acid is dimeric over the whole accessible range of concentrations. Recent data, however, suggest that the association constant of trichloroacetic acid in benzene is much lower, 5-20 1. mole-l. Accordingly, infra-red spectroscopy was used to study these equilibria. The absorbances due to the free and hydrogen-bonded carbonyl groups in the 5-6 pm region were measured. The results are given in table 2 and, in agreement with other recent results, show that trichloroacetic acid is essentially monomeric at concentrations not exceeding 0.01 M in benzene.

TABLE 2.-~SOCIA'ITON CONSTANTS FOR HALOACETIC ACIDS IN BENZENE AT 28°C

association constant 0. mole-1) acid

trichloroacetic acid 4.7 dichloroacetic acid 14.5 acetic acid 167

Evidence on the association of phosphorus acids in benzene is sparse, but Pep- pard 21 suggests, from an infra-red study, that dialkyl or diary1 phosphates may well be dirneric in solution even in low concentration.

USE OF INDICATORS TO F O L L O W ACID-BASE REACTIONS I N BENZENE

Although the formation of indicator-acid complexes of varying stoichiometry precludes the use of azoarylamine indicators for the comparison of the strengths of different acids in benzene, the indicators may nevertheless be useful in following the course of acid-base reactions in benzene. In this application, the requirement is that [HA] may be derived from a measurement of the indicator ratio in a mixture of HA

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2038 ACID-BASE REACTIONS IN B E N Z E N E

and added base B. The indicator must react with HA to form the same set of complexes as in the presence of HA alone : the method is not valid if mixed complexes are formed. For example, if a complex InH+A-BHA were formed, this would amount to a direct reaction of the indicator with the acid complex B .2HA acting as an independent acid, In+B .2HA = InH+A-BHA. The formation of such complexes has been reported,1° and seems to be common when HA is a p h e n 0 1 . ~ ~ * ~ ~ According- ly, to test the indicator method, three neutralization reactions were studied by both the indicator method and by infra-red spectroscopy.

0 0 c5 0 10

mine added, mole/l. FIG. 3.Neutralization of trichloroacetic acid in benzene at 25°C ; 0, with triethylamine ; 0 , with diethylamine ; 0 , with butylamine.

To obtain simple, clear-cut neutralization curves, having linear portions at the beginning and end of the neutralization, it is desirable to use relatively strong (0.1M) solutions of acid and of amine, and a weaker indicator base than PAlN. PAAB was satisfactory. The neutralization of trichloroacetic acid by triethylamine, diethylamine and n-butylamine is shown in fig. 3. The concentration of acid drops sharply at first, and the first small additions of base consume three moles of acid per mole of base. Subsequent additions of base consume less acid and, eventually, free acid disappears from the system when an equimolar quantity of amine has been added. The results show that the initial reaction is B + 3HA = B . 3HA, producing an acid- salt complex, and that the final reaction product at complete neutralization has the composition B . HA.

I N F R A - R E D S T U D Y OF ACID-BASE N E U T R A L I Z A T I O N I N B E N Z E N E

The changes in the infra-red absorption spectra of mixtures containing a total of 0.05 mole/l. of trichloroacetic acid and varying amounts of triethylamine can be seen in fig. 4. 0.05 M Trichloroacetic acid exhibits a carbonyl band vco attributable to monomeric acid at 5.62 pm, and a band characteristic of hydrogen-bonded carbonyl

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J . H. T. BROOK 2039

groups at 5.72 pm. There are gaps in the spectrum where the solvent benzene absorbs radiation completely. As triethylamine is progressively added, the 5.62 pm band diminishes until it is barely distinguishable at 40 % neutralization. A band appears at 6-17 pm which increases to a maximum at 35 % neutralization, and then disappears by merging into a new band at 5-97 pm. A band at 7.68 pm arises from about 40 % neutralization.

The 6.17 pm band is ascribed to a hydrogen-bonded carboxylate group and, from its peak intensity at about 33 % neutralization, it can further be attributed to the complex B .3HA. The bands at 5.97 and 7-68 pm having maximum intensity at 100 % neutralization are attributable to the complex B . HA, and they are associated with an asymmetric vibration of the carboxylic group.

L 1 s o 6 5 7 0 7 5 8 0 0 5

wavelength, microns FIG. 4.-Changes in IR spectra as 0-05 mole/l. trichloroacetic acid is neutralized by triethylamine.

An attempt has been made to estimate the absorption at 5-62, 5-97, 6.17 and 7.68 pm. The extent of the absorption at 5.62pm can only be guessed by sketching in the expected shape of the 5-71 pm band, as the 5.62 pm band is too close to a strong benzene absorption band to be measured by any other technique. The height of the 6.17 pm band can be measured, as can that of the 5.97 pm band. An approximate correction to the 6.17 pm absorption can be obtained by considering the absorption of the tail of the 5.97 pm band at 6.17 pm when the acid is completely neutralized, and the absorptions of the 6-17 and 5.8 pm bands at 5-97 pm can be estimated approxi- mately from the spectra at low degrees of neutralization. By a series of successive approximations, the absorptions of the 5-97 and 6-17 pm bands, and their mutual

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2040 ACID-BASE REACTIONS I N BENZENE

overlaps may be calculated. The results of such calculations are expressed in fig. 5, which also shows the rise of the 7.68 pm band.

The spectra obtained when trichloroacetic acid was neutralized by diethylamine

neutralization, % FIG. 5.-Neutralization of trichloroacetic acid (0-095 mole/l.) with triethylamine in benzene (cell

thickness 0.087 mm).

0 4

0 3

h

w 8 .* 0 g 0 2

.- 2 * + 8

01

0

neutralization, % FIG. 6.-Neutralization of trichloroacetic acid (0.10 mole/l.) with diethylamine in benzene (cell

thickness 0.169 mm).

or butylamine are similar, though the peaks are less sharp. Again, it is possible to correct the 6-15 pm band for overlap by the 5.97 pm band and vice versa and, more- over, this then shows that there is an extra absorption of some intensity of 6.02 pm.

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J . H. T. BROOK 2041

Fig. 6 shows a graph of these absorbances as a function of neutralization ; the 6.15 pm band reaches maximum intensity at about 25 % neutralization and the 6-02 pm band at about 50 % neutralization. The results with butylamine are similar.

The appearance and disappearance of the 6.15 or 6-17 pm band is good evidence of the formation of a complex B . nHA, and the appearance of a band at 6.02 pm, heavily overlapped by the strong band at 5.97pm, is perhaps some evidence of a further complex B . mHA. The existence of the complexes B .3HA and B .2HA is to be expected from the results of the indicator experiments, and since the 6.15 and 6.02 pm bands appear to reach maximum intensities at about 25-30 % neutralization and 50 % neutralization respectively, it seems reasonable to attribute them to these complexes. The asymmetric shape of the graph of the intensity of the 6.17 pm band in the neutralization with triethylamine suggests that there is a contribution to this absorption from both the B . 3HA and B .2HA complexes but, in this case, it was not possible to separate the contributions arithmetically.

ESTIMATION OF ASSOCIATION CONSTANTS

Both the indicator and the infra-red techniques show that a range of complexes are formed by the reaction between trichloroacetic acid and amine bases, and this conclusion is confirmed by the work of Bruckenstein and Saito lo who used a method based upon colligative properties. The sequence of reactions involved may be written

2HA = (HA)z HA$-CsH6 = C6H6. . . HA

HA+B = B . HA H A + B . HA = B . 2HA

H A + B . 2HA = B . 3HA. (4) (5)

At low concentrations in benzene, trichloroacetic acid appears to be present in the hydrogen-bonded form CgH6 . . . HA, and as there are no data for the magnitude of K,, it is best to regard this as the standard state. With respect to this state, trichloro- acetic acid associates relatively feebly with itself (Kl /Kz - 5 1. mole-') and more strongly with the complexes B . HA and B .2HA.

TABLE 3.-ASSOCIATION CONSTANTS FOR REACTION OF AMINE WlTH TRICHLOROACETIC ACID

triethylamine diethylamine butylamine

Ks& 1. mole-1

KdKZ 1. mole-1

530 35 30 170 45 80

The infra-red spectral data obtained are of too low an accuracy for the calculation of equilibrium constants, but some rough estimates may be made of K4 and K5 for the aliphatic amines. The concentration of the species B . HA can be determined with adequate precision from the absorptions at 7-5-707 pm, and [HA] can be deter- mined from the indicator ratio. Then, if material balances are taken for the total acid and total base added, [B .2HA] and [B . 3HA] may be estimated. The values for K,, obtained are not strictly constant, but the approximate values obtained (table 3) show the order of magnitude of the constants. The concentrations of B .2HA and B .3HA cannot be too grossly in error, but errors may appear from the assumption that the various species are monomeric in solution. The aliphatic amines are so

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2042 ACID-BASE REACTIONS I N BENZENE

much more basic than the azoarylamine indicators that it is not possible to estimate K3 by the indicator technique.

These experiments show that trichloroacetic acid associates strongly with the trichloroacetate ion in benzene, and the formulation of Bruckenstein and Saito for the B .3HA complex

CC13

C I

"1 \+-

I

CC13COOH. . . 0 0 . . . HOOCCC13 H

+NR3 explains adequately why this complex is stable. The reason for the unusual stoichio- metries observed with the azoarylamine indicators is that these are weak bases which are protonated under conditions where the acid is present in considerable excess over the base, and evidently K4 is of the same order of magnitude as K3. As a result, the neutral salt I n . HA is not observed, and with PAlN the formation of In . 2HA is complete. This leads to the expression [InH+J/[In] = K[HAI2 for the dependence of the indicator ratio upon [HA].

REACTION OF H Y D R O G E N - B O N D DONORS W I T H TRICHLOROACETIC A C I D

Strong organic acids are neutralized by the formation of strong hydrogen-bonded complexes with carboxylate ions, and indicator experiments were conducted with

0 0 2 ri 34 0 C 6 0 08 0 10

H-bond donor added, molell. FIG. 7.-Neutralization of trichloroacetic acid with amide and phosphine oxide in benzene.

other strong hydrogen-bond donors. The curves of the neutralization of trichloro- acetic acid by the hydrogen-bond donors, N,N-dimethylacetamide and tri-octyl- phosphine oxide are shown in fig. 7. The curves are similar to those found for the

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J . H . T. BROOK 2043

amine bases; N,N-dimethylacetamide forms a strong 1 : 1 complex with effectively complete neutralization of the acid and a complex of formula D .2HA is present when the acid is in excess. The phosphine oxide also forms a strong 1 : 1 complex and, a complex D .3HA is formed when the acid is in excess. The intermediate D .2HA may also be formed, but this cannot be demonstrated by the data from the neutralization curve.

These results are similar to those obtained with the aliphatic amines, but one should be cautious before attempting to explain the formation of D . 2HA and D . 3HA along the same lines. Hadzi has demonstrated 24 that there is no proton transfer in the 1 : 1 complexes of acids with phosphine oxides, and if the trichloro- acetate ion is not formed, it would only be possible for one extra acid molecule to complex with D . HA by simple hydrogen bonding. Moreover, since this bond would be formed to a relatively unpolarized C=O group, this bond should be weak, and this seems inconsistent with the results shown in fig. 7. Thus, we are unable to explain the nature of the higher D . nHA complexes on the basis of simple hydrogen bond formation.

The author thanks Dr. D. Stewart, and Messrs. G. J. Boyle, G. B. Nutt and G. Steel for contributions to the experimental work.

I. M. Kolthoff, S. Bruckenstein and M. K. Chantooni, J. Amer. Chem. Soc., 1961, 83, 3927.

J. F. Coatzee and W. S. Muney, J. Physic. Chem., 1962, 66, 89. R. P. Bell and J. W. Bayles, J. Chem. SOC., 1952,1518. J . W. Bayles and A. Chetwyn, J. Chem. Soc., 1958, 2328. J. W. Bayles and A. F. Taylor, J. Chem. SOC., 1961,417. ' R. P. Bell and J. E. Crooks, J. Chem. SOC., 1962,3513.

G. M. Barrow and E. A. Yerger, J. Amer. Chem. Soc., 1954,76, 5211 ; 1955, 77,4474, 6206. G. M. Barrow, J. Amer. Chem. SOC., 1956,78,5803.

' I. M. Kolthoff and M. K. Chantooni, J. Amer. Chem. Soc., 1963,85,426,2195.

lo S. Bruckenstein and A. Saito, J. Amer. Chem. SOC., 1965, 87, 698. l1 M. M. Davis and H. B. Hetzer, J. Res. Nat. Bur. Stand., 1958, 60, 569. l2 M. M. Davis and M. Paabo, J. Amer. Chem. Soc., 1960,82,5081. l3 V. K. La Mer and H. C. Dowaes, J. Amer. Chem. Soc., 1933,55,1840. l4 G. E. Lewis, Tetrahedron, 1960,10, 129. l5 T. E. Mead, J. Physic. Chem., 1962, 66,2149. l6 R. J. W. Le Fevre and H. Vine, J. G e m . SOC., 1938, 1795. '' G. Allen and E. F. Caldin, Quart. Rev., 1953, 7, 255. l8 E. Calvet and C. P. Paoli, Compt. rend., 1963, 257, 3376. l9 E. Constant and A. Lebrun, J. Chirn. physique, 1964,61, 163. 2o D. P. N. Satchel1 and J. L. Wardell, Trans. Faraday SOC., 1965,61,1199. 21 D. F. Peppard, J. R. Ferraro and G. W. Mason, J. Inorg. Nuclear Chem., 1958,7,231. 22 J. Steigmann and P. M. Lorentz, J. Amer. Chem. Soc., 1966,88,2093. 23 I. M. Kolthoff, M. K. Chantooni and S. Bhowmik, J. Amer. Chem. SOC., 1966, 88, 5430. 24D. Hadzi, J. Chem. SOC., 1962, 5128.

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