- Email: [email protected]
A study on the esterification of terephthalic acid
European PolymerJournal, 1972, Vol. 8, pp. 1171-1178. Pergamon Press. Printed in England.
A STUDY ON THE ESTERIFICATION OF T E R E P H T H A L I C ACID F . CAMPADELLI
and C. N1CORA
Montecatini Edison S.p.A., Centro Ricerche Bollate, Via San Pietro, 50, 20021 Bollate (Milan), Italy
(Received 25 November 1971) Akstraet--The esterifications of terephthalic acid and monomethylterephthalate with methanol have been studied in order to compare the reaction rate constants of the first and second carboxyl groups of terephthalic acid. The ratio of the two constants is found to be 1.18 ± 0.07 at 110 °. A value of the Hammett substituent constant for the group --COOCH3 at the para position is reported (acoocn 3
= 0.59). INTRODUCTION IN THE reactions of bifunctional compounds, it is generally assumed that the reactivities of the two functional groups in the same molecule do not differ appreciably when the groups are properly spaced. In the esterification of polymethylene dicarboxylic acids, equal rate constants were found for dibasic acids and for monoesters derived from them when at least one methylene group is placed between the reactive groups, tl) However, when functional groups are separated by a double bond or by an aromatic ring, they may be expected to be interdependent and the reactivity of one site may change after the other has reacted. This is true in the case of the reaction of aromatic diisocyanates with butanol, where the ratio of the rate constants of the first and second isocyanate groups in the para position was found to be 10.0 at 24°33) In other cases, the ratio is much smaller, as in the reaction of terephthalic acid with ethylene oxide, where the ratio of the rate constants for the nucleophilic addition of the first and second carboxylate ions was found to be 1.07 at 80°. C4~ While the present work was being carried out, some data were published giving some indication of the relative reactivity of the carboxyl groups of terephthalic acid during esterification. In a study concerning the esterification of terephthalic acid with ethylene glycol in the absence of added catalysts, ts) it was found that the behaviour of monoglycol terephthalate could be interpreted by assuming that it may react according to different mechanisms, one of which is catalysed by the undissociated form of terephthalic acid. Since terephthalic acid itself, according to the authors, is subject to the same type of catalysis, a comparison between the two second order rate constants should be possible. The value thus obtained for the relative reactivity of the carboxyl groups might, however, be affected by the complexity of the system and by the assumptions made. In the present work, the kinetics of the esterification of terephthalic acid with methanol has been investigated in order to elucidate this point. In the esterification o f dicarboxylic acids, differences in the first and second dissociation constants should be taken into account since they may influence the reaction rates to a different degree, t6) The autocatalytic effect of the carboxyl groups, whatever 1171
1172
F. CAMPADELLI and C. NICORA
m e c h a n i s m o f catalysis m a y be i n v o l v e d , m a y be d i s r e g a r d e d by s t u d y i n g t h e r e a c t i o n in t h e p r e s e n c e o f a n a d d e d a c i d catalyst, so t h a t a n y d i f f e r e n c e in r a t e c o n s t a n t s m i g h t b e a t t r i b u t e d t o a n intrinsic difference in reactivity. EXPERIMENTAL
Reagents Monomethylterephthalate (MMT) was prepared according to Otten tT) and purified by two crystallizations from methanol. Terephthalic acid (TA), fibre grade reagent, was crystallized from methanol using a continuous low temperature method. Methanol, Rudi-Pont reagent grade, was refluxed for 24 hr over magnesium turnings and fractionated on a 22 theoretical plates column. Pyridine, Carlo Erba reagent grade, was refluxed for 24 hr over potassium hydroxide and fractionated on a 22 theoretical plates column. p-Toluenesulphonic acid (monohydrate), Carlo Erba reagent grade, was used without further purification.
Analytical methods Carboxyl groups: when conversion was lower than 60-70 per cent, carboxyl groups were titrated with 0.1 N NaOH to phenolphthalein in a water-pyridine-methanol mixture. For higher conversions, a conductometric titration was adopted in water-pyridine-methanol mixtures using 0.02 N NaOH dissolved in the same mixture. Preliminary experiments showed that the two methods are equivalent in the range of 60--70 per cent conversion. Dimethylterephthalate (DMT): it was determined by a gas-chromatographic method using a 2-metre column packed with methylvinylsilicone SE 31 (Carlo Erba) over Aloport F (HewlettPackard) and using diphenyl as internal standard. Preliminary controls showed that the water formed during the esterification of M MT with methanol does not cause hydrolysis to TA under our conditions. From the analytical data for the carboxyl groups and DMT, conversions with respect to TA and MMT were calculated. Allowance was made for the change of methanol volume with temperature {s) in the derivation of the kinetic constants.
Procedure Reactions were carried out in sealed ampoules of about 30 ml at 90-120 ° using methanol as solvent and reagent concentrations of about 3-5 × 10 -2 eq]l. p-Toluenesulphonic acid concentrations of I eq/100 eq of acid reagent were used. Terephthalic acid was weighed directly into the ampoules; all other reagents were fed as methanol solutions through a burette. Ampoules were filled to about 80 per cent of their volume, frozen in liquid nitrogen, pumped out and sealed. They were stored at --5 o until ready to be used. The beginning of the reaction was taken when the ampoutes, after being preheated in water at 15°, were placed in a thermostatted vessel controlled at +0-2 °. Solutions containing MMT are homogeneous at room temperature; TA was completely dissolved by occasional shaking while heating the ampoules to the bath temperature. The time required to reach the reaction temperature was negligible compared with the overall reaction time. Reactions were stopped by quenching the ampoules at --78 °. They were opened and the contents analysed. Each conversion value is an average of at least three experiments. Preliminary runs showed that conversion is independent of the quantity of the reaction mixture in the ampoule within 20 and 80 per cent of its volume. THEORETICAL P r o t o n s f o r m e d in s o l u t i o n a r e d i s t r i b u t e d a m o n g t h e d i f f e r e n t c h e m i c a l species ( m e t h y l a l c o h o l , T A , M M T , D M T a n d w a t e r ) a c c o r d i n g to t h e r e s p e c t i v e p r o t o n a t i o n c o n s t a n t s . F r o m t h e k n o w n v a l u e s o f the e q u i l i b r i u m c o n s t a n t s f o r a c e t i c a c i d ¢9) in m e t h y l a l c o h o l , it s e e m s r e a s o n a b l e t o n e g l e c t t h e a m o u n t o f p r o t o n s c o o r d i n a t e d t o a c i d a n d ester m o l e c u l e s in c o m p a r i s o n w i t h t h e a m o u n t o f p r o t o n s coordinated to alcohol and water molecules. According to Goldschmidt "°) the
A Study on the Esterification of Terephthalic Acid
1173
effect of the proton distribution between alcohol and water on the reaction kinetics can be taken into account by the use of the ratio r = [CHaOH2 + ] [H20]/[HaO +] which is constant when excess alcohol is used; the value of r depends on the nature of the esterifying alcohol but not on the nature of the acid. Literature values show that r for methyl alcohol increases with temperature in the range 20--60 ° and is 0.56 mole/1, at 60°. t11~ On the other hand, a linear dependence of log r on the reciprocal of the absolute temperature has been reported tll.x2~ and a value of r greater than 5 may be obtained at 1 l0 ° by extrapolation. Since, in our case, the water concentration is not greater than 4 x 10 -2 mole/1., it can easily be seen that less than 1 per cent of all protons are associated with water and their amount can be neglected with respect to the amount of protons associated with methyl alcohol. Furthermore, because of the very low water concentration even at the end of the esterification reaction, the reverse reaction has been neglected. The first and second dissociation constant for T A at 16 ° in water and 1 : 1 watermethanol mixture are known, cl 3~ The dissociation constants of T A in methyl alcohol have not been reported; they can be extrapolated as a function of the dielectric constant of the medium, E, by the use of the relationship t14~ log K, = A/~ + B, where K~ is the ratio of the dissociation constant of TA to the dissociation constant of a reference acid (benzoic acid); A and B are constants. By the use of the known values of the dielectric constants of water-methyl alcohol mixtures t15~ and of the dissociation constants of benzoic acid in such media, t16~ the following values of the first and second dissociation constants of T A in methanol can be obtained: KI = ca. 10 -8, K2 = ca. 10 -11. By solving the equations relevant to the dissociation of a mixture of two acids and assuming: (1) (2) (3) (4)
dissociation constant ofp-toluenesulphonic acid = 2 × 10-2, (6) dissociation constant of T A = 10 -8, p-toluenesulphonic acid concentration = 4 × 10 -4 eq/l., TA concentration = 4 × 10 -2 eq/1.,
it can easily be shown that p-toluenesulphonic acid can be considered as completely dissociated and that the proton concentration derived from T A is negligible with respect to the concentration derived from p-toluenesulphonic acid. The same considerations hold for M M T , as its dissociation constant is less than 10-s. As a consequence of the foregoing considerations, in deriving the kinetic equation for the esterification reaction, the following assumptions were made: (1) (2) (3) (4) (5)
the alcohol concentration is constant throughout the reaction; all protons are associated with methyl alcohol; the effect of water formed during the reaction is negligible; the acid catalyst is completely dissociated; the acid reagents are undissociated.
1174
F. CAMPADELLI and C. NICORA
I f an acyl-oxygen cleavage mechanism is assumed for the reaction, (tT) the results can then be interpreted on the basis o f the equation --dx/dt
= kcx
where x = reagent concentration; c = catalyst concentration; k = rate constant. A linear dependence of the rate constants for TA and M M T on the concentration of p-toluenesulphonic acid in the range 1-9 × 10 -4 mole/l, has been verified, as shown in Fig. 1 where the relative reaction rates are plotted against the ratio of the catalyst concentrations. RESULTS A N D D I S C U S S I O N The esterification of terephthalic acid with methanol gives rise to two consecutive reactions. kl
TA 9- C H a O H
~
> M M T q- H 2 0 kz
M M T + CHaOH <
~ D M T -F H 2 0
where TA, MMT, and D M T mean terephthalic acid, monomethylterephthalate and dimethylterephthalate, respectively.
2"0
4o
a y
I-0
, 0
I I
,
Z 2
Relative c a t a l y s t concentration,
~
I 3
=
C/C o
FIG. 1. Relationship between relative reaction rates and relative catalyst concentrations at 110°. Co = 3- 5 x 10- 4 mole/l.; ro = rate constant when catalyst concentration is 3.5 × I 0-4 mole/L; slope of the straight line: 45°. ([3) MMT, (©) TA. In Fig. 2 the dependence of the concentrations of terephthalic acid, monomethylterephthalate and dimethylterephthalate on time is shown. For a direct determination of the reaction order and of the kinetic parameters o f the first and second carboxyl groups of terephthalic acid, the esterifications ofterephthalic acid and monomethylterephthalate with methyl alcohol were studied separately.
A Study on the Esterification of Terephthalic Acid
1175
2C
._o ,4--
E r0 o o rqx O 5
q IO
5 Time,
I 15
L
hr
FIG. 2. Variation of the concentrations of (O) terephthalic acid, ([]) monomethylterephthalate and (A) dimethylterephthalate as a function of time during the esterification of terephthalic acid.
./
/
1.0
f,'/° i v t~ o I
/ 0.5
0
I0
20
Time,
30
40
50
hr
FIG. 3. Dependence of the extent of reaction (p) on time for terephthalic acid and monomethylterephthalate at 110 °. O terephthalic acid; • monomethylterephthalate
1176
F. C A M P A D E L L I and C. N I C O R A
Figure 3 shows that in both cases the reaction at 110 ° is first order with respect to the carboxyl group, at least up to 90 per cent conversion. The following values for the rate constants can be derived from the slopes of the straight lines: k~ = 12.08 × 10 -2 l./mole sec kz = 5.11 × 10 -2 l./mole sec. Also, data obtained at 90 °, 100 ° and 120 ° for terephthalic acid and monomethylterephthalate fit first order kinetics. Table 1 shows kinetic parameters and standard deviations (8) calculated by the least squares method for both carboxyl groups at different temperatures. TABLE 1. DEPENDENCE OF THE RATE CONSTANTSON TEMPERATURE
Temperature (°C) 90 100 110 120
Terephthalic acid kl × 103 (1. mole -1 sec -~) n 38.8 64.9 120.8 185"7
4 4 8 4
8 × 10 3 0"3 0"7 3"2 4-7
Monomethylterephthalate k2 × I0 a (1. mole -~ sec -~) n c5 × 10 3 17"3 30.6 51.1 84-8
4 4 13 4
0"75 0"73 0"26 2.4
n = n u m b e r o f experiments.
Since one mole of terephthalic acid corresponds to two equivalents o f carboxyl groups, a factor o f 0.5 has to be used in order to change terephthalic acid rate constants from a molar basis into an equivalent basis. The relative reactivity o f terephthalic acid carboxyl groups, i.e. the ratio kl/k2, assumes the value I. 18 4- 0.07 at 110° (confidence limit 90 per cent). Values derived at the other temperatures are slightly different, because of uncertainty due to a smaller number of experiments. If allowance is made for this, no significant dependence of the relative reactivity on temperature is found over the range 90-120 °. F r o m the data by Mareg etal.(5~avalueof 5-7 × 10 -2 kgmole-1 min_iforthe rate constant of the esterification of terephthalic acid catalysed by its own molecule may be averaged at 197°; according to the same authors, the rate constant of the esteritication of monoglycolterephthalate catalysed by terephthalic acid is 2.38 4- 0.081 x 10 -2 kg mole- 1min- ~at the same temperature. If ( 1) any consideration about the mechanism of catalysis is disregarded, (2) differences in density are neglected, (3) a conversion factor of 0.5 is taken into account, the reactivity ratio would be 1.20 in excellent agreement with the value here reported. The activation energies for the reactions of the first and second carboxyl groups can be obtained by plotting log kl and log kz against 1/T°K (Fig. 4). The values 15.1 4- 0.75 kcal/mole and 15.0 4- 0.2 kcal/mole, respectively, are very similar to those for other esterification reactions. "s) No value of the Hammett substituent constant, cr, for the carbomethoxyl group at thepara position has been found in the literature; a value ~coon = 0.265 4- 0.126 for
A Study on the Esterification of Terephthalic Acid
1177
1.3
4--
~,
1.2
o o >(
0.8
o 0 0.4 o
0
i
2-5
~
i
I
I
r
t
I
I
2"6
I
I
i
i
I
2.7
IO 3 X 1"2T*K
FIO. 4. Arrhenius plot for terephthalic acid and monomethylterephthalate. C)Terephthalic acid; [] monomethylterephthalate. t h e c a r b o x y l g r o u p at the p a r a p o s i t i o n is given. (19) By a p p l y i n g the following relationship: log k l / k o
~cooH
log k2/ko
Ocoocn a'
where ko is the rate c o n s t a n t for the acid catalysed esterification o f benzoic acid with m e t h y l a l c o h o l at 110% the value o f Ocoocn3 c o u l d be o b t a i n e d . I f ko is e x t r a p o l a t e d f r o m the d a t a by C h a p m a n et al. (2°) (ko = 0.0693 1. m o l e - ~ sec -~ at l l 0 °) a value ~coocH3 ---- 0- 59 can be d e t e r m i n e d . REFERENCES (l) A. Kailan, Z. phys. Chem. 85, 706 (1913). (2) I. Vancsb-Szmercs/myi, K. Maros-Oreg6r and E. Makay-B/Sdi, Europ. Polym. J. 5, 155 (1969). (3) J. H. Saunders and K. C. Frisch, Polyurethans, Chemistry and Technology, Part I, Chemistry, 157, Interscience, New York (1962). (4) F. Mar~, J. Hetflej~ and V. BaJ~ant, Colln Czech. chem. Commun. 34, 3086 (1969). (5) F. Mar~, V. Bahant and J. Krupi~ka, Colin Czech. chem. Commun. 34, 2208 (1969). (6) I. Vancs6-Szmercs~lnyi and E. Makay-B(Sdi, J. Polym. Sci. P C, 16, 3709 (1968). (7) B. W. Otten, Ind. Engng Chem. 49 (10), 1691 (1957). (8) J. Timmermans, Physico-chemical Constants of Pure Organic Compounds, Elsevier, 302 (1950). (9) H. Zimmermann and J. Rudolph, Angew. Chem. 4, 40 (1965). (10) H. Goldschmidt and O. Udby, Z. Phys. Chem. 60, 728 (1906). (11) A. Hilton, A. Smith and H. Reichardt, J. Am. chem. Soc. 63, 605 (1941). (12) A. G. Gassmann and R. J. Hartman, J. Am. chem. Soc. 63, 2393 (1941). (13) R. Kuhn and A. Wassermann, Helv. chim. Acta 11, 44 (1928). (14) G. Kortum and J. O'M. Bokris, Textbook of Electrochemistry, Elsevier, 328 (1951). (15) J. Timmermans, Physico-chemical Constants of Binary Systems, Interscience, Vol. 2, 163 (1959). (16) A. L. BaccareUa, E. Grunwald, H. P. Marshall and E. L. Purlee, J. org. Chem. 20, 747 (1955). (17) E. N. Gur'yanova, P. I. Saukov, A. I. Kutepova, R. A. Soboleva and N. I. Grishko, Zh. Organ. Khim. 2, 493 (1966). (18) R. J. Hartman and A. M. Borders,./. Am. Chem. Soc. 59, 2107 (1937). (19) H. H. Jaffe, Chem. Rev. 53, 191 (1953). (20) N. B. Chapman, M. G. Rodgers and J. Shorter, J. chem. Soc. (B), 157 (1968). E.rJ. 8/IO--D
1178
F. C A M P A D E L L I and C. N I C O R A
R~sum6---On a 6tudi6 l'est6rification de l'acide t6r6phtalique et du monom&hyl t6r6phtalate par le m6thanol afin de comparer les constantes de vitesse de r6action du premier et du second groupement carboxylique. On a trouv6 que le rapport des deux constantes 6tait 6gal h 1,18 4- 0,07 h 110 °. On donne la valeur de la constante du substituant de Hammett pour le groupement ---COOCH3 en position para (ocoocn3 = 0,59). Sommario--Lo studio delle reazioni di ¢stcrificazione deli'acido tereftalico e del monometiltereftalato con metanolo ha permesso di confrontare le costanti di velocit/t di reazione del primo e del secondo gruppo carbossilico dell'acido tereftali¢o. II rapporto delle due costanti risulta uguale a 1,18 4- 0,07 a 110 °. Viene pure riportato un valore della costante di Hammett per il sostituente - - C O O C H 3 in posizione para (ocoocn a = 0,59). Zusammenfassung--Die Veresterung von Terephthals~iure und von Monomethylterephthalat mit Methanol wurde untersucht, urn die Reaktionsge.schwindigkeitskonstanten der ersten und zweiteu Carboxylgruppe der Terephthals~ure zu vcrgleichen. Das Verhiiltnis der beiden Konstanten betr~igt 1,18 4- 0,07 bei 110°. Fiir die Hammett-Konstante der Gruppe - - C O O C H 3 in para Stellung wird in der Literatur ein Wert yon aCOOCH3= 0,59 angegelaen.
A STUDY ON THE ESTERIFICATION OF T E R E P H T H A L I C ACID F . CAMPADELLI
and C. N1CORA
Montecatini Edison S.p.A., Centro Ricerche Bollate, Via San Pietro, 50, 20021 Bollate (Milan), Italy
(Received 25 November 1971) Akstraet--The esterifications of terephthalic acid and monomethylterephthalate with methanol have been studied in order to compare the reaction rate constants of the first and second carboxyl groups of terephthalic acid. The ratio of the two constants is found to be 1.18 ± 0.07 at 110 °. A value of the Hammett substituent constant for the group --COOCH3 at the para position is reported (acoocn 3
= 0.59). INTRODUCTION IN THE reactions of bifunctional compounds, it is generally assumed that the reactivities of the two functional groups in the same molecule do not differ appreciably when the groups are properly spaced. In the esterification of polymethylene dicarboxylic acids, equal rate constants were found for dibasic acids and for monoesters derived from them when at least one methylene group is placed between the reactive groups, tl) However, when functional groups are separated by a double bond or by an aromatic ring, they may be expected to be interdependent and the reactivity of one site may change after the other has reacted. This is true in the case of the reaction of aromatic diisocyanates with butanol, where the ratio of the rate constants of the first and second isocyanate groups in the para position was found to be 10.0 at 24°33) In other cases, the ratio is much smaller, as in the reaction of terephthalic acid with ethylene oxide, where the ratio of the rate constants for the nucleophilic addition of the first and second carboxylate ions was found to be 1.07 at 80°. C4~ While the present work was being carried out, some data were published giving some indication of the relative reactivity of the carboxyl groups of terephthalic acid during esterification. In a study concerning the esterification of terephthalic acid with ethylene glycol in the absence of added catalysts, ts) it was found that the behaviour of monoglycol terephthalate could be interpreted by assuming that it may react according to different mechanisms, one of which is catalysed by the undissociated form of terephthalic acid. Since terephthalic acid itself, according to the authors, is subject to the same type of catalysis, a comparison between the two second order rate constants should be possible. The value thus obtained for the relative reactivity of the carboxyl groups might, however, be affected by the complexity of the system and by the assumptions made. In the present work, the kinetics of the esterification of terephthalic acid with methanol has been investigated in order to elucidate this point. In the esterification o f dicarboxylic acids, differences in the first and second dissociation constants should be taken into account since they may influence the reaction rates to a different degree, t6) The autocatalytic effect of the carboxyl groups, whatever 1171
1172
F. CAMPADELLI and C. NICORA
m e c h a n i s m o f catalysis m a y be i n v o l v e d , m a y be d i s r e g a r d e d by s t u d y i n g t h e r e a c t i o n in t h e p r e s e n c e o f a n a d d e d a c i d catalyst, so t h a t a n y d i f f e r e n c e in r a t e c o n s t a n t s m i g h t b e a t t r i b u t e d t o a n intrinsic difference in reactivity. EXPERIMENTAL
Reagents Monomethylterephthalate (MMT) was prepared according to Otten tT) and purified by two crystallizations from methanol. Terephthalic acid (TA), fibre grade reagent, was crystallized from methanol using a continuous low temperature method. Methanol, Rudi-Pont reagent grade, was refluxed for 24 hr over magnesium turnings and fractionated on a 22 theoretical plates column. Pyridine, Carlo Erba reagent grade, was refluxed for 24 hr over potassium hydroxide and fractionated on a 22 theoretical plates column. p-Toluenesulphonic acid (monohydrate), Carlo Erba reagent grade, was used without further purification.
Analytical methods Carboxyl groups: when conversion was lower than 60-70 per cent, carboxyl groups were titrated with 0.1 N NaOH to phenolphthalein in a water-pyridine-methanol mixture. For higher conversions, a conductometric titration was adopted in water-pyridine-methanol mixtures using 0.02 N NaOH dissolved in the same mixture. Preliminary experiments showed that the two methods are equivalent in the range of 60--70 per cent conversion. Dimethylterephthalate (DMT): it was determined by a gas-chromatographic method using a 2-metre column packed with methylvinylsilicone SE 31 (Carlo Erba) over Aloport F (HewlettPackard) and using diphenyl as internal standard. Preliminary controls showed that the water formed during the esterification of M MT with methanol does not cause hydrolysis to TA under our conditions. From the analytical data for the carboxyl groups and DMT, conversions with respect to TA and MMT were calculated. Allowance was made for the change of methanol volume with temperature {s) in the derivation of the kinetic constants.
Procedure Reactions were carried out in sealed ampoules of about 30 ml at 90-120 ° using methanol as solvent and reagent concentrations of about 3-5 × 10 -2 eq]l. p-Toluenesulphonic acid concentrations of I eq/100 eq of acid reagent were used. Terephthalic acid was weighed directly into the ampoules; all other reagents were fed as methanol solutions through a burette. Ampoules were filled to about 80 per cent of their volume, frozen in liquid nitrogen, pumped out and sealed. They were stored at --5 o until ready to be used. The beginning of the reaction was taken when the ampoutes, after being preheated in water at 15°, were placed in a thermostatted vessel controlled at +0-2 °. Solutions containing MMT are homogeneous at room temperature; TA was completely dissolved by occasional shaking while heating the ampoules to the bath temperature. The time required to reach the reaction temperature was negligible compared with the overall reaction time. Reactions were stopped by quenching the ampoules at --78 °. They were opened and the contents analysed. Each conversion value is an average of at least three experiments. Preliminary runs showed that conversion is independent of the quantity of the reaction mixture in the ampoule within 20 and 80 per cent of its volume. THEORETICAL P r o t o n s f o r m e d in s o l u t i o n a r e d i s t r i b u t e d a m o n g t h e d i f f e r e n t c h e m i c a l species ( m e t h y l a l c o h o l , T A , M M T , D M T a n d w a t e r ) a c c o r d i n g to t h e r e s p e c t i v e p r o t o n a t i o n c o n s t a n t s . F r o m t h e k n o w n v a l u e s o f the e q u i l i b r i u m c o n s t a n t s f o r a c e t i c a c i d ¢9) in m e t h y l a l c o h o l , it s e e m s r e a s o n a b l e t o n e g l e c t t h e a m o u n t o f p r o t o n s c o o r d i n a t e d t o a c i d a n d ester m o l e c u l e s in c o m p a r i s o n w i t h t h e a m o u n t o f p r o t o n s coordinated to alcohol and water molecules. According to Goldschmidt "°) the
A Study on the Esterification of Terephthalic Acid
1173
effect of the proton distribution between alcohol and water on the reaction kinetics can be taken into account by the use of the ratio r = [CHaOH2 + ] [H20]/[HaO +] which is constant when excess alcohol is used; the value of r depends on the nature of the esterifying alcohol but not on the nature of the acid. Literature values show that r for methyl alcohol increases with temperature in the range 20--60 ° and is 0.56 mole/1, at 60°. t11~ On the other hand, a linear dependence of log r on the reciprocal of the absolute temperature has been reported tll.x2~ and a value of r greater than 5 may be obtained at 1 l0 ° by extrapolation. Since, in our case, the water concentration is not greater than 4 x 10 -2 mole/1., it can easily be seen that less than 1 per cent of all protons are associated with water and their amount can be neglected with respect to the amount of protons associated with methyl alcohol. Furthermore, because of the very low water concentration even at the end of the esterification reaction, the reverse reaction has been neglected. The first and second dissociation constant for T A at 16 ° in water and 1 : 1 watermethanol mixture are known, cl 3~ The dissociation constants of T A in methyl alcohol have not been reported; they can be extrapolated as a function of the dielectric constant of the medium, E, by the use of the relationship t14~ log K, = A/~ + B, where K~ is the ratio of the dissociation constant of TA to the dissociation constant of a reference acid (benzoic acid); A and B are constants. By the use of the known values of the dielectric constants of water-methyl alcohol mixtures t15~ and of the dissociation constants of benzoic acid in such media, t16~ the following values of the first and second dissociation constants of T A in methanol can be obtained: KI = ca. 10 -8, K2 = ca. 10 -11. By solving the equations relevant to the dissociation of a mixture of two acids and assuming: (1) (2) (3) (4)
dissociation constant ofp-toluenesulphonic acid = 2 × 10-2, (6) dissociation constant of T A = 10 -8, p-toluenesulphonic acid concentration = 4 × 10 -4 eq/l., TA concentration = 4 × 10 -2 eq/1.,
it can easily be shown that p-toluenesulphonic acid can be considered as completely dissociated and that the proton concentration derived from T A is negligible with respect to the concentration derived from p-toluenesulphonic acid. The same considerations hold for M M T , as its dissociation constant is less than 10-s. As a consequence of the foregoing considerations, in deriving the kinetic equation for the esterification reaction, the following assumptions were made: (1) (2) (3) (4) (5)
the alcohol concentration is constant throughout the reaction; all protons are associated with methyl alcohol; the effect of water formed during the reaction is negligible; the acid catalyst is completely dissociated; the acid reagents are undissociated.
1174
F. CAMPADELLI and C. NICORA
I f an acyl-oxygen cleavage mechanism is assumed for the reaction, (tT) the results can then be interpreted on the basis o f the equation --dx/dt
= kcx
where x = reagent concentration; c = catalyst concentration; k = rate constant. A linear dependence of the rate constants for TA and M M T on the concentration of p-toluenesulphonic acid in the range 1-9 × 10 -4 mole/l, has been verified, as shown in Fig. 1 where the relative reaction rates are plotted against the ratio of the catalyst concentrations. RESULTS A N D D I S C U S S I O N The esterification of terephthalic acid with methanol gives rise to two consecutive reactions. kl
TA 9- C H a O H
~
> M M T q- H 2 0 kz
M M T + CHaOH <
~ D M T -F H 2 0
where TA, MMT, and D M T mean terephthalic acid, monomethylterephthalate and dimethylterephthalate, respectively.
2"0
4o
a y
I-0
, 0
I I
,
Z 2
Relative c a t a l y s t concentration,
~
I 3
=
C/C o
FIG. 1. Relationship between relative reaction rates and relative catalyst concentrations at 110°. Co = 3- 5 x 10- 4 mole/l.; ro = rate constant when catalyst concentration is 3.5 × I 0-4 mole/L; slope of the straight line: 45°. ([3) MMT, (©) TA. In Fig. 2 the dependence of the concentrations of terephthalic acid, monomethylterephthalate and dimethylterephthalate on time is shown. For a direct determination of the reaction order and of the kinetic parameters o f the first and second carboxyl groups of terephthalic acid, the esterifications ofterephthalic acid and monomethylterephthalate with methyl alcohol were studied separately.
A Study on the Esterification of Terephthalic Acid
1175
2C
._o ,4--
E r0 o o rqx O 5
q IO
5 Time,
I 15
L
hr
FIG. 2. Variation of the concentrations of (O) terephthalic acid, ([]) monomethylterephthalate and (A) dimethylterephthalate as a function of time during the esterification of terephthalic acid.
./
/
1.0
f,'/° i v t~ o I
/ 0.5
0
I0
20
Time,
30
40
50
hr
FIG. 3. Dependence of the extent of reaction (p) on time for terephthalic acid and monomethylterephthalate at 110 °. O terephthalic acid; • monomethylterephthalate
1176
F. C A M P A D E L L I and C. N I C O R A
Figure 3 shows that in both cases the reaction at 110 ° is first order with respect to the carboxyl group, at least up to 90 per cent conversion. The following values for the rate constants can be derived from the slopes of the straight lines: k~ = 12.08 × 10 -2 l./mole sec kz = 5.11 × 10 -2 l./mole sec. Also, data obtained at 90 °, 100 ° and 120 ° for terephthalic acid and monomethylterephthalate fit first order kinetics. Table 1 shows kinetic parameters and standard deviations (8) calculated by the least squares method for both carboxyl groups at different temperatures. TABLE 1. DEPENDENCE OF THE RATE CONSTANTSON TEMPERATURE
Temperature (°C) 90 100 110 120
Terephthalic acid kl × 103 (1. mole -1 sec -~) n 38.8 64.9 120.8 185"7
4 4 8 4
8 × 10 3 0"3 0"7 3"2 4-7
Monomethylterephthalate k2 × I0 a (1. mole -~ sec -~) n c5 × 10 3 17"3 30.6 51.1 84-8
4 4 13 4
0"75 0"73 0"26 2.4
n = n u m b e r o f experiments.
Since one mole of terephthalic acid corresponds to two equivalents o f carboxyl groups, a factor o f 0.5 has to be used in order to change terephthalic acid rate constants from a molar basis into an equivalent basis. The relative reactivity o f terephthalic acid carboxyl groups, i.e. the ratio kl/k2, assumes the value I. 18 4- 0.07 at 110° (confidence limit 90 per cent). Values derived at the other temperatures are slightly different, because of uncertainty due to a smaller number of experiments. If allowance is made for this, no significant dependence of the relative reactivity on temperature is found over the range 90-120 °. F r o m the data by Mareg etal.(5~avalueof 5-7 × 10 -2 kgmole-1 min_iforthe rate constant of the esterification of terephthalic acid catalysed by its own molecule may be averaged at 197°; according to the same authors, the rate constant of the esteritication of monoglycolterephthalate catalysed by terephthalic acid is 2.38 4- 0.081 x 10 -2 kg mole- 1min- ~at the same temperature. If ( 1) any consideration about the mechanism of catalysis is disregarded, (2) differences in density are neglected, (3) a conversion factor of 0.5 is taken into account, the reactivity ratio would be 1.20 in excellent agreement with the value here reported. The activation energies for the reactions of the first and second carboxyl groups can be obtained by plotting log kl and log kz against 1/T°K (Fig. 4). The values 15.1 4- 0.75 kcal/mole and 15.0 4- 0.2 kcal/mole, respectively, are very similar to those for other esterification reactions. "s) No value of the Hammett substituent constant, cr, for the carbomethoxyl group at thepara position has been found in the literature; a value ~coon = 0.265 4- 0.126 for
A Study on the Esterification of Terephthalic Acid
1177
1.3
4--
~,
1.2
o o >(
0.8
o 0 0.4 o
0
i
2-5
~
i
I
I
r
t
I
I
2"6
I
I
i
i
I
2.7
IO 3 X 1"2T*K
FIO. 4. Arrhenius plot for terephthalic acid and monomethylterephthalate. C)Terephthalic acid; [] monomethylterephthalate. t h e c a r b o x y l g r o u p at the p a r a p o s i t i o n is given. (19) By a p p l y i n g the following relationship: log k l / k o
~cooH
log k2/ko
Ocoocn a'
where ko is the rate c o n s t a n t for the acid catalysed esterification o f benzoic acid with m e t h y l a l c o h o l at 110% the value o f Ocoocn3 c o u l d be o b t a i n e d . I f ko is e x t r a p o l a t e d f r o m the d a t a by C h a p m a n et al. (2°) (ko = 0.0693 1. m o l e - ~ sec -~ at l l 0 °) a value ~coocH3 ---- 0- 59 can be d e t e r m i n e d . REFERENCES (l) A. Kailan, Z. phys. Chem. 85, 706 (1913). (2) I. Vancsb-Szmercs/myi, K. Maros-Oreg6r and E. Makay-B/Sdi, Europ. Polym. J. 5, 155 (1969). (3) J. H. Saunders and K. C. Frisch, Polyurethans, Chemistry and Technology, Part I, Chemistry, 157, Interscience, New York (1962). (4) F. Mar~, J. Hetflej~ and V. BaJ~ant, Colln Czech. chem. Commun. 34, 3086 (1969). (5) F. Mar~, V. Bahant and J. Krupi~ka, Colin Czech. chem. Commun. 34, 2208 (1969). (6) I. Vancs6-Szmercs~lnyi and E. Makay-B(Sdi, J. Polym. Sci. P C, 16, 3709 (1968). (7) B. W. Otten, Ind. Engng Chem. 49 (10), 1691 (1957). (8) J. Timmermans, Physico-chemical Constants of Pure Organic Compounds, Elsevier, 302 (1950). (9) H. Zimmermann and J. Rudolph, Angew. Chem. 4, 40 (1965). (10) H. Goldschmidt and O. Udby, Z. Phys. Chem. 60, 728 (1906). (11) A. Hilton, A. Smith and H. Reichardt, J. Am. chem. Soc. 63, 605 (1941). (12) A. G. Gassmann and R. J. Hartman, J. Am. chem. Soc. 63, 2393 (1941). (13) R. Kuhn and A. Wassermann, Helv. chim. Acta 11, 44 (1928). (14) G. Kortum and J. O'M. Bokris, Textbook of Electrochemistry, Elsevier, 328 (1951). (15) J. Timmermans, Physico-chemical Constants of Binary Systems, Interscience, Vol. 2, 163 (1959). (16) A. L. BaccareUa, E. Grunwald, H. P. Marshall and E. L. Purlee, J. org. Chem. 20, 747 (1955). (17) E. N. Gur'yanova, P. I. Saukov, A. I. Kutepova, R. A. Soboleva and N. I. Grishko, Zh. Organ. Khim. 2, 493 (1966). (18) R. J. Hartman and A. M. Borders,./. Am. Chem. Soc. 59, 2107 (1937). (19) H. H. Jaffe, Chem. Rev. 53, 191 (1953). (20) N. B. Chapman, M. G. Rodgers and J. Shorter, J. chem. Soc. (B), 157 (1968). E.rJ. 8/IO--D
1178
F. C A M P A D E L L I and C. N I C O R A
R~sum6---On a 6tudi6 l'est6rification de l'acide t6r6phtalique et du monom&hyl t6r6phtalate par le m6thanol afin de comparer les constantes de vitesse de r6action du premier et du second groupement carboxylique. On a trouv6 que le rapport des deux constantes 6tait 6gal h 1,18 4- 0,07 h 110 °. On donne la valeur de la constante du substituant de Hammett pour le groupement ---COOCH3 en position para (ocoocn3 = 0,59). Sommario--Lo studio delle reazioni di ¢stcrificazione deli'acido tereftalico e del monometiltereftalato con metanolo ha permesso di confrontare le costanti di velocit/t di reazione del primo e del secondo gruppo carbossilico dell'acido tereftali¢o. II rapporto delle due costanti risulta uguale a 1,18 4- 0,07 a 110 °. Viene pure riportato un valore della costante di Hammett per il sostituente - - C O O C H 3 in posizione para (ocoocn a = 0,59). Zusammenfassung--Die Veresterung von Terephthals~iure und von Monomethylterephthalat mit Methanol wurde untersucht, urn die Reaktionsge.schwindigkeitskonstanten der ersten und zweiteu Carboxylgruppe der Terephthals~ure zu vcrgleichen. Das Verhiiltnis der beiden Konstanten betr~igt 1,18 4- 0,07 bei 110°. Fiir die Hammett-Konstante der Gruppe - - C O O C H 3 in para Stellung wird in der Literatur ein Wert yon aCOOCH3= 0,59 angegelaen.