Ind. Eng. Chem. Prod, Res. Dev. 1984, 23,625-629
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Organophosphates: Catalysis of Ligand Exchange Wllllam S. Wadsworth, Jr. Department of Chemistry, South Dakota State Universiw, Brookings, South Dakota 57007
There are three basic mechanisms for ligand exchange at phosphorus in cyclic phosphate esters all of which are dependent upon the leaving group, nucleophile, and catalyst. A one-step SN2 type mechanism which results in inversion is favored by a good leaving group and nucleophiles with low nucleophlcky. Strong (very basic) nucleophiles attack all possible faces of the phosphate ester indiscriminately. In contrast, metal salts which are capable of coordinating to phosphoryl oxygen lead to addition-elimination and overall retention. In this latter instance, lead acetate is particularly effective, not only in directing the stereochemistry, but by greatly enhancing the reactivity of phosphates and in some cases even allowing specific control of ligand exchange.
Introduction Scheme I There is a growing interest in the synthesis of new and novel phosphates, especially syntheses which control both / C V \ CH3C-CH 0-P + S02C12 j ligand exchange and the configuration about the phos\CH:O/ phorus atom (Stec, 1983). This interest is predicated upon c1 the industrial use of phosphates as catalysts and the growing realization that the chirality a t phosphorus may Table I. Chemical Shifts of have a direct bearing upon the reactivity of phospholipids 2-Substituted-5-(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaand of enzymes with phosphate functionality (Tsai et al., p hosphorinanes" 1983;Skarjune and Oldfield, 1979;Rajan et al., 1981). It cis trans has become a challenge for the chemist to design systems R CH3 CHzCl CH3 CHzCl which mimic biological processes and in this content to design phosphates which resemble their biological counOCH3 l.Olb 3.89 1.32 3.61 OH 1.03 3.80 terparts. To do so requires a thorough knowledge of the OCBH6 0.98 3.77 1.29 3.32 mechanisms by which ligand exchange takes place at the OCsHdOCH3 0.90 3.70 1.25 3.32 phosphorus atom and the stereochemical consequences of OCaHdCH3 0.93 3.71 1.28 3.29 the exchanges. OC6H4Br 0.94 3.94 1.30 3.32 For purposes of our studies we have designed and exOC6HdNOz-p 1.01 3.78 1.38 3.40 ploited a system which allows us to follow not only the O C ~ H ~ ( N O ~ ) - O1.03 ,~ 3.78 1.43 3.40 OC (O=)CeH5 0.97 3.75 1.36 3.45 ligand exchange pathway but also the stereochemistry of SCBHS 0.96 3.72 1.19 3.11 exchange without resorting to optically active substances NC6H11C 1.28 3.60 0.98 3.83 and to the uncertainties which very often accompany measurements of optical purity. The cis and trans isomers of 2-substituted-5-(chloromethyl)-5-methyl-2-0~0-1,3,2- a All samples run in CDC13 as solvent. In ppm downfield from TMS. e With amides the amido group is equatorial and the phosdioxaphosphorinanes can easily be distinguished, one from phoryl oxygen axial. OR
cH3Q% OR P=O
CIS
C1CH2F CH 3 -
=
O
trans
the other, by simple NMR measurements (Wadsworth et al., 1973; Wadsworth, 1973). The rings are conformationally immobile with the result that the methyl hydrogens have different chemical shifts than do those of the chloromethyl group. With this tool the course of substitutions at the phosphorus atom has been determined with relative ease (Wadsworth and Tsay, 1974;Wadsworth and Wilde, 1976;Bauman and Wadsworth, 1978). Although there is very little, if any, strain as a consequence of the six-membered ring, it remains to be seen whether our discoveries can be completely extrapolated to acyclic systems. The thermodynamic parameters, i.e., heat of hydrolysis, associated with the phosphorinane system are not unlike those of acyclic phosphates (Wadsworth and Wilde, 1976). Experimental Procedure The starting point for our synthesis is the 2-chlorophosphorinane, a phosphorochloridate (Wadsworth et al., 0196-432 118411223-0625$01.50/0
1973). It is prepared by slow addition of a carbon tetrachloride solution of bicyclic phosphite to sulfuryl chloride also dissolved in carbon tetrachloride (Scheme I). The order of addition is important for reverse addition leads to a polymeric intractable material. The structure and configuration of the product are deduced from analysis and spectra and are a consequence of the mode of formation. Various esters and amides are prepared by treatment of the chloridate with alcohols, phenols, or amines under conditions reported in previous publications (Wadsworth, 1973; Wadsworth and Tsay, 1974;Wadsworth and Wilde, 1976;Bauman and Wadsworth, 1978). Where mixtures of isomers are obtained, they can often be separated by simple fractional recrystallization. The products in all cases are crystalline stable solids. We have used the variation in chemical shifts of hydrogens at the 5 positions to distinguish between isomers and in turn as a diagnostic tool in our study of substitution. The hydrogens of an axial chloromethyl group are shifted downfield from those of an equatorial chloromethyl group. Likewise, the methyl hydrogens when axial are shifted downfield relative to those of an equatorial methyl group, Table I. 0 1984 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
Scheme I1
Table 11. Catalysis of the Methanolysis of cis -2-Chlorophosphorinane by Metal Salts. Isomer Ratios metal salt" inversion, 90 retention, %
Solution 0.1 M in both reactant and metal salt. Reactions run at room temperature.
6CH3
Mechanisms of Ligand Exchange Although ligand exchange mechanisms do overlap, we have, primarily with examples in which overlap is held at a minimum, concluded that substitution occurs by three basic routes. Direct backside attack by a nucleophile leads to inversion, a route which requires a good leaving group and weakly basic nucleophile. Attack by a strong negatively charged nucleophile leads to attack at three and possibly all four faces of the tetrahedral phosphate. Although we have numerous examples of these two routes, perhaps the most dramatic is the methanolysis of the cis-2-chlorophosphorinane(Scheme 11). Under neutral conditions, exclusive inversion is observed while methanolysis under basic conditions proceeds at a much faster rate and leads to exclusive retention. Apparently, as described below, retention in the latter case is observed as a consequence of the ring system and the lack, due to steric or electronic effects, of attack by the nucleophile at a position opposite the leaving group. Unshared electrons on all three oxygen atoms make attack of the negatively charged strongly basic nucleophile at this face unfavorable. Indeed, we have shown with experiments carried out in aprotic solvents with para-substituted phenoxide ions, that as a negative charge is dispersed, inversion is enhanced (Wadsworth, 1973). That methoxide ion attacks indiscriminately at at least three of the four possible phosphate faces was proven by a ring opening-closure sequence, (Scheme 111) (Wadsworth and Wadsworth, 1983). The methoxide ion displaces the ring, not the more basic isopropoxide ion, to give an isolatable and fully characterized acyclic phosphate. The latter ring closes in the presence of a stronger base to give, irrespective of the starting pure isomer, a mixture of isopropoxy ester isomers in a ratio identical with the thermodynamically controlled ratio, a ratio previously established by allowing a pure isomer of a reactive ester, i.e., p-nitrophenoxy, to come to equilibrium with its counterpart (Wadsworth, 1973). In all cases the cis isomer, chloromethyl group axial, predominates. To account for the lack of stereospecificity, it can be shown that methoxide ion must produce a set of diastereomers, not just a single pair of enantiomers, upon ring
opening. To do so, attack at three of the four faces, even assuming polytopal rearrangements (Westheimer, 1968; Gillespie et al., 1973),is required. Subsequent ring closure, also by a charged nucleophile, must likewise take place indiscriminately with the most thermodynamically stable isomer produced in excess regardless of the configuration of cyclic starting material. Metal Acetate Catalyzed Alcoholysis The third mechanism for ligand displacement entails an addition-elimination much like that observed for the hydrolysis or transesterification of carboxylic esters. With phosphates the procedure is strongly catalyzed by metal ions, ions capable of complexing with phosphoryl oxygen (Scheme IV). For methanolysis, we have found the most effective catalyst by far to be lead acetate (Wadsworth and Wadsworth, 1983). With a good leaving group, i.e., chloride ion, we were surprised in our early studies to find that lead acetate is most effective in diverting methanolysis under nonbasic conditions from inversion to partial retention (Table 11). Silver, zinc, and mercury salts, other than their acetates, act as electrophilic catalysts and greatly accelerate the normally slow uncatalyzed inversion process whereas as acetates mechanisms are partially diverted to give products of retention. Lead as its acetate is a powerful catalyst for the alcoholysis of phosphate esters, a property which is most likely due to its acidic, in the Lewis sense, properties and as a consequence its ability to coordinate with phosphoryl oxygen. Mercuric acetate does not coordinate well, whereas zinc acetate may be too effective. The latter is so tightly coordinated to solvent molecules that its effectiveness toward phosphoryl oxygen is held to a minimum. The effectiveness of lead acetate is unusual, for it catalyzes alcoholysis even when a good leaving group is not present and under circumstances where reaction does not ordinarily occur, i.e., methanolysis of phosphate triesters under neutral conditions. In such cases, for example the lead acetate catalyzed methanolysis of a 2-methylphosphorinane ester, ring opening is the sole route (Scheme V) (R-CHJ. As with the chloridate, the catalytic effect
Scheme I11 ? C H ( CH3)2 CH30CH3
trans
k = O . 154i;r-'
il
FH3 H°CH2CCH20P~OCH CH2C1O C H ( C H 3 ) 2
diastereomers
1
C1 CH2& o $ = O CH3
40
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 627 Scheme IV 0
Scheme VI1 P
CH,C1
Scheme V
Table 111. Effect of Metal Acetate Catalysis on Methanolysis of Phosphorinane Esters retention, inversion, R t1/% h % % cat.a OC6H4-pNo2 CH,COO-Na 97 100 0 73 27 190 OCBHI.PN~Z Hg(CzH30z)z 70 30 Mg(CzH302)2.4HZO 31 OC&.pNOz OC6H4.pN02 Zn(CzH30z)2.3Hz0 7 42 58 OC6H4.pNO2 Pb(C2H302)2*3HzO 1.25 12 88 52 48 Mg(CzH302)2*4H20 38 OC6H4pCOCH3 OC6H4Pb(CzH302)2.3H20 3.4 13 87 pCOCH3
Scheme VI
CH C1 c H 3 e : 3 = 0
PbZt CH30H ~ C ~ H ~ O C H ~
,p
Reactions 0.1 M in ester and catalyst.
y2C' HOCHzCCH20P(OCH3)2
Table IV. Effect of Metal Ions on the Stereochemical Pathway. The Reaction of cis-2-Chlorophosphorinane with p -Methoxyphenol F retention, inversion, of lead acetate is superior to that of zinc acetate, Figure solventa added salt % % 1, whereas mercuric acetate is ineffective. CHSCN 0 9 91 As seen in Figure 1,both cis and trans methyl esters are Mg(C104)z(1 equiv) 0 100 CH3CN CHXN KClO, (1 eauiv) 0 100 catalyzed at the same rate. Also, methanolysis of the CH~CN (CzH5jzN+(H)38 62 corresponding thio methyl esters, cis or trans-2-methCHzC6H5C1-(1 equiv) oxy-5-(chloromethyl)-5-methyl-2-thio-1,3,2-dioxaphos- CHICN LiCl (1 equiv) 88 12 phorinane, follows curve A, all of which is evidence for our CHBCN LiC10, (1 equiv) 96 4 purposed mechanism; the rapid equilibrium formation of tetrahydrofuran LiC104 (1 equiv) 79 21 an adduct followed by a rate-determining departure of the CHSCN LiC104 (l/zequiv) 88 12 CH3
leaving group (Scheme IV). That lead acetate catalyzed attack takes place directly opposite the phosphoryl oxygen bond is seen not only by the stereochemical results with a good leaving group (a diversion toward retention, Table 11)but also by means, as before of a ring opening-closure sequence (Scheme VI). Initially, ring opening is preferred to loss of the p-methoxyphenoxide ion; sequentially the latter ion is lost via ring closure to give the stable methyl esters. If attack occurs opposite the phosphoryl oxygen only, it can be shown that starting with a single isomer, either A or B, in each case only one set of enantiomers of C is produced which ring close, also by attack opposite the phosphoryl oxygen, to give product. The trans starting material must produce only the methyl ester of opposite configuration, D, while B produces only E. With both starting isomers the reactions can be easily followed by NMR, the acyclic intermediate, C, detected and the stereochemistry of the cyclic methyl esters ascertained. The reactions are entirely stereospecific as proposed (Wadsworth and Wadsworth, 1983). Whereas the final product is the dimethyl ester, F, D only is detected from A and E only from B. Again, pseudorotational permutations may or may not occur but, if so, they do not alter the argument. The structure of the intermediate trigonal bipyramid (Scheme IV)is not known with certainty, and thus whether the negative oxygen atom assumes an apical or basal position either initially or as a result of a permutation. Lead acetate has the ability not only to greatly increase the rate of alcoholysis and to direct substitution to retention but also to allow the displacement of ligands se-
Solutions 0.1 M in reactants. Reactions run at room temperature.
lectively. This little investigated asset may be most important and its ramifications are currently being pursued. In a single example, whereas proton catalyzed methanolysis of a 2-amidophosphorinane leads, by means of initial protonation at nitrogen, exclusively to P-N bond cleavage with inversion, lead acetate catalysis, by means of phosphoryl oxygen complexation, leads exclusively to ring opening by retention (Scheme VII). By "Ligand Selectivity" a number of new and unique phosphates should be capable of synthesis. As proposed, lead acetate is, in alcohol, a strong Lewis acid. It can complex not only with phosphoryl oxygen but also, as expected, with other basic groups as well. Thus, when the phosphate contains a p-nitro or p-carbonylphenoxy group, lead acetate a d s as an electrophilic catalyst by complexing with the nitro or carbonyl group. The reaction is diverted to direct displacement and inversion (Table 111) (Wadsworth, 1981). Again, lead acetate is most effective. Catalysis in Aprotic Solvents In aprotic solvents such as acetonitrile, benzene, etc., lead acetate is insoluble and thus not capable of acting as a catalyst and diverting substitution to retention. Under aprotic conditions we find that all soluble positive ions, with the exception of highly hindered quaternary salts, have an effect, with some ions more effective than others. In various solvents, with triethylamine as HC1 scavenger,
628
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 /&
n
% Reaction
/
5o
14
za
30
yo
so
60
io
m
90
dm
/IO
mo
NO
/vo
iro
/do
,70
I*
Hours
Figure 1. Lead acetate catalyzed methanolysis of 2-substituted phosphorinane esters: (A) cis- and trans-methyl; (B) cis- and trans-isopropyl; (C) cis- and trans-methyl catalyzed by zinc ion. Solutions 0.1 M in ester and catalyst. Reactions carried out at room temperature. Table V
added salt
0 KCIO, ( 4 equiv) LiC10, (1equiv) LiClO, ( 2 equiv) LiClO, ( 6 equiv) 18-crown-6 ( 2 equiv)b CH,C, H,O-N*( CH, ), b , c
time reaction, min
3 1
14 21 36
retention inversion (cisltrans) 1.o 1.o 4 .O 10.0 10.0 0.47
0.25
a Solutions 0.1 M in reactants. Ratios obtained by allowing reaction mixtures t o stand for 15 min before workup. Reactions are complete. Used in place of CH,C,H,OK.
Scheme VIII C1I.CI
the phosphorochloridate undergoes substitution with p methoxyphenol to give products of both retention and inversion (Scheme VIII). Added salts have an effect on product ratios with lithium ion being the most effective in diverting the mechanism (Table IV). A similar result is obtained in dimethylformamide (DMF) using a p nitrophenyl ester and potassium salt of the nucleophile (Table V) (Wadsworth and Wilde, 1976). The low retention/inversion ratio obtained in the presence of the crown ether is understandable. The ether removes the potassium ion, thus limiting its ability to complex with the phosphoryl oxygen. It is perhaps obvious that if the influence of the positive ion could be completely eliminated, inversion by direct substitution only would result. Lithium ion slows down the rate by forming a more covalent bond with the negative nucleophile than does potassium ion; it reduces
the nucleophilicity of the nucleophile. Conclusions In summary, our extensive experiments in this area shed light on some of the confusion found in the literature and brings out the importance of the positive counterion in any study pertaining to substitutions at phosphorus in phosphate esters. By means of our studies with the phosphorinane system we are approaching our goal to be able in the case of organophosphates to carry out ligand exchange both in a selective and stereospecific manner. Although much remains to be studied, our work to date, particularly catalysis by lead acetate, points the way. Acknowledgment Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. The author also acknowledges the Colorado State University Regional NMR Center supported by National Science Foundation Grant CHE 78-18581. Registry No. AgN03, 7761-88-8; ZnC12, 7646-85-7; HgC12, 7487-94-7;Hg(OAc),, 1600-27-7;~ ( O A C 557-34-6; )~, Pb(C&IsO2)2, 7319-86-0; Pb(OA&, 301-04-2; CH3COO-Na, 127-09-3; Mg(C2H302)2, 142-72-3; Mg(C104)2, 10034-81-8; KC104, 7778-74-7; (CzH6)2N+(H)CH2CBHsCl-, 51834-90-3;LiC1,7447-41-8;LiC104, 7791-03-9; CH3CBH40-N+(CH3)4, 63738-98-7; 18-crown-6,1745513-9; cis-2-chloro-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, 28097-07-6; 4-methyl-2,6,7-trioxa-l-phosphabi-
Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 629-634
629
cyclo[2.2.2]octane,1449-91-8; trans-2-chloro-5-(chloromethyl)-5- 36912-42-2; cis-2-thiophenoxy-5-(chloromethyl)-5-methyl-2-oxo21071-81-8; cis-2-meth- 1,3,2-dioxaphosphorinane, methyl-2-oxo-l,3,2-dioxaphosphorinane, 36912-43-3; trans-2-thiophenoxy-5(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, (chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, oxy-528097-12-3; trans-2-methoxy-5-(chloromethyl)-5-methyl-2-oxo- 36912-44-4; cis-2-(pentylamino)-5-(chloromethyl)-5-methyl-21,3,2-dioxaphosphorinane, 36912-27-3; cis-2-hydroxy-5-(chlorooxo-1,3,2-dioxaphosphorinane,92366-30-8;trans-2-(pentylmethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, 36912-29-5; amino)-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphoricis-2-phenoxy-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphos-nane,92366-31-9; trans-2-(4-acetylphenoxy)-5-(chloromethyl)-5phorinane, 36912-30-8; trans-2-phenoxy-5-(chloromethyl)-5- methyl-2-oxo-1,3,2-dioxaphosphorinane, 92366-32-0. methyl-2-oxo-1,3,2-dioxaphosphorinane, 36895-18-8; cis-2-(4methoxyphenoxy)-5-(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxa- Literature Cited Bauman, M.; Wadsworth, W. S.,Jr. J . Am. Chem. SOC. 1978, 100, 6380. phosphorinane, 36912-31-9;trans-2-(4-methoxyphenoxy)-5P.; Ramirez, F.; Ugi, 1.; Marquarding, D. Angew. Chem., Int. Ed. (chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, Gillespie, Engl. 1973, 12, 91. 36912-32-0;cis-2-(4-methylphenoxy)-5-(chloromethyl)-5- Rajan, S.; Kang, S.;Gutowsky, H.; Oidfield, E. J . 8/01, Chem. 1981, 256, 1160. methyl-2-oxo-1,3,2-dioxaphosphorinane, 36912-33-1; trans-2-(4methylphenoxy)-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxa- Skarjune, R.; Oidfieid, E. Bbchemlstry 1979, 78, 5903. Stec, W. J. Acc. Chem. Res. 1983, 16, 411. phosphorinane,36912-34-2; cis-2-(4-bromophenoxy)-5-(chloro- Tsai, M.; Jlang, R.; Bruzik, K. J . Am. Chem. SOC.1983, 105, 2478. methyl)-5-methyl-2-oxo-1,3,2-dioxaphosporinane, 36912-35-3; Wadsworth, W. S.,Jr.; Larsen, S.;Horten, J. L. J . Org. Chem. 1983, 38, 256. trans-2-(4-bromophenoxy)-5-(chloromethyl)-5-methyl-2-oxoWadsworth. W. S.,Jr.; Tsay, Y. G. J . Org. Chem. 1974, 39, 984. 1,3,2-dioxaphosphorinane,36912-36-4; cis-2-(4-nitrophenoxy)- Wadsworth, W. G.; Wadsworth, W. S., Jr. J . Am. Chem. SOC. 1983, 105, 5-(chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, 1031. 36912-37-5;trans-2-(4-nitrophenoxy)-5-(chloromethyl)-5- Wadsworth, W. S., Jr.; Wllde, R. L. Chem. Commun. 1978, 93. methyl-2-oxo-1,3,2-dioxaphosphorinane, 36912-38-6; cis-2-(2,4- Wadsworth, W. S.,Jr.; Wilde, R. L. J . Org. Chem. 1976, 41, 1264. W. S., Jr. J . Org. Chem. 1973, 38, 2921. dinitrophenoxy)-5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxa- Wadsworth, Wadsworth, W. S.,Jr. "Phosphorus Chemistry"; ACS Symp. Ser. 1981, No. phosphorinane, 36912-39-7;trans-2-(2,4-dinitrophenoxy)-5171, 548. (chloromethyl)-5-methyl-2-oxo-l,3,2-dioxaphosphorinane, Westheimer, F. H. Acc. Chem. Res. 1968, 1 , 70. 36912-40-0; cis-2-benzoyloxy-5-(chloromethyl)-5-methyl-2-oxo1,3,2-dioxaphosphosphorinane, 36912-41-1; trans-2-benzoyloxyReceiued for reuiew February 8, 1984 5-(chloromethyl)-5-methyl-2-oxo-1,3,2-dioxaphosphorinane, Accepted August 3,1984
Byproduct Identification in the Terephthalic Acid Production Process and Possible Mechanisms of Their Formation Paolo Roffla," Plerangelo Callnl, and Lulgl Motta Montedipe S.p.A., Research Center of Boliate, Via S.Pletro 50, 20021 Boliate MI, Italy
Serglo Tontl Montedlpe S.p.A., Research Center of Port0 Marghera, Via dell'ElettricltiS 4 1, 30 175 Port0 Marghera VE, Italy
Many byproducts are formed in the p-xylene oxidation process to terephthalic acid and they are present in the mother liquor solution. Their identification has been carried out by HPLC and GLC-MS. W e have found that most of these by products are derivativesof benzoic acid,phenol,terephthalic acid,diphenyl,fluorenone,benzophenone, anthraquinone,and aromatic esters. The formation of these compounds is due to radical or ionic reactions occurring in the oxidation process among p -xylene(and its impurities),oxidation intermediates,reaction solvent,and the catalytic system. Some possible mechanisms assumed for these side reactions are given.
Introduction The degree of purity of the terephthalic acid used in polyester fiber production must be very high and comparable to that of dimethyl terephthalate,which, unlike terephthalic acid,is easily purified by crystallizationwith commercial solvents and by distillation under specified conditions. Terephthalic acid is usually produced by oxidation of p-xylenein acetic acid solution at temperatures of 180-230 OC using Co,Mn, and Br as componentsof the catalytic system (Mid-CenturyProcess) (Burney et al., 1959; Vora et al.,1977; Nippon Chem.Consul.Inc.,1980). The impurities present in the terephthalic acid are formed through side reactions in the p-xyleneoxidation; they CM coprecipitate during the reaction or be embodied in the product in the final step of the mother liquor separation. The nature of these byproduds and their possible 0196-4321/04/1223-0629$01.50/0
presence as impurities depend both on the operating conditions adopted for the oxidation (i.e.,temperature, reaction time,catalytic system, etc.) and the process technology selected. The impurities present in the terephthalic acid used in polyester fiber synthesis slow down the polymerization rate or decrease the average molecular weight of the polymer. This is mainly due to the presence of monofunctional compounds,such as benzoic acid or p-toluicacid,but some other impurities can also cause coloring of the polymer as a consequence of their thermal instability during polyesterification (Nippon Chem. Consul. Inc.,1975). These by-productsare always present in the liquid reaction phase. Being highboiling products, they tend to build up in the solventwhen the latter is recycled without purification. The result is that the terephthalic acid can 0 1984 American Chemical Society
