ESTER I FI CA TI ON E. EMMET REID PO3 EAST 33RD ST., BALTIMORE 18, MD.
The most obvious trend during 1 9 5 0 in the field of esterification is to increase production, particularly of plasticizers and alkyds. Plasticizers have gone from 30,000,000 pounds in 1 9 3 9 to 190,OOO,OOO p o u n b in 1949 (733) and are still going UP. The quest is for new materials and new combinations to be used in old processes which, however, are being modified to take care of the less reactive materials. As mentioned in previous review, more and more use is being made of transesterification. In the esterification of cellulose the emphasis is on fundamental investigations of the factors involved (98). Great ingenuity i s being shown in making esters from starting materids other than alcohols and acids. M 8 n y more references are given than in previous yews, although a smaller proportion of those available have been put in. For centuries mankind has been eatinq esters. The bie news is that we will soon be wearing them. Fabrics are to be woven of polymeric ethylene terephthalate.
-
A
-
TTENTION has been given to the kinetics of esterification. In the acid-catalyzed esterification of the dimethylbenzoic acids in methanol the velocities for the 2,3-, 2,4-, and 2 , s acids are about one thud of that for benzoic and for the 2,6too small to measure. Two methyl groups in the 3,4- or 3,5positions make little difference (159). The primary and secondary reaction rates, K1 and Kz, for the three pairs of cis- and tramcyclohexanedicarboxylic acids have been measured. The K 1 values for the 1,3-and 1,4-acids are of the same order as that for hexahydrobenzoic acid but those for the 1,Z-acids are much smaller (168). Equilibria determinations for 85% lactic acid a n d methanol seem to show that K increases from 2.39 t o 4.96 as the ratio of methanol to the acid is lowered from 7.5 to 1 to 0.90 t o 1 (170). The polycondensation of alkyldicarboxylic acids Kith glywls follows the kinetics of Flory: Alkyl groups reduce the probability factor (73). The reactions of phthalic and tetrachlorophthalic anhydrides with pentaerythritol a t 180O C. are bimolecular. The phthalic reacts 53 times as fast as the tetrachloro (124). Hydrolysis rates for several alkyl acetates and benzoates have been determined in watrr and in 60% ethyl dcohol (86). The hydrolysis of tallow, coconut oil, and peanut oil is complete in 0.5 hour at 280" C. but requires 2.5 hours a t 225' C. (163). In the hydrolysis of coconut oil a t 200" C. intermediate products, supposed to be diglycerides, separate out. The rate of hgdrolysis of palm oil is altered only 'slightly a8 the ratio of oil and water changes from 1 to 2 (188). The hydrolysis of I-menthyl formate is recommended aa a laboratory experiment to illustrate kinetics and equilibrium, The reaction is rapid and can be followed by change in rotation. It is of the second order (160). The alkiiline hydrolysis of swollen cellulose acetates has been studied (128). The products of the partial saponification of polyvinyl acetate have been investigated (145). The addition of 1% of phenol, or a-naphthol, hastens the saponification of cottonseed oil (129,130). The oxygen isotope 0 1 8 has been used to show the presence of an intermediate involving the solvent in the hydrolysis of an ester (6). From the heats of formation of organic silicon compounds, it has been shown that the formation of methyl and ethyl silicate esters from silicon tetrachloride, or trichlorosilane, with alcohols is endothermic (137). By heating tall oil with pentaerythritol a t 180" C. for 6 hours the fatty acids are completely esterified, while the resin acids are untouched. The resin acids react rapidly at 250' to 275 O C. (38). The two types of acids may be separated quantitatively by boiling tall oil with a 50 to 50 mixture of benzene and butyl
alcohol and an mid catalyst with water take-off (96). The esterification of fatty acids by glycerol a t 2700 C . is not selective (81). The hydrolysis of methyl methanetrisulfonate in aqueous hydrochloric acid is pseudounimolecular (I@).
SPEEDING UP ESTERIFICATION
Catalysis by lipase is more efficient the longer the alkyl of the alcohol but is ihdependent of the acid (109). The strong acids, trifluoroacetic (13) and trichloroacetic ( I @ ) , require no additional catalyst. Subtilin has to be esterified below 50' C. A mineral acid swvw as catalyst (10, 93). The same is true of d-pinonic acid (62).
The ultimate in catalysts seems to be trifluoroacetic anhydride. When it is added to a mixture of an alcohol and fatty acid, esterification takes place spontaneously, with evolution of heat, and is complete by the time the mixture cools. It is particularly suitable for acylating acid-labile glycosides (8). The addition of phosphorus pentoxide to a methanol solution of methacrylic acid is said to give 98 to 99% of the ester (26). There is one mention af the use of an ion exchange resin in esterification (147). The usual acid catalysts are still popular, but space iF not available for even listing references to them. For high temperature esterifications and transesterifications various catalysts are used. Numerical data are still lacking but the impression is that these are of value, making the esterification go several times faster than i t would otherwise. A number of these are mentioned: ealcium hydroxide (66),calcium naphthenate (M), lead oxide (106, for),tin chloride (106), zinc oxide (138, 182), zinc stearate (183), clay (164),and an emulsifying agent (160). Boric acid is used in splitting glycerides ( 4 ) .
ESTERIFICATION PROCEDURES V A P O R PHASE
An alkyl acetate is hydrolyzed when ita vapor mixed with steam is passed over cadmium, zinc, or silver acetate on a s u p ' port, a t 230" to 300" C. (54). NITRILE T O ESTER
GChIorovaleronitrile is converted to the ester by refluxing i t in methanol saturated with hydrogen chloride (76). ALCOHOLYSIS
Rates and equilibria have been determined in the alcoholysis of olive oil by ten alcohols with 0.1 N sodium hydroxide as catalyst ('77). T o reproduce soaps exactly, blending purified methyl esters prepared by alcoholysis of glycerides is recommended (68). Alkyl esters of fatty acids can be obtained from low grade fats (98,111, 168). Powdered soy beans suspended in 0.05 N alcoholic alkali, a t room temperature, give a good yield of the ethyl esters (71). Various lactic esters are made from
1942
September 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
methyl lactate by alcoholysis ( 4 1 , 132, 135). Diallyl carbonttte can be made from ethyl carbonate with sodium ethylate as c a t a l p t (8). Alkyl zirconates are subject to alcoholysis (176). Polyvinyl alcohol is prepared from the acetate by alcoholysis with either acid (69) or alkaline catalyst ( 5 2 ) . Diethylaminoethyl esters are obtained from et.hy1 esters by alcoholysis with sodium ethylate as catalyst (151). Lime is the catalyst for making diglycerides from linseed oil and glycerol heated for an hour at 230' C. (174). Nitroalkyl acrylates, methacrylates, and crotonates can be prepared from the corresponding methyl esters with an acid catalyst (100). Sulfuric acid is used in making 8-hydroxypropionic esters from the linear polyester from 8propiolactone (63). Aluminum ethylate ( 2 7 ) , zinc (82), and zinc tetradecylmalonate (46) have been used as catalysts at higher temperatures. Pyrophoric lead was found to be superior t o litharge (101). Vinyl acetate and adipic acid give vinyl adipate when heated to 80' C. €or 20 houw, in presence of a polymerization inhibitor, with cuprous chloride and hydrogen chloride (60). This looks like ordinary acidolysis but vinyl interchange appears t o proceed through dissociation back t o acetylene which then combines with the other acid (1). ACID ANHYDRIM
The esterification of hydroquinone by acetic anhydride is catalyzed by a strong acid such a8 perchloric ( 1 5 ) . Diethylene glycol is acetylated by acetic anhydride in pyridine (94). Castor oil is acetylated continuously by passing it down a column heated to 190" C., against acetic anhydride vapor coming up (136). ACID CHLORIDES
Since the treatment of a n alcohol with an acid chloride k standard practice only a few selected examples are given. An ester is made from 2-phenyl-6-benzothiamolecarboxylic acid chloride and Zdiethylaminoethanol (48). Picryl methacrylate is from the acid chloride and picric acid Nicotinyl chloride is used with alcohols (184). A large number of potential plmticizers have been made from esters of lactic acid and chloroformates of alcohols (134) or of glycols (133) The hexa ester can be made from laumyl chloride and mannitol in pyridine (110). A number of testosterone esters have been prepared by using the acid chlorides in pyridine (105). Stearic and benzoic esters of polyvinyl alcohol are obtained when it k treated with the acid chlorides in pyridine. The benzoate is better than that made with sodium hydroxide (111). In making alkyl silicates from silicon tetravhloride and alcohols, the yields and qualities are improved by saturating the alcohol with hydrogen chloride before mixing (172). The acetates of methyl, ethyl, isobutyl, and isopropyl mercaptans (thiola) are made by treatment with acetyl chloride alone, but krt-butyl mercaptan requires the addition of pyridine ( 1 4 ) . Phenol dissolved in lauric acid is chlorinated. Phosphows oxychloride is added to produce the laurate of the chlorinated phenol (65).
(e).
1943
symmetrical ester results from the reaction of 2-chlorocyclohexanone and a potassium salt of a monoalkyl ester of a dibasic acid (87). Methyl-2,4,6-tribromobenzoic acid is made from the potassium salt and dimethyl sulfate (14). This hindered acid cannot be esterified in the usual way. Methyl angelate made from the potassium salt is pure, while t h a t from direct esterification is not ( 1 2 ) . .4n aldehyde is oxidized by silver oxide and the resultant silver salt treated with ethyl iodide (179). The chlorides, ClCH2(CH,CH,),CH,CC1, and CICH2(CH2CH2),CH:CC12, are converted into esters by reaction with sodium salts (148). Triphenylmethyl chloride and sodium benzoate give triphenylmethyl benzoate (60). Acetobromoglucose reacts with diphenyl silver phosphate, (PhO)zPOOAg, to give the phosphate ester (126). Esters are prepared from a 2-thenyl chloride and sodium salts (59). Silicon-containing esters are made from chloro- and dichlorotetramethylsilane, Me,SiC€I,CI and XI&i(CH2CI)2, and metal salts ( 3 6 ) . KETENE
Ketme passed into glycol gives a inlxture of mono- and diacetates. The proportion of the diacetate is increased in the presence of sodium acetate. Similar results are obtained with thiodiglycol (f 25). lerl-Butyl mercaptan treated with ketene a t 20" to 50" C. gives 89% of the acetate (33). Isopropenyl acetate, H,C: CMeOAc, is obtained from acetone a.nd ketene in the presence df phosphorus oxychloride ( 1 6 ) ,sulfuric or chlorosulfonic acid, or sulfur trioxide (187). Other methyl ketones (16) and cyclohexanone (187) react simjlarly. II-ith furfural the product is the beta lactone of p-hydroxy-2iui anepropionic acid (57). .4mixed anhydride is produced by the addition of ketene t o an organic acid (37). This is the acetylation of an acid. ADDlTlON OF A N ACID TO A N UNSATURATE
Acids can be added to vinyl alkyl ethem, CH,:CHOR (173). M-ith isoprene, acetic acid, and tert-butyl hypochlorite the addiI ion product is t h a t which should be from CH,COOCl (120).
METAL SALTS
-4 dialkylaminoethyl c
