Quantitative Saponification of Highly Hindered Esters by Perchlorate Anion Catalysis DONALD E. JORDAN Research and Development Dept., Continental Oil Co., Ponca City, Okla.
b A method i s presented for quantitative saponification of hindered esters. The reaction proceeds through perchlorate anion catalysis in alcoholic alkali solvent and the effect of perchlorate anion concentration on saponification i s described. Rapidity of saponifkation i s dependent on the size, number, and symmetry of the groups; the positions occupied by them on the ester skeletal backbone; and, to a lesser extent, on the size of the alkoxy moiety attached to the carboxyl group. Actual reaction times range from 6 minutes for nominally hindered esters to 13 hours for the most highly hindered ester studied. The proposed method i s superior to other published methods because it provides, for the first time, a relatively rapid way to study the reaction characteristics of esters previously considered hydrolytically stable. Simple esters can also be saponified with the described procedure and with less drastic solvent conditions.
S
of esters is commonly determined by alkali hydroxide hydrolysis. The ester is refluxed with alcoholic potassium hydroxide, the mixture is then cooled, and excess alkali is titrated with standard acid. A reference sample containing the same amount of alkali hydroxide but no ester sample is treated in the same manner. The extent of saponification is calculated from the difference between these two titrations. Xumerous methods for determination of saponification equivalents of esters are described in the literature. I n general these methods are applicable only to simple natural and synthetic esters, although occasionally a procedure is described which is applicable to certain esters difficult to saponify. S o n e of the described procedures are applicable, however, to the quantit,ative saponification of both highly hindered and simple esters described hy this work. Hall and Shaefer ( 2 ) prpsent an excellent review of the pertinent, literature through 1952. They describe various saponification techniques in addition to the most common one presented above. They include general chemical and physical, as well as very specific, procedures. APONIFICATION
2 134
ANALYTICAL CHEMISTRY
Tobey, MeGregor, and Cottrell (8) used alkali hydroxide in a 1:1 pdioxane-water solution instead of ethanol to determine the saponification equivalent of phenolic esters. Koroton and Byrodov (5) describe a specific procedure for saponification of isoborneol esters. They present two continuous reaction systems for these esters in which the saponification is determined in either the liquid phase above the melting point of the ester or in the vapor phase. Cations of strong organic bases, such as dimethylbenzyl trimethylammonium chloride and other similar quaternary salts, have been used by Lucius and Bruning (6) to accelerate the saponification rate of ester groups on high molecular weight polymers. The reaction proceeds in either aqueous alkali hydroxide or by placing a thickened saponifier onto the structure a t a temperature of 60' to 140' C. Suitable thickeners are described as methyl cellulose, or similar molecules. The use of ion exchange resins as catalysts for organic reactions, such as esterification, saponification, alcoholysis and acidolysis, and others, has been described by Isagulyants ( 3 ) . Further catalysis work on saponification and esterification with ion exchange resins has been done by ;Indreas ( I ) . He presents a continuous acid saponification procedure for various esters using "Wofatit P" cation exchange resin. The effects of resin particle size as well as other parameters are discussed. Shaefer and Balling ( 7 ) saponified the nominally hindered rosin esters using a diethylene glycol solution of alkali hydroxide to which phenetole was added. Diethylene glycol solvent allows a much higher reaction temperature than ethanol. Phenetole has a blanketing effect and improves the solvent power of the reagent. -1great deal of care i5 necessary, however, to avoid a side reaction between the solvent and alkali hydroxide. Johnson and Lawrence (4) also describe a rapid saponification of rosin esters using n-hexyl alcoholic alkali hydroxide containing hydrazine. The hydrazine acts both as catalyst and bleaching agent. They achieve theoretical saponification in about one hour, compared to two hours for the procedure utilizing phenetole. However, none of the foregoing catalyzed procedures is
applicable for esters more highly hindered than the rosin esters because of the prohibitive amount of time required for complete saponification. The present work demonstrates that by increasing the boiling point of the alkali hydroxide solvent and catalyzing the reaction with perchlorate, quantitative saponification of highly hindered esters is possible in a reasonable length of time. The time required for complete saponification of a hindered ester is a function of size, position, and number of hindering groups, the symmetry of the groups, and, to a lesser extent, the length of the alkoxy moiety connected to the carboxyl radical of the ester. Briefly, the ester is refluxed in a nhexyl alcohol solution containing 20 gramslliter of sodium hydroxide and 10 g r a m s liter of sodium perchlorate until saponification is complete. The per cent saponification is calculated in the usual manner. Comparative data are given for catalyzed n-butanol, Formula 30 alcohol, and hexyl alcohol, and for uncatalyzed Formula 30 and n-hexyl alcohol solvents containing 28 grams/ liter of potassium hydroxide. The present work is used primarily to saponify esters that are normally thermally stable a t 315' C. or higher, but it can also be used for the more common simple and complex esters. EXPERIMENTAL
Apparatus and Reagents. Sodium perchlorate purified, Fisher Scientific Co. Yo. S-360. Alkali resistant flasks, 200 to 250 ml., 24/40 standard taper, Corning S o . 7280. R a t e r condensers, 24/40 standard taper. Suitable hot plate. CATALYZED REAGENTS.Hexyl alcohol, Matheson Coleman & Bell S o . HX395 or Eastman Kodak Co. S o . P-825; 20 grams of sodium hydroxide and 10 grams of sodium perchlorate are dissolved in 1 liter of hexyl alcohol using a magnetic stirrer or mechanical shaking. Dissolution usually requires several hours. After standing overnight' decant or filter the solution froin the residue. n-Butanol, J . T. Baker Co., No. 9054; reaction mixture iiremred as outlined for hexyl alcohol. Formula 30 alcohol (95% ethanol, 5% methanol); reaction mixture prepared as outlined for hespl alcohol. UNCATALYZED REAGENTS.Hesyl and L
.
Table I.
Saponification of Hindered Esters by Proposed Catalyzed Hexyl Alcohol 0.5N in Sodium Hydroxide and 1 .O% in NaC104
Time for complete saponification, hr.
Ester sample
Di(2 methyl-2-ethylpenty1)2,2,S,S-tetraethylazelate
Di(2,2-dimeth) Ipentyl)-2,2,8,8tetraethvlazelate Di( 2,2-dimethylhexyl)-2,2,8,8tetraethylazelate Di(2,2-dimethyloctyl).2,2,8,8tetraethylazelate DI(2,2-dimethylhexyl)-2,8dimethyl-2,S-diethyl azelate Di( 2,2-dimethylhexyi)-2,8dimethyl-2,8-diprop,ylazelate Di( 2,2-dimethylhexyl)-2,2,6,6tetramethylpimelatcb Di( 2,2-diniethylhexyl)-3,3,6tetramethylsuberate 2 , Methyl-2-propyl-l,3-propanediol-di( 2,2-dimethyl tetradecanoate ) hlethyl-2-ethyl-2-methylhexanoate a
Saponification number
=
12 6
211
214
7.6
230
226
7.0
217
214
5.5
189
193
1.0
222
226
1.1
219
214
