Langmuir 1990, 6, 1797-1799
1797
Reactions in Monolayers: Base-Catalyzed Ester Hydrolysis Revisited Jamil Ahmad' and K. Brian Astin* Department of Chemistry, University of Bahrain, P.O. Box 32038, Isa Town, Bahrain Received March 6, 1990. In Final Form: June 1, 1990 The kinetics of the base-catalyzed hydrolysis of 2-(octadecylthio)ethylethanoate have been examined at the air-liquid interface. The rate of reaction was found to be independent of the area/molecule in the monolayer, in accordance with the notion that reactions occurring at a single center are insensitive to area/ molecule variation. In a number of previous studies,'+ we have sought to establish the utility of studying reactions in monolayer films, a t the air-liquid interface, as an effective probe of neighboring group and proximity effects. Reactivity differences may be produced by varying the surface pressure (or equivalently area/molecule) in the monolayer. For example, the rates of acid-catalyzed cyclization of the monoterpenoid alcohol nerol' and of acidare catalyzed dehydration of 1,l-diphenyl-1-octadecano12 strongly affected by surface pressure changes. It was suggested that these reactions require the participation of remote functionalities-the remote double bond in nerol and the &hydrogens in 1,l-diphenyl-1-octadecanol-for reaction. High surface pressures (low areas/molecule) disfavor this participation, and the rate is correspondingly lowered. By contrast, reactions taking place a t a single center, e.g., the chromic acid oxidation of 1-phenyl-1hexadecanol,3 are not significantly affected by surface pressure changes. An interesting exception to this behavior is observed in the acid-catalyzed hydrolysis of 1-phenylhexadecyl e t h a n ~ a t ewhere ,~ rate differences between various surface pressures have been ascribed to a change in mechanism between the different film types, namely, A A C in ~ compressed films and A A L in ~ more expanded films. In view of these results, we would predict that basecatalyzed ester hydrolysis proceeding by the B ~ c 2 mechanism would be essentially unaffected by surface pressure changes as it is a single-center reaction. The early work of Alexander et ale? quoted in well-known texts,*,g suggests that the base-catalyzed hydrolysis of (for example) ethyl hexadecanoate is markedly dependent upon surface pressure; the rate for an area/molecule of 0.72 nm2 is 8 times greater than the rate a t an area of 0.20 nm2 over 1 M hydroxide. This has been interpreted in terms of the ethyl groups "crowding" the carbonyl carbon at high surface pressures and thereby inhibiting nucleophilic attack by
OH-. We have examined the base-catalyzed hydrolysis of 2-(octadecylthio)ethyl ethanoate (1)to 2-(octadecy1thio)ethanol (2), a t the air-liquid interface, using modern (1) Ahmad, J.; Astin, K. B. J. Am. Chem. SOC.1986, 108,7434. (2) Ahmad, J.; Astin, K. B. J . Am. Chem. SOC.1988, 110,8175. (3) Ahmad, J.; Astin, K. B. Langmuir 1988,4, 780. (4) Ahmad, J.; Astin, K. B. Angew. Chem., Int. Ed. Engl. 1990,29,306. (5) Ahmad, J.; Astin, K. B. Langmuir 1990,6, 1098. (6) Ahmad. J.: Astin. K. B. Surf. Colloids In Dress. (7) (a) Alexander, A. E.; Schulan, J. H. Proc. R. SOC.London, Ser. A 1937,161,115. (b)Alexander, A. E.; Rideal, E. K. Proc. R. SOC.London, Ser. A 1937. 163. 70. (8) Adamson, A . W . Physical Chemistry of Surfaces, 3rd ed.; Wiley: New York, 1976; p 160. (9) Davies, J . T.; Rideal, E. K . Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1961; p 72.
0743-7463/90/2406-1797$02.50/0
analytical techniques to reinvestigate the ester hydrolysis reaction.
-I-
OAc
1
-I-
OH
2
Results and Discussion The choice of an ester of a long-chain alcohol, rather than one of a long-chain acid, is important, since in our hydrolysis the product is a surface-active, yet neutral, alcohol. Under the same hydrolysis conditions, the.esters of longchain acids afford the carboxylate anion, with its attendant problems of enhanced solubility with respect to the starting ester. In our study, both ester 1 and alcohol 2 are exceptionally stable. Films of either 1 or 2 compressed to 20 mN m-l show negligible surface pressure change (
