The Freundlich isotherms for the adsorption of sonle ortho-alkylated phenols on carbon from cyclohesar~e solution have been determined. The minimum surface area covered by the adsorbed phenols is compared with the area of the molecules themselves. Large allcyl groups ortho to the hyclroxyl group greatly reduce the adsorption. INTRODUCTIONThe importance of steric effects in reactiolls occurring in a homogeneous phase is now well understood ( I ) , but few data are available concerning the effect of steric hindrance in heterogeneous reactions. The kinetic rate-controlling step in such reactiolls is often the adsorption of the reactants on the surface of an adsorbent, and such a process will be considerably influenced by steric effects.The adsorption of a series of ortho-all<\-lated phenols on carbon from cyclohexane solutio~l has been studied. Phenols \irere chosen since they should be strongly adsorbed and can be easily analyzed. Carbon is a suitable adsorbent since it contains neither strongly acidic nor basic groups, and cyclohexane is a convcaient inert solvent which is available in a state of high purity.The effective surface area of the carbon used was first determined by the standard method (2) of adsorption of nitrogen a t liquid air temperature.The results of the adsorption of the phenols \irere expressed in the for111 of the Freundlich isother111 equation (x/m = k.c7" where x is the amount adsorbed by m grams of adsorbent, and G is the equilibriu~n concentratio~l of the adsorbate) as plots of x/m against c (Fig. I). In all cases the graphs show limiting values of the amounts of phenol adsorbed, which must correspond to the formation of unimolecular l a~w -s . E X P E R I M E N T A L ReagentsCarbon This was a sample of Darco G-60 adsorbent carbon supplied by the Atlas Po~vder Company of Canada, Ltd. The nlanufacturer's analysis was: ash content 2-5%, water solubles 0.1-0.2%, acid solubles O.5%, pH of water extract 5-6, moisture %lo%. The loss of weight of a sample dried to co~lstallt weight a t 145' C correspo~lded to a ~noisture content of 5.2%. The sample used was dried under these co~~ditions and stored over phosphorus pentoxide.Gases Nitrogen and heliu~n were Canadian Liquid Air Company's pure samples and were dried prior to use. Phenols Phenol (Allied Chemical and Dye Corporation, absolute), p-tert-butylphenol (Dow Chemical Company), and 2,G-di~nethylphenol (Aldrich Chenlical Company) were used without further purification. 2,6-Diisopropylpl~enol (Aldrich) was vacuum-distilled twice.'Manzrscript

I7apor pressure studies of trimethylamine and ketones in heptane solution have shown that unstable complexes are formed with cyclohesano~~e a t 0' and with cyclohexanone artd cyclope~~tanone a t -40'. No complex formation was observed with methylethyl ketone, diethyl ketone, or cyclopentar~one a t 0'.Robinson and co-workers (1, 2) originally suggested that there was some type of interaction between a lceto group and a tertiary arnine group in certain allcaloids having these groups in a large-membered ring, to explain the inasked reactivity of the lceto group (1) and the large displaceme~lt of its infrared stretching f r e q~~e n c y from its normal value (2). Later Leonard and co-workers (3, 4, 5) prepared a series of cyclic aza-acyloins and found evidence for transail~lular interaction in 8-(3, 5), 9-(4), and 10-(5) membered rings between the ketone and tertiary amine group. This was demonstrated by infrared and ultraviolet absorption (6) studies.Anet, Bailey, and Robinson (2) found no evidence for interaction between acetone and triethylamine or between cyclopentanone and N-methyl piperidine by infrared measurements in chloroform solution. However, the amine would preferentially interact with the chloroform (7) and hinder association between the amine and ketone, and the evidence of these worlcers cannot be considered conclusive. Thus, though interaction between lceto and amine groups within the same ring has been ainply proved, no sound evidence has been published concerning possible interaction between ketone and amine groups in separate molecules.Infrared and ultraviolet measurements are probably not the best method of observing weak interactions resulting in unstable coinplex formation, since such n~easurements must be carried out in dilute solution and cannot readily be made a t low temperatures. Moreover these measurements will only detect interactions which result in a change in the electron density of the keto group. Vapor pressure studies, however, are usually made with solutions of moderate concentration, and the lower limit of temperature is only governed by the measurable vapor pressure of one (or more) con~ponent, and the nlethod is not liinited to specific types of interaction. In this study the vapor pressure of trimethylamine above a n-heptane solution and above solutions of the lcetones, inethylethyl lcetone, diethyl ketone, cyclohexanone, and cyclopentanone in n-heptane has been measured. Trirnethylamine was chosen as a conveniently volatile, non-sterically hindered anline and ~nethylethyl lcetone and diethyl lcetone as examples of reactive and unreactive acyclic lcetones, and cyclohexanone and cyclopentanone as examples of a very reactive and a moderately reactive cyclic lcetone. The results are expressed in terms of Henry's law (8) (P solute = I<. N solute) and negative deviatioils are assumed to be due t o complex formation (8) caused by interaction between the ketones and the arnine.