Cyclohexanol Dehydration A Simple Experiment in Heterogeneous Catalysis A. Costa Department of Organic Chemistry. Palrna de Mallorca University, Balearic Islands, Spain A great number of important industrial chemistw reactions are hkterogeneous in which a gaseous stream of the reactants is flowed thrvugh the bed of a solid catalyst. This general picture has an enormous relevance hut, unfortunately, there are relatively few undergraduate laboratory experiments to illustrate this class of heteroeeneous catalvzed reactions ( I . 2 ) .There are three fundamen& reasons fo; this: (A) crudes of reaction form mixtures in which separation is not possible except when effective fractionating techniques are available. (B) Gaseous reaction products are obtained and it is necessarv to use special freezing traps with the consequent hazards that involvc the handling of liuuid gases. tC) Special unavailable materials are required. ~ d t h and'(^) only allow analysis by GC, but preparation purposes should be omitted. The proposed experiment avoids these difficulties, does not require a special preparation, and i t can be completed ina4-hr labopreviously. T h e inratory period if the catalyst is tramolecular dehydration of cyclohexanol on y-Alz03 is a classical heterogeneous process (3-5) that leads t o diverse cycloolefins in variable proportion according to the following reaction.
(A)
T h e appearance of methvlcvilo~entenesbv Waener-Meerwein rearrangement andhobble bond isomerizarion do not occur at temperatures below 300°C ( f i ) . Anti-elimination of the elementsof water seems to take place through an E2-like transition state (7) which is evidenced by kinetic isotope studies (8).The catalyst in this reaction acts by partial formation of a bond between the hydroxy group of cyclohexanol and the acidic sites that are present on the surface of y-Alz03 (9), thus converting the initial hydroxy group in a better leaving group.
Experimental Preparation of Gamma-Alumina Commrrcial -AlzO? can be used if desired, or alternatively, it can be prepared from the reaction of aluminium nitrate with aqueous ammonia, as the equations below indicate.
-
+ 3 NH~NOB 2 A l ( O H ) s d y-A1203 + 3 H20 mec
AlNOs + 3 NHdOH
AI(0H)d
A
In this case, the slowly precipitated aluminium hydroxide could be dried at llO0C and calcined at 600°C for 2 hr to yield y-AI2Oa.
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Journal of Chemical Education
Figwe 1. S c h e m a t i c d i i of flow reamused fadehydration of cyelchexanal in gas phase.
Reaction System Figme 1 shows the components of a verysimple flowreactor system. All parts are of Pyrexm glass with standard taper joints. The Pyrex@ halls inside the reactor act as a preheater ensuring complete vaporization of cyclohexanol. Nitrogen can he introduced into the system by means of a flexible plastic or rubber tube and its flow controlled directly by the pressure reducing regulator. To avoid overpressures a pressure release device is enclosed. The height of the mercury eolumn should be about 2 cm.The vertical electricalfurnaceisa standard one and it can he replaced easily by an electrical tube furnace with the consequent modification of the "U" tuhe in which the reaction takes place. Experimental Procedure Four grams of y-AlzOs are placed between two plugs of glass wool and Pyrex" balls are positioned as illustrated in Figure 1. The system is assembled, and 20 ml of cyclohexanol are introduced into the top container. The furnace is regulated at 3CODC,and a nitrogen flow-rate of 200 ml per min is established with the aid of a soap bubble flowmeter. There is a 30-min waiting period for the stabilization of the system. At the end of this period cool water for the condenser is circulated and a mixture of ice-salt is placed around the collector flask. At this moment, cyclohexanol is dripped into the "U"tuhe at a rate of six drops per minute during 2 hr. Finally when cyclohexanol is exhausted, the nitrogen flow is continued for an additional 30 min. The collector flask is disconnected, and the organic layer is dried with anhydrous sodium sulfate and distilled (hp 81-83'C). Average yield of cyclohexenc is greater than 70%.This yield is comparable to that obtained by the use of mineral acids as dehydrating agents (10). Dlscusslon Because of the high yields that this method offers, i t is an
ideal source of cyclohexene for use in other laboratory experiments. For instance, it can be used as a substrate in the permanganate oxidation to adipic acid using phase transfer catalysts (11) in a general context of catalysis in organic chemistry. Variants of this experiment could be tried with a laboratory group. For example, if kinetic data are of interest and GC analysis is available, this process follows a zero-order kinetic (3)over a wide range of experimental conditions and the kinetic constant can be measured from the slope obtained by plotting X (fractionary conversion of cyclohexanol) versus W/F (space velocity or contact time). Safety precautions include standard protection against gases and dling. Special precautions should also be taken to prevent
leakage of cyclohexene into the a i r by connecting the effluent to a hood. 'Ited (1) oat-, B. c . and sherman, J. D.. them. ~ n g~. d .124 , (1-6).
L,, , ~ ~ ; , 5 2 ~ ~ ~;;&D2~,"u~~ and Baant,
V,, Call.
~c h p m ~ corn., ~ 27,1199 ~ ~(1962). I ~ ~ . 14) ~ (5) Cnd~ z, iS., n gSantan~elo, cH.,A~@u. r. ~chrm. .,ao~ d unat~i.A,, , d Chim. .7,791 , ~ (1968). n d48.229 , (1968). c
(6) ~
~
~
K..~J. , c o. t o ~ 24.57 ., (1972). 5H . . H~~ M ,H i ., ~ K ~~ ~~ M
(7) Blmc, E. J. and Pinea, H . , L 018. Chom, 33.2035 (1968). (8) Knbzinger, H. and Scheglia.A..J. Cotol.. 17,252(1970). ( 9 ) Peri, J.B., J. Phyr. Chem., 69,211(1965). (10) v&,A. ~ . " ~ ~ e x t b oof~raetieal ok organicchemistw"3rd ~d.Longman,1974,~. 243. (11) Field. K.W.,Glover, A. D., M o m , J. S., Callander, D. J.,and Kolb. K. E.,J. C m . EDUC..~6,209(19'19).
Volume 59
Number 12 December 1982
1067
