EXPERIMENTS WITH CYCLOPENTADIENE E. C. WAGNER and WILLIAM C. HUNT' University of Pennsylvania, Philadelphia, Pennsylvania
Tm
1. Interconversion of Cyclopentadiene and Diyclopentadiene
experiments outlined below serve to illustrate the processes of polymerization and depolymerization, the Diels-Alder reaction, and the presence of a reactive methylene group in cycl~pentadiene.~The experiments are simple, requiring no expensive or unusual equipment or chemicals, and the results are striking and convincing demonstrations of the well-defined characteristics of the compounds. Dicyclopentadiene (dimeric cyclopentadiene) is present in coal tar and is available and inexpensive. At its boiling point (170") it depolymerizes to cyclopentadiene (b. p. 41°), which is readily obtained by distillation (Experiment 1). Upon storage, cyclopentadiene gradually reverts to dicyclopentadiene by dimerization. The change can be followed by determining the change in specific gravity through a period of some days (sp. gr. of monomer: 0.80540/190;of dimer, 0.97fi3j0)(Experiment 2). The dimerization is probably a Diels-Alder self-condensation which can be reversed by heat:
Dioyclopentadiene
-
4,7-endomethylene-3a,4,7,7a-tetrahydroindene
Both compounds are rapidly and extensively polymerized by concentrated sulfuric acid, with blackening and evolution of heat (Experiment 3). Cyclopentadiene and dicyclopentadiene are used in manufacture of certain commercial resins, in which polymerization is probably linear.3 Cyclopentadiene, as a conjugated diene, readily enters into Diels-Alder condensations, of which the spontaneous reaction with maleic anhydride is given below as an illustrative example (Experiment 4). The activity of the methylene hydrogens of cyclopentadiene is illustrated by the formation of diphenylfulvene in Experiment 5. The chemicals required for these experiments are dicyclopentadiene, henzophenone, maleic anhydride, sodium, and ethanol. Present address: Department of Chemistry, Washington and Lee University, Lexington, Virginia. a The essential chemistry of oyclopenhdiene and dicyolopenta"The Chemistry of the Cardime is given compactly in RICHTER, bon Compounds," Nordeman Publishing Co., New York, 1939, Val. 11,pp. 50, 188. a CAUMODY, et at.,Ind. Eng. Chem., 30, 245 (1938).
Apparatus. For the depolymerization and distillation use a 250-ml. modified Claisen flask.' Through the straight main neck introduce a thermometer the bulb of which reaches nearly to the bottom of the flask, where it will be immersed as far as possible in the boiling liquid. Into the side neck, a t the head of the Vigreux insert, introduce a second thermometer in position for the distillation. Support the distillation flask on a perforated asbestos board above a burner and connect with the flask a water-cooled condenser. Use as receiver a 50ml. Erlenmeyer flask, supported on a funnel stand. The receiver need not be weighed. To follow the repolymerization of cyclopentadiene to dicyclopentadiene there is needed equipment for the determination of specific gravities. A Fisher "Gravitometer" is the most satisfactory apparatus for the purpose, as it requires only small amounts of material and is easy and rapid to use. The Becker No. 110 Chainomatic Specific Gravity Balance, with 2-ml. plummet, is excellent, and by use of a cut-off test tube of minimal bore can he used with about 15 ml. of liquid. The usual Westphal balance, because of the size of the plummet, requires more liquid than will be available in this experiment. The use of a pycnometer is not recommended, owing jointly to its inconvenience, to volatility of the monomer, and to the unpleasant odors of the hydrocarbons. Since high accuracy is not essential (the increase in specific gravity may reach 0.17) a small hydrometer, requiring 10 to 15 ml. of liquid to float, and calibrated for the range 0.8 to 1.0, may be used if other equipment is not at hand. Procedure. I t is advisable to complete the distillation during the early portion of a laboratory period, and to start Experiments 2, 3, 4, and 5 during the same period, so as to use the cyclopentadiene while repolymerization is trivial. Transfer to the distillation flask about 30 ml. of dicyclopentadiene and introduce several boiling chips. Heat with a small flame, and note that as distillation starts the inside thermometer (bulb in liquid) indicates a temperature of about 170° (the boiling temperature of dicyclopentadiene), but that the second thermometer (bulb in distilling vapor) shows an initial temperature "Organic Syntheses," John Wiley snd Sons, New York, 1941, Coll. Val. I, p. 130, Fig. 9 (a). I t is interesting to record that the use of a 3-ball Snyder column led to unsatisfactory results, the prolonged and repeated heating8 of condensed liquid about the floating balls leading to partial repolymerization within the column
308
JOURNAL OF CHEMICAL EDUCATION
310
of about 41' (the boiling temperature of cyclopentadiene). Adjust the flame so that distillation is steady and a t a rate of about 25 drops per minute. At intervals take readings from both thermometers. The inside temperature will rise slowly to perhaps 176"; the temperature of distillation will rise also, but may remain below 50' and should not exceed 65°.5 Stop the distillation when the liquid in the distillation flask has decreased to about 5 ml. This liquid can be left in the flask as part of the next run by another student. Stopper the receiver tightly and keep the hydrocarbon in a fairly cool place, to minimize loss by volatilization. 2. Spontaneous Repolymerization of Cyclopentadiene
Determine the specific gravity of the freshly distilled cyclopentadiene and that of the original dicyclopentadieue and record the values and the temperatures. Thereafter determine and record the specific gravity of the distillate a t intervals of one or two days, until the value approaches or equals the specific gravity of the dimer (0.976 at 35'). Plot the results: specific gravity versus time. The rate of dimerization is of course affected by temperature, but if no large fluctuation in temperature occurs a smooth curve should be obtained. 5. Extensive Polymerization of Cyclopentadiene and of
small portions 1ml. of freshly distilled cyclopentadiene. Reaction is immediate and evolves heat, which may cause some of the hydrocarbon to boil off.? The entire mixture solidifies on cooling. To it add 4 ml. of 95 per cent ethanol, effect solution by brief warming on a water bath (the solution will probably be turbid), and filter through a small fluted paper into a second 50-ml. Erlenmeyer flask. Rinse the first flask with 0.5 ml. of hot ethanol and use this also to wash the filter. Chill the solution in ice and collect the crystals in a Hirsch funnel or similar device, using the filtrate to transfer remaining crystals to the filter. Wash the product with 0.5-1 ml. of chilled ethanol. Dry the crystals in air or in a desiccator and determine the melting point. The yield is about 0.4 g. The pure product melts at 164165'; as obtained here it may melt several degrees lower. 6. Preparation of Diphenylfulvene
This condensation illustrates the activation of the methylene hydrogens of cyclopentadiene. Diphenylfulvene, a stable derivative of the hypothetical fulvene
Dicyclopentadiene by Acid (a) Diyelopentadiene. To one drop of concentrated sulfuric acid on a small watch glass add one drou of dicyclopentadiene, and stir with ;short glass rod. Note the darkening and resinification as the hydrocarbon polymerizes. (b) Cyelopentadiene. Spread a drop of concentrated sulfuric acid on a small watch glass, and bring over it and close to it the mouth of the (inclined) flask containing the freshly distilled cyclopentadiene, so that the vapor of the hydrocarbon comes into contact with the acid. Note that darkening occurs. Then add several drops of cyclopentadiene. Polymerization occurs actively, with blackening and evolution of heat.
has a deep orange color, though it is a hydrocarbon and is derived from two colorless compounds. The reason for this strong color becomes clear when there is considered the high degree to which the unsaturation of the molecule is involved in extended conjugation. Pro~edure.~Dissolve 0.5 g. (0.02 mol) of freshly cut sodium in 15 ml. of 95 per cent or absolute ethanol in a 50-ml. Erlenmeyer flask, while cooling under the tap. To this liquid add 3.6 g. (0.02 mol) of benzophenone, most of which dissolves. Now introduce 1.6 ml. (1.3 g.; 0.02 mol) of freshlv distilled cvclouentadiene. while " swirling the mixture, and after several minutes warm it 4. The Diels-Alder Reaction gently. The mixture develops a deep red color. Chill Cyclopentadiene and maleic anhydride condense to- in an ice bath; if this fails to cause separation of solid gether to form 3,G-endomethylene-l,2,3,6-tetrahydro-material evaporate some of the alcohol by gentle warmphthalic anhydride. ing and again chill. To assure separation of the product keep the mixture in an ice bath or refrigerator for some hours or overnight. Collect the crude product in a Biichner funnel, pressing well and drawing the mother liquor through as completely as possible. Wash the solid with 2 to 3 ml. of chilled alcohol, and dry in the air or in a desiccator. The cmde diphenylfulvene weighs about 3 g. and should melt at about 81' to 82'. The appearance of the product is much improved by Procedure.Vransfer 0.5 g. of maleic anhydride (0.005 mol; m. p. 57-60') to a dry 50-ml. Erlenmeyer recrystallization from alcohol. For this purpose disflask and chill in ice. While chilling and shaking add in solve it in 20 ml. of hot 95 uer cent ethanol. filter hot A
'When a 3-ball Snyder column was used, the temperature of distillation rose quickly to 115'. DIELSAND ALDER,Ann., 460, 98 (1928).
' This procedure is not a good one for preparative purposes, since a. large excess of cyclopentadiene is specified to compensate for the loss by volatilization during the exothermic reaction. THIELE, Ber., 33,672 (1900).
JUNE, 1951 through a fluted paper (some black tarry material is thus removed), and use about 5 ml. more of hot ethanol to wash flask and iilter paper. Chill the filtered solution in ice, collert the crystals in a Biichner funnel, and
311
wash with 5 ml. of chilled ethanol. The product is thus obtained as deep orange leaflets; recovery is about 80 per cent. The melting point is 81" to 82'; the m. p. of pure diphenylfulvene is82'.
