Isaac Angres and Herman E. Zieger Brooklyn College o f CUNY Brooklyn, New York 11210
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I
A Crossed Ald0l Condensation for the Undergraduate Laboratory
The aldol condensation is a widely studied reaction in undergraduate organic laboratories because it provides a starting point for study of nucleophilic attack a t cebonyl carbon as illustrated by the benzoin condensation, the Cannizzaro reaction, and several others.' Student perception of the formation of a carbanion intermediate which performs nucleophilic attack at carbonyl carbon is a primary teaching objective. Although students rapidly become aware of the acidity of hydrogens alpha to a carbonyl group, they are less appreciative of the acidity of hydrogens in hydrocarbons such as cyclopentadiene. Consequently, we thought that a crossed aldol condensation involving reaction of a carbanion from a hydrocarbon with a model carbonyl compound would show students the special acidic properties of benzylic hydrogen in hydrocarbons such as fluorene or 9,lO-dihydroanthracene. We chose the reaction of fluorene carhanion with benzaldehyde in benzyl alcohol solution because the reaction proceeds smoothly to 9-benzalfluorene (eqn. (1)) on a water bath a t 95-100'C in 90 min.
benzyl alcohol (or more accurately potassium benzylate) produces 1 mole of benzaldehyde for each molecule of 9benzalfluorene that is reduced to 9,benzylfluorene. Students secured (11) in yields of 62-91% using Fisher certified henzyl alcohol with no provision for exclusion of atmospheric oxygen. In summary this two-step experiment for undergraduate organic chemistry students illustrates three basic ideas: (1) crossed aldol condensation; (2) the acidity of benzylic hydrogen in hydrocarbons; and (3) reduction of a double bond by hydride transfer. Experimental
Preparation of 9-Benzalfluorene In a 50-ml round-bottom flask, a mixture of 4.15 g (0.025 mole) of flu~rene,~ 4 ml of freshly distilled benzaldehyde, 20 ml of benzyl alcohol and 2.0 g of potassium hydroxide are heated at IW'C (in a water hsth) for 90 min. After cooling, the mixture is transferred to a 500-ml round-bottom flask and treated with 200 ml of water. The product crystallizes slowly and about 'h hr must be allowed for complete crystallization.* The solid is then filtered and washed with water. The yields of benzalfluorene ranged from 32-66%. The melting point ranges from 72-80'C; lit., 74-76°C. The solid after recrystallization from heptane melts from 7576°C. ~
A second advantage to this experiment is the possibility for students to study hydride transfer reductions by carrying out the facile reduction of 9-henzalfluorene, (I), to 9benzyltluorene, (11). by means of potassium hydroxide in refluxing benzyl alcohol (eqn. (2)).
This two-step synthesis of 9-benzylfluorene was studied by Sprinzak who demonstrated that the rate of the reaction (eqn. (2)) is accelerated by the addition of benzaldehyde.2 The fact that the benzaldehyde formed in eqn. (2) is a better bydride donor than potassium benzylate may be seen by the appearance of a copious precipitate of potassium benzoate which forms via eqn. (3).
This two-step sequence can be accomplished easily in two 3-hr laboratory periods. In a section of sophomore organic chemistry students, the yields of (I) ranged from 32-66% of products having a two degree melting range. The yields of (II)from (I) were 69-88%. It is also possible to carry out both steps in a single laboratory period by operating a t the boiling point of benzyl alcohol. Only trace amounts of benzaldehyde are required to initiate t h e two-step sequence because oxidation of
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Journal of Chemical Education
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Reduction of 9-Benzaifluorene to 9-Benzyifluorene In a 25-ml round-bottom flask, 2.00 g of 9-benzalfluorene,6 ml of henzyl alcohol and 0.5 g of KOH are heated under reflux for 15 min. A voluminous precipitate of potassium benzoate appears. The mixture is allowed to coal and then 20 ml of water is added and the flask swirled vigorously. The white crystals are filtered and washed with 15 ml of water. The crystals are dried and a melting point taken. Yields of 9-henzylfluorene ranged from 6288%.The dried crystals melt at 132-134'C (range 105-137'C). Preparation of 9-Benzylfluorene from Nuorene in one Laboratory Period In a 50-ml round-bottom flask, 4.15 g (0.025 mole) of fluorene, 10 ml of henzyl alcohol, and 2 g of KOH are refluxed for 2 hr. The mixture is allowed to cool, treated with 10 ml of water and then the crystals of 9-henzylfluarene are filtered and dried. Recrystallization from heptane gives a mp of 134-135'C. Yields ranged from 52-91%. Melting points ranged from 129-137% The pmr spectrum of 9-benzylfluorene shows a simple triplet (6 4.2) for the hydrogen at Cs and a doublet at (6 3.01) for the methylene hydrogens characteristic of an AX2 case ( A YIJ= 59 Hz17 Hz). The aromatic protons occur in two groups at 6 7.2 and 6 7.6
'Adams, R., Johnson, J. R., and Wilcox, Jr., C. F., "Laboratory Experiments in Organic Chemistry," 6th Ed., The Macmillan Co., 866Third Ave., New York, 1970, pp. 355-82. Sprinzak, Y., J Amer. Chem. Soc, 78,466 (1956). Technical grade fluorene from Matheson, Coleman, and Bell was used
'Some students art glubule.i of oil rather than crystals; huwever. t h m difficulty is ovrrcnme by telling the students to decanr the water and add 5 10 ml of 95% ErOH. This msults in rmmediate crystallization.
