An Aldol Condensation Experiment Using a Number of Aldehydes and Ketones Bnue A. Hathaway Southeast Missouri State University, Cape Girardeau, MO 63701 The aldol condensation is an extremely useful carboncarbon bond-forming reaction in organic chemistrv. The re. action is usually easy to perform, and therefore most organic laboriltory texts include ar lrast one.'l'woof these have been used in the organic chemistry laboratories a t my institution: preparation of benzalacetophenone (I) and preparation of dihenzalacetone (2). The ease of the latter preparation led me to investigate whether it could be expanded to other aldehyde and ketone combinations. The general reaction is 0
R'
0
R'
1I R4H-C-CX-CHR
I
I
R'
R'
+ 2H,O
anisaldehyde are quite slow. Heating a slow-reacting mixture on a steam bath for a short while will speed i t up. It is important to remove all the sodium hydroxide from thecrude product; hence, the washings with95% ethanol and the acetic acid-ethanol mixture. If the sodium hydroxide is not removed, i t can promote the reverse reaction during the recrystallization, thus leading to lower yields. Crude yields of the products range between 30 and 90%, depending upon which combination of aldehyde and ketone was used (see table). In most cases, the crude products are reasonably pure. Recrystallization of some products may require large amounts (200-300 mL) of solvent. Experimental Procedure In a 125-mL Erlenmeyer flask are placed the ketone (1.0 mL), the aldehyde (4.0 mL), 95% ethanol (20 mL), and 2N aqueous sodium hydroxide (15 mL). The flask is stirred or shaken a t room temperature for 15 min or until no more precipitate is observed to form. In some cases, no precipitate may have formed. These can he allowed to stand with occasional shaking or stirring (2-3 h may be required for complete precipitation), or heated on a steam bath for 15 min, then allowed to cool to room temperature. The flask is then cooled in ice, and product is collected by suction filtration. The crude product is washed consecutively with ice-cold, 10mL portions of (1) 95% ethanol, (2) 4% acetic acid in 95% ethanol, and (3) 95% ethanol. If the product is not to be recrvstallized. i t is allowed to drv. then it is weighed and a mel;ing point is taken. If some o; all of the product is to be recrvsrallized, then sdubilities in 93"' ethanol and toluene shoild he checked and the better solvent used.
Four aldehydes and four ketones have been used (see table), which gives a total of 16 different products that can be produced. Other aldehydes and ketones were investigated (for example, furfural and cycloheptanone), but solid products were not obtained in all cases; therefore, these were omitted. One main purpose for this laboratory is t o encourage each student to work independently. Each student can be assigned a particular aldehydeketone combination. Therefore, the student will have to make his or her own observations, and make individual decisions as to which recrystallization solvent to use and how much, for example. This is a Acknowledameni very positive aspect of this laboratory, and most students enjoy doing "their own experiment", rather than the same The aurhor would like to thank Bjorn Olesen for helpful one everyone else is doing. comments and for testing this laboratory in his class and to thank the organic chemiLtry students at Southeast Missouri A few comments on the procedure are in order. The rates StateUniversity. who tested thisexperiment and madesome of reaction vary dramatically. Reactions involving cyclnpentanone or cinnamaldehyde are very fast, while those with phelpful suggestions. Melting Points PC), Literature References(in Parentheses), and Typlcal Percent Yields (In Brackets) for the Products Derived from Each Aldehyde and Ketone Comblnatlon Ketone
A&
Aldehyde
113i3 150-60461 175 (41 [50-60%]
189(7) [BO-90%] 235-236 (6) [60-90%]
118(7) [SO-SO%] 170 (81 [40-60%]
Volume 64
Number 4
96-99 (IQ [50-60%) 133-135 ( 1 1 ) [60-go%]
April 1987
367
388
Journal of Chemical Education
