Byron L. Hawbecker, David W. Kurtz, Theodore D. Putnam, Patricia A. Ahlers, a n d Gregory D. G e r b e r Ohio Northern University Ada. Ohio 45810
I II
The AMO~Condensation: A Simple Teaching M0dd for Organic Laboratory
A thorough discussion of the organic reaction known as the aldol condensation can h e found in almost any standard organic textbook and it is not t h e purpose of this paper t o elaborate on the basic process. However, illustrating t h e r e action in the laboratory in a simple, yet impressive manner is more difficult. Many laboratory texts contain experiments which employ the aldol condensation, h u t the combination of a readily workable experimental procedure and a very definite teaching goal is frequently lacking. T h e following teaching model for the organic laboratory was developed for the Ohio Northern Universitv lahoratorv to meet . oroeram . .. thvsc criterta. T h r experiment itself can he accomplished in on,, 3-hr lohoracory period, and it very c l ~ a r l yillustrates a number of concepts which serve to enrich the student's understanding of organic reaction chemistry. Ketones and aldehydes possessing no a-hydrogens (benzaldehyde, for example) cannot enter into a self-condensationofthe aldol type. However. such comoounds can be attacked bv carbanions from other suurcrs and participate in a crowed aldol condmsatim. Thi5 is the situation l o br exammed in the model experimmr. One uf the difficultiesof m a d aldd rea~.tionsis the self-condensntionnf t h ~ a c t i w methylene reactant. In this instance the side reaction is suppressed by the choice of acetone as the acidic reactant. The well-known retro aldol reaction of the self-condensation product (diacetone alcohal) prevents its accumulation in appreciable quantity. Another interestine..feature in this exoeriment is the fact that the active methvlene curnpuund, acetone, hnz t u o sets uf acidic ( 0 1 hydrogrn*. l'hws, two differentaldol eondpnrnt~,nprrrducts are pussihle from rhr Inlerartim uf hemaldehyde and aretune under hasic conditions. dep~ndmg upon the relative concentrations of the two organic reactants. This is demonstrated in eqns. (1)and (2) which are indicative of experiments A and B, respectively. Part A: Benzaldehyde in excess
1.
@cHo,oH-(~~)
2
-H,O
0
0-
II
CH=CH-C-CH=CH
a
(1)
dibenzalacetone; mp, 110-112°C (formula weight 234) Part B: Acetone in excess n ~,
11 + CH-,+-Ulj
benzalacetone; mp, 41°C (often forms an oil) The object ofthe experiment, then, is to demonstrate the validity of the reactions shown above by isolating and comparing the products from the crossed aldol condensation of benzaldehyde and acetone. In part B it is desirable toascertain the structure of the product appearing as a crude oil (which cannot be readily purified) by converting it into a solid derivative which can be identified via its melting point (eqn. (3)).This, of course, adds still another dimension to theexperiment as one approach for identifying a very impure product is demonstrated. Even the recovery of a pure 2,4-dinitrophenylhydrazone is difficult unless the ~eroxidetreatment described in Part B is employed to rid the oily h;nzalaeetone of unreacted benzaldehyde.
n
2 -H,O
540 1 Journal of Chemical Education
benzalacetone
2,4-dinitrophenylhydrazine
ml of ethanol.) Any unrenrted benzaldehyde s h d d now be oxidized and the product, henxalaretone, may be ~solatedhy ether eatmrtlon. Be sure m w l the solution 10 nwm temperature in an ice-bath before adding the ether. Pour the cold reaction mixture into a separatory funnel. Wash the beaker with 5 ml of ether and add this to the seoaratorv funnel. Extract the mixture two times with 10-ml p r t i o n s of ether. Isolate the combined ether lavers which contain the henzalacetone..drv,for 5 min ,~ over anhydrous MgSO,, filter.and evaporate the filtrate m a n oil (the benralaretone) in a 50-ml beaker on a steam bath. Nuw convert the derivative as dehenralacetone to ita 2.4.dnnntrophcnylhyd1a7~~ne scribed below. ~
~
"2,4-dinitrophenylhydrazone"
~~~
~~
~
Part A Carefully mix 40 ml of 1.0 M ethanolic benzaldehyde and 40 ml of 0.5 M aqueous NaOH in a 250-ml beaker. Add to this 0.5 ml of acetone and allow this mixture to stand for about 20 min. (Now begin port B.) Periodicstirring and scratchingof the sides of the beaker may aid in formation of the dibenzalacetone crystals. These should be recovered hy vacuum filtration. A second crop of crystals can often be collected from the filtrate. Now dissolve the crystals in a minimum amount (15-25 ml usually) of hot ethanol (in a 50-ml beaker). Once all the organic solid is in solution, add 1-2 ml of cold water t o produce turbidity and place the container in an ice bath. Again scratching the sides of the beaker may facilitate crystal formation. Collect the crystals on a Buchner funnel, dry, weigh and calculate the % yield based upon 0.40 g of acetone (density 0.79 glml). Typical student yields are in the 40-60% range.
2,4-Dinitrophenylhydrazone Derivative The 2,4-dinitrophenylhydrazinereagent is prepared as follows: Dissolve 12 g of 2,4-dinitrophenylhydrazinein €4ml of eoncd H2S01. Add this solution with stirring to 80 ml of H20and 280 ml of earbonyl free ethanol. Mix the solution thoroughly and filter throughafritted glass funnel. This produces about 400 ml of reagent. Dissolve 0.5 ml(10-12 drops) of the henzalacetone oil in 20 ml of 95% ethanol. T o this alcoholic solution of the oil add 15 ml of 2,4dinitrophenylhydrazine reagent with thorough mixing. Allow the mixture to stand a t room temperature. Crystallization readily oeeurs within 10 min. Recover the precipitate by vacuum filtration and bail it with vigorous stirring in 10 ml of ethanol for about 5 min. Cool the mixture, filter, and repeatthe treatment with another 10 ml of ethanol. The final, filtered product can now be air-dried prior to obtaining the melting point. In some instances it may he preferable t o let the derivative dry until the next laboratory period before taking the melting point.
Part B Carefully mix 10 ml of 1.0 M cthanolic ben7aldehyde and 10 ml of 0.5 wueou.; NaOH in a IW-ml beaker. Add to this4 rnl of awtone and allow-this mixture to stand for a t least 20 min. (Now continue part A.) ~~.After the 20 min have elaosed remove the unreacted benzaldehyde ax id low^: Add 20 ml of fr& .I% hydrogen perioxide solutwn and warm the mixture to 65-75Y on hot plate (low heat! o r a watpr-bath fur 15 mm. ( I f turbidity da\,tlopr it can hedischarged by adding 2-3
A Suggedion for Additional Work If an additional laboratory period is available, hoth of the condensation products may be characterized spectroscopically. The product from part A is essentially pure trans, trans-1,5-diphenylpenta-1.4-diene-3-one (C17Hl40). The oil obtained in part B before (CloHloO) with a derivatization is trans-l-phenylbut-l-ene-3-one small amount of the cis isomer. Standard spectra data far hoth compounds are available in the Sadtler Spectral Indices.
m p , 223°C
Volume 55. Number 8, August 1978 1 5 H
