Synthesis of Dibenzalacetone by Aldol Condensation of Benzaldehyde and Acetone - Lab Report Example

? Chemistry Laboratory Report Synthesis of Dibenzalacetone by Aldol Condensation of Benzaldehyde & Acetone By (i) Synthesis of Dibenzalacetone by Aldol Condensation of Benzaldehyde and Acetone (ii) Introduction: Aim: To understand mixed aldol condensation reactions by performing the synthesis, purification and characterization of dibenzalacetone from acetone and benzylaldehyde Overview: This laboratory report details the synthesis of dibenzalacetone using benzaldehyde and acetone. The reaction involves an aldol condensation reaction between the two reactants in presence of a basic catalyst, NaOH. Aldol condensation proceeds via an addition reaction between an aldehyde and a ketone, resulting in the formation of an aldol. The aldol product undergoes dehydration to give an enone product. In the present experiment, benzaldehyde, an aldehyde, reacts with acetone, a ketone, to give dibenzalacetone. Benzalacetone is the intermediate product formed during this reaction. Dibenzalacetone precipitates from the reaction medium and can be purified by filtration. Overall Reaction: The aldol condensation reaction between benzaldehyde and acetone is given below: CH3 – CO – CH3 + 2 – CHO ? – CH = CH – CO – CH = CH – Acetone Benzaldehyde Dibenzalacetone Reaction Mechanism: Under basic conditions, the carbonyl group of acetone having an alpha hydrogen atom is converted to an enolate ion (Mc Murry, 1999, pp. 939). The enolate ion thus produced is a strong nucleophile and attacks the carbonyl group of the “accepting partner”, which in this case is benzaldehyde (Mc Murry, 1999, pp. 939). An alkoxide is formed as a result of this nucleophilic attack. The alkoxide then undergoes protonation by H2O, forming a “neutral condensation product”, hydroxyketone. This undergoes dehydration in presence of NaOH, forming an enolate ion, hydroxyenolate, which then forms Benzalacetone by loss of a hydroxyl group. Benzalacetone also has alpha hydrogen, which again forms enolate ion under basic conditions, similar to the first step. This enolate nucleophilically attacks another Benzylaldehyde molecule. Subsequent reactions are similar to those already discussed, leading to the formation of dibenzalacetone, the final product. Thus, two molecules of benzaldehyde and one molecule of acetone are required to form one molecule of dibenzalacetone. O O O CH3 – C – CH2 – H CH3 – C– CH2 – H – C – OH- Acetone Enolate ion- nucleophilic attack on benzaldehyde O OH O O- CH3 – C – CH2 – CH – CH3 – C – CH2 – CH – OH- ?-Hydroxyketone Alkoxide O OH O CH3 – C – CH- – CH – CH3 – C – CH = CH – Hydroxyenolate Benzalacetone O – CH = CH – C – CH = CH – Dibenzalacetone (iii) Experimental: Benzaldehyde (80 µl), acetone (29 µl), and ethanolic NaOH (1.0 ml) were used in this experiment. The reaction was carried out in a conical vial containing a magnetic spinvane, according to the procedure specified in the SC214 practical manual, page 41. Filtration of the product was done using a Hirsch funnel, and the Craig tube method was used for purification and recrystallization of the product as per the SC214 practical manual, page 42. Melting point and IR spectrum were also obtained for the characterization of the product. (iv) Results: The weight and yield of the product (dibenzalacetone) are calculated as follows: (1) Calculation of mass of the product: Weight of glass= 37.458 g Weight of glass along with Dibenzalacetone crystals= 37.502g Therefore, mass of Dibenzalacetone crystals= 37.502 – 37.458 = 0.044g (2) Calculation of yield of the product: % Yield of Dibenzalacetone = Benzaldehyde is the limiting reactant in this experiment No. of moles of Dibenzalacetone= = = 0.001877 = 1.87 ? 10-4 moles No. of moles of Benzaldehyde= = = 0.000784 =7.84 ? 10-4 moles Since 2 benzaldehyde molecules are involved in each reaction, =7.84 ? 10-4/2 = 3.92 moles % yield of Dibenzalacetone= = = 0.477?100 = 47.7% Therefore, % yield of Dibenzalacetone = 47.7% (3) Melting point of Dibenzalacetone = 109.5-111.6?C Standard melting point= 111 ?C (4) IR spectrum of the product: Peaks were observed to identify those belonging to alkene groups, aromatic benzene rings and carbonyl group. All the peaks are to the left of 3000 cm-1, with the highest peak at 691 cm-1, indicating the presence of aromatic groups, i.e. benzene rings. There are no peaks in the 1760 - 1690 cm-1 region, especially 1720 cm-1 that is the standard carbonyl frequency (Furniss, Hannaford, Smith and Tatchell, 1989, 297). However, a band appears at 1648 cm-1 that is quite close to the carbonyl range. Minor peaks are also found in the 3000 cm-1 range. (v) discussion and conclusions: The reaction involved in this experiment was a mixed aldol condensation reaction between an aldehyde and a ketone. One reactant, acetone, has alpha hydrogen atoms and serves as the nucleophilic donor and is converted to an enolate ion. The other reactant, benzaldehyde, acts as the electrophilic acceptor and is attacked by the nucleophile generated from acetone. The reaction is catalyzed by a base, NaOH and requires an acidic alpha hydrogen atom at which the base hydroxyl group can attack. The alpha hydrogen is lost in the process and becomes an enolate ion, a strong nucleophile, which attacks the carbonyl group of benzaldehyde forming an alkoxide intermediate. The alkoxide undergoes protonation, yielding aldol and the base is regenerated in this process. The aldol product is then dehydrated under basic conditions to yield the final product – Dibenzalacetone. Of the two reactants involved in the reaction, only acetone possesses alpha hydrogens, while benzaldehyde does not. This is beneficial for the reaction as multiple products will not be formed since only one reactant, acetone, is serving as the nucleophilic donor. Self-condensation of acetone is prevented by addition of excess benzaldehyde. Thus only one product, Dibenzalacetone, is formed in the reaction. The yield of the product obtained in this experiment is 47.7%. Some of the product may have been lost while filtering and washing with water. In addition, some of the product may have been lost while re-washing to make it neutral and also during air drying. Another possible reason may be that the reaction was stopped before it reached completion. The melting point of the product is found to be 109.5-111.6?C. The melting point of Dibenzalacetone in literature is 111?C. Although the melting point of the product is almost equal to that given in literature, the minor variations may be due to improper drying of the product or due to errors during measurement. Comparison of the IR spectra of the product and dibenzalacetone from literature reveals differences in certain peaks. The IR spectrum from literature is given below (SDBS) The peak at 3000 cm-1 in the literature IR spectrum of dibenzalacetone is absent in the IR spectrum obtained in the experiment. This peak corresponds to C – H stretch that is absent in the product. Minor peaks at this position are however observable. The peaks corresponding to the alkene, benzene and the carbonyl groups of the product correlate well with those given in literature. Other peaks may correspond to contamination or incomplete utilization of reactants. Conclusion: The aldol condensation of acetone and benzaldehyde was successfully carried out in this experiment. Yellow colored dibenzalacetone was obtained in the reaction. However, the product yield was low. IR spectrum revealed the necessary peaks at required regions of functional groups and the melting point also correlated well with that given in literature. Thus, the product obtained was characterized as dibenzalacetone. It was purified and stored. (vi) Questions: (1) Mechanism for the base-catalyzed aldol condensation of ethanal with itself Overall reaction: 2 CH3CHO ? CH3 CH (OH) CH2 CHO Ethanal aldol Reaction mechanism: H O H O O O O- H – C H H—C- H + H3C – C H H H H OH- O OH Dehydration O OH H CH-H Base H CH2 CH3 + OH- CH3 CH (OH) CH2 CHO Addition product (Aldol) OHC-C=C-CH3 (Enone) (2) Cross or mixed aldol condensations are practical for synthesis if one of the aldehydes or ketones has no ?-hydrogen atoms. Explain. If both the carbonyl reactants undergoing cross or mixed aldol condensations have an ?-hydrogen atom, multiple products will be formed, as the reactants can undergo addition reactions in various combinations. For instance, if a carbonyl A having an ?-hydrogen atom reacts with another carbonyl B having an ?-hydrogen atom, the following possible combinations occur: A+A, B+B, B+A and A+A. Thus, if cross or mixed aldol condensations are to be carried out, one of the reactants should be lacking in ?-hydrogen atoms so that only one of the reactants can act as an electrophile to form an enolate ion and carry out the condensation reaction. For instance, benzaldehyde has no ?-hydrogen atoms, and so, when it is reacted with another carbonyl reactant having ?-hydrogen atoms, the second reactant will react with benzaldehyde or with it itself. Only two products are possible. The yield of the required product can be raised by increasing the concentration of benzaldehyde so that only it will react with the second reactant and prevent self-condensation of the second reactant. (3) Structure and names of the products formed from mixed aldol condensations of : (i) Propanal with (CH3)3C – CHO Two possible products are formed: Self-condensation of propanal: O CH3 – CH2 – CH = C– C – H (2-Methyl 2-Pentenal) CH3 Condensation of Propanal with (CH3)3C – CHO CH3 (CH3)3 C – CH = C –CHO (2-Methyl 3-trimethyl 2-Propenal) (ii) Propanal with C6H5 – CHO Two possible products are formed: Self-condensation of propanal: O CH3 – CH2 – CH = C– C – H (2-Methyl 2-Pentenal) CH3 Condensation of Propanal with C6H5 – CHO C6H5CH = C – CHO (2-Methyl 3-Phenyl 2-Propenal) CH3 (iii) Propanal with H2CO Formaldehyde H2CO does not have alpha carbon and hydrogen atoms. Yet it undergoes aldol condensation reactions, as it is a very reactive electrophile (Bruckner and Wender, 2010, p. 568). It undergoes multiple aldol condensation reactions instead of the ordinary aldol condensation reactions. Self-condensation of formaldehyde: In alkaline conditions, formaldehyde undergoes aldol condensation to yield CH3 OH (National Research Council (U.S.). Committee on Aldehydes, 1981, p. 23). H2CO + H2 CH3OH (Methanol) Self-condensation of propanal: O CH3 – CH2 – CH = C– C – H (2-Methyl 2-Pentenal) CH3 Condensation of propanal with formaldehyde CH2=CH – CH2 – CHO [butenal (crotonaldehyde)] (4) Three possible products in the given reaction: O O The above structure can be written as HC – CH2 – CH2 – CH2 – C – CH3 The three acidic hydrogen atoms are highlighted below: O O H C – CH2 – CH2 – CH2 – C – CH3 The three possible products that can be formed from addition of OH- to the above compound are given below: (i) O- O HC = CH – CH2 – CH2 – C – CH3 (ii) O O- HC – CH2 – CH2 – CH = C – CH3 (iii) O O- HC – CH2 – CH2 – CH2 – C = CH2 The first compound will be the major product, as in the second and third product, two substitutions are possible on either side of the carbonyl group that will result in steric hindrance. (5) Mechanism of the reactions: (i) O O O O HC – CH2 – CH2 – CH2 – C – CH3 H – C – CH – CH2 – CH2 – C – CH3 O- O HC = CH – CH2 – CH2 – C – CH3 (ii) O O O O HC – CH2 – CH2 – CH2 – C – CH3 H – C – CH2 – CH2 – CH – C – CH3 O O- HC = CH – CH2 – CH = C – CH3 (iii) O O O O HC – CH2 – CH2 – CH2 – C – CH3 H – C – CH2 – CH2 – CH2 – C – CH2 O O- HC = CH – CH2 – CH – C = CH2 References: Bruckner, R. and Wender, P. A. 2010, Organic mechanisms: reactions, stereochemistry and synthesis, Springer, Missouri. Furniss, B. S., Hannaford, A. J., Smith, P. W. G. and Tatchell, A. R. 1989, Vogel’s Textbook of Practical Organic Chemistry (5th Ed.), John Wiley and Sons, New York. Mc Murry, J. 1999, Organic Chemistry (5th Ed.), Brooks/Cole, California. National Research Council (U.S.). Committee on Aldehydes. 1981. Formaldehyde and other aldehydes, National Academy Press, Washington D.C. SDBS. Spectral Database for Organic Compounds. National Institute of Advanced Industrial Science and Technology (AIST), Japan. Available at: [Accessed 4 October 2011]. Read More