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Autor: anton 07 March 2011
Words: 1231 | Pages: 5
Experiment2: Preparation of Dibenzalacetone
Using the cabon-cabon bond making ability in carbonyl chemistry, Dibenzalacetone is synthesized from 2 equivalent of benzaldehyde and 1 equivalent of acetone in a base catalyzed reaction.
Physical Data1: *detailed risk and safety phrases are attached.
substance Hazards, risks and safety practices MW (g/mol) Amt. Used Mol. mp (K) bp (K) density(g/cm^3)
acetone R11, R36, R67, S9, S25, S26 58.08 0.24 g 0.004 178.2 329.4 0.79
benzaldehyde R22, S24 106.13 0.82 g 0.008 247 451.1 1.0415
ehtyl acetate R11, R36, R66, R67, S16, S26, S33 88.11 2 ml per gram n/a 189.55 350.25 0.897
NaOH R35,S22, S26, S38, S45, S62, s24/25 39.997 0.4 g 0.01 591 1663 2.1
Ethanol R11, S2, S7, S16 46.07 2 ml n/a 158.8 351.6 0.789
Dibenzalacetone n/a 234 g/mol n/a n/a 379 unknown unknown
Limiting reagent: acetone (0.004 mol)
Presuming 100 % of limiting agent makes dibenzalacetone
Dibenzalacetone (100% yield) = 0.004 mol * 234 g/mol = 0.936 gram
The theoretical yield of dibenzalacetone is 0.936 gram
In a 50 ml conical flask sodium hydroxide (0.4g, 0.01 mol), distilled water (2 ml) and ethanol (2ml, 95%) were mixed into a clear solution. The solution was then cooled to room temperature. Benzaldehyde (0.8ml; 9.82g, 0.008mol) was then added to the solution followed by addition of acetone (0.3ml; 0.24g, 0.004mol) and formed into a thick yellow solution. The flask was then swirled gently and constantly for 5 minutes. This turned into a fluffy precipitate as the flask was swirled. After 5 minutes of constant swirling, the flask was swirled once or twice every minute for 10 minutes thereafter. Ethanol (95%) was cooled in ice bath while the fluffy precipitate in the 50 ml conical flask was collected using a small buchner funnel. The fluffy precipitate was washed with distilled water (approximately 500 ml) followed by ethanol (approximately 2ml, 95%). The washing produced a clear filtrate solution and a yellow precipitate. The washed precipitate was then left to air dry for a week in a clean 50 ml beaker forming a lumpy yellow solid. Which was then weighted and recrystallised in ethyl acetate (approximately 2ml) to afford dibenzalacetone, a shiny powedery yellow solid ( 0.4377g, 46.8%, m.p 107-109, literature m.p. 110-1112).
Product calculations for Adol formation of Dibenzalacetone
Weight of the receiver flask = 16.2662 g
Received flask + product = 16.7039 g
Nett weight = 0.4377 g
Theoretical yield =0.936 g
Percentage yield = 46.76%
BP = 107 Ð²Ð‚â€œ 109 (Celsius) Lit. BP = 110 Ð²Ð‚â€œ 111 (Celsius)2
Infrared spectrum table3:
*the IR spectrum of dibenzalacetone is attached
Absorption (per cm) 3024 2363 1649 1600 1338-981
Intensity medium, sharp medium, sharp very strong, sharp very strong, very sharp all strong and sharp
Description sp and sp2 C-H streches, alkene, arene. This confirms the two rings and 2 double bonds in our molecule aldehyde C-H strech, aldehyde shouldn't be present but aldehydes are very similar to ketone which is present. C=O stretch. This is a very useful peak as it is very diagnostic which also confirms the presence of ketone. C=C stretch, This could be from the alkene and benzene ring in the molecule. A very strong intensity indicates presence. A lot of different things are absorbed at this region thus although the signals are strong, it's best ignored.
The key to this experiment is the aldol reaction4 that results in a C-C bond forming reaction. From observation it seems this reaction can be used to synthesis very large organic molecules. The concept of this reaction revolves around the idea having the ÐžÂ±-carbon of an aldehyde or ketone attacking the carbonyl carbon of another aldehyde or ketone. The result of this attack is a new C-C bond being formed.
The ÐžÂ±-carbon of the acetone in our experiment gets deprotonated easily in NaOH revealing an ÐžÂ±-carbon with a lone pair of electron attached to it. This ÐžÂ±-carbon is a very good nucleophile, a very good lewis acid and is extremely reactive. In other word we have turned our acetone into an anion. This anion likes to attack and form a covalent bond with a carbonyl carbon. This is due to the positive nature of carbonyl carbon and the electronegativity of the oxygen , most of the electron in a carbonyl molecule is around the oxygen thus leaving the carbon bare and susceptible to nuecleophilic attack. As a result of this attack, a molecule is yielded with both aldehyde and alcohol functional groups, hence the name aldo reaction. The product is a ÐžÐ†-hydroxyaldehydea (or ÐžÐ†-hydroxyketonea), this molecule is then easily dehydrated in the presence of an acid to form a separate water molecule.
In our experiment, the reaction involving an aromatic aldehyde and a ketone is referred to as a Claisen-Schmidt condensation4. The Claisen-Schmidt condensation always involves dehydration of the mixed aldol condensation product to yield a chemical in which the double bond (produced during dehydration) is conjugated to both the aromatic ring and the carbonyl group. Since we have 2 alpha hydrogens on the acetone and thus yield a symmetrical product with the same conjugated formation on the other side. The equilibrium in this reaction is shifted toward the product because the compound precipitates from the reaction mixture as it is formed.
The order of addition of reagent in our experiment is very important. Benzalaldehyde must be added before the acetone. Due to the benzene ring, benzaladehyde does not have an alpha hydrogen. Thus in base the benzaladehyde will not get deprotonated and is stable. If acetone is added first in our experiment it has an alpha hydrogen on each side, those alpha hydrogen will get deprotonated and go through aldol reaction with other acetones in the solution.
Another important aspect of this experiment is the stoichiometry of reagents. Excess acetone in our solution will perform aldol reactions with itself and our reaction will yield a lot of impurities. Excess benzaladehyde will be wasted as it will not react. An interesting note for the use of bezalaldehyde is that since it is an aldehyde but lacks an alpha hydrogen we yield one product (dibenzalacetone). A mixture of four products will be yielded if the benzalaldehyde is replaced with a carbonyl with an alpha hydrogen as they will react with themselves as well as each other.
Our experiment appears to be successful. The IR of our product has shown all the major functional groups of Benzaldehyde. These include An alkene peak around 3024, a ketone peak around 1649 and a group of peaks around 1600 which indicates the presence of benzene in our molecule. Our productÐ²Ð‚â„¢s melting point of 107 Ð²Ð‚â€œ 109 degrees Celsius is comparable with the reference M.P2 of dibenzalaldehyde 110 Ð²Ð‚â€œ 111 degrees Celsius. Although there is an indication of impurities present due to the difference in M.P, we are fairly confident our product is dibenzalacetone, given the similarities in functional groups and M.P.
One disappointment from our experiment is the low yield of final product. Three major factors influenced the result of our yield. The first factor is the stoichiometry of the regent used. As mentioned earlier excess acetone will result acetone doing aldol reaction with itself and a lack of it will result in unreacted benzaladehyde. In future extra care needs to be applied to the stoichiometry of regent used. The second factor is the mixing of the regent, constant mixing is needed for the nucleophilic acetone anion to react with the benzalaldehyde. The third factor is the washing of crude product with ethanol. Washing with too much ethanol will dissolve our crude product into the ethanol and then washed away into the filtered solution. Paying extra care to these three factors will increase the yield in future experiment.
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4. Organic chemistry, 7th edition. Brooks/Cole Thomson Learning,2000.