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Autor: anton • March 7, 2011 • 1,231 Words (5 Pages) • 2,122 Views
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‚" 109 (Celsius) Lit. BP = 110 Ð²Ð‚" 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 Ðž±-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 Ðž±-carbon of the acetone in our experiment gets deprotonated easily in NaOH revealing an Ðž±-carbon with a lone pair of electron attached to it. This Ðž±-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