Experiment 2 Solvent-free Aldol Condensation between 3,4-dimethoxybenzaldehyde and 1-indanone Chemical Concepts Carbonyl chemistry, base catalyzed aldol reaction, melting point, recrystallization Green Chemistry Concepts Concepts: solvent free reaction, solid-solid reaction at room temperature, high atom economy, and ease of separation and waste prevention. Green chemistry principles Atom economy, safer solvents and auxiliaries, use of selective catalyst, safer chemistry. The Aldol condensation is one of the few reactions and perhaps the most powerful and useful reaction, used to form new carbon- carbon bonds. It is widely used in synthetic chemistry. Many biological systems utilize the aldol condensation or one of its variations to build up the carbon skeletons of many complex naturally occurring compounds. Under the influence of a dilute base or dilute acid, two molecules of an aldehyde or ketone may combine together to form a b hydroxy aldehyde or b-hydroxy ketone. The reaction is called aldol condensation. In every case the product results from addition of one molecule of aldehyde (or ketone) to second molecule in such a way that a-carbon of the first becomes attached to the carbonyl carbon of the second. 9
(b) This reactant is mixed with a catalyst (c) A carbonyl compound that contains a-hydrogen is added slowly to this mixture. In this experiment, aldol condensation reaction of 3,4-dimethoxybenzaldehyde and 1-indanone is done. Conventional method of preparation The reaction is carried out in absolute ethanol according to the method of Waltanasin and Murphy. Powdered reactants are dissolved in absolute ethanol and powdered sodium hydroxide is added. The reaction mixture is stirred overnight during which time products separate from the reaction mixture as solids. 1M aqueous HCl is added to the slurry to quench the reaction and this leads to further precipitate. The precipitation is filtered off and where necessary, recrystallized in appropriate solvent. Green Method of preparation Some solvent-free aldol condensation reactions between acetophenone derivatives and benzaldehyde are known. In many of these reactions one of the reagents is a liquid at room temperature and in all cases aldehydes without a-hydrogen are used to ensure that only a single aldol condensation process is possible. In the reaction between 3,4-dimethoxybenzaldehyde and 1-indanone, also a single condensation product is expected as the aldehyde bears no a-hydrogen and cannot act as a carbon nucleophile. Another advantage is that both reactants are solid at room temperature yet react under solvent-free conditions at room temperature. This is done simply by grinding the solid reagents together with solid NaOH. Moreover, remarkably a high degree of conversion is achieved and 11
a single major product results. It produces only a small quantity of aqueous acidic waste. Although such reactions are frequently referred to as solid state reactions. It has been noted that in many cases mixture of the solid reactants results in melting so that the reaction actually occurs in the liquid albeit solvent-free state. The aldol condensation if affected without dehydration has an atom economy of 100% and requires only a catalytic amount of acid or base and even with dehydration; the atom economy remains very high. Chemicals and equipment Sodium hydroxide 3,4-dimethoxybenzaldehyde 1-indanone 10% aqueous HCl ph paper 90% ethanol Glass rod Watch glass Mortar and pestle Filtration flask Vacuum pump Test tube Filter paper 12 Procedure Separately grind 0.25 g of 3,4-dimethoxybenzaldehyde and 0.20 g of 1-indanone in a pestle & mortar. Mix them together in a test tube and crush the two solids together with a glass rod until they become brown oil (mix carefully to avoid breaking the tube). Add 0.05 g of finely ground solid NaOH to the reaction mixture and continue mixing until it becomes solid, and allow the mixture to stand for 15 minutes. Add about 2 ml of 10% aqueous HCl solution. Check the ph of solution to make sure it is acidic, filter, wash with 1 ml of water and dry to yield 94% of the compound. Recrystallise from 90% ethanol. This remarkably simple yet effective methodology is further enhanced by the stability of the powdered intermediate formed after grinding. The solid material obtained at this stage may be stored, prior to work up, for a lengthy period of time. No deterioration in product quality was noted after storing of solid reaction mixture, without protection from either light or atmospheric oxygen or water for 1 month. This advantage makes this methodology attractive in industrial application.
REFERENCES Raston, C.L. and Scott, J.L. Green Chemistry, 2000, 2, 49. Wattansin,S. and Murphy, W.S. Synthesis, 1980, 647. 14
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