Experiments

EXPERIMENT 1: REACTIONS OF ENOLATE IONS WITH CARBONYL GROUPS Aims In this experiment we used two techniques for the reactions of enolate ions with carbonyl groups. One technique used was Doebner reaction and the other technique used was Claisen-Schmidt reaction. Therefore the aim of this experiment is to synthesize trans p-methoxycinnamic acid and to synthesize dibenzalacetone via an aldol condensation reaction between acetone and benzaldehyde. The products would be recrystallized using ethanol, then characterized using melting point analysis. Introduction:

In this experiment we learnt on carbonyl compounds, enols and enolates. We also learnt two different techniques to test the reactions of enolate ions with carbonyl groups which are discussed below. I. Carbonyl Compounds, Enols and Enolates. Some of the most suitable reactions of carbonyl compounds contain carbon hydrogen bonds adjacent to the carbonyl group. Such reactions, which can be observed as the strength of much synthetic organic chemistry, usually result in the replacement of the hydrogen by some other atom or group, as in the H-C-C=O -+ X-C-C=O.

Transformation of a carbonyl compound to an enol at a suitable rate typically needs either a basic catalyst or an acidic catalyst and, of course, at least one hydrogen on the a carbon. Even though these reactions lead to numerous varied products subjecting to the reagents and conditions, they have one feature shared- they proceed by way of the en01 or the enolate anion of the parent carbonyl compound as shown below. II. Knoevenagel Condensation Reaction The Knoevenagel condensation reaction is an organic reaction named after Emil Knoevenagel. It is a reform of the Aldol condensation.

A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule of water is eliminated (hence condensation). The product is often an alpha, beta conjugated enone. An example is shown below. In this reaction the aldehyde or a ketone is an carbonyl group. The catalyst is generally a weakly basic amine. The active hydrogen component has the form •Z-CH2-Z or Z-CHR-Z for diethyl malonate, Meldrum’s acid, ethyl acetoacetate or malonic acid. •Z-CHR1R2 for nitromethane. where Z is an electron removing functional group.

Z must be dominant to enable hydrogen abstraction to the enolate ion even with a mild base. With a strong base in this reaction would make self-condensation of the aldehyde or ketone. The Hantzsch pyridine synthesis, the Gewald reaction and the Feist-Benary furan synthesis all comprise a Knoevenagel reaction step. The reaction also steered to the finding of CS gas. III. Doebner Reaction From the above reaction, The Doebner change of the Knoevenagel condensation. Acrolein and malonic acid react in pyridine to give trans-2,4-pentadienoic acid with the loss of carbon dioxide.

With malonic complexes the reaction product can miss a molecule of carbon dioxide in a later step. In the so-called Doebner modification the base is pyridine. For instance, the reaction product of acrolein and malonic acid in pyridine is trans-2,4-Pentadienoic acid with one carboxylic acid group and not two. Experimental: There were two parts to this experiment. As mentioned under the aims, we tested the reactions using two techniques: a)We placed p-anisaldehyde (p-methoxybenzaldehyde), malonic acid and pyridine in a 100ml round bottom flask, and fit with a reflux condenser.

In the table below the amount of p-anisaldehyde, malonic acid and pyridine are shown. P-anisaldehyde3. 30ml Malonic acid6. 505g pyridine6ml Table 1 We then moved to the fume hood to setup and conduct the experiment. In the fume hood, 5 drops of pyridine was added into the round bottom flask and heated in an oil bath under reflux for 90 minutes. During this period, bubbles of carbon dioxide evolved were seen. After 90 minutes under reflux, the flask was cooled and the contents were diluted with an equal amount of water. Concentrated hydrochloric acid was added to acidify the solution and precipitate the p-methoxycinnamic acid.

A blue litmus paper was used to test. The blue litmus paper turned red when tested. The flask with the solution was cooled again. The organic acid was then collected using vacuum filtration. Once collected, the organic acid was washed with cold water and air dried briefly. Lastly, the product was recrystallized from ethanol. The mass of the crude product, mass of p-methoxycinnamic acid and the melting point range of p-methoxycinnamic acid was measured and it is shown below. Mass of crude product4. 44g mass of p-methoxycinnamic acid3. 72g melting point range of p-methoxycinnamic acid169. ,172. 8 Table 2 b)A solution of sodium hydroxide (3g) in water(30ml) an ethanol (25ml) was prepared and stirred in a 250ml conical flask that was maintained at 20-25? in water bath. Benzaldehyde and A. R acetone were mixed and divided into two portions. The accurate masses of the starting materials, benzaldehyde and A. R acetone are shown below. benzaldehyde3. 2g acetone1. 1g Table 3 Half of the benzaldehyde- acetone mixture was added in one portion while stirring the solution vigorously. At this stage, a precipitated was formed after a few minutes.

After 15 minutes, the remaining benzaldehyde- acetone mixture was added and the mixture was stirred for a further 30 minutes. After 30 minutes, the precipitate was collected by vacuum filtration and washed with cold water until the washings were neutral. To test for neutralisation, a red litmus paper was used. The red litmus turned blue once tested. The precipitate was washed one more time to confirm neutralisation. The crude product was air dried and weighed. Lastly the crude product was recrystallized from methanol and the weight and melting point was determined.

The table below shows the mass of the crude product, mass of dibenzaldehyde and the melting point rage of dibenzaldehyde. mass of the crude product3. 33g mass of dibenzalacetone2. 4g the melting point rage of dibenzalacetone108. 7,110. 1 Table 4 Results and Discussions: Results and discussions for experimental part a and b are discussed below. Experimental part a: Q1. Comment on the temperature of the reaction solution when it is heated under reflux (i. e. at its boiling point) in an oil bath at 120? C vs a Bunsen burner, with temperature of 4oo? C.

ANS: When heated with Bunsen burner will not be effective and the temperature will not be constant throughout as compared to when heating with reflux at 120 degrees Celsius. Heating under reflux is more constant . Q2. Why does p-methoxycinnamic acid precipitate upon acidification? ANS: it is because the precipitate contains water and thus when acid was added to p-methoxycinnamic acid it reacts to remove the water molecules in the form of precipitate to make the solution more acidic. Experimental part b: Q1. Explain the principal of recrystallization as a purification technique.

ANS: Crystallization depends on the values of solubility: compounds (solutes) tend to be more soluble in hot liquids (solvents) than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be detached from the dissolved impurities by filtration. Q2. Why is dibenzalacetone coloured? ANS: It is because of the conjugate dienes. Such as the double bond C single bond C double bonds. C=C-C=C etc.

These conjugated dienes absorb light in the visible spectrum and in this case it happens to be pale yellow. Q3. Identify the absorption associated with the carbonyl stretch in the IR spectrum of dibenzalacetone. ANS 🙁 See page behind for answers). However, there were two peaks observed at around 3020. 00, which matches the positions of alkenes. Mechanism of the reaction for dibenzalacetone: The IR spectrum of our products can be seen behind. The functional groups that characterised dibezalacetone are aromatics and carbonyl compounds. There were peaks at 1445. 01-1590. 0, which indicates there are benzene rings in the final product. However the band positive carbonyl compounds 1670-1780 could not be observed. There was a peak at 1645. 04 which is close to the band position of carbonyl compounds. There was a peak observed at 3346. 00 which may be the band position of alkenes. Conclusion: The observed melting point for methoxycinnamic was 169. 3-172. 8 [Table 2], compared to a literature value of 173-175 ? C. The lower and broader melting point detected may be due to the product still being wet or some impurities present within.

However the observed melting point was close to the literature value and thus it can be concluded that the product was methoxycinnamic. Thus the synthesis of trans p-mthoxycinnamic was successful. The experimental melting point for dibenzalacetone was 108. 7 – 110. 1°C [Table 4], compared to a literature value of 110. 5°C. The lower and broader detected melting point could have been due to the product still being wet. It may also be due to unevaporated ethanol or other impurities in the product. However, the observed melting point was close to the literature value, and it can thus be concluded that the product was dibenzalacetone.

Thus, the aldol condensation reaction was successful. References: 1. ^ Jones, G. Org. React. 1967, 15. 2. ^ Emil Knoevenagel (1898). “Condensation von Malonsaure mit Aromatiachen Aldehyden durch Ammoniak und Amine”. Berichte der deutschen chemischen Gesellschaft 31 (3): 2596–2619. doi:10. 1002/cber. 18980310308. 3. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed. ), New York: Wiley, ISBN 0-471-85472-7 4. ^ O. Doebner (1902). “Ueber die der Sorbinsaure homologen, ungesattigten Sauren mit zwei Doppelbindungen”.

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