EXPERIMENT 1: REACTIONS OF ENOLATE IONS WITH CARBONYL GROUPS
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.
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.
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.
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.
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.
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...
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