EXPERIMENT 7 Organic Chemistry II Pahlavan Cherif Aldol Condensation Synthesis of Dibenzalacetone Chemicals Materials Methanol CH3OH Acetone CH3COCH3 10 NaOH Benzaldehyde C6H5 CHO 95 Ethanol C2H5OH Hot water bath ice bath large tube 10 ml graduated cylinder Buchner funnel B chner funnel thermometer mel temp Purpose The objectives of this experiment are to learn aldol condensation mixture of aldehydes and ketones which used extensively in organic synthesis to form C C bonds and make bigger molecules Introduction T Like the Grignard reaction the Aldol Condensation is an extremely useful carbon carbon bond forming reaction in organic chemistry Under the reaction conditions in the experiment two equivalents of aldehyde will react The aldol condensation is a reaction that is named based on the type of product formed when two aldehydes or ketones in the presence of dilute base yields a molecule having both alcohol and aldehyde functional groups An example of the type of base catalyzed aldol condensation that you will perform is shown below Reaction Scheme O O O 2 H Benzaldehyde H3C 2 H2O CH3 Acetone Dibenzalacetone 1 5 diphenyl 1 4 pentadien 3 one These products are a hydroxyaldehyde or a hydroxyketone This reaction is used extensively in organic synthesis to form C C bonds and make bigger molecules In every case the product results from the addition of one molecule of an aldehyde or ketone to a second molecule in such a way that the a carbon of the first becomes attached to the carbonyl carbon of the second 1 MECHANISM OF THE ALDOL CONDENSATION The acidity of the alpha carbon makes beta dehydration of aldols an easy reaction This is of course quite different than the chemistry of normal alcohols This conjugated enone synthesis is catalyzed by both acids and bases This shows the mechanism of the experiment performed The reaction proceeds by an aldol condensation Step 1 First an acid base reaction Hydroxide functions as a base and removes the acidic hydrogen giving the reactive enolate Step 2 The nucleophilic enolate attacks the aldehyde at the electrophilic carbonyl C in a nucleophilic addition type process giving an intermediate alkoxide Step 3 An acid base reaction The alkoxide deprotonates a water molecule creating hydroxide and the hydroxyaldehydes or aldol product MECHANISM OF THE DEHYDRATION OF AN ALDOL PRODUCT Step 1 First an acid base reaction Hydroxide functions as a base and removes an acidic hydrogen giving the reactive enolate Step 2 The electrons associated with the negative charge of the enolate are used to form the C C and displace the leaving group regenerating hydroxide giving the conjugated aldehyde 2 Dehydration generally occurs under slightly more vigorous conditions such as higher temperature than the condensation reaction Thus at higher temperature in base the aldol reaction will go directly to the conjugated enone without any isolation of the aldol intermediate The aldehyde and ketone used for aldol reaction will be selected from the following group Notice that none of the aldehydes shown above contains an enolizable hydrogen so they cannot act as the nucleophilic species in the aldol reaction Ketones in general are less susceptible to nucleophilic attack than aldehydes so in a reaction mixture containing both an aldehyde and a ketone the aldehyde will react faster with nucleophiles Thus it is possible to perform a crossed aldol reaction in which the enolate formed by abstraction of the alpha hydrogen on the ketone attacks the carbonyl of the aldehyde Since we are working with conjugated aldehydes the resulting beta hydroxyketones readily eliminate water to form enones Under the conditions used in this experiment an excess of aldehyde a double condensation occurs by reaction on both sides of the ketone to give the products shown below Although a hydroxyaldehyde or a hydroxyketone is produced in an aldol condensation the ultimate product of these reactions as shown above is often the unsaturated aldehyde and a separate molecule of water Upon heating the hydroxy aldehyde product of an aldol condensation easily undergoes dehydration to yield an unsaturated aldehyde or ketone Conjugation of the newly formed double bond with the carbonyl group and of the benzene ring as shown in the example below stabilizes the product and provides the thermodynamic driving force for the dehydration process In the present case the reaction a mixed or crossed aldol condensation involving an aromatic aldehyde is referred to as a Claisen Schmidt condensation The Claisen Schmidt condensation always involves dehydration of the product of the mixed addition to yield a product in which the double bond produced during dehydration is conjugated to both the aromatic ring and the carbonyl group 3 Because this aromatic aldehyde lacks a hydrogens only one product is formed rather than a mixture of four different compounds as long as the concentration of the second aldehyde is carefully controlled In this experiment we will prepare the dibenzalacetone 1 5 diphenyl 1 4 pentadien 3 one The equilibrium is shifted toward the product because the compound precipitates from the reaction mixture as it is formed 1 5 diphenyl 1 4 pentadien 3 one Dibenzalacetone is a common ingredient in sunscreen This is because dibenzalacetone absorbs UV light and helps to protect the skin from the sun s damaging rays The properties that are most valuable in a compound that is used in sunscreen are the compound s abilities to absorb reflect or even scatter the harmful UV rays Another importance is for the compound to not cause an allergic reaction on a person s skin This experiment was being performed so that dibenzalacetone could be synthesized from benzaldehyde and acetone This experiment was performed to show how a ketone and an aldehyde could be added together through the aldol condensation The aldol condensation is extremely important because it can form a b hydroxy aldehyde or ketone from two carbonyl compounds This type of reaction proceeds through the creation of a resonance stabilized enolate ion from one of the carbonyl groups The enolate ion can then act as a strong nucleophile and add to another carbonyl group It is extremely important that one of the carbonyl groups has an acidic alpha hydrogen one adjacent to a carbonyl group so that the enolate ion can be formed Aldol products can be formed through either acidic or basic conditions and since they are usually exothermic the reaction will be driven to