H - AldolCondensation

H - AldolCondensation - 31 6 Part Four Preparations and...

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Unformatted text preview: 31 6 Part Four Preparations and Reactions of Organic Compounds Experiment 35 The Aldol Condensation Reaction: Preparation of Benzalacetophenones (Chalcones) Aldol condensation Crystallization Benzaldehyde reacts with a ketone in the presence of base to give a, fi-unsaturated ketones. ’ This reaction is an example of a crossed aldol condensation where the intermediate under- goes dehydration to produce the resonance-stabilized unsaturated ketone. it H 0 H 0 H c / | OH— I ll 4,20 \ / \ C6H5—C\ + CH3—C—R —* C6H5—C “CHZ—C—‘R —"—’ /C=C\ R 0 6H C6H5 H Intermediate Crossed aldol condensations of this type proceed in high yield, because benzaldehyde cannot react with itself by an aldol condensation reaction because it has no oc-hydrogen. Likewise, ketones do not react easily with themselves in aqueous base. Therefore, the only possibility is for a ketone to react with benzaldehyde. In this experiment, procedures are given for preparing benzalacetophenones (chal- cones). You should choose one of the substituted benzaldehydes and react it with the ke- tone acetophenone. All the products are solids that can be recrystallized easily. Benzalacetophenones (chalcones) are prepared by the reaction of a substituted benz- aldehyde with acetophenone in aqueous base. Piperonaldehyde, p-anisaldehyde, and 3—nitrobenzaldehyde are used. Ar\ (1? _ Ar\ H /c=o + CH3—C—C6H5 & /C=C,< i+ H20 H H C—C6H5 II o A benzaldehyde Acetophenone A benzalacetophenone (trans) o 0 II M i C C C \H or \H o CH3O N02 | CH2— 0 Piperonaldehyde p-Anisaldehyde 3-Nitrobenzaldehyde An optional molecular modeling exercise is provided in this experiment. We will ex- amine the reactivity of the enolate ion of a ketone to see which atom, oxygen or carbon, is Experiment 35 The Aldol Condensation Reaction: Preparation of Benzalacetophenones (Chalcones) 317 more nucleophilic. The molecular modeling part of this experiment will help you to ratio- nalize the experimental results of this experiment. It would be helpful to look at Experi- ment 17E, starting on page 187, in addition to the material given in this experiment. REQUIRED READING Review: Technique 4 Sections 4.3 and 4.7 Technique 5 Section 5.4 SPECIAL INSTRUCTIONS Before beginning this experiment, select one of the substituted benzaldehydes. Alternatively, your in— structor may assign a particular compound to you. WASTE DISPOSAL All filtrates should be poured into a waste container designated for nonhalogenated organic waste. _/ PROCEDURE Running the Reaction. Choose one of three aldehydes for this experiment: piperonaldehyde (solid), 3-nitrobenzaldehyde (solid), or p—anisaldehyde (liquid). Place 0.150 g of piperonaldehyde (3,4—methylenedioxybenzaldehyde, MW: 150.1) or 0.151 g of 3-nitrobenzaldehyde (MW: 151.1) into a 5-mL conical vial. Alternatively, transfer 0.13 mL of p-anisaldehyde (4—methoxybenzaldehyde, MW = 136.2) to a tared conical vial and reweigh the vial to determine the weight of material transferred. Add 0.12 mL'ofacetophenone (MW= 120.2, d: 1.03 g/mL) and 0.80 mLof95% ethanol to the vial containing your choice of aldehyde. Place the conical vial into a 50-mL beaker. Stir the mixture with a microspatula to dissolve any solids present. You may need to warm the mixture on a hot plate to dissolve the solids. lfthis is necessary, then cool the solution to room temperature before proceeding with the next step. Add 0.10 mL of sodium hydroxide solution1 to the aldehyde/acetophenone mixture. Stir the mixture with your microspatula until it solidifies or until it becomes very cloudy (approximately 3 minutes). Isolation of the Crude Product. Add 2 mL ofice water to the vial. lfa solid is present at this point, stir the mixture with a spatula to break up the solid mass. lfan oil is present, stir the mix- ture until the oil solidifies. Transfer the mixture to a small beaker with 3 mL ofice water. Stir the precipitate to break it up, and then collect the solid on a Hirsch funnel. Wash the product with cold water. Let the solid air-dry for about 30 minutes. Weigh the solid, and determine the per- centage yield. Crystallization of the Benzalacetophenone (Chalcone). Crystallize part of the chalcone using a Craig tube as follows: 3,4-methylenedioxychalcone (from piperonaldehyde). Crystallize a 0.040-g sample from about 0.5 mL ofhot 95% ethanol; literature melting point is 122°C. 4-methoxychalcone (from p-anisaldehyde). Crystallize a 0.075-g sample from about 0.3 mL of hot 95% ethanol. Scratch the tube to induce crystallization while cooling; litera- ture melting point is 74°C. ___’_——— 1The instructor should prepare this reagent in advance, in the ratio of 0.60 g of sodium hydroxide to 1 mL of water. 318 Part Four Preparations and Reactions of Organic Compounds 3-Nitrochalcone (from 3-nitrobenzaldehyde). Crystallize a 0.025-g sample From about 1 mL of hot methanol. Scratch the tube gently to induce crystallization while cooling; litera- ture melting point is 146°C. Laboratory Report. Determine the melting point of your purified product. At the option of the instructor, obtain the proton and/or carbon-13 NMR spectrum. Include a balanced equa- tion for the reaction in your report. Submit the crude and purified samples to the instructor in la- beled vials. MOLECULAR MODELING (optional) v In this exercise we will examine the enolate ion of acetone and determine which atom, oxygen or carbon, is the more nucleophilic site. Two resonance structures can be drawn for the enolate ion of acetone, one with the negative charge on oxygen, structure A, and one with the negative charge on carbon, structure B. H2C=C—CH3 4—» HZC—C—CH3 A B The enolate ion is an ambident nucleophile—a nucleophile that has two possible nu- cleophilic sites. Resonance theory indicates that structure A should be the major contribut- ing structure because the negative charge is better accommodated by oxygen, a more electronegative atom than carbon. However, the reactive site of this ion is carbon, not oxy- gen. Aldol condensations, brominations, and alkylations take place at carbon, not oxygen. In frontier molecular orbital terms (see the essay on page 174), the enolate ion is an elec- tron pair donor, and we would expect the pair of electrons donated to be those in the high- est occupied molecular orbital, the HOMO. In the structure-building editor of your modeling program, build structure A. Be sure to delete an unfilled valence from oxygen and to place a —1 charge on the molecule. Re- ' quest a geometry optimization at the AM1 semiempirical level. Also request the HOMO surface and maps of the HOMO and the electrostaticxpotential onto the electron density surface. Submit your selections for computation. Plot the HOMO on the screen. Where are the biggest lobes of the HOMO, on carbon or on oxygen? Now map the HOMO onto the electron density surface. The “hot spot,” the place where the HOMO has the highest den- sity at the point where it intersects the surface, will be bright blue. What do you conclude from this mapping? Finally, map the electrostatic potential onto the electron density. This shows the electron distribution in the molecule. Where is the overall electron density high- est, on oxygen or on carbon? I Finally, build structure B and calculate the same surfaces as requested for structure A. Do you obtain the same surfaces as for structure A, or are they different? What do you con— clude? Include your results, along with your conclusions, in your report on this experiment. QUESTIONS 1. Give a mechanism for the preparation of the appropriate benzalacetophenone using the aldehyde and ketone that you selected‘in this experiment. 2. Draw the structure of the cis and trans isomers of the compound that you prepared. Why did you obtain the trans isomer? Experiment 35 The Aldol Condensation Reaction: Preparation of Benzalacetophenones (Chalcones) 31 9 3. Using proton NMR, how could you experimentally determine that you have the trans isomer rather than the cis one? 4. Provide the starting materials needed to prepare the following compounds: ‘1’ (a) CH3CH2CH=C|I— —H ' CH3 3 . : ‘ "4' 3 its! CH3\ ' tit (b) C: CHC— CH3 ‘ 34515: CH / " 3 0 Haw ‘ Ph 0 ‘ 1‘ \ » t (c) /C= CH— C— Ph r t CH3 ‘ o a ll 3 (d) CH3OOCH=CH—C—CH=CH OCH3 I (u) (e) OZNO—CH=CH—C©Br t (u) (D C1©CH=CH—CQ N02 5. Prepare the following compounds starting from benzaldehyde and the appropriate ketone. Provide reactions for preparing the ketones starting from aromatic hydrocarbon compounds (see Experi- ment 58). 0 CH3 / \ " '7 CH=CH—p CHZCH3 CH=(|3—C CH3 ' " 0 CH3 1 ...
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This note was uploaded on 01/21/2012 for the course ECON 2a taught by Professor Xi during the Spring '11 term at Cornell College.

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H - AldolCondensation - 31 6 Part Four Preparations and...

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