- 62 Experiment 30 Aldol Condensation Purpose To Demonstrate A Laboratory Preparation Using A Base Promoted Aldol Reactio 1 (62.73 KiB) Viewed 26 times
- 62 Experiment 30 Aldol Condensation Purpose To Demonstrate A Laboratory Preparation Using A Base Promoted Aldol Reactio 2 (58.07 KiB) Viewed 26 times
62 EXPERIMENT 30 ALDOL CONDENSATION PURPOSE: To demonstrate a laboratory preparation using a base promoted Aldol reaction. To demonstrate the use of physical and/or spectroscopie methods to determine the identity of a synthetic product. READ ABOUT: crossed Aldol reaction, Aldol condensation mechanism. EQUIPMENT: 10 mL Erlenmeyer flask, Hirsch funnel and vacuum filtration apparatus. CHEMICALS: one aldehyde and one ketone from Table 3,95% ethanol, potassium hydroxide (2M aqueous solution), 10% aqueous acetic acid, toluene, isopropyl alcohol. HAZARDS: The reactants and product of this reaction may be a skin irritant. Do not allow this product in contact with skin or eyes. It is recommended that the student wear gloves during this preparation. All manipulations should be completed in one lab period and the product turned in at the end of the period. DISCUSSION: The Aldol reaction carried out in this experiment is an example of the use of enolate anions as nucleophiles. Similar reactions include the Claisen and Michael condensations Reactions of this sort require that one reactant possess hydrogens on the carbon a to the carbonyl group. These protons can be removed relatively easily to form a resonance-stabilized carbonion. In the Aldol condensation, the carbonion/enolate then attacks the carbonyl group of another reactant molecule via a nucleophilie addition mechanism. The alcohol which results often undergoes rapid dehydration leading to an a, - unsaturated carbonyl system, especially when conjugation with an aromatic ring is possible. الياء ndan R4 HHH KOH OH -CH-R + H-C-AT - BON R-C CH-CH-AT R -CH-AT --- H-C-AV R 74-ig ch AT A crossed Aldol reaction occurs between two different carbonyl compounds. The crossed Aldol works best when only one of the reactants is capable of forming an enolate, that is, has a-hydrogens. If this is not the case, then four possible products can occur, leading to a complex mixture In this experiment, a crossed Aldol will be conducted using one or more unknown starting materials. The ketones used in this experiment are capable of reacting on both sides. This, combined with the reactant ratio, will lead to formation of double Aldol adducts. The student will then determine the identity of the product using the physical data from Table 3 and, if time allows, spectroscopic analysis, 96
PRE-LAB: The reactants for this experiment will be taken from Table 3. The instructor may inform you in advance of the identity of one or both of these starting materials. If a reactant is unknown, you should look up all the possible "unknowns" to determine physical properties and hazards. If the identity of both reactants is not known, calculation of the theoretical yield is not possible. Otherwise, this experiment requires all the usual items in the pre-lab, including as specific an equation as is possible. Check iLearn for this information. Be sure to investigate the health and safety properties of both reactants and products. 47.680 TABLE 3 MELTING POINTS("C) OF CROSSED ALDOL PRODUCTS BI Ketone acetone cyclopentanone 192 cyclohexanone 120 112 131 212 159 Aldehyde benzaldehyde p-anisaldehyde p-chlorobenzaldehyde p-ethylbenzaldehyde p-tolualdehyde 2,4-dimethoxybenzaldehyde 195 225 148 125 150 127 177 238 173 139 187 177 PROCEDURE: 24 drops 1 Piace 600 pil (or about 700 mg, if a solid) of the aldehyde into a tared 10 mL Erlenmeyer flask and determine its exact mass. ) Add 2.5 ml. of 95% ethanol and 2 mL of 2M KOH solution and swirl to dissolve. 6 Add 200 pil of the ketone and agitate vigorously, Vif no precipitate forms after 10 minutes, heat the solution gently on a steam bath until precipitation occurs. Cool in an ice bath to insure complete crystallization Filter the crystals using a Hirsch funnel and wash with small portions of ice-cold ethanol. Rinse the crystals with a small portion of 10% aqueous acetic acid, then rinse once more with cold ethanol. Air dry the crystals by continuing to pull a vacuum over them. 97