Question

Organic Chemistry 2

Experimental Outline In its simplest form, the aldol condensation is a self-addition involving two molecules of the same aldehyde or ketone, in the presence of a base, and results in the formation of a new carbon-carbon bond joining the carbonyl carbon of one molecule to the a-position of the second. In many cases - if the reaction conditions aren't sufficiently mild - the initially formed aldol product reacts further in an elimination reaction to give what is called an a,ß-unsaturated carbonyl compound, or enone. Both of these reactions are shown below for the self-condensation of acetaldehyde. OH O catalytic H O heat aldol product α.ßunsatur aldehvde The requirement for this reaction is that the molecule contains a reactive carbonyl group and one or more reactive hydrogens in the α-position. An aromatic aldehyde has no hydrogen atoms in the a-position, but it is capable of participating in a mixed (crossed) aldol condensation with another aldehyde or ketone that can furnish an α-hydrogen. If a ketone has reactive hydrogens in both a-positions (like in acetone), the reaction is complicated by the possible formation of multiple products. In today's experiment you will allow p-methoxybenzaldehyde (commonly named p- anisaldehyde) to react with acetone under two different sets of experimental conditions and you will compare the products formed using proton NMR spectroscopy. experimental condition #1 Product A experimental Product B PROCEDURE B NOTE: acetone is a reagent in this reaction, thus using equipment contaminated with traces of acetone (incomplete drying) could influence your yield and even the success of your experiment. You must rinse the pipettes with methanol to avoid contamination with acetone. Add 6 mL of 10% aqueous NaOH solution to your 50 mL Erlenmeyer flask, followed by 10 mL of acetone and a clean stir bar. Place this flask on one of the magnetic stirrers in your hood and vigorously stir the solution. Dissolve 0.5 mL ofp-anisaldehyde in 2 mL of 95% ethanol in your 10 mL beaker and swirl it vigorously to mix the reactants. Add the p-anisaldehyde solution drop-wise with your pipette to the aqueous NaOH/acetone solution, waiting a few seconds before adding the next drop in order to insure good mixing of the reactants. Poor mixing during this addition could lead to formation of undesired product due to locally high concentration of the reactants. Stir the reaction vigorously at room temperature for 25 minutes. A color change of the reaction mixture to yellow will provide a visible indication of product formation. At this point follow the instructions in the section 'Isolation and Purification of Products from Procedures A and B' as given below to perform your work-up. 2. What feature of the experimental conditions followed in procedure B dictate the structure of the product you obtain? (Use an electron pushing mechanism to rationalize your answer for this question; hint: what is the limiting reagent and why does this make a difference?) (2 points)

Answer 1

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