Read and be familiar with the experimental procedure in the lab manual. You will

Question

Read and be familiar with the experimental procedure in the lab manual. You will analyze your reaction by IR spectroscopy and melting point determination. You should recrystallize your crude product from ethanol as directed in your laboratory manual. Why is benzaldehyde a particularly suitable reactant for the crossed aldol condensation? How will IR spectroscopy help you determine if your reaction is complete? What IR stretches can help you to distinguish an aldehyde from a ketone? What changes would you expect in the^1 H NMR of the product versus starting material in this reaction? How many unique signals should appear in the^1 H NMR of starting material and product? Note any observations you make during the entire course of the experiment. Determine the limiting reagent and calculate the percent yield of your purified product. Obtain an IR spectrum of your recrystallized product and benzaldehyde. Dissolve about 50 mg of your purified product or benzaldehyde) in 3 mL of CHCl_3 (chloroform) and then place 3-4 drops of this solution on an IR plate. Let the solvent evaporate after each drop. Then obtain the IR spectrum. Obtain a melting point of your purified product after recrystallization from EtOH. What information from your IR spectrum indicates that was formed and benzaldehyde was completely consumed? Provide a table, as below, (for both benzaldehyde and) showing the pertinent IR absorptions and their assignment. What information about the purity of the product could you infer from the melting point? A HNMR of starting material and product is attached. Prepare a table similar to the one shown below in your notebook for both benzaldehyde and. What is the coupling constant for the hydrogens? Label the hydrogens on your compound, H_ H_ H_ etc. to correspond with your table assignments.

Explanation / Answer

A: Pre-lab questions: Answer

(1) Benzaldehyde is favoring the dehydration step of Aldol condensation due to the extended conjugated system. That complete conjugated system will be more stable than other Aldol products, which can form by aliphatic/other aldehydes.

(2)-(i) Disappearance of C=O stretching at near 1700 cm-1 (aromatic CHO present in reactant) and (ii) disappearance of C=O stretching at 1715 cm-1 (CO present in reactant, acetone) in IR spectrum will confirm the completion of reaction.

(3)-(i) In 1H-NMR of reactant acetone will appear as singlet of 6 Hs, because all Hs are equivalent; (ii) In 1H-NMR of benzaldehyde will appear in four set peaks-

Singlet, 1H, CHO; doublet, 2Hs, ortho Hs, doublet of doublet (dd); 2Hs, meta Hs, multiplet; and 1H, para H in aromatic ring.

(ii) In the aldol condensation product, additional two peaks will appear as a doublet of 1 H each with coupling constant J= 15-20 Hz (due to trans olefin Hs, these are the unique peaks for product). Rest of the peaks of product in aromatic region will be same as benzaldhyde. But the intensity will be double due to two aromatic rings.

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