1. Calculate the volume required to produce 0.0125 mol of acetone.2. Calculate the volume of 2.2 “equivalents” of benzaldehyde. (In other words, 2.2 times as many moles of benzaldehyde as of acetone.) Note: the equation involves a simple 2:1stoichiometry.By using an actual 2.2:1 ratio, it ensures that the benzaldehyde is surplus and that the acetone is limiting. This is helpful for several reasons:
a. Aldehyde oxidation. Aldehydes are often impure because oxidation to carboxylic acid is fairly facile. By using 2.2 equivalents of benzaldehyde, then even if 10% of the benzaldehyde is corrupt we ensure that we still have enough to fully react with the acetone.
b. Reaction Time. By having an excess of benzaldehyde, it makes it easier for the reaction to go to completion. Otherwise late in the reaction there isn’t much benzaldehyde left to react, so the reaction slows down a lot. By intentionally putting in some extra, it maintains at least a minimal concentration of electrophilic till the very end, such that getting 100% conversion of the ketone isn’t so hard and doesn’t take so long. Ease of Product Purification: Disubstitution versus monosubstitution. Enabling complete conversion greatly simplifies purification. If complete conversion does not occur, either because benzaldehyde runs out or because insufficient time is used, the desired “disubstitution” product “dibenzalacetone”, in which two benzaldehydes have been incorporated, is contaminated by “benzal acetone”, the “monosubstitution” product in which only one benzaldehyde has been incorporated. Since the mono- and disubstituted products aren’t that different, it’s not that easy to remove the undesired side-product from the main desired product. But if you just make sure the reaction goes all the way to the desired product, then you don’t need to worry about it! Doing the Reaction:1. Use a 125-mL Erlenmeyer flask with a magnetic stirring bar.
2. Add 50 mL of the NaOH-Ethanol-Water solution mixture.
3. Place the solution on the magnetic stirrers and adjust the stirring dial to get a nice, even stirring action.
4. To this add the calculated amount of benzaldehyde by syringe
5. Add the calculated amount of acetone by syringe, last. (The acetone should go in last after the benzaldehyde electrophile is already available. If the acetone goes in first, it could do aldol condensation on itself, in which enolate anions just attack neutral acetone carbonyls. Ketone carbonyls aren’t competitive with aldehyde carbonyls as electrophiles, but if there are no aldehydes available, ketones are better than nothing!)
6. Watch the solution carefully, with a watch, at the beginning of the reaction, so that you can keep good observational records. How long does it take for the solution to turn yellow? Given that all the reactants are colorless, what does the yellow color mean?