Transcription
Welcome, Alinga viewers. In this video, I will be synthesizing 1-phenyl-2-nitropropane from benzaldehyde and nitroethane. Similar to the previous video, this reaction is called the Henry reaction, or the nitroaldol reaction. The Henry reaction has often been used due to its highly functional scaffolding, universal synthetic capabilities, by easy manipulation of both the nitro and hydroxy functional groups, to make beta-nitro alcohols, which is the conventional product of the Henry reaction. These nitro alcohols can be easily converted to nitroalkenes via a dehydration pathway. These intermediates have found use in various pharmaceutical applications.
To start the experiment, add 5 mL of ethanol into a round bottom flask. Start stirring. On top of that, add 2.02 mL of nitroethane, then add 0.3 mL of enamine. After that, add 2.88 mL of benzaldehyde slowly over 2 minutes. After all of the reagents have been added, attach a Liebig condenser to the round bottom flask and attach a calcium chloride drying tube to the end of it. Connect a cold water pump to the condenser. Don't forget to turn the water pump on. Turn the hot plate on and set the knob ¼ away. Start timing the experiment and leave the mixture to stir at reflux for 8 hours. Make sure to wrap the flask with aluminum foil as well. After 4 hours, we can see that the mixture has turned into a yellow-orange color. After 8 hours, turn the heating off and remove the aluminum wrapped around the flask. As we can see, the mixture has taken a red-orange color.
Now, time for the chemistry in brief. Benzaldehyde and nitroethane react in the presence of an enamine base to form 1-phenyl-2-nitropropane. Interestingly, 1-phenyl-2-nitropropane is the only major product formed when an enamine base is employed. To elaborate, enamine is a primary amine which is organic-soluble. Thus, the requirements of minimal steric hindrance around the amino and high concentration of this reagent in the organic phase are easily met. Hence, enamine facilitates the formation of 1-phenyl-2-nitropropane, the base-catalyzed pathway.
Now, let the mixture cool to room temperature. Turn the water pump off and disassemble the reflux condenser. Then remove the solvent under vacuum, or pour the contents into a beaker and evaporate the solvent under low heat. After a while, you will be left with a mass of yellow crystals. This should be crude 1-phenyl-2-nitropropane. Get the crude 1-phenyl-2-nitropropane and carry out vacuum filtration in order to get the remaining crystals out of the beaker. Add a small amount of cold ethanol, swirl, and pour the contents into the Büchner funnel. Leave the crystals to dry on the pump for a few minutes.
Now we need to carry out recrystallization in boiling ethanol. Add the crude crystals into a beaker. Place the beaker onto a hot plate and add around 6 mL of ethanol. Make sure to add in a stir bar as well. Turn the hot plate and stirring on. After a short while, the crystals have all dissolved in the boiling ethanol. The solution has a clear yellow color. After the solvent has evaporated to around half its volume, turn the hot plate off and carry out a quick hot filtration. Leave the filtered solution to cool down to room temperature. Now we have to induce crystallization on the solution. Get a speck of 1-phenyl-2-nitropropane crystal on the end of a glass rod or glass thermometer and poke the solution. Immediately, we see a rapid formation of a yellow crystalline mass. This should be pure 1-phenyl-2-nitropropane. It was absolutely beautiful observing this. Crush and filter around the crystals. Place the crystals on a watch glass.
Okay, now we have to carry out a melting point test to check the purity of the product. Get a small amount of crystals and drop it into a glass tube. Insert a thermometer directly down into the crystals. Place the glass tube onto the hot plate and start heating very slowly. After a short while, we see the crystals starting to melt at around 60°C. Then we see that all of the crystals have melted at around 64°C. This means that our 1-phenyl-2-nitropropane has a melting point range of 60 to 64°C. The literature has it at 64 to 66°C. This test confirms that our material is relatively pure. The yield of the nitropropane material into a glass vial and seal it off with Parafilm. Store the material in a cool, dark, and dry place.