Question

Methanol and water mixtures were found to be suitable for recrystallization based upon solubility test results for the reaction product mix. Explain why this might not be a good idea besides the yield may suffer (why?) or mixed solvents are difficult to work with.

Here are the procedures for the lab if this helps:

Synthesis of Nitrated Phenylacetic Acid

The instructor will assign each student to either 90% fuming nitric acid or 70% nitric acid.

**** 90% fuming nitric acid was used****

1. Place a magnetic stir bar in a small round-bottom flask, and obtain approximately 5.0 mL of either 90% fuming nitric acid or 70% nitric acid from the instructor.
   1. THIS STEP REQUIRES ADDITIONAL PPE AND SHOULD BE DONE IN A HOOD.
2. Obtain 0.5 g of phenylacetic acid and add slowly to the nitric acid while maintaining a temperature between ~25–35 °C.
   1. PHENYLACETIC ACID HAS AN ODOR THAT CAN CLING TO CLOTHING AND OTHER SOURCES. YOUR INSTRUCTOR MAY ADVISE MEASURING IN A HOOD AND DISPOSING OF USED WEIGH BOATS IN A SOLID WASTE CONTAINER.
3. Once the phenylacetic acid is completely dissolved, reflux the solution for 1 hour. At the top of the condenser column, place septa with a needle to vent any NO₂ gases.
4. After refluxing for 1 hour, allow the round-bottom flask to cool to room temperature.
5. Pour the contents over ~10 mL of ice water.
6. Collect the precipitate using vacuum filtration, washing any flasks and the product with small amounts of cold DI water.
7. After drying for 15–20 minutes, record the yield of the product.

Recrystallization and Characterization

1. Recrystallize the product using DI water.
2. Obtain the melting point, IR, and proton NMR of the product.
3. If known nitrated phenylacetic acid reagents are available, obtain the melting point, IR spectrum, and proton NMR spectrum of those for comparison.

Answer 1

Methanol and water mixtures were found to be suitable for recrystallization based upon solubility test results for the reaction product mix.

But this does not produce good yield because dissolution of product in the solvent and this is lost during the separation of solvent and crystals.

Sometimes solids are dissolved below the boiling point of the solution that requires the more solvent and produce less yield.

Usage of Mixed solvents for recrystallisation can be used for recrystallisation where one solvent is used for solubilisation of product whereas other solvent is used for stimulating the crystallisation after saturation of solid solution with primary solvent.