Matthew McBride


To recrystallize benzoic acid from water with a pentane wash using the sintered glass funnel method to determine whether pentane can remove the solvent water.


Benzoic acid (0.2584 g) was added to a 10 mL screw cap test tube and water was added until all of the benzoic acid went into the solution around 100°C to create a 1.35 M solution. The solution was cooled on ice for 15 minutes and the transferred to a sintered glass funnel using pentane. The crystals were filtered using suction filtration and washed with an excess of pentane (3x). The crystals were allowed to air dry for ten minutes over the suction and the purity was determined by HNMR.


Recrystallization Sheet
Concentration: 1.35 M
Amount of Product Recovered: 0.1725 g (recovered on sintered glass funnel)
Yield: 67%
HNMR Spectrum
Water present in the sample due to the water peak at 1.5ppm.

Picture #1: The crystals passing through the filter after the pentane washes


At a concentration of 1.35 M, the temperature curve predicts that the recrystallization yield of benzoic acid in water is 98%. This experiment only found a recrystallization yield of 66%. This was because the benzoic acid was slightly soluble in the pentane and consequently the product was removed by the pentane washes. As can been seen on the solvent selector, the solubility of benzoic acid in pentane at room temperature is 0.053 M. This caused the pentane washes to remove some of the product. The pentane washes are only effective on compounds that have extremely low solubility in pentane.


The recrystallization yield of benzoic acid in water at a concentration of 1.35 M was 67%, but it was determined that the pentane washes cannot remove water, as water was still present in the product following the three washes.



11:20 The ethylene glycol bath was placed on the hot plate and set to 250°C.
11:24 The weight of the empty 10 mL screw cap test tube with the cap was 11.4609 g. The weight of the tube with benzoic acid was weighed to be 11.7193 g (0.2584 g of benzoic acid). 0.8432 g of distilled water was added to the test tube.
11:36 The tube was vortexed and placed in the hot bath at 80°C.
11:42 The solution began to boil and a few crystals were still visible. A small amount of distilled water was added to the solution.
11:43 All of the crystals were observed to go into solution. The tube was removed from the hot bath. Crystals were observed to come crashing out of solution immediately.
11:45 The tube with the crystals and the water was weighed to be 13.0670 g.
11:50 The tube was placed in an ice bath to cool.
11:56 The weight of the empty sintered glass funnel was 33.8688 g.
12:06 The tube was removed from the ice bath and the crystals were transferred to the sintered glass funnel using pentane and filtered using suction. The crystals were then washed with pentane 3x. Each wash consisted of turning off the vacuum, adding an excess of pentane to completely cover the crystals, and moving the crystals around in the pentane, before turning back on the vacuum and filtering. Some of the white pentane crystals passed through the filter during the pentane washes. Picture #1 was taken.
12:15 The crystals were allowed to dry over the suction.
12:20 The weight of the empty 5-dram vial was 11.7892 g.
12:25 The crystals were removed from the suction. The weight of the sintered glass funnel with the crystals was 34.0413 g.
12:30 The weight of the 5-dram vial with product was 11.9273 g. Of this, 0.0344 g was added to another 5-dram vial. Approximately 1 mL of acetone was added to these crystals and all the crystals dissolved. This was added to the inner coaxial NMR tube. This inner HMR tube was placed in an our coaxial NMR tube containing CDCl3. The sample was then analyzed by 500 MHz HNMR and the spectrum saved as ONSEXP359_A.