Separation of Three Compounds

Experiment 1

Separations in a Separating Funnel

Objective

The aim was to separate, using a separating funnel only, the three compounds benzimidazole, benzoic acid and diphenylethanedione in solution. The experiment was based on the principles that carboxylic acids can dissolve in water when ionised first (into salts) using a base such as NaOH or NaHCO3 due to its acidic nature; amines are basic and are, therefore, ionised by mineral acids like HCl to dissolve in water. Neutral organic compounds such as ketones can then be recovered directly from the organic layer. The principles distinguish between the organic substances (in this experiment, an acid, a diketone and an amine). The substances separated were purified by recrystallisation.

Method

Extraction - benzimidazole

The pre-weighed starting mixture (3g) was transferred into a separating funnel (250ml); the funnel was secured uprightly in a retort ring on a stand. Solutions of diethyl ether (30ml) and dilute HCl (3M, 4ml) were added to dissolve the solids, and the pressure was equalised by removing the stopper after briefly shaking the funnel. This was repeated and, with the stopper removed, the contents of the funnel was left to separate. The aqueous (lower) layer was drained off into a conical flask (25ml). To this water (3ml) was added, and the rest of the aqueous layer was removed after shaking. The PH of the solution was recorded using a universal PH test paper.

NaOH (3M, 4ml) was mixed into the solution and then added dropwise to give a crystallised, alkaline precipitate (PH paper turned blue). The flask and a test-tube of water were cooled in an ice-bath; then the benzimidazole was filtered by suction on a Hirsh funnel and washed with the cooled water, with recycling of the filtrate. The solid was dried in the 80˚C oven on a pre-weighed watch glass. After the drying process, the weight and the m.p. of the solid was recorded.

Extraction - Benzoic acid

The same method was used with the funnel using NaHCO3 (12ml) in diethyl ether. This time, because of the evolution of CO2, the funnel was shaken gently the first time and the pressure was released before shaking again. The pressure was equalised at frequent intervals. The aqueous phase was then drained into a conical flask (100ml) and re-drained with water as before. The PH of benzoic acid was recorded twice as with the benzimidazole, the second time upon addition of HCl (1ml, then dropwise) until acidic (PH paper turned red). The flask was cooled and the solid was drained with suction, dried in the oven; the weight and m.p. were then determined.

Recovery - diphenylethanedione (benzil)

The diethyl ether phase was removed from the funnel into a conical flask (100ml), rinsing the funnel with more diethyl ether (5ml). Two large spatula-tip loads of anhydrous sodium sulphate was added and shaken in for a few minutes to dry the mixture. It was then transferred to a pre-weighed B14 round flask (100ml) after 2 washes with some diethyl ether (5ml), with all of the sodium sulphate left behind. The diethyl ether was then removed on a rotary evaporator, weighed and, its m.p. was determined.

Recrystallisation - benzimidazole and benzoic acid

The dried benzimidazole was transferred into a conical flask (25ml), into which were also added anti-bumping granules (2) and water (10ml). The mixture was brought to a boil with a heat gun, whilst more water (approx. 1ml at a time) was being added until all the solid was dissolved, leaving behind the granules and any impurities. The solution only was then decanted into another flask and allowed to cool without aid. The suspension of the crystals formed was cooled in an ice bath. Using the filtration technique as with the extraction procedures, the crystals were collected. Once the crystals were washed and drained, they were dried in the oven and weighed. The m.p. was re-determined.

Recrystallisation - diphenylethanedione

Ethanol (3ml) was added to the diphenylethanedione (round bottomed flask) and brought to boil to dissolve the solid. The solution was cooled slightly and then pipetted into a conical flask (25ml). More ethanol (1ml) was added to the round bottomed flask, which was then heated to collect and pipette into the conical flask any residue left behind. The solution was cooled in air until crystals formed and then in an ice-bath with some ethanol. The crystals were filtered with suction on a Hirsh funnel, and washed with cold ethanol. The drained solid was then dried in the oven, weighed; its m.p. was determined for the second time.

Results

approx. PH after extraction approx. PH after recovery

Benzimidazole 1 9

Benzoic acid 8 1

WEIGHT / g M.P. / ˚C Text M.P. / ˚C (Sigma-Aldrich)

Crude Recrystallised crude Recrystallised Recrystallised

Benzimidazole 0.80 0.73 171-172 171 169-171

Benzoic acid 1.07 0.89 120-121 121 121-125

Benzil 0.69 0.57 92-95 92-93 94-95

Percentage yields

Crude / % Recrystallised / %

Benzimidazole 80 73

Benzoic acid 107 89

Benzil 69 57

Benzimidazole

Moles: m/Mr = 1.00/210.23 = 4.76 x 10-3 mol

Theoretical mass = 3/1 = 1.00g

% yield = empirical mass obtained (g) x 100

Theoretical mass (g)