Distillation & Recrystallization experiment

Experiment Overview

• Experiment 2 involves distillation of 1,4-dimethoxybenzene and recrystallization of benzoic acid.

Distillation of 1,4-Dimethoxybenzene

• This experiment isolates 1,4-dimethoxybenzene by distilling off the remaining DCM (dichloromethane).
• The solution containing 1,4-dimethoxybenzene and DCM is heated until boiling.
• Vapors condense in the condenser and DCM is collected in a receiving flask.
• 1,4-dimethoxybenzene is left in the round bottom flask.
• The round bottom flask containing 1,4-dimethoxybenzene in DCM is obtained from the previous week.
• The TAs evaporate the solvent using the Rotovap, ensure the flask is well labeled,
• After the dichloromethane is removed, the flask is weighed to determine the mass of 1,4-dimethoxybenzene.
• The percent yield is calculated.

Recrystallization of Benzoic Acid

• Recrystallization removes impurities from a crystalline substance using a recrystallization solvent.
• Recrystallization works because substances are more soluble in hot solvents and impurities have different solubilities.
• Benzoic acid is recrystallized using water.
• A hot plate is set up on medium heat with an Erlenmeyer flask (~250 mL) half-filled with deionized water, heated to ~90 °C.
• Dry flasks should never be placed directly on a hot plate as the glass may crack.
• The weight of the dried benzoic acid and 2-naphthol extracts from the previous experiment is recorded, removing any labeling tape.
• Clumps of benzoic acid are crushed with a metal scraper and placed into a 250 mL Erlenmeyer flask with a powder funnel and scraper, a magnetic stir bar and ~25 mL hot deionized water are also added to the flask.
• The flask containing benzoic acid and water is placed on the hot plate with constant stirring until the benzoic acid starts to dissolve.
• Hot water from the 250 mL Erlenmeyer flask is added to the benzoic acid solution using a Pasteur pipette until dissolved.
• A small amount (~0.15 g) of decolorizing charcoal is added to the flask, stirring for ~2-3 minutes at medium-low speeds.
• Another 250 mL Erlenmeyer flask with a small amount of deionized water is placed on the hot plate.
• A hot gravity filtration apparatus is set up.
• Crucially, everything must be kept hot to prevent crystal formation on the filter paper.
• Filter paper is wetted with hot water using a Pasteur pipette before hot filtration.
• The contents of the benzoic acid flask is slowly poured into the filtration funnel, using a paper towel if the flask is too hot.
• The flask is rinsed with hot water, and the contents poured into the filter.
• The flask is removed from the hot plate, and filter containing the solids (charcoal) is discarded.
• The slower the flask cools, the larger and purer the crystals formed will be.
• A vacuum filtration apparatus is set up, and the workstation is cleaned while the flask cools.
• A beaker and filter paper is weighed separately.
• Once the benzoic acid cools and begins to crystallize, the flask is placed in an ice bath and swirled.
• Another flask of deionized water is placed into the ice bath.
• The crystals are collected by vacuum filtration on the pre-weighed filter paper.
• The flask is rinsed with ice-cold water to collect any remaining crystals, retaining the solids this time.
• The crystals are allowed to dry for a few minutes with the vacuum running.
• The crystals and filter paper are transferred to a pre-weighed beaker, labelled with the group number, and stored by the TAs.

Distillation Considerations

• The round bottom flask, or RBF, should never be more than half full.
• RBFs should contain boiling chips to prevent bumping.
• A thermometer monitors the boiling point of the vapors for dichloromethane.
• The thermometer is installed in the distillation head, centered, to ensure accurate boiling point readings.

Gravity Filtration

• Gravity filtration removes a solid from a liquid by pouring the liquid through filter paper in a funnel.
• Gravity filtration removes impurities, activated carbon, or drying agents during recrystallization.
• The liquid collected after filtration is called the filtrate.
• The filter paper must be folded correctly.
• The funnel is heated during recrystallization, ensuring the solvent is hot, which is known as “hot gravity filtration”.

Recrystallization Defined

• Recrystallization purifies a substance by dissolving it in a hot solvent, then re-crystallizing it.
• Soluble colored impurities are removed using a decolorizing agent (activated carbon, NORIT).
• Only a small amount of decolorizing agent should be used to avoid product loss.
• The solution is cooled slowly, first at room temperature and then in ice, to induce crystal formation.
• Slow cooling results in larger crystals, while impurities may be trapped in the crystals when cooling rapidly.
• Crystals are collected through vacuum filtration.

How Recrystallization Works

• Recrystallization works by heating the solvent to its boiling point, where impurities are often insoluble.
• Impurities are removed via hot gravity filtration.
• Too much recrystallization solvent reduces the yield and can prevent recrystallization.
• Activated carbon (Norit) removes colored impurities.
• Never add Norit to a solution at or near boiling point or explosion may occur.
• Scratching the flask or "seeding" with a pure crystal can induce crystallization if no crystals form after cooling.
• The ideal recrystallization solvent dissolves a large amount of the substance in hot solvent and only a small amount in cold solvent.
• A solvent's volatility is important, with low boiling points being easier to evaporate.
• High boiling point solvents require heating under a vacuum.
• If the melting point of the recrystallized substance is significantly lower than the boiling point of the recrystallization solvent the substance being recrystallized will melt.
• The substance will form a liquid and "oil out" and not recrystallize..

Vacuum Filtration

• Vacuum filtration isolates a solid from a solid/liquid mixture or removes impurities from a large quantity of liquid.
• Vacuum filtration uses a Buchner funnel, a vacuum filter flask, thick vacuum tubing, and a vacuum aspirator.
• A filter flask is clamped to stand and connected to the vacuum aspirator via a rubber tube.
• A circular filter paper is placed on the funnel, and the vacuum aspirator is turned on
• The filter paper is moistened with ice-cold solvent, and the solid/liquid mixture is poured onto it.
• The vacuum aspirator forces the liquid through the filter paper and leaves the solid behind to compact and air-dry.
• To turn off the vacuum, disconnect the rubber tubing and then close the inlet.

Heating Under Reflux

• Heating increases the rate of most organic reactions, while reflux prevents the loss of solvent.
• Heating under reflux involves heating a liquid at its boiling and using a condenser to flow the vapors back into the reaction vessel.
• Refluxing time refers to the time after the reaction begins to boil.
• The heat source should be set to a level where one drop of solvent drops back into the flask per second.

Reflux Setup Procedure

• The heat source should be positioned on a stand.
• Select correct size RBF and ensure it is only halffull
• The flask should be clamped securely to the stand.
• Add reactants using a funnel, along with boiling chips or a magnetic stirring bar.
• Tubes should be connected to the condenser, and the condenser placed onto the RBF.
• Connect the lower tube of the condenser to a cold water source.
• Place the upper tube into the drain of sink and secure it to prevent water from splashing out.
• The heat source should be turned on and the solvent should boil gently, dripping back at approximately 1 drop per second.
• A clearly visible reflux ring of condensing vapors should be present.
• Reduce the heat if the ring appears more than halfway up the condenser.
• Never heat the reaction before the condenser has water running through it, or solvent vapors can escape and cause a fire.

Magnetic Stirring

• Stirring is conducted to ensure heat transfer and efficient contact between reactants.
• Magnetic stirrers are found on hot plates, using a magnetic stir bar to mix the solution.

Melting Point Determination

• A substance’s melting point is the temperature at which the solid and liquid phases.
• Melting point helps determine the purity and identity of a substance when compared with the theoretical melting point.
• The substance is pulverized and tapped into a melting point capillary tube to a height of 1-2 mm.
• The tube is tapped to move the solid to the closed end.
• The capillary tube is placed in a Mel-Temp apparatus, the heating rate is adjusted using the dial.
• Set the dial to 7 to determine approximate melting temperature and then cool the machine to fifteen degrees below the orientation melting point and then slowly heated up again
• The temperature range between the start and end of melting is recorded.
• Pure substances usually melt within a 1-2 °C range.

Rotary Evaporation

• The Rotovap removes volatile solvents from non-volatile samples by (1) reducing pressure, (2) rotating the sample, and (3) heating the solution.
• Ensure the power is on and the apparatus has enough space above the water bath.
• The temperature and the water level of the water bath must be checked, and topped with distilled water if necessary or cooled with ice.
• The cooling water to the condenser coils is turned on, then the vacuum.
• Check to ensure the stopcock is lined up with the vent hole.
• A round bottom flask (no more than half full) with sample solution is attached to the ground glass joint with a clip.
• A reduction adaptor may be needed if the flask neck size does not match the joint on the rotary evaporator.
• The flask is set to rotating.
• Close the stopcock (keepng hand on the tap) to generate a vacuum and the evaporation flask lowered into the water bath.
• Release the vacuum if it boils vigorously, once it is set turn on heat to the thermostatic water bath to 40-50°C.
• After evaporation, the apparatus out of the water is raised, stopping the rotation.
• The flask is held and the vacuum is released, carefully removing it.
• A white flask remover is used to help push the flask off if stuck.
• The water aspirator and condenser water should only be turned off after the flask is removed, remembering to empty any waste solvent.