Chemistry lab techniques

• Recrystallization with a mixed solvent system (water and methanol): the initial step involves dissolving the compound in a minimum amount of one of the hot solvents.
• Liquid solvent acetone (C3H6O) should be disposed of in the non-halogenated waste container.
• A high and broad melting point for a known substance indicates that the ramp rate of the Meltemp instrument was too fast, or there was too much sample.
• Filter paper is fluted for gravity filtration to allow more surface area of the paper to come into contact with the mixture being filtered.
• A sign that a solid sample may contain an impurity is a broad melting point range (>2°C span) and a value lower than the literature value.
• An impure solid sample typically has a broader than expected melting point.
• Organic chemists rinse glassware with organic solvents like acetone or alcohol to remove nonpolar or slightly polar residues after soap and water washing.
• Vacuum filtration is used to collect purified crystals.
• Melting point is a physical property that can help determine the identity of an unknown solid.
• Cold solvent is used to wash collected crystals because it is less likely to re-dissolve the solid.

Recrystallization Techniques

• If a dissolved colored impurity and solid contaminants are present, add activated charcoal to the solution of the unknown compound in the solvent before gravity filtration.
• Dichloromethane (DCM, CH2Cl2) should be disposed of in the halogenated waste container.
• In recrystallization with a mixed solvent system like ethanol/water, heat the sample in a minimal amount of hot ethanol and add drops of hot or cold water until the cloud point is reached, then allow to cool.
• To remove an insoluble solid like sand from an impure solid sample during recrystallization, use a solvent to dissolve the desired compound, then perform hot gravity filtration.
• A good recrystallization solvent dissolves the solid of interest when hot, but not when cold or at room temperature.
• If crystals are forming during gravity filtration when removing activated charcoal, rinse the filter paper with hot methanol.
• Solvents used in crystallization should be miscible.
• During microscale boiling point determination, the bubbles coming out of the capillary tube are actually the vapor of the unknown liquid.
• Insoluble impurities are removed from the solution during crystallization via filtration.
• In microscale boiling point determination, the capillary tube should be inserted with the open end down in the liquid.
• When separating a solid from a liquid via filtration, the liquid is called the filtrate.
• For crystallization, the solution should be saturated.
• Activated charcoal is used as a decolorizing agent.
• Using an excessive amount of solvent when trying to recrystallize a solid may result in few or no crystals forming.
• During vacuum filtration after recrystallization with Dichloromethane (DCM), the filtrate should be placed in the halogenated waste container.
• After collecting crystals, they should be washed with cold solvent.
• For microscale boiling point determination, the thermometer bulb should be aligned with the curved bottom of the test tube.
• A good practice for obtaining an accurate melting point of a solid is to increase the temperature at a rate of 2°C once the temperature reaches 20°C below the expected melting point.
• In Siwoloboff's method, the thermometer is read to determine the boiling point when the entire setup is cooled, and the liquid unknown begins to enter the capillary tube.

Distillation Techniques

• An azeotrope is a mixture of two or more liquids with a constant boiling point.
• Precipitation is not a step in the process of distillation.
• In a distillation setup, water should enter from the bottom of the condenser and exit from the top.
• When conducting a "safe" distillation: a boiling chip should be placed in the distillation flask and the glassware joints should be lightly greased.
• Fractional distillation under atmospheric pressure most likely would be used to separate ethyl acetate (boiling point of 77 °C) and ethanol (boiling point of 80 °C).
• Boiling stones or stir sticks are used when heating a liquid to provide a surface on which gas bubbles can form, preventing superheating.
• A forerun is a low boiling liquid that sometimes distills over long before the boiling point of the liquid you want has started to boil.
• The bulb of the thermometer should be aligned slightly below the opening of the 3-way adapter leading to the condenser for accurate reading
• During distillation, temperature is constant while a component is distilling as long as the composition of the mixture isn't changing.
• The distillate is a liquid that condenses after distillation.
• During distillation, the first boiling point range should be recorded when the vapor condenses to form a drop of liquid on the thermometer bulb and the temperature stabilizes.
• In fractional distillation, a large surface area for condensation is provided through a fractionating column.

Extraction techniques

• Products will be more soluble in water than in an organic solvent.
• Forgetting to heat the solution before gravity filtration will cause the product to precipitate and remain in the residue.
• Extractions should be performed with multiple small volumes.
• An acidic compound is being extracted into the sodium bicarbonate solution.
• The organic layer will be at the bottom of the separatory funnel.
• Smaller amounts of compound are expected with each successive extraction.
• Melting points will be broader and lower than expected.
• A saturated sodium bicarbonate solution is NaHCO3 dissolved in water to a point that no more can dissolve.
• Benzoic acid is being extracted into the sodium bicarbonate solution.
• Check that the separatory funnel is properly assembled.
• When mixing hexane with water, expect to see two layers: the top layer will be hexane, as hexane is less dense than water.
• To separate a water-insoluble organic base from a water-insoluble neutral organic compound, dissolve both compounds in ether and extract it with an acid.

Chromatography Techniques

• The pH of the solution during the addition of 6M HCl to the aqueous layer in an extraction is checked to verify that the water was acidic so that all of Compound A would be in its protonated form and precipitate from the water.
• Organic substances with acidic functional groups may dissolve in water if treated with base.
• The Rf value for a spot that travelled 2.0 cm up a TLC plate if the solvent front travelled to 8.0 cm is 0.25.
• All of the practices should be followed when preparing a gravity chromatography column.
• The first step of grinding spinach leaves in acetone with a mortar and pestle is to solubilize the organic plant pigments in the spinach leaves.
• To "dry" the organic layer by absorbing any water that remains in the hexane.
• An extraction with ether and washing the ether with a base such as saturated sodium bicarbonate solution.

Reaction Mechanisms

• For the reaction of Sodium Ethoxide with 1-Bromobutane the Reaction Mechanism is SN2.
• The precipitate is sodium bromide and it dissolves in the aqueous layer during quenching and is removed during the extraction work up.
• During the workup of the reaction, diethyl ether is washed with brine solution to help removes traces of water in the ether.
• Using multiple smaller volume extractions extracts a greater mass of the compound than does one larger volume extraction.
• Diethyl ether will be the top layer because it is less dense than water