Organic Chemistry Lab Techniques

Here is a cheat sheet you can use to study organic chemistry lab techniques tested on the DAT:

Summary of Purification Methods

Extraction: separates dissolved subs. Based on differential solubility in aqueous vs. organic solvents

Filtration: separates solids from liquids

Recrystallization: separates solids based on diff. solubilities; temperature is important

Sublimation: separates solids based on their ability to sublime

Centrifugation: separates large things (ex. cells, organelles, macromolecules) based on mass and identity

Distillation: separates liquids based on boiling point (depends on intermolecular forces)

Chromatography: uses stationary and mobile phases to separate compounds based on how tightly they adhere (generally due to polarity but sometimes size as well)

Electrophoresis: used to separate biological macromolecules (such as proteins or nucleic acids) based on size and sometimes charge

Extraction

• Water = aqueous layer; ether = organic layer
• Like dissolves like
• Three IMF that affects solubility:

1. Hydrogen bonding – ex. Alcohols & acids will move into aq. layer‍
2. Dipole-dipole interactions – less likely to move in aq. layer‍
3. van der Waals (London dispersion) – nonpolar molecules (does not go into aq. layer)
   

• When ACID dissociates, resulting anion formed is more soluble
• ***ADDing a BASE helps EXTRACT ACID into the aq. layer

Simple Distillation

• Separate liquids that boil BELOW 150°C (at least 25°C apart)

Vacuum Distillation

• Separates liquids that boil ABOVE 150°C
• Reduced P, lowering the BP of liquids (preventing their decomposition typical at high T)

Fractional Distillation

• Separates liquids that boil LESS than 25°C apart
• Near the top of the column, vapor is composed solely of 1 component, which will condense and collect in the receiving flask
• Can be thought of as repeated distillation of same vapor

Thin Layer Chromatography

• Used to isolate individual compounds from a complex mixture
• Stationary phase (solid medium) & mobile phase (liquid)
• Diff. compounds will adhere to stationary phase w/ diff. strengths

1. POLAR compounds bound TIGHTly to the silica gel – eluting poorly into the less polar solvent

• Rf = dist. compound / dist. of solvent
• Reverse phase chromatography – very nonpolar stationary phase instead of silica gel

Electrophoresis

• Separates macromolecules based on isolectric point
• If pH = isoelectric point --> protein doesn’t move
• If pH > isoelectric point --> protein deprotonated

SDS & Agarose Gel Electrophoresis

• Separates molecules based on SIZE

MP and BP trends

BP

• Increases with chain length due to dispersion forces
• Decreases with branching

MP

• Increases with branching because the molecules can pack tightly
• Increases with chain length again due to dispersion forces

Occasionally, on the DAT Organic Chemistry section, you’ll be asked about one of the chemical lab tests you likely used in your lab sections to identify the functional groups in a molecule.

Here is a list of the most common lab tests, which functional group they test for, and what a positive result looks like:

*Fun fact: The Jones Test is used in breathalyzers to test for drunk drivers.