Foundation: Solvents

What are solvents and why do we use them?

Solvents are widely used throughout the chemical industry for example in:

  • Synthetic Chemistry: as a reaction medium on both laboratory and industrial scale and in work-up and purifications.
  • Analytical Chemistry: for sample extraction and preparation (spectroscopy) and chromatography mobile phase (HPLC, TLC etc.)
  • Crystallography: for recrystallisation to purify compounds and prepare crystals suitable for analysis.

Solvents have many other applications other than synthetic chemistry, for example in paints and adhesives, for cleaning (e.g. dry cleaning) and for extraction (e.g. decaffeination of coffee).

Solvents are used as a reaction medium for various reasons including:

  • To bring reactants together at suitable concentrations;
  • For energy control:
    • Endothermic reactions require energy – heat can be supplied by heating solution;
    • Exothermic reactions – solvent acts as a heat sink preventing runaway reactions;
  • For efficient mixing and stirring;
  • To allow addition of solid reagents as a solution;

Solvent extraction is also vital part of purification process.  Often more solvent is used in work-up than as reaction medium.  Partitioning products between an organic phase and an aqueous phase is a very important way of extracting products, although this generates aqueous waste as well as organic waste, both of which require disposal.

Some common classes of solvents are given below.

Solvent Class

Example

Alkanes

Hexane

Aromatics

Toluene

Alcohols

Ethanol

Ethers

Diethyl ether

Polar aprotic

Acetonitrile

Chlorinated

Dichloromethane

Ketones

Acetone

Acids

Acetic acid

Bases

Pyridine