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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 ClassExample
Alkanes Hexane
Aromatics Toluene
Alcohols Ethanol
Ethers Diethyl ether
Polar aprotic Acetonitrile
Chlorinated Dichloromethane
Ketones Acetone
Acids Acetic acid
Bases Pyridine