3.3C: Determining Which Solvent to Use

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The most important factor in the success of crystallization is probably the chosen solvent. Besides having the crucial solubility properties for crystallization (the compound should be soluble in the hot solvent and as insoluble as possible in the cold solvent), there are other factors that determine an appropriate solvent.

An ideal crystallization solvent should be unreactive, inexpensive, and have low toxicity. It is also important that the solvent have a relatively low boiling point (b.p. often \(< 100^\text{o} \text{C}\) as it's best if the solvent readily evaporates from the solid once recovered. Table 3.1 shows a list of common solvents used with crystallization. Toluene has the highest boiling point \(\left( 111^\text{o} \text{C} \right)\) of the list, and should be avoided if alternatives exist for this reason (as well as its toxicity and smell). Along with fast evaporation, a relatively low boiling solvent is also ideal for crystallization as it minimizes the probability of a compound "oiling out", where material comes out of solution above its melting point and forms a liquid instead of a solid. When a compound liquefies first, it rarely crystallizes well.

Table 3.1: Boiling point of common solvents in crystallization. Note: petroleum ether is a mixture of hydrocarbons, and is very nonpolar. The term "ether" comes from its volatility, not the functional groups present.
Solvent	Boiling Point (°C)
Diethyl ether	35
Acetone	56
Petroleum Ether (low boiling)	30-60
Ligroin (high boiling petroleum ether)	60-90
Methanol	65
Hexanes	69
Athyl acetate	77
Ethanol	78
Water	100
Toluene	111

Solvents with very low boiling points (e.g. diethyl ether, acetone, and low-boiling petroleum ether) are highly flammable and can be difficult to work with as they readily evaporate. They can still be used with care, but if alternatives exist, they are often preferable.

There are some general trends in predicting the appropriate solvent for a particular compound. As the compound needs to be soluble in the boiling solvent, it helps if the compound and solvent have similar intermolecular forces. For example, if a compound can hydrogen bond (alcohols, carboxylic acids, and amines), it sometimes can be crystallized from water. If a compound has moderate polarity it sometimes is crystallized from ethanol. If a compound is mostly nonpolar, it sometimes is crystallized from petroleum ether or hexanes, or may require a mixed solvent.

As it can be difficult to predict the ideal solvent for crystallization, often a published procedure from a journal article will indicate the appropriate solvent. If no solvent is listed, an appropriate solvent can be predicted using solubility data (next section), determined experimentally, or chosen from Perrin's Purification of Organic Chemicals,\(^4\) a reference book which lists purification procedures for roughly 5700 known compounds.

\(^4\)D.D. Perrin, W.L.F. Armarego, Purification of Organic Chemicals, Pergamon Press, 3\(^\text{rd}\) edition, 1988.