Recrystallization

Most products contain impurities. One way to purify these products is by recrystallization. Recrystallization begins with dissolving the impure product in just enough hot solvent to form a saturated solution, where as much solute is dissolved in the solvent as possible. Any additional solute will not seem to dissolve.

A hot solvent is used because solubility typically increases with temperature. As temperature increases, the amount of solute that can be dissolved in the solvent increases. As the solution cools, the solubility of the product decreases, and solute molecules come together to form small stable crystals called nuclei.

This is the first step of crystallization, called nucleation. Additional crystal growth happens on the nuclei because solute molecules have a greater affinity for joining existing solute crystals than forming new crystals. Soluble impurities are left in solution.

Crystallization can happen spontaneously or can be encouraged by scratching the inside of the flask, agitating the solution, or adding a seed crystal of the compound, all of which provide a surface for further growth.

Recrystallization into large, pure, regularly shaped crystals only works when an appropriate solvent is used. The compound should be insoluble in the solvent at room temperature and soluble at high temperatures. Ideally, the impurities should be either insoluble in the solvent at high temperature or soluble in the solvent at room temperature.

If the impurities are insoluble in hot solvent, they are filtered out before crystallization. After recrystallization, the crystals are filtered out and washed with cold solvent to remove impurities from the surfaces. Now, the purity of the crystals can be analyzed.

When choosing a solvent, keep in mind that the greater the difference in solubility between high temperatures and low temperatures, the more likely the solute will come out of solution as it cools to form crystals. The rate of cooling is also important in recrystallization.

Rapid cooling favors the formation of many nucleation sites and the growth of smaller crystals, whereas cooling slowly induces the formation of fewer nucleation sites and the growth of fewer, larger but purer crystals. So, slower cooling is preferred.

In this lab, you will recrystallize two impure organic compounds, acetanilide and trans-cinnamic acid, and then assess the purity of the recovered compounds by comparing their melting point range to values in the literature.