Crystallization of Impure Sulfanilamide using 95% Ethyl Alcohol

Solution Preview

---

Please find attached the modified lab report. Answers/Comments are in blue ink. See attached for full solution and proper format.

Lab 4: Recrystallization

Objectives:
- Purify an impure sample of an antibiotic.
- Practice the crystallization technique.

Introduction:
The purpose of this experiment is to introduce the technique of crystallization, a very common procedure used to purify crude solids in the organic laboratory. The general technique involves dissolving the material to be crystallized in a hot solvent and cooling the solution slowly. The dissolved material has a decreased solubility at lower temperatures and will separate from the solution as it is cooled. This phenomenon is called crystallization if the crystal growth is relatively slow and selective or precipitation if the process is rapid and nonselective. Since the impurities are usually present in much smaller amounts than the compound being crystallized, most of the impurities will remain dissolved in the solvent even when it is cooled. The purified substance can then be separated from the solvent and impurities by filtration.

In this experiment, you will carry out a crystallization of impure sulfanilamide using 95% ethyl alcohol as the solvent. Sulfanilamide is one of the sulfa drugs, the first generation of antibiotics to be used in successfully treating many major diseases such as malaria, tuberculosis, and leprosy.

Sulfanilamide

The solubility of sulfanilamide in 95% ethyl alcohol is given in the following table:

Temperature Solubility (mg/mL)

0°C 14
20°C 24
40°C 46
60°C 88
78°C 210

Notice that the solubility increases significantly as the temperature increases. Therefore, 95% ethyl alcohol is an excellent solvent for crystallizing sulfanilamide.

A graph of these data is shown below:

You will crystallize a sample of impure sulfanilamide by dissolving it in the minimum amount of boiling 95% ethyl alcohol (78 °C) and then cooling the solution, first to room temperature, and then to 0 °C in an ice-water bath.
The purity of the final material after crystallization will be determined by observing the color of your crystals and by performing a melting point on your sample. You will also weigh your sample and calculate the percent recovery. It is not likely that you will obtain a 100% recovery. This is true for several reasons: experimental loss, the original sample is not 100% sulfanilamide, and some sulfanilamide is soluble in the solvent even at 0 °C. Because of this latter factor, some sulfanilamide will remain dissolved in the mother liquor (the liquid remaining after crystallization has taken place).

The melting point of a substance is often used to determine purity. The melting point of a sample is usually expressed as two numbers called the melting point range, such as 112 - 114°C. The first number is the temperature at which the substance begins to melt (when liquid is first observed) and the second number is the temperature at which the sample has completely melted (no solid left). A very pure sample will have a narrow melting point range that will be close to the literature value (supposedly determined on a very pure sample). An impure sample will have a lower melting point and the range will be bigger. The literature melting point of sulfanilamide is 164.5 - 166.5°C.

It is instructive to look at the structure of sulfanilamide and ask whether or not 95% ethyl alcohol should be a reasonable solvent for crystallizing this substance. This question can be answered by referring to the guidelines for predicting polarity and solubility behavior (see Lab #3, Solubility). There are several polar bonds in sulfanilamide, the NH and the SO bonds. Because of these bonds, sulfanilamide has some polar character. In addition, the NH2 groups and the oxygen atoms in sulfanilamide can form hydrogen bonds with ethyl alcohol. So even though, it is likely that sulfanilamide would be soluble in 95% the benzene ring part of sulfanilamide is quite nonpolar, sulfanilamide has an intermediate polarity because of the polar groups. Ethyl alcohol also has an intermediate polarity. Therefore ethyl alcohol since they have similar polarities. (Note that the other 5% in ethyl alcohol is usually a substance such as water or isopropyl alcohol that does not alter the overall polarity of the solvent.)

Although this kind of analysis is a good first step in determining an appropriate solvent for crystallization, without more information it is not possible to predict the shape of the solubility curve for the temperature vs. solubility data, as shown ...