1. What is the major purpose of doing distillations in organic chemistry? What is a secondary purpose?
The main purpose of distillation in organic chemistry is to purify liquids. As a secondary benefit, the boiling point of the liquid can be determined. Boiling point, along with other physical and chemical properties, can be used to help name the compound.
2. Under what conditions can simple distillation (vs. fractional distillation) be performed?
According to the lab manual, simple distillation can be used for a pure or “almost pure” substance (in which there is less than 10% impurities), a mixture in which the boiling points of the impurities, are very high, or a mixture in which there is a large difference between the boiling points of the constituents. However, fractional distillation is best when the differences between boiling points is 10-100 ?C.
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3. Explain how your distillation curves indicate the relative efficiencies of the three distillations: a) simple, b) fractional with an empty column, and c) fractional with a Vigreux column.
Simple distillation is efficient when it meets the criteria above. However, overall it is less efficient than fractional distillation with a packed or Vigreux column. Fractional distillation with an empty column is not much different from simple distillation, since there are no surfaces inside the column that will allow the compounds to condense, evaporate, and re-condense repeatedly. The Vigreux column does provide surfaces for repeat condensation, so it is more efficient.
4. Explain the purpose of boiling chips (or stones). Explain how they work.
Boiling chips or stones are used to prevent explosive boiling. They are made of a porous, chemically inert substance and produce many small bubbles when air trapped inside them is heated. This stream of small bubbles encourage even boiling instead of uneven boiling which can result in “bumping.” If bumping occurs it will cause liquid to be lost as it splashes.
5. A liquid compound has a boiling point of 150 ?C. Unfortunately it is not thermally stable and decomposes at 110 ?C. How could this compound be distilled without decomposition?
This compound can be distilled using vacuum distillation, in which external pressure is reduced. Lower external pressure allows the liquid to boil at a lower temperature than it would otherwise. Therefore, the compound will boil before it decomposes (Weaver, 2014).
6. Explain how a Vigreux column works to effect better separation than an empty column of the same length (at least in theory):
The Vigreux column has indentations or “ledges” along its inside which provide theoretical plates for condensation, evaporation, and re-condensation of vapors from the boiling flask below (Weaver, 2014). As the number of condensation cycles increases, the better the efficiency of the distillation. An empty column of the same length does not have theoretical plates, so it is less efficient.
- Weaver, S.L. (2014). Fractional distillation notes. Retrieved from http://www.xula.edu/
