The Determination of Boiling Points and Melting Points of Organic Compounds
During this laboratory session, we will practice determining the boiling point of a liquid organic compound and the melting point (or we could call it the freezing point) of a solid organic compound. The boiling point of a liquid is the temperature at which the pressure of the vapor above a liquid equals the existing pressure. As we heat a liquid, the pressure of the vapor above the liquid slowly increases. When this pressure equals the pressure existing in the container, the liquid begins to boil– the liquid turns to vapor. If not contained, the vapor will “escape” into the atmosphere. In other words, the liquid evaporates. If the vapor is contained, and then passed through an area where the temperature is lower, the vapor will “condense” and enter the liquid state again. This would be a distillation. Atmospheric pressure is approximately 760 mm Hg; i.e., the weight of a column of mercury measuring 1 mm2 and 760 mm tall. This equates to about 29.7 inches of mercury. If we reduce the pressure above the sample that we are heating, we can reduce the boiling point of the liquid. This is referred to as a vacuum distillation or carrying out a distillation in vacuo. For example, while water boils at 100oC (or 212° F) at 760 mm Hg, it boils around 22oC at 20 mm Hg. The boiling point of a liquid is a physical characteristic of a compound. Many factors go into the estimation of the boiling point of a liquid such as the shape (round, oval, elongated), the mass and most importantly, hydrogen bonding (H-bonding). Methane with a mass of 16 does not engage in H-bonding and is a gas at room temperature while water (mass 18) boils at 100oC. While we can often guess the relative boiling points of a series of compounds, it is rather difficult to calculate a boiling point based on first principals. Similarly,