Organic chemistry experiment

Author: Fu Chunling Compiler/Nationality:
Publisher:
Publishing Time: 2006-01-01
Features: 3. Separation and Purification of Organic Compounds
In organic chemistry and life science research, the structure determination of compounds is frequently encountered. Whether it is the determination of physical constants, functional group analysis, or spectral analysis, pure substances are required. However, in practice, the compounds to be identified are often impure, which necessitates the purification of the sample. Moreover, in organic reactions, apart from the desired products, by-products from side reactions, as well as unreacted starting materials and organic solvents, are often present. These together form a mixture, all of which require the separation of the desired product from the mixture. Methods for the separation and purification of organic compounds include distillation, fractional distillation, reduced pressure distillation, steam distillation, extraction, recrystallization, and chromatography. This chapter focuses on the principles and applications of these methods for separating and purifying organic compounds, and through experimental operations, students will become familiar with and master modern techniques for the separation and purification of organic compounds.
3.1 Distillation
Distillation is one of the most commonly used methods for separating and purifying liquid organic compounds. It can also be used to determine the boiling point of liquid compounds (gravimetric method), which holds significance for identifying pure liquid organic compounds.
3.1.1 Simple Distillation [Experiment 9]
Simple Distillation
1. Objective
(1) To understand the principle of distillation for separating and purifying liquid organic compounds.
(2) To master the experimental setup, operational techniques, and applications of distillation.
2. Principle
Due to molecular thermal motion, liquid molecules have a tendency to escape from the surface, and this tendency increases with temperature. When a liquid is heated, its vapor pressure increases with temperature (Figure 3.1). When the vapor pressure of the liquid reaches equilibrium with external atmospheric pressure, a large number of bubbles escape from the liquid, causing it to boil. The temperature at this point is known as the boiling point of the liquid. Heating the liquid to boiling, vaporizing it, and then condensing the vapor back into a liquid are combined operations known as distillation. Clearly, distillation can separate volatile and non-volatile substances, as well as liquid mixtures with different boiling points. If the boiling points of the liquid mixture differ significantly (at least by 30°C or more), the high-boiling component remains in the distillation flask, achieving the goal of separation and purification. Pure liquid organic compounds have a constant boiling point at a certain pressure, but not all liquids with a constant boiling point are pure compounds, as some compounds often form binary or ternary azeotropic mixtures with other components, which also have constant boiling points. For example, a binary azeotropic mixture of 95.57% ethanol and 4.43% water has a boiling point of 78.17°C, so it cannot be assumed that a constant boiling point indicates purity.
3. Apparatus and Reagents
(1) Apparatus: Heating mantle, thermometer (100°C), round-bottom flask (50 mL), distillation head, straight condenser, receiving adapter, conical flask, rubber tubing, iron stand, iron clamp, and iron ring.
(2) Reagents: Industrial alcohol.
4. Experimental Procedure
(1) Distillation setup and assembly. The laboratory distillation setup primarily consists of a heating and vaporization unit, a condensation unit, and a receiving unit. Figure 3.2 shows a commonly used distillation setup, composed of a distillation flask, thermometer, condenser, receiving adapter, and receiving flask.
① Distillation flask. The distillation flask is a commonly used instrument in distillation, where the liquid is heated and vaporized inside the flask, and the vapor enters the condenser through the distillation head. Choose an appropriate distillation flask (round-bottom flask) based on the volume of the distillate. The volume of the distillate should generally not exceed two-thirds of the flask's capacity and should not be less than one-third, otherwise, the liquid is prone to splash out or be lost during boiling.
② Thermometer. A ground glass thermometer can be directly inserted into the distillation head, while a regular thermometer is usually fixed in the upper part of the distillation head with a wooden or rubber stopper. The upper part of the mercury bulb of the thermometer should be at the same horizontal level as the lower part of the side arm of the distillation head. An incorrect position of the thermometer will result in a lower or higher boiling point reading.
③ Condenser. The vapor is condensed into a liquid in the condenser. An air condenser (Figure 3.2b) is used when the boiling point of the liquid is above 140°C, while a straight condenser (Figure 3.2a) is used when it is below 140°C. The lower side arm of the condenser is the water inlet, and the upper outlet should be upward to ensure that the inner tube is fully filled with water.
④ Receiving adapter. Composed of a receiving adapter and a conical flask or round-bottom flask. When using an adapter without a side arm, the adapter and the receiving flask should not be connected with a stopper to avoid creating a closed system, which could lead to excessive pressure and potential explosion. For distilling low-boiling, flammable liquids, a side-arm adapter with a rubber tube connected to the drain should be used. The order of assembling the apparatus is generally from bottom to top and left to right. It must be accurate, upright, and perpendicular. Whether viewed from the front or side, the axis of the entire apparatus should be in the same plane. The iron stand should be neatly placed behind the apparatus. The installation of the apparatus can be summarized in four words: stable, secure, upright, and correct. Stable means firm and reliable; secure means properly installed to eliminate all safety hazards; upright means straight and neat; and correct means using and selecting the apparatus appropriately.
(2) Distillation operation.
① Adding the sample. After the distillation setup is completed, first check whether all connections are tight and airtight. Then, add a few boiling chips to the flask and pour the liquid to be distilled into the distillation flask through a long-neck funnel to prevent the liquid from flowing out of the side arm.
② Heating. Before heating, turn on the cooling water for the condenser first, then heat. Adjust the heat to make the distillate flow at a rate of 1–2 drops per second.
③ Observing the boiling point and collecting the distillate. At least two receiving flasks should be prepared before distillation, as lower-boiling liquids often distill first before reaching the desired boiling point. This initial distillate is called the "fore-run." After the fore-run is distilled off and the temperature stabilizes, the more pure substance will distill. At this point, replace with a clean and dry receiving flask and record the temperature range. If specified, collect according to the prescribed temperature range. Even with minimal impurities, do not distill to dryness. When the flask contains only a small amount (about 0.5–1 mL) of liquid, stop distillation to prevent the flask from breaking or causing an explosion. After distillation is complete, first stop heating, then turn off the cooling water, and disassemble the apparatus in the reverse order of assembly.
(3) Distillation of industrial alcohol. Set up the experiment setup as shown in Figure 3.2(a). Carefully add 30 mL of pale yellow, turbid industrial alcohol to a 50 mL round-bottom flask through a long-neck funnel, add 2–3 boiling chips, and fit the flask with a stopper containing a thermometer. Connect the condenser and heat with a heating mantle. Initially, heat more quickly and observe the changes in the distillation flask and the thermometer reading. When the liquid in the flask begins to boil, the vapor front gradually rises, and when it reaches the thermometer, the reading rises sharply. Adjust the heating speed to control the distillate flow at 1–2 drops per second. When the thermometer reading reaches 77°C, replace with a pre-weighed dry conical flask as the receiving flask and collect the distillate at 77–79°C. Stop distillation when only a small amount (about 0.5–1 mL) of liquid remains in the flask. Weigh the collected distillate and calculate the recovery rate.
Annotations
[1] If there is almost no air in the liquid and the bottle wall is very clean and smooth, forming bubbles becomes very difficult. During heating, the liquid temperature may rise significantly above the boiling point without boiling, a phenomenon known as superheating. Under such conditions, once a bubble forms, due to the vapor pressure of the liquid at this temperature being much higher than atmospheric pressure, the rising bubbles increase rapidly, and the liquid may even be splashed out of the flask, a phenomenon called "bumping." To prevent superheating during heating, substances such as boiling chips, porcelain chips, or one-end-sealed capillaries are often added. These materials produce tiny bubbles upon heating, serving as centers for liquid vaporization and preventing bumping. If boiling chips are not added after heating, the liquid should be cooled first before adding them, otherwise, bumping may occur.
[2] The flow rate of the cooling water should be sufficient to ensure complete condensation of the vapor. A gentle stream of water is usually sufficient.
[3] Organic solvents should be distilled using small-mouth receiving flasks, such as conical flasks.
[4] 95% ethanol is an azeotropic mixture, not a pure substance, with a constant boiling point and composition, and cannot be separated by ordinary distillation.
Reflection Questions
1. Why are boiling chips added when distilling liquids?
2. Why should the upper part of the mercury bulb of the thermometer be at the same horizontal level as the lower part of the distillation head's side arm?
3. Why does cooling water flow into the condenser from the lower end and out from the upper end?