Chapter 12.19 - Purification of Organic Compounds

In the previous section, we saw the hyperconjugation in isopropyl cation and propene. In this section, we will see purification of organic compounds.

Some basics about purification can be written in 7 steps:
1. Organic compounds can be obtained from natural sources.
• For example,
   ♦ Sugar is obtained from sugar cane.
   ♦ Gasoline is obtained from crude oil.
2. Organic compounds can be produced in the laboratory also.
• For example,
   ♦ Teflon used as coating in some non-stick pans is a man made organic compound.
3. Whether it is natural or man made, an organic compound when produced, will contain some impurities. So it is essential to purify it.
• There are various methods available for purification. We must choose the most appropriate method. The choice will depend on two factors:
   ♦ The nature of the organic compound.
   ♦ The nature of the impurities present in that organic compound.
4. The common methods used for purification are:
   ♦ Sublimation
   ♦ crystallization
   ♦ Distillation
   ♦ Differential extraction
   ♦ Chromatography
5. Once the impurities are removed, we must use appropriate methods to confirm that the compound obtained is indeed pure.
• The most common method is to check the melting or boiling point.
6. Many organic compounds have sharp melting points and boiling points.
• That means,
   ♦ If the organic compound is a solid, it will melt at a definite temperature when heated.
   ♦ If the organic compound is a liquid, it will boil at a definite temperature and pressure when heated.
7. From the newly obtained organic compound, we collect a small sample and heat it. We must closely monitor the temperature while heating.
• The exact temperature at which melting or boiling occurs must be recorded. If the recorded value matches with the value in the data book, we can confirm that, the compound is pure.
• Different chromatographic and spectroscopic techniques can also be used to check the purity of organic compounds. We will see those methods in later sections.

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Now we will see the various methods of purification in detail.

Sublimation

• This can be explained in 3 steps:
1. We know that sublimation means direct change from solid state to gaseous state when a substance is heated. The substance will not pass through a liquid state. (Details can be seen below the video at the beginning of section 7.1)
2. Suppose that:
   ♦ The organic compound that we want is sublimable.
   ♦ Also the impurities present are non-sublimable.
3. Then, the impurities will be left behind when heated. We can collect the vapors.
• Those vapors will undergo deposition (reverse of sublimation) to give the pure solid compound.

Crystallization

• This can be explained in 5 steps:
1. We select a suitable solvent in such a way that:
   ♦ The impure solid (mixture of the pure organic compound and the impurities), is sparingly soluble at room temperature.
   ♦ The same impure solid is appreciably soluble at higher temperatures.
2. So we heat the solution. Due to the higher temperature, we will be able to dissolve more and more quantities of the impure solid.
3. We continue dissolving the substance until the hot solution becomes saturated.
(Saturation point is reached when any more solid added will stop to dissolve)
4. When the saturation point is reached, we begin to cool the solution. The cooling must be done slowly.
• Due to the cooling, the pure substance will crystallize. Those crystals can be removed by filtration.
5. After filtration, the mother liquor will contain impurities and a small quantity of pure substance. So repeated crystallization will be necessary.

Distillation

Distillation can be used in five cases:
Case 1:
This can be explained in 2 steps:
1. Suppose that, the pure organic substance that we want is a liquid. Also it is volatile.
(A liquid is said to be volatile if it passes from liquid state to vapor state even when small amount of heat is supplied at normal atmospheric pressure)
• Also suppose that, the impurities are non volatile.
2. Then, the impurities will be left behind when heated. We can collect the vapors. Those vapors can be condensed to obtain the pure liquid.

Case 2:
This can be explained in 4 steps:
1. Suppose that, the pure organic substance that we want is a liquid. It will have a certain boiling point.
• Also suppose that the impurity present is also a liquid. This liquid will also have a certain boiling point.
2. This case 2 is applicable when there is a big difference between the two boiling points.
• When the mixture is heated, the liquid having lower boiling point will boil first. The vapors of only this liquid will be present at this initial state. All the vapors so formed can be collected and condensed. The liquid so obtained will not contain the other liquid which has higher boiling point. Thus the two liquids can be easily separated.
3. The mixture of chloroform and aniline can be purified using this method.
   ♦ Chloroform has a boiling point of 334 K
   ♦ Aniline has a boiling point of 457 K
• So the vapors of chloroform will be formed first. They can be collected and condensed. Aniline will remain in liquid form.
4. If there are more than two components in the mixture, we must continue heating. When the temperature reaches a higher level, the component with higher boiling point will turn into vapors. Those vapors can be collected separately.

Case 3:
This can be explained in 7 steps:
1. Suppose that there are two or more components in the liquid mixture.
• Also suppose that the boiling points of those components do not differ much from each other.
2. In such a situation, all the liquid components will turn into vapors at the same time. Also during condensation, all the vapor components will become liquids at the same time.
• Thus the liquid obtained after condensation will again be a mixture, same as the original. So it will be impossible to use ordinary distillation.
3. Here we use fractional distillation.
• The mixture is taken in a round bottomed flask (R.B flask). It is heated from the bottom.
4. A fractionating column is fitted at the top of the R.B flask.
   ♦ The bottom end of the column is closer to the heat source.
   ♦ The top end of the column is away from the heat source.
   ♦ So a temperature gradient will be set up in the column.
         ✰ Bottom end will be hotter.
         ✰ Top end will be cooler.
5. Due to the heat, the components in the R.B flask change into vapors.
• Consider the vapors of that component which has low boiling point.
   ♦ Those vapors will condense only if the surrounding temperature is low.
   ♦ That means, those vapors will ascend all the way up to the top of the column.
   ♦ (Recall that temperature at the top is lower)
   ♦ They can be collected through an outlet at the top.
• Consider the vapors of that component which has high boiling point.
   ♦ Those vapors while ascending, will be getting lesser and lesser heat.
   ♦ Lesser heat will not be sufficient to keep them in gaseous state.
   ♦ So they will condense and fall back into the flask.
• While falling back, they will transfer some of their heat to the rising vapors of the low boiling point category. This will accelerate the fall back.
6. Once component with the lowest boiling point is completely removed from the top, the temperature is adjusted in such a way that, the vapors of the next higher boiling component reach the top of the column.
• Thus through multiple steps, all components can be separated.
7. In another type of fractional distillation, the column will have multiple outlets along it’s height.
• The components with lower boiling points will condense at top portions. They can be collected through outlets near the top.
• The components with higher boiling points will condense at bottom portions. They can be collected through outlets near the bottom.
• The components of crude oil are separated by this method. Some of the commercially important components of crude oil are: petrol, diesel, kerosene, bitumen etc.,

Case 4:
This can be explained in 4 steps:
1. Suppose that, a liquid has a very high boiling point.
• Also suppose that, this liquid is not able to take large quantities of heat. That is., if large quantities of heat is supplied to this liquid, it’s molecules will decompose. The molecules will decompose into new and different types of molecules. If the original molecules change to different types of molecules, the liquid itself will change. We will no longer have the required liquid.
2. If the liquid cannot take large quantities of heat, it can never reach it’s high boiling point.
• This is because, large quantities of heat is required to obtain high temperature.
• So it is clear that, the liquid will decompose before reaching the boiling point.
3. In such a situation, we use distillation under reduced pressure.
• In this method, we use a water pump or vacuum pump to reduce the pressure above the surface of the liquid.
• If this pressure is reduced, the the liquid will be able to boil at a lower temperature. Then the liquid will not need to take large quantities of heat. (Recall why boiling point of water becomes low at mountain tops. Details here)
4. Glycerol can be separated from spent-lye in soap industry by this method.

Case 5:
This can be explained in 7 steps:
1. Suppose that, the liquid is steam volatile. That is., if steam is passed through the liquid, the liquid will change into vapor state. This is because, the heat available in the steam is sufficient to transform the liquid into gaseous state.
• Also suppose that, the liquid is immiscible with water.
2. In such a situation, we use steam distillation.
• In this method, we pass steam through a flask containing the liquid. This flask is heated externally also, using a burner.
• So the liquid gets heat from two sources: the steam and the burner. This helps to reduce the quantity of steam required to effect the distillation process.
3. When the steam passes through the liquid, some of the steam loses heat and becomes water. So what we have in the flask, will be a mixture of water and the liquid.
• Also, as we continue to pass the steam, the mixture gets agitated. So the molecules of both liquid and water, will be in contact with the atmosphere.
4. Since both molecules are in contact with the atmosphere, the vapor pressure (p) above the surface of the mixture will be the sum of p_w and p_l.
• That is., p = p_w + p_l
• Where,
   ♦ p_w = partial pressure due to water
   ♦ p_l = partial pressure due to the liquid.
5. We know that, when the vapor pressure becomes equal to the atmospheric pressure, the mixture will begin to boil.
• Here we see that, both pwq and plq are contributing to build up p.
• If steam and water were not present, the p_l must solely become equal to the atmospheric pressure. This will happen only at a high temperature.
• Since we have both pwq and plq, the mixture will boil at a low temperature.
6. Once the boiling begins, the liquid will transform to gas and move out of the flask together with steam.
• So what we get from the flask is a mixture of steam and the required liquid in gaseous form.
• The inorganic impurities will remain in the flask.
7. This mixture is passed through a condenser. The resulting liquid will not be a mixture because, the liquid is immiscible with water. Water and the liquid will be present in two separate layers. The two layers can be separated using a separating funnel.

Differential extraction

1. Suppose that, the organic compound is present in an aqueous medium. That means, the organic compound is in a mixed state with water. We want to extract the pure organic compound from that mixture.
2. In such a situation, we use differential extraction.
In this method, we take the mixture in a separatory funnel. This is the bottom layer (dark blue layer) in fig.12.113(a) below:

Fig.12.113

   
3. Next we pour a suitable organic solvent over this mixture.
• This organic solvent should be immiscible with water.
   ♦ So it forms a separate layer above the dark blue layer.
   ♦ This new layer is shown in yellow color.
• Also this organic solvent should be such that, the organic compound (present in the dark blue layer) is more soluble in it than in water.
4. Now we shake the funnel thoroughly.
• The organic compound gets extracted out from the dark blue layer.
   ♦ It dissolves in the yellow layer.
   ♦ So the yellow layer now becomes green in color as shown in the fig.12.113(b) above.
• Also the dark blue layer has now become light blue.
   ♦ This is because, the organic compound is now removed from that layer.
5. We allow the funnel to stand still for some time. After that, we open the tap at the bottom. The bottom light blue layer flows out. It is collected in a container. The top green layer is collected in a separate container.
• The green layer can be subjected to distillation to obtain the organic compound.
6. In some cases, the organic compound may not be appreciably soluble in the yellow layer in fig.(a)
• Then while shaking, only a small quantity of the compound will be extracted into the yellow layer.
• In such cases, we will need to repeat the procedure several times.

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In the next section we will see chromatography.

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