Grade 11 → Organic Chemistry - Some Basic Principles and Techniques ↓
Purification and qualitative analysis of organic compounds
Organic compounds form the basis of life on Earth, and their study is essential to understanding chemistry. Organic chemistry is primarily concerned with the structure, properties, and reactions of organic compounds, which contain carbon in covalent bonds. This document explains the processes of purification and qualitative analysis, which are essential skills in organic chemistry.
Purification of organic compounds
Organic synthesis often produces products with impurities, which can interfere with their properties or further reactions. Therefore, purification is an important step. Common methods of purification include:
1. Filtration
Filtration separates solids from liquids or gases using a filter medium. It is often used when a reaction produces a solid precipitate. An example of this would be separating PbCl 2
crystals from a liquid mixture.
2. Crystallization
Crystallization helps to purify solid compounds. It involves dissolving a solid substance in a hot solvent to form a solution and slowly cooling it to form pure crystals. These crystals can then be collected through filtration. For example, benzoic acid can be purified using this method.
3. Distillation
Distillation is used to separate components based on different boiling points. For example, a mixture of acetone (CH 3 COCH 3
) and water can be separated by distillation because of their different boiling points.
Distillation Equipment: , , | Condenser | , , Boiling Flask Receiver
4. Sublimation
Sublimation is used for substances that change from a solid to a gas without passing through the liquid state. It is useful for purifying substances such as iodine (I 2
) and naphthalene.
5. Chromatography
Chromatography separates mixtures based on differences in their speed through the stationary phase. Thin layer chromatography (TLC) is a simple method where components move at different rates on a coated glass plate.
Chromatography Example: |-------------| (starting line) , | AB | (spots of different samples) , |................ (Solvent Front) , | A' B' | (spaces separated)
Qualitative analysis
Qualitative analysis identifies the components in a given organic compound. The main steps and methods are as follows:
1. Trace elements
Common elements in organic compounds are carbon, hydrogen, oxygen, nitrogen, sulfur, and halogens. Several tests help determine their presence:
Detection of carbon and hydrogen
Carbon and hydrogen can be tested by heating the compound in the presence of CuO
. Carbon is converted to CO 2
, which turns lime water milky, while hydrogen is converted to H 2 O
, which condenses.
Nitrogen detection
The Lassaigne test involves melting the organic compound with sodium. A typical test would involve:
Na + C + N → NaCN (sodium cyanide)
Prussian blue colour indicates the presence of nitrogen when NaCN
reacts with ferrous sulphate and ferric chloride.
Detection of halogens
Copper wire is used in the Beilstein test. It turns green in the flame due to the presence of halogens such as chlorine, bromine, or iodine.
Detection of sulfur
Sulfur is detected using sodium fusion and sodium nitroprusside, which turns purple in the presence of sulfur.
2. Identification of functional groups
Functional groups define the class and properties of organic compounds. Here are some common groups and their tests:
Functional group: Alcohol
Alcohols such as ethyl alcohol react with sodium to form hydrogen gas, which is embrittled.
Functional group: Aldehyde
Silver nitrate and ammonia are used in the Tollens test. In the presence of aldehyde, a silver mirror is formed inside the test tube.
Functional group: Carboxylic acid
These acids react with sodium bicarbonate to produce carbon dioxide.
Conclusion
Understanding the purification and qualitative analysis of organic compounds is a cornerstone of organic chemistry. Proper purification ensures that compounds retain desired properties, while qualitative analysis reveals element composition and functional groups, providing insight into chemical behavior and reactions.