What is a Chemical Compound?

A compound is a pure substance (different from elemental matter) composed of two or more different elements. A compound has certain characteristics, which are different from the elements or ions it contains, as well as other compounds, and usually have a certain composition.

Compounds have certain properties and usually have a certain composition.

Example: Water is a compound and liquid at normal temperature. It has a boiling point of 100 ° C and a freezing point of 0 ° C. It is composed of hydrogen and oxygen. One water molecule H ₂ O is composed of two hydrogen atoms and one oxygen atom.

example:

The material world is colorful, from the most primitive classifications of ancient times (gold, wood, water, fire, earth) to modern (as of December 12, 2013) there are hundreds of thousands of compounds with defined composition, and there are still a large number of New compounds were discovered.

The number of known compounds is not quite consistent in all aspects. More generally, the United States "

The method for identifying the purity of compounds starts from the requirements of fast, cheap and simple, mainly from the following points:

Identification of TLC purity:

1 Selection of development solvent, not only requires at least 3 deployment systems with different polarities. The first step is to select three solvent systems with different intermolecular forces, such as chloroform \ methanol, cyclohexane \ ethyl acetate, and n-butanol. \ Acetic acid \ water, expand separately to determine whether the component is a single spot. The advantage of this is obvious. The components are separated by various differences between the components. It is possible that several similar components are single spots in a solvent system, because the solvent system is different from these components. There is no significant difference in intermolecular force, which is not enough to distinguish between TLC. When another solvent system with a different intermolecular force is used, it is possible to separate. This cannot be achieved with three different polarity deployment systems.

2 For a solvent system, as wxw0825 said, at least 3 deployment systems of different polarities are required. One of the polar deployment systems pushes the Rf of the target component to 0.5, and the other two polar deployment systems push the target component. Rf pushed to 0.8, 0.2. The effect is to check for impurities that are more or less polar than the target component.

3 Color development methods, light expansion is not enough, but also various color development methods. Generally, it is necessary to use a general-purpose developer, such as 10% sulfuric acid and iodine, because each developer (whether it is a general-purpose developer or a proprietary developer) encounters a compound that does not develop in the work When), according to the situation that the components may contain mixed components, the exclusive developer is selected. Only when there is a single spot under multiple developers, can we conclude that the sample is thin and pure

Melt range judge purity:

The principle is simple. Pure compounds have a short melting range, 1,2 degrees. The melting point of the mixture decreases and the melting range becomes longer.

Purity identification by HPLC:

For HPLC, because there are fewer commonly used systems, and its good separation effect, it is generally not required to choose three solvent systems with different intermolecular forces. Only three solvent systems with different polarities are required to keep the target peaks in different retention. Time peaks.

Purity identification of soft ionization mass spectrometry:

Such as ESI-MS, APCI-MS. ESI-MS is used for large polar compounds, and APCI-MS is used for very polar compounds. These soft ionization mass spectrometers are characterized by giving only the excimer ion peaks of the compound, and the molecular weight is determined by the mutual communication of positive and negative ions. If the sample is not pure, multi-aligned molecular ion peaks are detected, which not only confirms the purity, but also the molecular weight of the hybrid.

Purity identification of nuclear magnetic resonance:

If there are many small peaks with less than one integrated in the hydrogen spectrum, it is likely that the sample is an impurity in the sample. Gated decoupling can also be used to identify purity by quantifying the carbon spectrum.

Each method has its own limitations, such as purity identification based on hydrogen spectrum. If many small peaks with less than one are found, it is also possible to make the active protons in the sample based on the purity identification of soft ion mass spectrometry. The same molecular weight as the target cannot be detected.

For compound purity, there is no 100% pure compound in the world. How high the purity is hoped should be related to the purpose. For example, if you want to measure the structure of nuclear magnetic resonance, the purity is generally 95%. If you want to measure EI-MS, the higher the purity, the better. More than 99%.

None of the above methods can distinguish the corresponding isomers.