chemical compound

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Pure substance composed of two or more different elements

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Compounds are composed of two or more different element Constituent Pure substance (different from Simple substance ）。 Compounds have certain characteristics, which are different from the elements or ions contained in them, as well as from other compounds, and usually have certain composition.

Chinese name: chemical compound

Foreign name: compound

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definition

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chemical compound

Compounds are pure substances composed of two or more different elements. The difference in composition of this compound atom It must exist in a certain proportion. In other words, compounds have a certain composition regardless of their source. In daily life, sodium chloride , and distilled water (water) are common compounds. From these compounds, people found that their properties are different from each other, and salt is composed of sodium ions and chloride ions; Sugar is carbon hydrogen And oxygen atoms; Hydrogen burns in oxygen and reacts to form water. These facts indicate that two or more substances can react to form a new substance, which is a compound. The nature of the new substance is completely different from that of the original substance. It is usually used chemically to determine whether the property of a substance is a compound. If a pure substance can be decomposed into two or more substances, the original substance must be a compound. For example, molten salt can be completely decomposed into sodium and chlorine atoms with current, so salt is a compound.

Compounds in molecular state are called molecular compounds, such as water, sugar, etc. Compounds that are combined by ions are called Ionic compound , such as salt 、 Mirabilite Etc. Compound available chemical formula The chemical formula is composed of the symbols of the elements contained in the compound. For example, when writing the chemical formula of a binary compound composed of two elements, it is like giving its English name. Metal elements are written first, followed by less metallic ones. For example, the chemical formula of table salt (sodium chloride) is NaCl. When the number of atoms between different elements in a compound is not equal, the ratio can be written under the symbol and expressed with numbers. For example, the chemical formula of sucrose is

, indicating that sugar consists of twelve carbon atoms, twenty-two hydrogen atoms, and eleven oxygen atoms.

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Hybrid compound^[1]

The classification of compounds is one of the main contents of the classification of chemical substances. The current method of classifying compounds is to classify them according to their different molecules.^[2-3]

Classification by composition

Compounds can be divided into organic compounds and inorganic compounds.

Organic compound ： Organic compound Carbon containing compounds (but carbon containing compounds are not necessarily organic substances). Compounds containing only carbon and hydrogen are called hydrocarbons. As methane (CH four ）It is alkane, ethylene (C two H four ）It is alkene, acetylene (C two H two ）It is alkyne, benzene (C six H six ）It is aromatic hydrocarbon. Organic compounds are compounds containing carbon elements (except CO two 、CO、H two CO three And carbonate) such as CH four 、C two H five OH、CH three COOH contains carbon (C).^[4]

inorganic compound: inorganic compound Is hydrocarbon free, such as H two O、KClO three 、MnO two 、KMnO four , NaOH, etc. are all inorganic substances.^[4] Inorganics can be divided into the following categories:

Acid: A substance consisting of hydrogen and acid radical ions is an acid. E.g. HCl, HNO three 、H two SO four 。 It should be noted that aminosulfonic acid, acetic acid, etc. are organic substances, which belong to sulfonic acid and carboxylic acid respectively. They are also regarded as generalized acids, instead of distinguishing whether they are inorganic substances.

Alkali: A substance consisting of cations and hydroxyl ions is a base. E.g. LiOH, NaOH, Ca (OH) two 、NH three ·H two O、NH two OH, etc.

Salt: the substance composed of cation and acid radical ion is salt, such as K two SO four 、HgCl two 、Ba(NO three ) two Etc. They have K + 、Hg 2+ 、Ba 2+ Ions and corresponding acid radical ions SO four 2- , Cl - and NO three - 。 Cu(CH three COO) two yes Cupric acetate Although acid radical containing acetic acid (organic acid), it is still considered as inorganic substance. (Only those with carbon metal covalent bond are considered as organometallic compounds)

Oxides: compounds consisting of two elements and one of which is negative oxygen are oxides, such as CeO two 、MnO two 、K two O、N two O、XeO four Etc. For H two O two 、Na two O two First class peroxide In middle school, it is still considered as oxide. In fact, H two O two It is a weak acid, O two 2- Is its acid radical ion, so Na two O two It's salt.

Carbides: compounds composed of two elements and one of which is negative carbon are carbides, such as WC and CaC two 、Fe three C, etc. CO two Although it is composed of two elements, carbon is positive, so it is not carbide, but oxide.

Nitride: Compounds consisting of two elements and one of which is negative nitrogen are nitrides, such as BN, Si three N four 、Mg three N two Etc. It should be noted that, Azide It does not belong to nitride, but to salt, because azide is Hydrogen azide (an acid) acid radical ion.

Classification by chemical bond

Ionic compound : Sodium is metallic element , Chlorine is Nonmetallic element 。 Both sodium and chlorine are easy to react with other substances. When they react with each other, they form compounds sodium chloride 。

Compound molecular formula

From the atomic structure of sodium and chlorine, there is one electron in the outermost electron layer of sodium atom, which is easy to lose, and there are seven electrons in the outermost electron layer of chlorine atom, which is easy to get one electron, so that the outermost layer reaches the stable structure of eight electrons. So when sodium reacts with chlorine, one electron in the outermost electron layer of the gaseous sodium atom is transferred to the outermost electron layer of the gaseous chlorine atom, so that the outermost electron layer of both atoms becomes a stable structure of eight electrons.

In this process, the sodium atom loses one electron and carries a positive charge of one unit; The chlorine atom carries a negative charge of one unit because it gets one electron. Such charged atoms are called ions. Positive ions are called cations, such as Sodium ion （Na + ）； The negatively charged ions are called anions, such as chloride ions (Cl -). These two oppositely charged ions interact to form the compound sodium chloride. It is electrically neutral.

Normally, sodium chloride is a solid. Compounds such as sodium chloride, which are formed by the interaction of anions and cations, are ionic compounds. Such as potassium chloride (KCl), magnesium chloride (MgCl two ）, calcium chloride (CaCl two ）, calcium fluoride (CaF two ）Wait Ionic compound 。 Charged atomic groups are also called ions, such as sulfate ion, hydroxyl ion (OH -), etc. Zinc sulfate (ZnSO four ）, sodium carbonate (Na two CO three ）, sodium hydroxide (NaOH), etc. are also ionic compounds.

Covalent compounds: Hydrochloric acid is Hydrogen chloride Aqueous solution of gas. Hydrogen can be combined with chlorine to form hydrogen chloride.

Chlorine and hydrogen are non-metallic elements. Not only is it easy for chlorine atom to obtain one electron to form a stable structure of the outermost eight electrons, but also it is easy for hydrogen atom to obtain one electron to form a stable structure of the outermost two electrons. The atoms of these two elements have a similar degree of difficulty in obtaining electrons, so they failed to seize the electrons of each other. The result of the interaction of atoms of two elements is that each side forms an electron pair with one outermost electron. This electron pair is shared by two atoms and moves in space outside the two atomic nuclei, so that the outermost structure of both sides can reach stability. This electron pair is called the shared electron pair. The common electron pair is attracted by two nuclei, so that two atoms form a molecule of a compound. In the hydrogen chloride molecule, because the chlorine atom has a slightly stronger attraction to the electron pair than the hydrogen atom, the electron pair is biased towards the chlorine atom, so the chlorine atom is slightly negative and the hydrogen atom is slightly positive, but as a whole the molecule is still electrically neutral.

Compounds such as hydrogen chloride that form molecules by sharing electron pairs are called covalent compounds. For example, water and carbon dioxide are covalent compounds.

Coordination compound: A complex molecule or ion formed by the combination of a central atom (or ion) and several ligand molecules (or ions) with a coordination bond is usually called a coordination unit. All compounds containing coordination units are called coordination compounds, also called complexes for short complex compound 。 Coordination compound is a kind of covalent compound.

Ni(CO) four They are coordination units, called coordination cations, coordination anions and coordination molecules.

[ Co(NH three ) six ]Cl three ， K three [Cr(CN) six ]， Ni(CO) four Are coordination compounds. [Co(NH three ) six ] [Cr(CN) six ]It is also a coordination compound. The key to judge is whether there is coordination unit.^[5]

Features

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Compounds have certain characteristics and usually have certain compositions.

For example, water is a compound and a liquid at room temperature, with a boiling point of 100 ℃ and a freezing point of 0 ℃. It is composed of hydrogen and oxygen. 1 water molecule H two O consists of two hydrogen atoms and one oxygen atom.

Example: sodium chloride (sodium chloride, NaCl) is a compound formed by the chemical action (neutralization reaction) of hydrochloric acid (HCl) and sodium hydroxide (NaOH). HCl(aq) + NaOH(aq) → NaCl(aq) + H two O(l)^[6]

Difference from mixture:

chemical compound

(1) The properties of the constituent elements of a compound when they no longer remain in a simple state; The mixture has no fixed properties, and each substance maintains its original properties (such as no fixed melting and boiling points).

(2) The constituent elements of compounds must be separated by chemical methods;

(3) The composition of the compounds is usually constant. The mixture is a mixture of different substances. It has no definite composition and cannot be used in one way chemical formula express.^[3]

Main differences of elements, simple substances and compounds:

It should be clear that simple matter and compound are two concepts derived from the perspective of elements, that is, pure matter composed of the same element is called simple matter, and pure matter composed of different elements is called compound. Whether in a single substance or a compound, as long as it is a type of atom with the same nuclear charge number, it can be called an element.

The main difference between the three is that elements are the components of constituent substances, while simple substances and compounds refer to the two existing forms of elements, which are specific substances. Elements can form simple substances and compounds, while simple substances cannot form compounds.^[3]

Difference between compound and solid solution

Same: all are single-phase materials.

Difference: A and B form Solid solution Later, its structure is consistent with that of the main crystal, and there is no definite molar ratio between A and B, which can fluctuate within a certain range. For example, ruby, after forming the compound AmBn between A and B, the structure of the product is different from that of A and B, which is a new structure, and there is a certain molar ratio between A and B.

Non proportional compound

In general people's impression, compounds are composed of several fixed elements in simple mathematical proportions, but more compounds are Non proportional compound , especially inorganic compound 。 The so-called non proportional compounds are compounds whose composition is non stoichiometric or non integral ratio, that is, the composition atoms of these compounds are not small integer ratio, and the composition can change within a certain range, which does not conform to Law of definite composition 。

The latest research shows that these compounds generally include the following categories:

(1) An alloy system formed by co melting two or more metals. Because various alloys may have multiple phase equilibrium points, a variety of alloy compounds with variable composition are formed. For example, in the binary system of steel composed of Fe and C austenite , ledeburite, martensite and pearlite.

(2) On rock salt In the system, compounds with special colors are formed in the lattice due to the formation of a small part of defects. As follows, sodium chloride is heated in sodium vapor to form Na+δ Cl (δ<<1), which is a light green compound.

(3) Many transition metal Oxide, sulfide nitrides 、 carbide or hydride , which has rich physical and chemical property The compounds of are generally non proportional compounds.

There are many kinds of compounds, up to more than 10 million, some of which are composed of Anion and cation Composition, such as sodium chloride NaCl, ammonium sulfate (NH4) two SO four Etc; Some compounds are composed of molecules, such as ammonia NH three Methane CH four 、 Phosphorus pentoxide P two O five 、 carbon disulfide CS two Etc; Some compounds consist of atoms, such as silicon dioxide SiO two , silicon carbide SiC, etc. Compounds can be divided into inorganic compounds (compounds without carbon) and organic compounds (compounds with carbon, except CO and CO two 、H two CO three And carbonate). According to different chemical properties, compounds can be divided into oxides, acids, bases and salts.

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The material world is colorful. From the most primitive classification in ancient times (gold, wood, water, fire, earth) to modern times (as of December 12, 2013), there are hundreds of thousands of compounds with definite composition, and a large number of new compounds are found every year.

The statistics of the number of known compounds are not quite consistent. It is generally recognized that the United States《 Chemical Abstracts 》According to the statistics of the editorial department, more than three million natural and synthetic compounds have been found. Some of these compounds are composed of two elements, and some are composed of three, four or more chemical elements. There are still more than 300000 newly synthesized compounds every year, of which more than 90% are organic compounds.^[6]

Identification method

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The identification method of compound purity, starting from the requirements of fast, cheap and simple, mainly comes from the following points:

Identification of TLC purity:

1. The choice of development solvent is not only three different Polarity To deploy the system, you must first select three types Intermolecular force Different solvent systems, such as chloroform methanol, cyclohexane\ ethyl acetate , n-butanol acetic acid water, developed separately to determine Components Whether it is a single spot. The benefits of doing so are obvious Components The various differences between them separate the components. It is possible that several similar components are single spots in a solvent system, because the solvent system is different from the Intermolecular force There is no significant difference in effect, which is not enough to distinguish in TLC. If another solvent system with different intermolecular forces is used, it may be separated. This is not possible with three different polarity deployment systems.

2 For a solvent system, as wxw0825 said, at least three different polarity development systems are required. One polarity development system pushes the Rf of the target component to 0.5, and the other two polarity development system pushes the Rf of the target component to 0.8, 0.2. The function is to check whether there are impurities with greater or smaller polarity than the target component.

3 Color development methods, light expansion is not enough, but also use various color development methods. Generally, universal type must be used Chromogenic agent For example, 10% sulfuric acid, iodine, because each color developer (whether it is a general color developer or a special color developer, they all have a compound that does not develop color in the work), and then select the special color developer according to the condition that the component may contain mixed components. Only when there is a single spot under multiple color developing agents can we conclude that the sample is thin layer pure

Determination of purity by melting process:

The principle is very simple, pure compound, Melting range Very short, 1, 2 degrees. The melting point of the mixture decreases and the melting range increases.

Purity identification of HPLC:

For HPLC, because there are few commonly used systems and its separation effect is good, it is generally not required to select three solvent systems with different intermolecular forces, as long as these three solvent systems with different polarities are selected to make the target peak appear at different retention times.

Purity identification of soft ionization mass spectrometry:

For example, ESI-MS and APCI-MS. ESI-MS is selected for high polar compounds and APCI-MS is selected for low polar compounds Soft ionization The characteristic of mass spectrometry is that it only gives the quasi Molecular ion Peak, passing positive anion To determine molecular weight 。 If the sample is impure, multiple alignments will be detected Molecular ion peak , not only the purity but also the molecular weight of the impurities are determined.

Purity identification of nuclear magnetic resonance:

If there are many small peaks less than one integral in the hydrogen spectrum, it is possible that the sample is an impurity in the sample. The purity identification can also be achieved by quantifying the carbon spectrum using the gated decoupling technology.

Each method has its own limitations, such as purity identification based on hydrogen spectrum. If there are many small peaks with less than one integration, it may make the sample active proton , based on Soft ionization For the purity identification of mass spectrometry, if the molecular weight of the impurity is the same as that of the target, it cannot be detected.

There is no 100% pure compound in the world. The desired purity should be related to the purpose. For example, if you want to test the structure by NMR, it is generally required to have a purity of 95%. If you want to test EI-MS, the higher the purity, the better. More than 99%.

None of the above methods can distinguish correspondence Isomer 。

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