Alkane is an open-chain saturated hydrocarbon chain (saturated group), the carbon atoms in a molecule are a single bond connected to, the remaining valences are bonded hydrogen compounds. The general formula is C_nH_2n+2, (Alkane Functional Group).

Which is the simplest organic compound? The main sources of alkanes are oil and natural gas, which are important chemical raw materials and energy materials.

Definition

A compound consisting of only two elements, hydrocarbons, is called a hydrocarbon, or simply a hydrocarbon. According to the different molecular structure of hydrocarbons, hydrocarbons can be divided into two types: Chain hydrocarbons ( aliphatic hydrocarbons ) and Cyclic hydrocarbons ( alicyclic hydrocarbons ). 

Chain hydrocarbons can be divided into saturated hydrocarbons and unsaturated hydrocarbons. Most of the overall configuration of carbon only hydrogen atoms to carbon-carbon single bond and an organic compound consisting of a hydrocarbon bond, and means a saturated carbon atom bound to other atoms in the molecule reaches the maximum.

Strictly speaking, alkanes do not include naphthenes, so this entry focuses on cyclic alkanes.

In addition, the alkane is a kind of saturated hydrocarbon, and saturated hydrocarbon includes cycloalkane and alkane.

Physical Properties

1. When the number of carbon atoms is less than or equal to 4, the alkanes are gaseous at normal temperature, and other alkanes are solid or liquid at normal temperatures.

2. Not soluble in water, easily soluble in organic solvents.

3. As the number of carbon atoms increases, the boiling point gradually increases.

4. As the number of carbon atoms increases, the relative density gradually increases. The density of alkanes is generally less than the density of water.

Micro Structure

The alkane is not a planar structure drawn by the structural formula, but a three-dimensional shape. All carbon atoms are sp ³ hybrids. Each atom is connected by a σ bond, and the bond angle is close to 109°28 ‘. The length is 154 pm, and the average bond length of the CH bond is 109 pm. Since the σ-bond electron cloud is distributed along the axis of the axis, the two bonding atoms can “freely” rotate about the axis of the bond.

Chemical Formula

Starting from methane, two carbon atoms are added for each carbon atom. Therefore, the general formula of an alkane is C_nH_{2n + 2}, where n is the number of carbon atoms (n = 1,2,3, ···) In theory, n can be large, but the known alkane n is within about 100.

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A series of compounds with the same molecular formula and structural characteristics are called homologous series. The homologue of alkane is CH₂, and alkanes with different numbers of C atoms are homologous to each other. The homologs in the same series have similar structures, similar chemical properties, and regular changes in physical properties with increasing carbon atoms.

Naming rules

There are three commonly used nomenclatures of alkanes, which are described below (only in China):

Common Nomenclature

Ordinary nomenclature, also known as customary nomenclature, is applicable to simpler alkanes. Alkanes with a carbon number of less than 10 are represented by the number of carbon atoms using the names of A, B, C, D, P, P, H, G, S, N, and Dec.

For example, CH₄ is called methane, and C₂H₆ It is called ethane, C₃H_{8 is} called propane, and the rest can be deduced by analogy when the number of carbon atoms is 10 or more, it is represented by Chinese numerals. For example, C₁₁H_{24 is} called undecane, and C₁₈H_{38 is} called octadecane.

In order to distinguish isomers, they can be expressed by prefixes such as “positive”, “different”, “new”, etc. E.g:

N-hexane

CH ₃ —CH ₂ —CH ₂ —CH ₂ —CH ₂ —CH ₃

Iso- hexyl alkoxy

CH ₃ —CH—CH ₂ —CH ₂ —CH ₃
　　∣
　　CH ₃

New hexyl alkyl

CH ₃
　　|
　　CH ₃ —CH—CH—CH ₃
　　|
　　CH ₃

2,2-dimethylbutane

CH ₃

|

CH ₃ —C—CH ₂ —CH ₃

|

CH ₃

Derivative nomenclature

Derivative nomenclature uses methane as the parent and treats other alkanes as alkyl derivatives of methane. When naming, choose the carbon atom with the most alkyl groups. The alkyl groups are arranged in order of size, with the smaller ones in front. E.g:

CH ₃ — CH —CH ₂ —CH ₃
　　∣
　　CH ₃
　　dimethyl ethyl methane

Dimethyl ethyl isopropyl methane

Although this nomenclature can reflect the molecular structure of alkanes, it is still not suitable for the construction of more complex alkanes.

System nomenclature

This is a systematic nomenclature formulated by adopting the universal IUPAC naming principles and combining the characteristics of Chinese characters. The naming of straight-chain alkanes is basically the same as that of ordinary nomenclature, except that the word “normal” is omitted; and branched-chain alkanes are regarded as alkyl derivatives of straight-chain alkanes, and are named according to the following rules:

(1) The longest carbon chain in the molecule is selected as the main chain, and the branched alkyl group is regarded as a substituent on the main chain. The alkane is called according to the number of carbon atoms contained in the main chain.

(2) Starting from the end closest to the branch chain, the carbon atoms of the main chain are numbered with Arabic numerals, and the position of the branch chain is indicated by the number of the carbon atom to which it is connected.

(3) Write the name of the substituent in front of the name of the alkane. If the main chain contains several different substituents, they are arranged in ascending order; if they contain several identical substituents, they can be listed in The name is indicated by two, three, four …

If the position of the first substituent is the same from either end of the carbon chain, the sum of the numbers representing the positions of all substituents is required to be the smallest number.

Preparation

The main sources of hydrocarbons are natural gas and petroleum. Although the natural gas composition varies from place to place, almost all of them contain 75% methane, 15% ethane, and 5% propane, and the rest are higher-level alkanes. 

The most alkane-containing types are petroleum. Petroleum contains chain alkane and some cyclic alkanes with 1 to 50 carbon atoms. Cyclopentane, cyclohexane and their derivatives are the main types of petroleum. It contains aromatic hydrocarbons. 

The petroleum produced in different parts of China has different compositions, but they can be fractionated into different fractions and applied as required. Alkanes are not only an important source of fuel but also a feedstock for the modern chemical industry. In addition, alkanes can also be used as food for some bacteria.

After the alkanes are consumed by the bacteria, many useful compounds are secreted, that is, the alkanes can become more useful compounds after being “processed” by the bacteria.

The above situation shows that the development of the petroleum industry is very important for the development of the national economy and organic chemistry.

Although petroleum is rich in various alkanes, this is a complex mixture. Except for C1 to C6 alkanes, it is difficult to completely separate them into extremely pure alkanes due to the small molecular weight differences between the components and similar boiling points. 

Although gas chromatography can be used for effective separation, it is only suitable for research and cannot be used for mass production. Therefore, in use, only petroleum is separated into several fractions for application. Sometimes pure alkane is used as a reference in petroleum analysis, which can be prepared by synthetic methods.

Gasoline (petrol) burns in an internal combustion engine and deflagrates or knocks, which reduces the power of the engine and damages the engine. The tendency of the fuel to cause knocking is expressed as an octane value.

The octane number of 2,2,4-trimethylpentane is set to 100 in the gasoline combustion range. The higher the octane number, the stronger the ability to prevent knocking. The linear alkane with more than six carbons has a low octane number, and branched, unsaturated alicyclic rings, especially aromatic rings, are the most ideal, some more than 100. Most modern equipment requires an octane number between 90 and 100. 

Naphtha, atmospheric residue, and sometimes gas oil can be processed to increase the octane number to about 95, and then mixed into gasoline for use. One of the processing methods is catalytic reforming, which mainly aromatizes more than C6 components in naphtha, that is, becomes aromatic hydrocarbons. 

In addition to increasing the octane number of naphtha, this method is mainly used in the chemical industry to produce aromatic hydrocarbons. The second method is the catalytic cracking. In addition to increasing the octane number, this method is mainly used in the production of propylene and butadiene Ene. 

Physical properties

Physical state

The physical properties of alkanes change regularly with the increase in the number of carbon atoms in the molecule.

At room temperature 25°, alkanes containing 1 to 4 carbon atoms are gases.

Alkanes containing 5 to 17 carbon atoms are liquid. However, alkanes containing 10 to 19 carbon atoms can be solid at normal temperatures.

Containing 18 or more carbon atoms as solid n-alkanes, but then-alkanes containing up to 60 carbon atoms (melting point of 99 deg.] C) melting point (melting point) does not exceed 100 ℃.

Alkane is non-polar molecules (non-polar molecule), dipole moments (dipole moment) is zero, but the molecular charge distribution is not uniform, may be generated in the movement transient dipole moment, the dipole moment has between instantaneous Interaction force ( dispersion force ). In addition, there are Van der Waals forces between the molecules.

These intermolecular forces are one or two orders of magnitude smaller than chemical bonds. The energy required to overcome these forces is also low. Therefore, the melting point and boiling point of organic compounds rarely exceed 300°C.

Boiling point

The boiling point of n-alkanes increases with the increase of carbon atoms. This is because of the energy required for molecular movement increases, the contact surface between molecules increases, and the van der Waals force increases. For each additional CH₂ in lower alkanes, the relative molecular mass changes greatly, and the boiling point also varies greatly; the difference in boiling points of higher alkanes gradually decreases. Therefore, lower alkanes are easier to separate, and higher alkanes are much more difficult to separate.

In the isomer, the molecular structure is different, the molecular contact area is different, and the interaction force is also different. The boiling point of n-pentane is 36.1°C, the boiling point of 2-methylbutane is 25°C, and the boiling point of 2,2-dimethylpropane is only 9°C. Fork chain molecules tend to be spherical. Due to the steric hindrance of the branch chain, the contact area is reduced, which reduces the intermolecular forces and has a lower boiling point.

Melting Point

The melting point of solid molecules also increases with the increase of carbon atoms, but it is not as regular as the change in boiling point. It is less regular with the same series of C₁ – C₃, but those above C₄ increase with the number of carbon atoms. 

This is due to the interaction between the molecules of the crystal, which is not only determined by the relative molecular mass, but also by the arrangement of the molecules in the crystal lattice. High molecular symmetry, the tighter the alignment, the more intermolecular absorption

The higher the gravity, the higher the melting point. Among normal alkanes, alkanes containing odd carbon atoms have a lower melting point increase than even carbon atoms. Even in the melting point curve of a linear alkane, an alkane containing an odd number and an even number of carbon atoms each constitutes a melting point curve, the even number is above and the odd number is below.

According to X-ray diffraction analysis, the solid n-alkane crystals are sawtooth-shaped, and both ends of the methyl groups in the dentate chain of the odd carbon atoms are on the same side, such as n-pentane. 

The methyl groups at both ends of the even carbon chain are not on the same side. For example, n-hexane, the even carbon chains are closer to each other, and the interaction force is large, so the increase in melting point is larger than that in the single carbon chain.

Density

The density of alkanes increases as the relative molecular mass increases. This is also the result of the intermolecular interaction force. The intermolecular gravity increases, the distance between molecules decreases accordingly, the relative density increases, and the density increases to a certain level. After the value, the relative molecular mass increases with little change in density. The maximum is approximately 0.8 g · cm ^-3, so all alkanes are lighter than water.

Solubility

The sigma bond in alkane has a very small polarity, and its molecular dipole moment is zero, which is a non-polar molecule. According to the principle of similar compatibility, alkanes are soluble in non-polar solvents such as carbon tetrachloride and hydrocarbon compounds ( ether, benzene ) and insoluble in polar solvents such as water.

Compared with paraffin with the same number of carbon atoms, the boiling point, melting point, and density of naphthenes are slightly higher. This is because the chain-shaped compound can be shaken relatively freely, and the “pulling” between molecules is not tight and it is easy to volatilize, so the boiling point is lower.

Because of this shaking, it is difficult to make an ordered arrangement in the crystal lattice, so the melting point is also lower. Because there is no ring restriction, the arrangement of chain compounds is looser than that of cyclic compounds, so the density is lower. Isomers and the cis and trans isomers have different physical properties. The following table shows the physical constants of several alkanes and naphthenes.

Chemical Properties of Alkane

Because the alkane contains only CC single bond and CH single bond, the strength of these two types of bonds are very large, and the electronegativity between carbon and hydrogen is very small. Therefore, the polarity of the CH bond is very small and belongs to a weakly polar bond. Compared to other organic substances, alkane ion reagents have considerable chemical stability.

In general, alkanes do not react with most reagents such as strong acids, strong bases, and strong oxidants. However, under certain conditions, such as at high temperatures or in the presence of a catalyst, alkanes can also interact with some reagents.

Halogenation Reaction

A reaction in which a hydrogen atom in an alkane is replaced with a halogen atom (ie, a seventh main group element) is called a halogenation reaction. But the practical halogenation reactions are chlorination and bromination.

Chlorinated

Alkanes do not react with chlorine at room temperature and in the dark, but can undergo substitution reactions under sunlight or ultraviolet light or under high temperature (250-400°C). The hydrogen atoms in the alkanes can be gradually replaced by chlorine, resulting in different A mixture of chloroalkanes.

For example, methane reacts with chlorine to form a mixture of four chlorinated products.

If the amount of chlorine is controlled, a large amount of methane is used to obtain mainly methyl chloride; if a large amount of chlorine is used, carbon tetrachloride is mainly obtained. Industrially, the mixture is separated one by one by rectification. The above-chlorinated products are all important solvents and reagents.

The facts of the methane chlorination reaction are:

① No reaction occurs in a dark place at room temperature.

② The reaction occurs at 250°C.

③ can react under the effect of light at room temperature.

④ Use light to initiate the reaction and absorb a photon to generate thousands of methyl chloride molecules.

⑤ If there is oxygen or some impurities that can capture free radicals, the reaction has an induction period. The length of the induction period is related to the presence of these impurities. According to the characteristics of the above facts, it can be judged that the chlorination of methane is a radical type of substitution reaction.

2. Halogenation of methane

In the same type of reaction, you can understand the difficulty of the reaction by comparing the activation energy that determines the reaction rate in one step.

The reaction between fluorine and methane is exothermic, but it still requires +4.2 KJ / mol activation energy. Once the reaction occurs, a large amount of heat is difficult to remove, destroying the generated fluoromethane, and carbon and hydrogen fluoride are obtained, so the reaction of direct fluorination hard to accomplish. 

The reaction between iodine and methane requires activation energy greater than 141 KJ / mol, and the reaction is difficult to proceed. The activation energy of chlorination only needs +16.7 KJ / mol, and the activation energy of bromination only needs +75.3 KJ / mol, so the halogenation reaction is mainly chlorination and bromination. Chlorination is easier than bromination.

Iodine cannot react with methane to form methyl iodide, but its reverse reaction is easy to proceed.

Adding iodine to the base chain reaction can stop the reaction.

3. Halogenation of higher alkanes

Under the action of ultraviolet light or heat (250-400 ° C), chlorine and bromine can react with alkanes. Fluorine can be fluorinated under the dilution of inert gas, but iodine cannot.

Free Radical Reaction

1. Definition and structure of carbon radical

When a bond is split, an atom or group with a lone electron is generated, which is called a free radical. The radical of a lone electron on a hydrogen atom is called hydrogen radical. 

The radical of a lone electron on a carbon atom is called a carbon radical. When the carbon and hydrogen bonds in the alkane are split, hydrogen radicals and an alkyl radical, that is, carbon radicals are generated. 

Free radical carbon sp² hybrids, three sp² hybrid orbitals have a flat triangle structure, each sp² hybrid orbital, and other atomic orbitals form a σ bond by axial overlap, and a pair of spins on the bond orbitals are opposite Electronics. A p-orbit is perpendicular to this plane, and the p-orbit is occupied by a solitary electron.

2. Bond dissociation energy and stability of carbon radicals

(1) Bond dissociation energy

Atoms in molecules always make small vibrations around their equilibrium positions. Molecular vibrations are similar to the movement of spring-connected spheres. At room temperature, the molecules are in the ground state.

At this time, the amplitude is small, and the molecules absorb energy and the amplitude increases. If sufficient energy is absorbed and the amplitude is increased to a certain degree, the bond is broken.

The heat absorbed at this time is the enthalpy of the bond dissociation reaction (ΔH), which is the bond energy of the bond, or the bond dissociation energy ( bond-dissociation energy), expressed in Ed.

(2) Stability of carbon radicals

The stability of a free radical means that compared with the stability of its parent compound, the energy of the free radical is much more unstable than that of the parent compound and less stable. 

From the dissociation energy data of the C-H bond above, it can be seen that the dissociation energy of the C—H bond in CH₄ has the largest dissociation energy, and the first compound in the homologous series is often special; CH₃CH₃ and In CH₃CH₂CH₃, the hydrogen on the first-order carbon is slightly lower than that of CH₄ and all the first-order free radicals are formed.

In CH₃CH₂CH₃, the hydrogen on the second-order carbon is broken. The dissociation energy is lower, forming a secondary radical, the hydrogen on the tertiary carbon atom in (CH₃)₃CH is broken, and its dissociation energy is the lowest, forming a tertiary radical. 

In these bond dissociation reactions, one of the products is the same, so the difference in bond dissociation energy reflects the different stability of carbon radicals. 

Carbon radicals with lower dissociation energy are more stable. So the stability order of carbon radicals is

3 ° C ·> 2 ° C ·> 1 ° C ·> H3C ·

In alkane molecules, C-C bonds can also be dissociated.

3. The commonness of Free Radical Reaction

Chemical bonds are split to produce free radicals. A reaction initiated by a free radical is called a free radical reaction, or a free radical type chain reaction. Free radical reactions generally go through three stages: chain initiation (propagation, or chain generation), and chain termination. 

The chain initiation stage is the stage that generates free radicals. Since the home cracking of the bonds requires energy, the chain initiation phase requires heating or light.

Some compounds are very active and easily generate active particle-free radicals. These compounds are called initiators. In some cases, free radicals can also be generated by a single-electron transfer redox reaction. 

The chain transfer stage is a stage in which a free radical is transformed into another free radical. Like a relay race, free radicals are continuously passed on, like a chain after chain, so it is called a chain reaction. The chain termination stage is the stage where free radicals disappear. Free radicals are combined in pairs. All free radicals are gone and the free radical reaction is stopped.

The characteristic of free radical reaction is that there is no obvious solvent effect, and catalysts such as acid and alkali have no obvious effect on the reaction. When there is oxygen in the reaction system (or some impurities that can capture free radicals), the reaction often has an induction period. (induction period).

Thermal cracking reaction

In the absence of oxygen, alkanes undergo carbon-carbon bond cleavage at high temperatures (about 800°C), and large molecules become small molecules. This reaction is called pyrolysis. 

In addition to gasoline after petroleum processing, there are alkanes with a relatively high molecular weight such as kerosene and diesel, through thermal cracking reactions, they can be converted into small molecule compounds such as gasoline, methane, ethane, ethylene and propylene, and the process is very complicated.

The product is also complex, both carbon-carbon bonds and carbon-hydrogen bonds can be broken, and the break can occur in the middle of the molecule or on the side of the molecule, the larger the molecule, the easier it is to break, and the thermally cracked molecule can also be thermally cracked. 

The reaction mechanism of the thermal cracking reaction is a free radical reaction under the action of heat, and the raw materials used are mixtures.

Free radicals generated after thermal cracking can be combined with each other. Free radicals generated by thermal cracking can also be cleaved by hydrocarbon bonds to produce olefins.

The overall result is that alkanes and alkenes with smaller molecules are thermally cracked. This reaction is difficult to perform in the laboratory but is very important in the industry. In industrial thermal cracking, alkane mixed with water vapor is passed through a heating device at about 800°C in a tube and then cooled to 300-400°C.

These are completed in less than one second, and the thermally cracked product is then Separate one by one by freezing. Raw materials such as plastic, rubber, and fiber can be obtained through this reaction.

The world-scale production of ethylene is tens of millions of tons per year, and it is still growing. Different alkane raw materials used in different countries also have different products.

For example, naphtha can be used as a raw material to obtain 15% methane, 31.3% ethylene, 3.4% ethane, 13.1% propylene, 4.2% butadiene, and butane. 2.8%, gasoline 22%, fuel oil 6%, there are still a few other products.

It is generally easier to break in the middle of the carbon chain, and then a series of β-breaks occur.

Naphtha also contains branched alkanes, naphthenes, and aromatic hydrocarbons, such as thermal cracking of naphthenes to obtain ethylene and butadiene.

Aromatic hydrocarbons only react on the side chains and remain unchanged because the aromatic ring is stable. Therefore, for example, the production of ethylene is preferably a petroleum fraction containing the most linear paraffin.

If the catalyst is used to perform the thermal cracking reaction, the temperature can be reduced, but the reaction mechanism is not a radical reaction but an ionic reaction.

Oxidation and Combustion

People often encounter such phenomena in life. People have wrinkles on their skin when they are old, rubber products have become hard and sticky over time, plastic products have become hard and crackable over time, and cooking oil has deteriorated over time.

These phenomena are called aging. The aging process is very slow. The reason for aging is first that oxygen in the air enters various molecules with active hydrogen and autoxidation occurs, and then other reactions occur.

All alkanes can be burned. When completely burned, the reactants are completely destroyed, generating carbon dioxide and water, and emitting a lot of heat at the same time.

When burning, the flame is light blue and not bright.

Nitrification

Alkanes react with nitric acid or nitrous oxide in the gas phase (400-450°C) to form nitro compounds (RNO₂). This reaction that directly generates a nitro compound is called nitration, and it is an important reaction in the industry. 

It is important because nitroalkanes can be converted into many other types of compounds, such as amines, hydroxylamines, nitriles, alcohols, aldehydes, ketones, and carboxylic acids. In addition, nitroalkanes can undergo a variety of reactions, so reports of the use of nitroalkanes in modern literature are increasing. 

The use of gas-phase nitration in the laboratory has great limitations, so the nitroalkanes are mainly prepared in the laboratory by indirect methods. Gas-phase nitration method is used to prepare nitroalkanes, and often a mixture of multiple nitro compounds is obtained.

Sulfonation and Chlorosulfonation

Alkanes react with sulfuric acid at high temperatures, similar to the reaction with nitric acid, to form alkyl sulfonic acids. This reaction is called sulfonation.

The sodium salt of long-chain alkyl sulfonic acid is a detergent, called a synthetic detergent, such as sodium dodecyl sulfonate.

The reaction of higher alkanes with sulfuryl chloride (or a mixture of sulfur dioxide and chlorine gas) to form alkyl sulfonyl chloride under the irradiation of light is called Chlorosulfonation. 

The name sulfonyl chloride is derived from sulfuric acid. The group that is left after sulfuric acid removes a hydroxyl group is called a sulfonic acid group. Compounds in which a sulfonic acid group is connected to an alkyl group or other hydrocarbon group are collectively called a sulfonic acid. 

After the hydroxyl group in the sulfonic acid is removed, a sulfonyl group is obtained, which is combined with chlorine to obtain a sulfonyl chloride.

Sulfonyl chloride is hydrolyzed to form alkylsulfonic acid, and the sodium or potassium salt thereof is the above-mentioned detergent. The reaction mechanism is very similar to the chlorination of alkanes.