alkane

Organic compound

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Alkanes (w á n t ī ng) are a class of organic compounds carbon atom All in carbon Single bond Connected, the rest Valence bond Are combined with hydrogen, divided into Naphthenic hydrocarbon and Alkane Two types.^[1] The general formula of alkane is C n H 2n+2 , the general formula of naphthenic hydrocarbon is C n H 2n , is the simplest type Organic compound 。 The main sources of alkanes are petroleum and natural gas, which are important chemical raw materials and energy materials.

Alkanes are open chain saturated groups. The carbon atoms in the molecule are connected by single bonds, and the other valence bonds are combined with hydrogen.

Chinese name: alkane
Foreign name: Alkane

Composition: Hydrocarbon
Chemical bond: Single bond (carbon carbon single bond and hydrocarbon single bond)

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Physical properties

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definition

Only by Hydrocarbon two types Element composition The organic compounds of hydrocarbon 。 According to the different molecular frameworks of hydrocarbons, hydrocarbons can be divided into chain hydrocarbons（ Aliphatic hydrocarbon ）And cyclic hydrocarbon （ Alicyclic hydrocarbon ）Two categories. Chain hydrocarbon can be divided into saturated hydrocarbon and Unsaturated hydrocarbon 。 The overall structure is mostly composed of carbon and carbon atoms Single bond Saturation of organic compounds with hydrocarbon single bond means that the combination of carbon atoms and other atoms in the molecule has reached the maximum.^[1]

In addition, chain alkanes are one kind of saturated hydrocarbons, which include Naphthenic hydrocarbon And chain alkanes.

1. When carbon Atomic number When it is less than or equal to 4, alkanes are gaseous at room temperature, carbon atoms are liquid at 5 to 16, and solid at 17 or more（ Neopentane It is gaseous at normal temperature).

2. They are insoluble in water, Soluble to Organic solvent 。

3. The boiling point increases with the increase of carbon atom number.

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

5. The melting point of straight chain alkanes increases with the increase of carbon atoms, but the melting point of even number of carbon atoms increases more than that of odd number of carbon atoms.^[5]

Microstructure

Hybrid orbitals of carbon atoms

Alkanes are not Structural Painted Plane structure The carbon skeleton is in a zigzag shape. All carbon atoms are sp three Hybridization, between atoms σ bond to be connected, Bond angle Near 109 ° 28 ', the average bond length of C-C bond is 154 pm, and the average bond length of C-H bond is 109 pm Electronic cloud along Key shaft Axiform Symmetrical distribution Two bonding atoms can rotate "freely" around the bond axis.^[3]

chemical formula

from methane At the beginning, each additional carbon atom will be correspondingly increased by two hydrogen atom Therefore, the general formula of alkane is C n H two n +2 , n is the number of carbon atoms（ n =1, 2, 3,...), theoretically n Large, but known alkanes n About within 100.^[4] A series of compounds with the same general formula and structural characteristics Same series , alkane homologous difference is CH two Alkanes with different number of C atoms homologue 。 Homologous Structural similarity ， chemical property close, physical property With the increase of carbon atoms Regularity Change.

smell

Alkanes with low boiling point are colorless liquid , with special smell; High boiling point alkanes are viscous oily liquids, odorless.

State of matter

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

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

Alkanes containing 5~16 carbon atoms are liquids. But in fact, alkanes containing 10-19 carbon atoms can be solid at normal temperature.

The n-alkanes containing more than 18 carbon atoms are solid, but the melting point of the n-alkanes containing 60 carbon atoms (melting point 99 ℃) is not more than 100 ℃.^[1] Alkanes are Nonpolar molecule （non-polar molecule）， dipole moment (dipole moment) is zero, but the distribution of charge in the molecule is not very uniform, which can produce instantaneous dipole Moment, instantaneous dipole moment Interaction force （ Dispersion force ）In addition, there are very weak intermolecular Induced force 。 these ones here Intermolecular force than Chemical bond One or two children of Order of magnitude , overcome these Force The required energy is also low, so the melting point and boiling point of general organic compounds rarely exceed 300 ℃.^[2]

boiling point

The boiling point of n-alkanes increases with the increase of carbon atoms, because Molecular motion The energy required increases, and the contact surface between molecules increases, Van der Waals It is enhanced accordingly. Each increase of CH for low-grade alkanes two ， relative molecular mass The change is large, and the difference of boiling point is also large; The boiling point difference of higher alkanes gradually decreases. Therefore, it is easier to separate low-grade alkanes and much more difficult to separate high-grade alkanes.

stay Isomer Medium, molecular structure Different, molecular contact area Different, the interaction force is also different. N-pentane Boiling point 36.1 ℃, 2-Methylbutane Boiling point 25 ℃, 2,2-dimethylpropane The boiling point is only 9 ℃. Cross chain molecules tend to be spherical because Branched chain Of Steric hindrance Function to reduce its contact area, thereby reducing Intermolecular force , the boiling point is low.

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 of boiling point one -C three Not so regular, but C four The above increases with the increase of the number of carbon atoms. This is because crystal The intermolecular force not only depends on the relative molecular weight, but also on the arrangement of molecules in the lattice. Molecular symmetry High, the more orderly the arrangement is, the closer it is, the greater the intermolecular attraction is, and the higher the melting point is.^[3] In normal alkanes, the melting point of alkanes with odd carbon atoms increases less than those with even carbon atoms. So that in the melting point curve of straight chain alkanes, alkanes containing odd and even carbon atoms form a melting point curve respectively, with the even number at the top and the odd number at the bottom.

By X-ray diffraction analysis, the solid n-alkane crystal is Zigzag In the toothed chain of odd carbon atoms, the methyl groups at both ends are at the same side, such as N-pentane 。 even numbers Carbon chain The methyl groups at both ends are not on the same side, such as N-Hexane Even number carbon chains are closer to each other, and the interaction force is large, so the rise value of melting point is larger than that of single number carbon chains.

density

The density of alkanes increases with the increase of relative molecular weight, which is also Intermolecular interaction As a result, the intermolecular attraction increases, the distance between molecules decreases correspondingly, and the relative density increases. When the density increases to a certain value, the relative molecular weight increases Density change Very small. Maximum close to 0.8g · cm -3 So all alkanes are lighter than water.^[3]

solubility

Alkane σ bond The polarity is very small, and its molecular dipole moment is zero, yes or no Polar molecule 。 according to Similitude compatibility In principle, alkanes are soluble in Non-polar solvent as carbon tetrachloride 、 Hydrocarbons In compounds, insoluble in Polar solvent , such as water.^[3] When liquid alkanes are used as solvents, weak polar compounds can be dissolved, but strong polar compounds cannot be dissolved.^[7]

Equal to the number of carbon atoms Alkane In contrast, the boiling point, melting point and density of naphthenes are higher. that is because Chain shape The compound can be shaken freely, and the intermolecular "pull" is not tight, so it is easy to volatilize, so the boiling point is lower. Because of this shaking, it is difficult to arrange orderly in the lattice, so the melting point is also lower. Since there is no ring, the arrangement of chain compounds is looser than that of ring compounds, so the density is also lower. Isomer And cis trans isomers also have different physical properties. The following table shows the Physical constant 。

Physical constants of some alkanes and cycloalkanes
name	Molecular formula	Boiling point/℃	Melting point/℃	relative density
methane	CH four	-161.7	-182.6	——
ethane	C two H six	-88.6	-182.8	——
propane （ Cyclopropane ）	C three H eight (C three H six )	-42.2(-32.7)	-187.1(-127.6)	zero point five zero zero five
butane （ Cyclobutane ）	C four H ten (C four H eight )	-0.5(12.5)	-138.4(-80)	zero point five seven eight eight
Pentane （ Cyclopentane ）	C five H twelve (C five H ten )	36.1(49.3)	-129.3(-93.9)	0.6264(0.7457)
hexane （ cyclohexane ）	C six H fourteen (C six H twelve )	68.7(80.7)	-94.0(6.6)	0.6594(0.7786)
Heptane （ Cycloheptane ）	C seven H sixteen (C seven H fourteen )	98.4(118.5)	-90.5(-12.0)	0.6837(0.8098)
octane （ Cyclooctane ）	C eight H eighteen (C eight H sixteen )	125.6(150)	-56.8(14.3)	0.7028(0.8349)
Nonane	C nine H twenty	one hundred and fifty point seven	-53.7	zero point seven one seven nine
Decane	C ten H twenty-two	one hundred and seventy-four	-29.7	zero point seven two nine eight
Undecane	C eleven H twenty-four	one hundred and ninety-five point eight	-25.6	zero point seven four zero four
Dodecane	C twelve H twenty-six	two hundred and sixteen point three	-9.6	zero point seven four nine three
Tridecane	C thirteen H twenty-eight	two hundred and forty-five point four	-6	zero point seven five six eight
Tetradecane	C fourteen H thirty	two hundred and fifty-one	five point five	zero point seven six three six
Pentadecane	C fifteen H thirty-two	two hundred and sixty-eight	ten	zero point seven six eight eight
Hexadecane	C sixteen H thirty-four	two hundred and eighty	eighteen point one	zero point seven seven four nine
Heptadecane	C seventeen H thirty-six	three hundred and three	twenty-two	zero point seven seven six seven
Octadecane	C eighteen H thirty-eight	three hundred and eight	twenty-eight	zero point seven seven six seven
Heptadecane	C nineteen H forty	three hundred and thirty	thirty-two	zero point seven seven seven six
Eicosane	C twenty H forty-two	three hundred and forty-three	thirty-six point four	zero point seven eight eight six
Triacontane	C thirty H sixty-two	four hundred and forty-nine point seven	sixty-six	zero point seven seven five zero
Tetradecane	C forty H eighty-two	——	eighty-one	——

^[2]

chemical property

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As alkanes only contain C-C single bond and C-H single bond, the strength of these two bonds is very large, and the carbon and hydrogen Electronegativity The difference is very small, so the polarity of C-H bond is very small, belonging to weak Polar bond Therefore, compared with other organic substances, alkane ionic reagents have a considerable chemical stability In general, alkanes and most reagents such as strong acid , strong alkali Strong oxidant Couldn't respond to waiting. However, under certain conditions, such as high temperature or the presence of catalyst, alkanes can also interact with some reagents.^[1]

Halogenation reaction

The hydrogen atom in alkanes is replaced by the halogen atom (i.e. the seventh Main family elements ）The substitution reaction is called Halogenation reaction （ halogenation ）。 But the halogenation reaction with practical significance is chlorination and Bromination 。

1. Chlorination

Alkanes at room temperature and in the dark chlorine Not responding, but in sunlight or UV irradiation Or under the action of high temperature (250~400 ℃) Substitution reaction The hydrogen atom in the alkane molecule can be gradually replaced by chlorine to obtain different chloroalkane A mixture of.

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

Chlorination of alkanes

If the amount of chlorine is controlled and a large amount of methane is used Methyl chloride ； If a large amount of chlorine is used, carbon tetrachloride is mainly obtained. In industry, the mixture is separated one by one through distillation. The above chlorination products are all important solvents and reagents.

methane Chlorination reaction The fact is:

① No reaction occurs in dark place at room temperature;

② React at temperatures higher than 250 ℃;

③ It can react under the action of light at room temperature;

④ Using light to initiate reaction, absorbing one photon can produce thousands of methyl chloride molecules;

⑤ If there is oxygen or some can catch Free radical There is an impurity of induction period The length of induction period is related to the amount of these impurities. According to the characteristics of the above facts, it can be judged that the chlorination of methane is a radical type substitution reaction.

2. Halogenation of methane

In the same type of reaction, it can be determined by comparison reaction rate One-step activation energy Size, understand the difficulty of the reaction.

The reaction of fluorine with methane is exothermic, but still needs+4.2 kJ/mol activation energy. Once the reaction occurs, a large amount of heat is difficult to remove, and the generated Fluoromethane , and get carbon and Hydrogen fluoride Therefore, direct fluorination is difficult to achieve. The reaction of iodine with methane requires an activation energy greater than 141 kJ/mol, which is difficult to carry out. Only activation energy+16.7 kJ/mol is required for chlorination and+75.3 kJ/mol for bromination, so halogenation reaction is mainly chlorination and bromination. Chlorination is easier than bromination.

Iodine cannot be generated by substitution reaction with methane Iodomethane , but its Reverse reaction It's easy to do.

Iodine is added to the base chain reaction, which can stop the reaction.

3. Halogenation of higher alkanes

stay ultraviolet Or heat (250~400 ℃), chlorine and bromine can react with alkanes, and fluorine can inert gas Fluorination of alkanes is carried out under dilution, but iodine cannot.^[2]

Free radical reaction

1. Definition and structure of carbon free radicals

A key Homolysis Free radicals are the atoms or groups with solitary electrons that will be generated when. The free radicals of lone electrons on the hydrogen atom are called hydrogen radicals. The radicals of lone electrons on carbon atoms are called carbon radicals. One hydrogen radical and one alkyl radical, namely carbon radical, will be produced when the hydrocarbon bond in alkanes is split evenly. Free radical carbon sp two Hybrid, three sp two The hybrid orbit has a plane triangle structure, and each sp two Hybrid orbitals pass through other atomic orbitals axial Overlap to form σ bond, Bonding orbital There is a pair of electrons with opposite spin on it. One P Orbit Perpendicular to this plane, the p orbital is occupied by an lone electron.

two Bond dissociation energy And the stability of carbon free radicals

(1) Bond dissociation energy

Atoms in molecules always surround them Balance position Make small vibration, Molecular vibration Similar to the motion of a spring connected ball, at room temperature, the molecules are ground state , at this time amplitude Very small, the molecule absorbs energy, and the amplitude increases. If sufficient energy is absorbed and the amplitude increases to a certain extent, the bond will be broken. At this time, the heat absorbed is the enthalpy of bond dissociation reaction (Δ H ）Is the bond energy of this bond, or Bond dissociation energy (bond association energy), represented by Ed.

(2) Stability of carbon free radicals

The stability of free radicals refers to the Parent compound Compared with the stability of the parent compound, the one with much higher energy is more unstable, and the one with less energy is more stable. From the C-H key above Dissociation energy It can be seen from the data that: CH four The dissociation energy of C-H bond is the largest, and the first compound in the homologous system is often special; CH three CH three And CH three CH two CH three The dissociation energy of hydrogen on the primary carbon is higher than that of CH four Slightly lower, all of them are primary free radicals; CH three CH two CH three The dissociation energy of hydrogen on the secondary carbon atom is lower, forming secondary free radicals; (CH three ) three The dissociation energy of hydrogen on the third carbon atom in CH is the lowest, forming the third radical. In these bond dissociation reactions, one of the products is that is the same, so the different bond dissociation energies reflect the different stability of carbon radicals. The lower the dissociation energy, the more stable the carbon radicals are. Therefore, the stability order of carbon free radicals is

3°C·＞2°C·＞1°C·＞H three C·

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

3. Common characteristics of free radical reaction

The chemical bond splits to produce free radicals. The reaction initiated by free radicals is called free radical reaction, or free radical type chain reaction (chain reaction)。 Free radical reactions generally go through Chain initiation (initiation)、 chain transfer (promotion, or chain generation) Chain termination (termirrntimi) Three stages. chain Initiation phase It is the stage of free radical production. Since the bond splitting requires energy, the chain initiation phase requires heating or lighting.

Some compounds are very active and easy to produce living radicals. These compounds are called Initiator ( initiator )。 Sometimes through Single electron transfer Of Redox reaction To produce free radicals. The chain transfer stage is the stage of transforming from one free radical to another, as if relay race Similarly, free radicals are transmitted continuously, like chain after chain, so it is called chain reaction. chain Termination phase It is the stage of the disappearance of free radicals. Free radicals form bonds in pairs. All free radicals disappear, and the free radical reaction stops.

The characteristics of free radical reaction are not obvious solvent effect The acid, base and other catalysts 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 a induction period （induction period）。^[2]

Hot cracking reaction

In the absence of oxygen, the carbon carbon bond of alkanes breaks at high temperature (about 800 ℃), Macromolecular compound Becomes Small molecule Compounds, this reaction is called hot cracking（ pyrolysis )。 In addition to gasoline, there are kerosene , diesel and other alkanes with relatively high molecular weight; It can be converted into gasoline, methane ethane 、 ethylene and propylene The process and products of small molecule compounds are very complex; Both carbon carbon bond and hydrocarbon bond can be broken, and the breaking can occur in the middle of the molecule or on one side of the molecule; The larger the molecule, the easier it is to break, and the molecule after hot cracking can be hot cracked again. The reaction mechanism of hot cracking reaction is free radical reaction under thermal action, and the raw materials used are mixtures.

The free radicals produced after hot cracking can combine with each other. The free radicals generated by thermal cracking can also be broken through the hydrocarbon bond to produce olefin 。

The overall result is macromolecule Alkanes are thermally split into smaller alkanes and olefins. This reaction is difficult in the laboratory, but very important in industry. In industrial hot cracking, alkanes are mixed with steam Pass a heating device at about 800 ℃ in the tube, and then cool it to 300~400 ° C, which is completed in less than one second, and then use Freezing method Separate them one by one. Plastic rubber , fiber, etc. can be obtained through this reaction.

If the catalyst is used for hot cracking reaction, the temperature can be reduced, but reaction mechanism It's not a free radical reaction Ionic reaction 。^[2]

oxidation reaction And combustion

We often encounter such a phenomenon in our life that people have wrinkles when they get old, Rubber products It becomes hard and sticky after being used for a long time, and plastic products become hard and easy to crack after being used for a long time, Edible oil These phenomena are called aging. The aging process is very slow. The first reason for aging is that the oxygen in the air enters various molecules with active hydrogen to produce automatic oxidation reaction, and then other reactions occur.^[2]

All alkanes can burn, Complete combustion When, reactant All destroyed, generated carbon dioxide And water, releasing a lot of heat at the same time.

When burning, the flame is Light blue , not bright.^[2]

Nitrification

Alkanes and nitric acid or Nitrogen tetroxide Conduct gas phase (400~450 ℃) reaction to generate Nitro compound （RNO two ）。 This reaction to directly generate nitro compounds is called nitration, which is a very important reaction in industry. It is important because nitroalkanes can be transformed into a variety of other types of compounds, such as amines hydroxylamine , nitrile, alcohol, aldehyde, ketone, carboxylic acid, etc. In addition, nitroalkanes can react in many ways, so there are more and more reports about the application of nitroalkanes in modern literature. Gas phase nitration has great limitations in the laboratory, so nitroalkanes are mainly prepared by indirect methods in the laboratory. Nitroalkanes are prepared by gas phase nitration, and the mixture of many nitro compounds is often obtained.^[2]

Sulfonation and Chlorosulfonation

Alkanes and sulphuric acid Reaction, similar to the reaction with nitric acid, generates alkyl sulfonic acid , this reaction is called sulfonation（ sulfonation ）。

Long chain The sodium salt of alkyl sulfonic acid is a detergent called Synthetic detergent , for example Sodium dodecyl sulfonate It is one of them.

Higher alkanes and Thionyl chloride (or sulfur dioxide And chlorine) under the irradiation of light, produce alkyl Sulfonyl chloride The reaction is called chlorosulfonation. The name sulfonyl chloride is derived from sulfuric acid. The groups left after sulfuric acid removes one hydroxyl group are called sulfonic (acid) group, sulfonic (acid) group and alkyl group or others Alkyl group The connected compounds are collectively called sulfonic acid. After the hydroxyl group in the sulfonic acid is removed Sulfonyl group It combines with chlorine to get sulfonyl chloride.

Sulfonyl chloride is hydrolyzed to form alkyl sulfonic acid, its sodium salt or sylvite That is, the above detergent. The reaction mechanism is similar to that of chlorination of alkanes.^[2]

Natural distribution

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Alkanes are widely distributed in the universe, of which methane is the most widely distributed, and alkanes composed of more than 50 carbon atoms are rarely seen. Alkanes are distributed in the atmosphere of many planets in the solar system, some accounting for a larger proportion, such as Uranus (2.3%) and Titan (5%), but they are less distributed on most planets, such as Earth, Mars, Saturn, etc. Most of the alkanes on the earth are methane, and the concentration of methane decreases with the decrease of the earth's latitude, and it is at 40 north latitude. And near the equator. The average concentrations in the Northern Hemisphere and the Southern Hemisphere are 1 65mg/L and 1.55mg/L.^{[8 ]}

The alkanes used by humans mainly come from oil and natural gas. Natural gas contains about 75% methane, 15% ethane, 5% propane, and other higher alkanes.^{[8 ]} Petroleum contains the most kinds of alkanes, which can be fractionated into different fractions for application as required.^{[8 ]}

Some animals and plants also have a small amount of alkanes, such as the wax on tobacco contains heptadecane and triacontane, the wax on Chinese cabbage leaves contains twenty-nine alkanes, and the wax on apple peel contains heptadecane and twenty-nine alkanes. In addition, some insect pheromones are alkanes. The so-called "insect pheromone" is a chemical substance secreted by the same kind of insects to transmit messages. For example, there is an ant that transmits warning information by secreting an odor substance. After analysis, the odor substance contains n-undecane and n-tridecane. Another example is that the sex pheromone used by female tiger moths to lure male moths is 2-methylheptadecane.^{[9 ]}

Preparation method

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The main sources of hydrocarbons are natural gas and petroleum. Although the composition of natural gas varies from place to place, almost 75% methane , 15% ethane And 5% propane The rest are higher alkanes. The largest number of alkanes is petroleum, which contains 1 to 50 carbon atoms of chain alkanes and some cyclic alkanes Cyclopentane 、 cyclohexane And its derivatives, and the oil from some places also contains Aromatic hydrocarbon 。 The oil from different places in China has different components, but they can be fractionated into different fraction Apply. Alkanes are not only an important source of fuel, but also modern chemical industry Raw materials. In addition, alkanes can also be used as food for some bacteria. After eating alkanes, bacteria secrete many useful compounds, which means that alkanes can become more useful compounds after being "processed" by bacteria.

The above situation shows that, petroleum industry The development of national economy as well as Organic chemistry The development of is very important.

Although petroleum is rich in various alkanes, it is a complex mixture, except C one ～C six In addition to alkanes, it is difficult to completely separate them into extremely pure alkanes because of the small difference in the relative molecular weight of each component and the close boiling point. use Gas chromatography Although it can be effectively separated, it is only applicable to research, not mass production. Therefore, in use, petroleum is only separated into several fractions for application, and pure alkanes are sometimes required for petroleum analysis Benchmark , which can be prepared by synthesis.

Petroleum fraction
fraction	Fractionation range	Main components	Fuel application
Gas	Bp below 20 ℃	C one ～C four	Refinery fuel, liquefied petroleum gas
gasoline	bp 30℃～75℃	C four ～C eight	Low octane number, used as the blending component of motor gasoline
naphtha	bp 75℃～190℃	C eight ～C twelve	The octane number is too low to be directly used as vehicle gasoline
kerosene	bp 190℃～250℃	C ten ～C sixteen	Household fuel, jet fuel, tractor fuel
Gas oil	bp 250℃～350℃	C fifteen ～C twenty	Diesel, fuel for central heating
Atmospheric residue	Bp above 350 ℃	C twenty above	Fuel for power plants, ships and large heating equipment

The gasoline (petroleum) burns in the internal combustion engine and causes deflagration or knock, which will reduce the power of the engine and damage the engine. The tendency of fuel to cause detonation, using Octane number (octane value) means that within the range of gasoline combustion 2,2,4-Trimethylpentane The octane number of is set as 100. The higher the octane number, the stronger the ability to prevent detonation. The octane number of straight chain alkanes with more than six carbons is very low, and branched unsaturated aliphatic rings, especially aromatic rings, are the most ideal, some of which exceed 100. Most modern equipment requires an octane number between 90 and 100. You can change naphtha 、 Atmospheric residue , sometimes used Gas oil After processing, the octane number is increased to about 95, and then mixed with gasoline for use. One of the processing methods is catalytic reforming (catalytic reforming), mainly mixing C in naphtha six The above ingredients Aromatization (aromatization), that is Aromatic hydrocarbon 。 In addition to increasing the octane number of naphtha, this method is mainly used to produce aromatic hydrocarbons in chemical industry. The second processing method is catalytic cracking. In addition to increasing the octane number, this method is mainly used to produce aromatic hydrocarbons in chemical industry propylene 、 Butene 。^[2]

Main purpose

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Fuel

Natural gas and biogas, which are widely used as clean energy, contain a large amount of methane. Gasoline is mainly composed of alkanes with 5-11 carbon atoms, and diesel is mainly composed of alkanes with 14-20 carbon atoms, both of which can be used as fuel.^{[10 ]}

solvent

Petroleum ether is mainly a mixture of pentane and hexane, which can be used as solvent and extractant in the separation of natural products.^{[10 ]}

chemical industry

With oil and natural gas as raw materials, chemical processing can produce products necessary for national defense and national economy, such as plastics, rubber, synthetic fibers, detergents, medicines, pesticides and explosives.^{[11 ]}

medicine

Liquid paraffin is mainly composed of straight chain alkanes containing 18-30 carbon atoms, which can be used as a laxative.^{[10 ]}

product

In addition to gasoline, kerosene and diesel oil, the commonly used alkane mixture also has the following products.^{[11 ]}

name	chemical property	Main purpose
Petroleum ether	The mixture of low-grade alkanes, and the mixture of pentane and hexane with a boiling point of 30~60 ℃; The boiling point range is 90 - 120 ℃, which is a mixture of heptane and octane.	It is mainly used as an organic solvent, which is highly inflammable and toxic
Liquid paraffin	The main component is a mixture of liquid alkanes with 18-24 carbon atoms, which is transparent liquid. Insoluble in water and alcohol, but soluble in ether and chloroform.	Stable and refined liquid paraffin is commonly used as a relief agent for intestinal lubrication in medicine
Vaseline	A mixture of liquid paraffin and solid paraffin, in the form of ointment semi-solid, insoluble in water, soluble in ether and petroleum ether	It is not easy to react with the drugs in the ointment, and is often used as the ointment matrix in medicine
paraffin wax	C twenty-five ～C thirty-four Mixture of solid hydrocarbons	Used in medicine as wax therapy, pill coating, bottle sealing, physiotherapy, etc

Naming rules

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Alkane Nomenclature There are three commonly used ones, which are described as follows (only in China):

Common nomenclature

Name of linear saturated hydrocarbon. Source: Naming Principles for Organic Compounds 2017

Common nomenclature Also known as customary nomenclature, it is applicable to relatively simple alkanes. carbon Atomic number For alkanes below 10, the number of carbon atoms is indicated by the names of A, B, C, D, E, H, G, Xin, None, Kui and other heavenly stems, such as CH four be called methane ，C two H six be called ethane ，C three H eight be called propane , and so on; For carbon atoms above 10 Chinese Numeric representation, e.g. C eleven H twenty-four be called Undecane ，C eighteen H thirty-eight be called Octadecane 。

In order to distinguish isomers, "positive", "different" and "new" can be used as prefixes.

Derivative nomenclature

The derivative naming method takes methane as the parent and regards other alkanes as methane alkyl Derivatives. When naming, select the carbon atom with the most alkyl group, and the alkyl group is based on the size Sequential arrangement , the smaller ones are at the front.

Although this naming method can reflect the molecular structure of alkanes, it is still not applicable to alkanes with more complex structures.

System nomenclature

Alkanes and others Organic compound The system naming of Chinese Chemical Society The newly released Naming Principles for Organic Compounds 2017, which is in line with the internationally accepted IUPAC The naming principle is fully in line with the practice, and also basically conforms to the characteristics of Chinese characters.

（1） Nomenclature of straight chain alkanes

In order to express the number of carbon atoms Cardinal number word Add the word "alkane" after the name

(II) Branched chain Nomenclature of alkanes

(1) Selection of main chain: general branched alkanes, if there is no choice, Should be With the longest Carbon chain As the main chain. When a branched alkane has multiple carbon chains of the same length that can be used as the main chain, it should be selected to have Side chain The carbon chain with the largest number is the main chain.

(2) The rank number of alkanes: the main chain of alkanes is numbered from one end to the other, and the rank is represented by 1,2,3. When several numbers are possible, you should select Substituent The number with "lowest series". The so-called "lowest series" means that the main chain is numbered from different directions to get two or more series with different numbers, and then the different positions of each series are compared item by item. The first one with the smallest position is defined as the "lowest series". When different substituents have the same number, the smaller number of the substituent in front should be given according to the English naming alphabetical order of the substituent.

(3) Determine the name of the alkane: take the name of the main chain as the main name, plus the name of the branch chain or substituent and the position of the main chain Prefix To name. When naming, a half word line "-" is added between the bit symbol and the name, and "bit" words can be added when reading, such as one bit, two bits, etc. The same branch chain or substituent is represented by the corresponding multiple (i.e. cardinal number) words two, three, four, etc. in the same combination principle, and the position symbols are separated by commas ",".

(4) Branched chain or Substituent Listing order: When there are several branch chains or more than two substituents in the alkane molecule at the same time, the substituent shall be listed in the order of the English name letter of the substituent, so that the number of the substituent at the front of the letter is as small as possible; In English, di, tri, tetra, etc affix Does not participate in alphabetic sorting.^[6]

Safety matters

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Fire hazard

Alkanes have relatively stable properties, but under appropriate conditions, oxidation, halogenation, nitrification, pyrolysis and other reactions can also occur. Their density, boiling point and melting point increase with the increase of relative molecular weight, and their fire risk decreases with the increase of relative molecular weight. In case of fire, gaseous alkanes can be used to put out powder and haloalkanes, and liquid and solid alkanes can be put out with foam and mist water.^{[12 ]}

Health hazards

Alkanes can produce anesthetic effect on animals when they are exposed to extremely high concentrations in the air or for a long time within the general concentration range. In some small chemical plants, due to poor ventilation in the workplace, the alkane organic solvents used cannot be dissipated from the workshop or other working environment in time, which directly causes the staff to suffer from diseases due to long-term inhalation.^{[14 ]}

Methane is basically non-toxic to people, but when the concentration is too high, the oxygen content in the air will be significantly reduced, causing people to suffocate. When the concentration of methane in the air reaches 25%~30%, it can cause headache, dizziness, fatigue, inattention, accelerated breathing and heartbeat, and supply imbalance. If it is not kept away in time, it may cause suffocation and death. Skin contact with liquefied methane may cause frostbite.^{[13 ]}

Ethane has simple asphyxia at high concentration. When the concentration in the air is greater than 6%, people will have dizziness, mild nausea, anesthesia and other symptoms; When it reaches more than 40%, it may cause convulsion, or even suffocation.^{[13 ]}

environmental pollution

The pollutants of alkanes appear with the development and application of petroleum products. Surface water pollution caused by oil leakage and oil spill during the transportation and transmission of crude oil on water. In addition, due to road erosion in cities, the sewage contains a certain amount of alkanes. In soil, the pollution of alkanes is caused by the application process of oil extraction, oil refining and petroleum.^{[14 ]}

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