Alkane is any of a series of saturated aliphatic hydrocarbons CnH2n+2 (such as methane, ethane, etc.). This type of compound is the main component of petroleum. Fire axane, fire. ——"Ji Yun" Alkane is a saturated hydrocarbon, a chain hydrocarbon with only carbon-carbon single bonds, and is the simplest type of organic compound. In the alkane molecule, the number of hydrogen atoms reaches the maximum, and its general formula is CnH2n+2. Every carbon atom in the molecule is sp3 hybridized. The simplest alkane is methane. In alkanes, each carbon atom is tetravalent and uses sp3 hybrid orbitals to form strong σ bonds with the surrounding four carbon or hydrogen atoms. The carbon atoms connected to 1, 2, 3, and 4 carbons are called primary, secondary, tertiary, and quaternary carbons respectively; the hydrogen atoms on the primary, secondary, and tertiary carbons are called primary, secondary, and tertiary hydrogen respectively. To minimize bond repulsion, four atoms attached to the same carbon form a tetrahedron. Methane is a standard regular tetrahedron with a bond angle of 109°28′ (accurate value: arccos(-1/3)). Theoretically, all alkanes can exist stably due to their stable structure. However, the alkanes that exist in nature do not have more than 50 carbon atoms at most, and the most abundant alkane is methane. Since the carbon atoms in alkanes can be arranged randomly according to rules, there are countless structures of alkanes. Straight-chain alkanes are the most basic structure, and theoretically this chain can be extended indefinitely. It is possible to produce branched chains on the straight chain, which undoubtedly increases the types of alkanes. Therefore, starting from a 4-carbon alkane, the same molecular formula of an alkane can represent multiple structures. This phenomenon is called isomerism. As the number of carbon atoms increases, the number of isomers increases rapidly. Alkanes may also undergo optical isomerism. When the four atomic groups connected to a carbon atom are different, the carbon is called chiral carbon, and this material is optically active. The remaining part of an alkane after losing one hydrogen atom is called an alkyl group, generally represented by R-. Therefore, alkanes can also be represented by the general formula RH. Alkanes were first named using customary nomenclature. However, this nomenclature is difficult to use for alkanes with a large number of carbon atoms and many isomers. So someone proposed a derivative nomenclature, treating all alkanes as derivatives of methane. For example, isobutane is called 2-dimethylpropane. The current nomenclature uses the IUPAC nomenclature. The systematic naming rules for alkanes are as follows: Find the longest carbon chain as the main chain, and name the main chain according to the number of carbons. The first ten are represented by heavenly stems (A, B, C...) Number, when the number of carbons is more than ten, it is named with Chinese numbers, such as: undecane. Number from the nearest substituent position: 1, 2, 3... (make the position number of the substituent as small as possible). The position of the substituent is represented by a number. Numbers and Chinese numbers are separated by -. When there are multiple substituents, use the carbon chain with the smallest and longest substituent number as the main chain, and list all substituents in the order of methyl, ethyl, and propyl. When more than two substituents are the same, add Chinese numbers in front of the substituent: one, two, three..., such as: dimethyl, its positions are separated by , and are listed together in front of the substituent. The structural formula of isooctane (2,2,4-trimethylpentane). Iso-octane is a standard for gasoline anti-knock, and its octane number is set at 100. For some alkanes with simple structures or commonly used ones, common names are often used. For example, it is customary to add the word "normal" in front of the name of a straight-chain alkane, but this word is not included in the system name. Those with one methyl group at position 2 of the main chain are called "iso", and those with two methyl groups at position 2 are called "new". Although this only applies to butane and pentane with few isomers, it is still retained out of habit. Even 2,2,4-trimethylpentane, which should not be called "iso", is also labeled "iso". Octane" name. Chemical properties Alkanes are very stable because C-H bonds and C-C double bonds are relatively stable and difficult to break. In addition to the following three reactions, alkanes can hardly undergo other reactions. Oxidation reaction R + O2 → CO2 + H2O All alkanes can burn, and the reaction is extremely exothermic. Complete combustion of alkanes produces CO2 and H2O. If the amount of O2 is insufficient, toxic gases carbon monoxide (CO) and even carbon black (C) will be produced.
Take methane as an example: CH4 + 2O2 → CO2 + 2H2O When the supply of O2 is insufficient, the reaction is as follows: CH4 + 2/3 O2 → CO + 2H2O CH4 + O2 → C + 2 H2O Alkanes with large molecular weights often cannot be completely burned. When burned, black smoke is produced, which is carbon black. The same goes for the black smoke in car exhaust. Halogenation reaction R + X2 → RX + HX Because the structure of alkanes is too strong, general organic reactions cannot proceed. The halogenation reaction of alkanes is a free radical substitution reaction. The initiation of the reaction requires light energy to generate free radicals. The following are the steps by which methane is halogenated. This highly exothermic reaction can cause an explosion. Chain initiation stage: Two Cl free radicals are formed under the catalysis of ultraviolet light Cl2 → Cl* / *Cl Chain growth stage: One H atom is detached from methane; CH3Cl begins to form. CH4 + Cl* → CH3+ + HCl (slow) CH3+ + Cl2 → CH3Cl + Cl* Chain termination stage: the two radicals recombine Cl* and Cl*, or R* and Cl*, or CH3* and CH3*. Cracking Reaction cracking reaction is a process in which large molecular hydrocarbons are split into small molecular hydrocarbons under high temperature, high pressure or catalyst conditions. The cracking reaction is an elimination reaction, so the cracking of alkanes always produces olefins. For example, hexadecane (C16H34) can be cracked to obtain octane and octene (C8H18). Since the environment of each bond is different, the probability of breaking is also different. Let’s take the cracking of butane as an example to discuss this: CH3-CH2-CH2-CH3 → CH4 + CH2=CH-CH3 During the process, the CH3-CH2 bond breaks, The possibility is 48%; CH3-CH2-CH2-CH3 → CH3-CH3 + CH2=CH2 The CH2-CH2 bond is broken during the process, the possibility is 38%; CH3-CH2-CH2-CH3 → CH2=CH-CH2-CH3 The probability of C-H bond breaking during + H2 process is 14%. In the cracking reaction, different conditions can trigger different mechanisms, but the reaction processes are similar. Carbon radicals are generated during thermal decomposition, and carbocation and hydrogen ions are generated during catalytic cracking. These extremely unstable intermediates undergo steps such as rearrangement, bond cleavage, and hydrogen transfer to form stable small molecular hydrocarbons. In industry, deep cracking is called cracking, and the products of cracking are gases, called cracking gas. The main function of alkanes is to act as fuel. Natural gas and biogas (mainly composed of methane) are clean energy sources that have been widely used recently. The various fractions obtained by petroleum fractionation are suitable for various engines: C1~C4 (the fraction below 40°C) is petroleum gas and can be used as fuel; C5~C11 (the fraction at 40~200°C) is gasoline and can be used as fuel , can also be used as chemical raw materials; C9~C18 (fraction at 150~250℃) is kerosene and can be used as fuel; C14~C20 (fraction at 200~350℃) is diesel oil and can be used as fuel; Fractions above C20 are Heavy oil can be distilled under reduced pressure to obtain lubricating oil, asphalt and other substances. In addition, the reaction of cracking alkanes to produce olefins has become an important method for producing ethylene in recent years.