f4da; The homologous group of alkanes | 烷烃同系物
Alkanes form a fundamental homologous series in organic chemistry, consisting solely of carbon and hydrogen atoms linked by single covalent bonds. Understanding their systematic patterns, nomenclature and reactivity is essential for A-Level Chemistry.
烷烃是有机化学中最基础的同系物，仅由碳和氢原子通过单键连接而成。理解其系统性规律、命名法及反应活性，对A-Level化学至关重要。
1. Definition of a Homologous Series | 同系物的定义
A homologous series is a family of organic compounds that share the same general formula, functional group and similar chemical properties. Each successive member differs by a —CH2— unit, leading to a gradual change in physical properties.
同系物是一类具有相同通式、相同官能团和相似化学性质的有机化合物家族。相邻成员之间相差一个—CH2—单元，因此物理性质呈现规律性递变。
Members of a homologous series can be prepared by similar general methods, and they exhibit a gradation in melting and boiling points as the relative molecular mass increases.
同系物成员可以通过类似的通用方法制备，并且随着相对分子质量的增加，其熔点和沸点呈梯度变化。
2. The General Formula and Bonding in Alkanes | 烷烃的通式与键合
The general formula for alkanes is CnH2n+2, where n is the number of carbon atoms. Each carbon atom forms four sigma (σ) bonds, either with hydrogen or with neighbouring carbon atoms, giving a tetrahedral geometry with bond angles of approximately 109.5°.
烷烃的通式为 CnH2n+2，其中 n 是碳原子数。每个碳原子形成四个 σ 键，与氢原子或相邻碳原子相连，呈四面体构型，键角约为 109.5°。
The carbon atoms in alkanes are sp³ hybridised, and all bonds are strong σ bonds formed by the head-on overlap of orbitals. This saturation makes alkanes relatively unreactive under normal conditions.
烷烃中的碳原子为 sp³ 杂化，所有化学键均为轨道正面重叠形成的牢固 σ 键。这种饱和性使烷烃在通常条件下相对不活泼。
3. Characteristics of the Alkane Homologous Series | 烷烃同系物的特征
Alkanes exhibit the classic characteristics of a homologous series:
烷烃表现出同系物的典型特征：
1. All members obey the same general formula CnH2n+2.
1. 所有成员都符合相同的通式 CnH2n+2。
2. They have similar chemical properties, dominantly combustion and free-radical substitution.
2. 它们具有相似的化学性质，主要是燃烧和自由基取代反应。
3. Physical properties such as boiling point, viscosity and density show a steady gradation with increasing chain length.
3. 沸点、粘度、密度等物理性质随着碳链增长呈现稳定的渐变趋势。
4. Consecutive members differ by a —CH2— group and by a molecular mass increment of 14.
4. 连续成员相差一个 —CH2— 基团，分子量增加 14。
4. Nomenclature and Naming Rules | 命名法与命名规则
Alkanes are named according to IUPAC rules. The stem indicates the number of carbon atoms in the longest continuous chain: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10). The suffix ‘-ane’ denotes the alkane family.
烷烃依据 IUPAC 规则命名。词干表示最长连续碳链的碳原子数：meth-（1）、eth-（2）、prop-（3）、but-（4）、pent-（5）、hex-（6）、hept-（7）、oct-（8）、non-（9）、dec-（10）。后缀 “-ane” 代表烷烃家族。
For branched alkanes, the longest carbon chain is identified as the parent. Substituent alkyl groups (e.g. methyl, ethyl) are named and numbered to give the lowest possible locants. Numbers are separated from words by hyphens, and multiple identical substituents use prefixes di-, tri-, tetra-.
对于支链烷烃，找出最长的碳链作为主链。取代基烷基（如甲基、乙基）需命名并编号，使位次之和尽可能小。数字与名称间用连字符分隔，多个相同取代基使用前缀 di-、tri-、tetra-。
An example is 2,4-dimethylhexane, where a six-carbon parent chain carries two methyl groups on carbons 2 and 4.
例如 2,4-二甲基己烷，主链为六个碳，在碳 2 和碳 4 上各有一个甲基取代基。
5. Structural Isomerism in Alkanes | 烷烃的结构异构现象
Structural isomers are molecules with the same molecular formula but different arrangements of atoms. For alkanes, chain isomerism occurs when the carbon skeleton can be arranged in different ways.
结构异构体是指分子式相同但原子排列方式不同的分子。烷烃的链异构现象发生在碳骨架可以不同方式排列时。
Methane (CH4), ethane (C2H6) and propane (C3H8) have no isomers. Butane (C4H10) has two: n-butane and 2-methylpropane (isobutane). Pentane (C5H12) has three isomers: n-pentane, 2-methylbutane and 2,2-dimethylpropane. The number of possible isomers rises sharply with increasing carbon atoms.
甲烷（CH4）、乙烷（C2H6）和丙烷（C3H8）无异构体。丁烷（C4H10）有两种：正丁烷和 2-甲基丙烷（异丁烷）。戊烷（C5H12）有三种异构体：正戊烷、2-甲基丁烷和 2,2-二甲基丙烷。随着碳原子数增加，可能的异构体数量急剧上升。
Branching does not alter the molecular formula, so all isomers of a given alkane share the same combustion products, but they differ in boiling and melting points due to varied surface contact and packing.
支链不会改变分子式，因此给定烷烃的所有异构体燃烧产物相同，但由于分子间接触面积和堆积方式不同，它们的沸点和熔点有所差异。
6. Physical Properties: Boiling and Melting Points | 物理性质：沸点与熔点
The only intermolecular forces in alkanes are London (dispersion) forces. As the molecular size and relative molecular mass increase, the strength of these temporary dipole–induced dipole interactions increases, requiring more energy to separate molecules. Hence, boiling and melting points rise with chain length.
烷烃中只存在伦敦（色散）力。随着分子尺寸和相对分子质量增加，这些瞬时偶极–诱导偶极作用力增强，分离分子需要更多能量，因此沸点和熔点随碳链增长而升高。
Branching lowers the boiling point because branched isomers have more compact, spherical shapes with smaller surface areas, reducing the magnitude of London forces compared with straight-chain isomers of the same molecular formula.
支链会降低沸点，因为与相同分子式的直链异构体相比，支链异构体形状更紧凑、近球形，表面积较小，伦敦力较弱。
| Alkane | Formula | Boiling point (°C) |
|---|---|---|
| Butane | C4H10 | −0.5 |
| 2-Methylpropane | C4H10 | −11.7 |
| Pentane | C5H12 | 36.1 |
| 2,2-Dimethylpropane | C5H12 | 9.5 |
Alkanes are non-polar and insoluble in water but dissolve in non-polar organic solvents. Density remains below that of water, typically around 0.6–0.8 g cm−³ for liquids.
烷烃为非极性分子，不溶于水，但可溶于非极性有机溶剂。其密度均低于水，液态烷烃的密度通常在 0.6–0.8 g cm−³ 范围。
7. Combustion of Alkanes | 烷烃的燃烧
Alkanes readily undergo complete combustion in an excess of oxygen, producing carbon dioxide and water. The reaction is highly exothermic, which makes alkanes important fuels.
烷烃在充足氧气中容易发生完全燃烧，生成二氧化碳和水。该反应高度放热，使得烷烃成为重要的燃料。
CnH2n+2 + excess O2 → n CO2 + (n+1) H2O
If the oxygen supply is limited, incomplete combustion occurs, producing carbon monoxide (a toxic gas) and/or carbon (soot, C). This releases less energy and can be hazardous.
若氧气供应不足，则发生不完全燃烧，生成一氧化碳（有毒气体）和/或碳（烟灰，C）。这释放的能量较少，且可能带来危险。
To compare the energy released by different alkanes, chemists use the standard enthalpy change of combustion (ΔcH°). The longer the carbon chain, the greater the absolute value of ΔcH° per mole of alkane.
为比较不同烷烃释放的能量，化学家使用标准燃烧焓变（ΔcH°）。碳链越长，每摩尔烷烃的标准燃烧焓变的绝对值越大。
8. Halogenation and Free-Radical Substitution | 卤代反应与自由基取代
Alkanes react with chlorine or bromine in the presence of ultraviolet (UV) light or at high temperatures. This reaction is a free-radical substitution, where a hydrogen atom is replaced by a halogen atom.
烷烃在紫外光照射或高温下与氯或溴反应。这一反应为自由基取代反应，氢原子被卤原子取代。
The overall equation for the monochlorination of methane can be written as:
甲烷一氯取代的总反应方程式可写为：
CH4 + Cl2 → CH3Cl + HCl
Further substitution can produce CH2Cl2, CHCl3 and CCl4. For higher alkanes, a mixture of structural isomers is obtained because different hydrogen atoms can be replaced. Bromination is slower and more selective than chlorination.
继续取代可生成 CH2Cl2、CHCl3 和 CCl4。对于高级烷烃，由于不同类型的氢原子均可被取代，会得到结构异构体的混合物。溴化反应比氯化反应慢，但选择性更高。
9. Mechanism of Free-Radical Substitution | 自由基取代反应机理
The mechanism proceeds through three stages: initiation, propagation and termination. All steps involve unpaired electron species called free radicals, shown using a single dot (e.g. Cl⋅).
反应机理分为三个阶段：引发、增长和终止。所有步骤都涉及含未成对电子的物种——自由基，用单点表示（如 Cl⋅）。
Initiation: UV light breaks the Cl—Cl bond by homolytic fission, producing two chlorine radicals.
引发：紫外光使 Cl—Cl 键发生均裂，生成两个氯自由基。
Cl2 → 2 Cl⋅
Propagation: A chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical. The methyl radical then reacts with a chlorine molecule, generating chloromethane and a new chlorine radical that continues the chain.
增长：氯自由基从甲烷中夺取一个氢原子，生成 HCl 和甲基自由基。甲基自由基再与氯分子反应，生成氯甲烷和一个新的氯自由基，使链增长得以继续。
CH4 + Cl⋅ → ⋅CH3 + HCl
⋅CH3 + Cl2 → CH3Cl + Cl⋅
Termination: Two free radicals combine to form a stable molecule, removing radicals from the system. Possible termination steps include:
终止：两个自由基结合形成稳定分子，将自由基从体系中移除。可能的终止步骤包括：
2 Cl⋅ → Cl2
2 ⋅CH3 → C2H6
Cl⋅ + ⋅CH3 → CH3Cl
The overall rate depends on the strength of the C—H bond broken and the halogen’s reactivity.
总反应速率取决于被断裂的 C—H 键的强度以及卤素的反应活性。
10. Reactivity and Bond Strength | 反应活性与键能
Alkanes are generally unreactive because the σ bonds are strong and non-polar. The C—C bond enthalpy is around 350 kJ mol−¹ and the C—H bond enthalpy is about 410 kJ mol−¹. This makes alkanes stable towards polar reagents, acids and bases under normal conditions.
烷烃通常不活泼，因为 σ 键强度大且非极性。C—C 键焓约为 350 kJ mol−¹，C—H 键焓约为 410 kJ mol−¹。这使烷烃在常规条件下对极性试剂、酸和碱稳定。
The reactivity of halogens towards alkanes decreases in the order: F2 > Cl2 > Br2 > I2. Fluorine reacts explosively, iodine is effectively unreactive. Chlorine and bromine are the most practical halogens for substitution.
卤素对烷烃的反应活性顺序为：F2 > Cl2 > Br2 > I2。氟反应剧烈可爆炸，碘几乎不反应。氯和溴是实际取代反应中最常用的卤素。
The selectivity of bromination is higher because the C—H bonds on tertiary carbons are broken more easily than those on secondary or primary carbons; this difference is more pronounced with the less reactive bromine radical.
溴化反应的选择性更高，因为叔碳上的 C—H 键比仲碳或伯碳上的键更容易断裂；这种差异性在活性较低的溴自由基作用下更为显著。
11. Sources and Industrial Importance | 来源与工业重要性
The primary source of alkanes is crude oil (petroleum) and natural gas. Fractional distillation of crude oil separates alkanes into fractions of similar chain lengths, including refinery gases, petrol, kerosene, diesel and lubricating oils.
烷烃的主要来源是原油（石油）和天然气。原油通过分馏被分离为不同链长的馏分，包括炼厂气、汽油、煤油、柴油和润滑油等。
Shorter-chain alkanes are also produced by cracking, in which long-chain hydrocarbons are broken into smaller, more useful molecules, often producing alkenes as co-products.
短链烷烃也可通过裂解产生，在此过程中长链烃断裂为更小、更有用的分子，常联产烯烃。
Alkanes are vital as fuels in domestic heating, electricity generation and transport. They also serve as feedstocks for the petrochemical industry to produce plastics, solvents and synthetic fibres.
烷烃作为燃料，在家庭供暖、发电和交通中至关重要。它们还作为石化工业的原料，用于生产塑料、溶剂和合成纤维。
12. Environmental Considerations | 环境考量
The complete combustion of alkanes produces carbon dioxide, a greenhouse gas that contributes to global warming. Rising CO2 levels from fossil fuel combustion are a major environmental concern.
烷烃完全燃烧产生二氧化碳，这是一种导致全球变暖的温室气体。化石燃料燃烧造成的 CO2 水平上升是重大的环境问题。
Incomplete combustion releases carbon monoxide, which binds irreversibly to haemoglobin, reducing the blood’s oxygen-carrying capacity. Particulate carbon (soot) can cause respiratory problems and blackening of buildings.
不完全燃烧释放的一氧化碳与血红蛋白不可逆结合，降低血液携氧能力。颗粒碳（烟灰）会引发呼吸系统问题，并使建筑物变黑。
Unburnt hydrocarbons from vehicle emissions contribute to photochemical smog in the presence of sunlight and nitrogen oxides. Consequently, catalytic converters are used to promote complete combustion and reduce harmful emissions.
机动车排放的未燃烧烃在阳光和氮氧化物作用下会形成光化学烟雾。因此，催化转化器被用来促进完全燃烧，减少有害排放。
Catalytic cracking and the use of biofuels are among the strategies to mitigate the environmental impact while meeting energy demands.
催化裂解和生物燃料的使用是减轻环境影响同时满足能源需求的策略之一。
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