7.1
烯烃的结构和成键
烯烃是一类含有一个或多个碳碳双键的不饱和烃，大致分为烯烃和环烯烃。 烯烃的化学通式为CnH2n。
双键碳是 sp2 杂化的并且具有三角形平面几何形状。 双键由杂化轨道重叠形成的 σ 键和两个碳上未杂化 2p 轨道横向重叠形成的 π 键组成。 每个碳原子通过 sp2–s 轨道重叠与两个氢原子键合。 由…
烯烃（又称链烯烃），通式为CnH2n，是含有碳-碳双键的烃类化合物。其环状类似物，即在环内含有一个双键的化合物，称为环烯烃。
以最简单的烯烃乙烯为例，双键两端的碳原子为sp2 杂化。杂化轨道以头对头方式重叠形成σ键，而两个p轨道则通过侧向重叠形成π键。根据分子轨道理论，π键的电子密度集中在分子平面的上方和下方。
尽管sp2 碳原子之间的双键明显比sp3碳原子之间的单键更短且更强，但π键比σ键弱。这一点可由碳-碳双键的键能并非单键键能的两倍得以证明。参与成键轨道的重叠效果较差，以及2p电子的能量高于sp2 杂化轨道中的电子，导致了π键较弱。
由于sp2 杂化轨道相比sp3含有更多的s轨道成分，乙烯中由sp2–s轨道重叠形成的碳–氢键比乙烷中由sp3–s 轨道重叠形成的碳–氢键更短。
同样，在丙烯中，由sp2–sp3 杂化轨道重叠形成的碳–碳单键比在丙烷中由sp3–sp3 杂化轨道重叠形成的碳–碳单键更短。
H–C–C键角偏离预期的120°是由于双键两侧取代基之间的排斥性非键相互作用所产生的张力所致。
碳-碳双键具有吸电子效应。未杂化的2p轨道中的电子密度分布不均，因此对碳原子核的屏蔽作用不如杂化轨道有效。因此，某些烯烃尽管较弱，但仍表现出偶极矩。
由于烯烃分子中缺乏强偶极，其主要的分子间作用力为伦敦色散力。这种作用力随分子质量的增加和分子表面积的增大而增强。因此，在室温下，较小的烯烃呈气态，而碳原子数超过四个的烯烃则为液态，且沸点随分子量增加而升高。
由于烯烃为非极性分子，可溶于非极性溶剂，但不溶于水。
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Q1: What is the molecular structure of an alkene?
Alkenes are hydrocarbons with the general formula CnH2n containing a carbon–carbon double bond. The doubly bonded carbons are sp2 hybridized with trigonal planar geometry. The double bond consists of a sigma bond from head-to-head overlap of hybrid orbitals and a π bond from sideways overlap of unhybridized 2p orbitals, with electron density concentrated above and below the molecular plane.
Q2: Why is a carbon–carbon double bond stronger than a single bond but weaker than expected?
Although a double bond between sp2 carbons is significantly shorter and stronger than a single bond between sp3 carbons, the π bond is weaker than the σ bond. The double bond energy is not twice that of a single bond because the π bond has less effective orbital overlap and higher energy 2p electrons compared to sp2 electrons, reducing its overall strength.
Q3: How do sp2 and sp3 hybridization affect bond lengths in alkenes?
Because sp2 orbitals have more s character than sp3 orbitals, carbon–hydrogen bonds formed by sp2–s overlap in ethylene are shorter than those created by sp3–s overlap in ethane. Similarly, carbon–carbon single bonds from sp2–sp3 overlap in propylene are shorter than those from sp3–sp3 overlap in propane.
Q4: What intermolecular forces exist between alkene molecules?
Because alkenes are nonpolar, the key attractive forces between alkene molecules are London dispersion forces. These forces strengthen with increasing molecular mass and larger surface area. Small alkenes are gaseous at room temperature, while those with more than four carbons are liquids with increasing boiling points.
Q5: Why do electrophiles preferentially react with alkenes?
Electrophiles react preferentially with the π bond electrons of alkenes because unhybridized p electrons have higher energy than electrons in hybrid orbitals. The π bond electrons generally have higher energy than σ bond electrons, making them more susceptible to electrophilic attack during introduction to electrophilic addition reactions of alkenes.
Q6: What are the physical states of alkenes at room temperature?
Ethylene, propylene, and butylene exist as colorless gases. Alkenes containing 5 to 14 carbon atoms are liquids, and those containing 15 or more carbon atoms are solids. Being effectively nonpolar, alkenes are insoluble in water but soluble in nonpolar solvents.
Q7: Where do alkenes occur naturally and what roles do they play?
Alkenes occur abundantly in nature. Ethylene, the simplest alkene, functions as a plant hormone affecting fruit ripening. Lycopene and carotenes are polyenes responsible for red and orange colors in fruits and vegetables like tomatoes and carrots. Alkenes also form the structural frames of plant essential oils and insect pheromones.