9.1
炔烃的结构和物理性质
介绍:
在自然界中，同时含有碳和氢的化合物被称为"碳氢化合物"。 脂肪烃是分子中含有饱和单键（即烷烃）或不饱和双键或三键的化合物。 烯烃含有碳-碳双键，结构式为CnH2n。 含有碳碳三键的不饱和烃称为"炔烃"，结构式为CnH2n-2。
最简单的炔烃是乙炔，一种无色气体，在高温下燃烧，用作焊接燃料。…
含有碳-碳三键的不饱和烃称为 "炔烃" 在结构上表示为CnH2n-2.
乙炔是最简单的炔烃。它是一种无色气体，可在高温下燃烧，常用于焊接。
炔基在治疗帕金森病的药物（如司来吉兰）以及口服避孕药（如炔雌醇）中作为具有生物活性的官能团具有重要意义。
乙炔中的碳-碳三键由一个σ键和两个π键组成。碳原子的sp杂化轨道相互重叠形成一个σ键，而两个碳原子的2py和2pz轨道通过侧向重叠形成两个π键。每个碳原子的另一个sp杂化轨道与氢原子的1s轨道重叠，形成碳-氢σ键。
因此，炔烃是 sp 具有线性几何结构和180°键角，导致分子结构刚性。
炔烃的产率显示出百分之五十 s 给定特征的 sp 杂交。当电子在杂化轨道中时 s 轨道更靠近原子核 p 轨道能量较低，与原子核结合更紧密。因此，结合能增加 s 该特征导致更强且更短的化学键。
这体现在乙炔的碳-碳三键键长为 1.21 Å，碳-氢键键长为 1.06 Å。这两个键均比相应烯烃和烷烃中的键更短。
除了更高的 s 特性外，两个碳原子之间化学键数量的增加也有助于形成更短且更强的化学键。
因此，炔烃的键解离能明显高于烯烃或烷烃。
炔烃是非极性化合物，不溶于水，但可溶于苯等非极性有机溶剂。
低分子量的炔烃（如乙炔）为气体；中等分子量的炔烃（如1-辛炔）为液体。相比之下，高分子量的炔烃在室温下为固体。
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Q1: What is the molecular structure of alkynes and how do they differ from alkenes?
Alkynes are unsaturated hydrocarbons with carbon-carbon triple bonds, represented by the formula CnH2n-2. They differ from alkenes, which contain double bonds and follow CnH2n. The triple bond in alkynes consists of one sigma bond and two pi bonds, making them more unsaturated and structurally distinct from alkenes and alkanes.
Q2: Why are alkyne bonds shorter and stronger than those in alkenes and alkanes?
Alkynes are sp hybridized with 50% s character, meaning electrons occupy lower-energy s orbitals closer to the nucleus and are more tightly bound. The increased s character and the presence of three bonds between carbons result in shorter, stronger bonds. Acetylene's C-C triple bond measures 1.21 Å, significantly shorter than alkene (1.34 Å) or alkane (1.53 Å) bonds.
Q3: What is the molecular geometry of alkynes and what determines their shape?
Alkynes have linear geometry with a 180° bond angle due to sp hybridization. The lateral overlap of p orbitals and the sp orbital arrangement create this rigid, linear structure. This linear geometry is a defining structural feature that distinguishes alkynes from other hydrocarbons with different hybridization states.
Q4: How does the physical state of alkynes vary with molecular weight?
Lower molecular weight alkynes like acetylene and propyne exist as gases at room temperature. Medium molecular weight alkynes such as 1-octyne are liquids, while higher molecular weight alkynes exist as solids. This variation in physical state depends on intermolecular forces increasing with molecular weight.
Q5: What are the solubility properties of alkynes and why?
Alkynes are nonpolar compounds that are insoluble in water and polar solvents but soluble in nonpolar organic solvents like benzene. Their nonpolar nature and lower density than water determine these solubility characteristics. This makes alkynes compatible with organic reaction media rather than aqueous systems.
Q6: What are some important applications of alkynes in pharmaceuticals and industry?
Acetylene, the simplest alkyne, is used as a fuel for welding due to its high combustion temperature. Alkyne-containing compounds serve as bioactive moieties in drugs like selegiline for treating Parkinson's disease and ethynylestradiol in oral contraceptives. Natural alkynes also occur in compounds like South American tree frog poisons.
Q7: How can alkynes be converted into other functional groups?
Alkynes undergo various transformations including electrophilic addition to alkynes halogenation, hydrohalogenation, and reduction reactions. They can be reduced to alkenes through catalytic hydrogenation or converted to aldehydes and ketones via acid-catalyzed hydration. These reactions expand the synthetic utility of alkynes in organic synthesis.