Bonding in Organic Molecules | 有机分子中的化学键

📚 Bonding in Organic Molecules | 有机分子中的化学键

Understanding how atoms are held together in organic molecules is central to explaining structure, reactivity and physical properties. This article reviews covalent bonding, hybridisation, polarity, resonance and intermolecular forces, with particular attention to carbon frameworks.

理解有机分子中原子如何结合在一起，是解释结构、反应活性和物理性质的核心。本文梳理共价键、杂化、极性、共振和分子间作用力，重点关注碳骨架。

1. The Covalent Bond in Organic Molecules | 有机分子中的共价键

An organic molecule is held together mainly by covalent bonds, in which pairs of valence electrons are shared between non-metal atoms. Carbon has four valence electrons and commonly forms four single bonds, giving a full outer shell of eight electrons.

有机分子主要通过共价键将原子连接在一起，即非金属原子之间共享价层电子对。碳有四个价电子，通常形成四个单键，使最外层达到八个电子的稳定结构。

Lewis structures are a useful shorthand: a single line represents one shared electron pair, a double line represents two shared pairs, and a triple line represents three shared pairs. Formal charge helps in choosing the most plausible resonance contributors when more than one structure is possible.

路易斯结构是一种简洁表示法：一根线代表一对共享电子；双线代表两对；三线代表三对。当可能存在多个结构时，形式电荷有助于选出最合理的共振式。

H₃C–CH₃ : one C–C σ bond

2. Sigma and Pi Bonds | σ 键与 π 键

A sigma bond is formed by head-on overlap of atomic orbitals along the internuclear axis. The electron density is concentrated between the two nuclei and is cylindrically symmetrical, so a single bond allows free rotation.

σ 键由原子轨道沿键轴方向头对头重叠形成。电子云集中在两个原子核之间，并具有圆柱对称性，因此单键可以自由旋转。

A pi bond arises from sideways overlap of two p orbitals. The electron density lies above and below the plane of the bond axis, and the p orbitals must remain parallel, so rotation around a double bond is restricted.

π 键由两个 p 轨道侧向重叠形成。电子云分布在键轴平面的上方和下方，p 轨道必须保持平行，因此双键的旋转受到限制。

A double bond consists of one sigma bond and one pi bond; a triple bond consists of one sigma bond and two pi bonds. The pi system is delocalised in aromatic molecules such as benzene.

双键由一个 σ 键和一个 π 键组成；三键由一个 σ 键和两个 π 键组成。在苯等芳香分子中，π 体系是离域的。

C=C: one σ bond + one π bond

3. Hybridisation of Carbon: sp³, sp² and sp | 碳的杂化：sp³、sp² 和 sp

Carbon hybridises its 2s and 2p orbitals to form equivalent bonding orbitals. In an sp³ carbon, one s and three p orbitals mix to give four identical sp³ orbitals arranged tetrahedrally at 109.5°.

碳将 2s 和 2p 轨道杂化，形成等价的成键轨道。在 sp³ 杂化碳中，一个 s 轨道和三个 p 轨道混合，形成四个相同的 sp³ 杂化轨道，呈四面体排列，键角为 109.5°。

In an sp² carbon, one s and two p orbitals mix, leaving one p orbital unhybridised. The three sp² orbitals lie in a plane at 120°, and the remaining p orbital forms a pi bond. In an sp carbon, one s and one p orbital mix, giving a linear geometry with bond angles of 180°.

在 sp² 杂化碳中，一个 s 轨道与两个 p 轨道杂化，剩余一个未杂化的 p 轨道。三个 sp² 轨道在同一平面内，夹角为 120°，剩余的 p 轨道形成一个 π 键。在 sp 杂化碳中，一个 s 轨道与一个 p 轨道杂化，形成直线形结构，键角为 180°。

4. Bond Length, Bond Energy and Reactivity | 键长、键能与反应性

As the number of shared pairs increases, bond length decreases and total bond energy increases. Typical values are C–C 154 pm and 348 kJ mol⁻¹, C=C 134 pm and 612 kJ mol⁻¹, and C≡C 120 pm and 837 kJ mol⁻¹.

随着共享电子对数目增加，键长减小，总键能

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