π键Pi bond

σ 键由轨道沿键轴头碰头重叠形成，重叠程度大，较牢固，可绕键轴旋转。π 键的侧面重叠程度较小，所以一般比 σ 键弱：C=C 的平均键焓约 612 kJ mol⁻¹，C–C 约 348 kJ mol⁻¹，可估算 π 键部分约 260 kJ mol⁻¹。旋转会破坏 p 轨道的平行重叠，因此 π 键阻止旋转，这是顺反异构存在的原因。

A σ bond forms by end-on overlap along the bond axis, which is extensive, so it is strong and allows rotation. Sideways overlap in a π bond is smaller, so π bonds are generally weaker: the mean bond enthalpy of C=C is about 612 kJ mol⁻¹ and of C–C about 348 kJ mol⁻¹, putting the π part at roughly 260 kJ mol⁻¹. Rotation would destroy the parallel overlap, so π bonds prevent rotation, which is why cis–trans isomers exist.

π 电子离原子核较远、暴露在分子平面两侧，容易被亲电试剂进攻，所以烯烃易发生亲电加成反应，π 键断开、形成两个新的 σ 键。羰基 C=O 中 π 电子偏向电负性大的氧，碳带部分正电荷，因而羰基易受亲核试剂进攻。

π electrons are further from the nuclei and exposed on either side of the molecular plane, so they are readily attacked by electrophiles; alkenes therefore undergo electrophilic addition, in which the π bond breaks and two new σ bonds form. In the carbonyl group C=O the π electrons are drawn towards the more electronegative oxygen, leaving carbon partially positive, so carbonyls are attacked by nucleophiles.

当多个相邻原子都有平行的 p 轨道时，π 电子可以分布在多个原子之上，形成离域 π 键。苯分子中 6 个碳原子的 p 轨道连续重叠，6 个 π 电子在环上下离域，使苯特别稳定，倾向于发生取代而不是加成。石墨、羧酸根离子和共轭二烯中也存在离域 π 键。

When several neighbouring atoms all have parallel p orbitals, the π electrons can spread over all of them, forming a delocalised π system. In benzene the p orbitals of the six carbons overlap continuously and six π electrons are delocalised above and below the ring, making benzene especially stable and inclined to substitution rather than addition. Graphite, carboxylate ions and conjugated dienes also contain delocalised π bonding.