定义Definition
椅式环己烷每个碳上的两个 C–H 键：一个平行于环的对称轴、上下交替，称直立键（a 键）；另一个大致沿环的“赤道”向外伸出，称平伏键（e 键）。
Each carbon of chair cyclohexane has two bonds: one parallel to the ring’s symmetry axis, alternating up and down (axial, a); the other pointing outward roughly along the ring’s ‘equator’ (equatorial, e).
1,3-双直立作用1,3-Diaxial interactions
直立键取代基与同侧 C-3、C-5 上的直立氢靠得很近，产生空间排斥（相当于两个邻位交叉丁烷作用），所以取代基一般偏爱平伏键。
An axial substituent crowds the axial hydrogens on C-3 and C-5 on the same side (equivalent to two gauche-butane interactions), so substituents generally prefer equatorial positions.
A 值A-values
取代基由直立键变为平伏键的自由能降低：–CH₃ 7.3、–CH(CH₃)₂ 9.2、–C(CH₃)₃ 约 20、–OH 约 3、–Cl 约 2.2 kJ/mol（数值随溶剂略有变化）。
The free-energy preference for equatorial: –CH₃ 7.3, –CH(CH₃)₂ 9.2, –C(CH₃)₃ ~20, –OH ~3, –Cl ~2.2 kJ/mol (slightly solvent-dependent).
化学后果Chemical consequences
E2 消除要求离去基团与 β-H 都在直立键（反式双直立）；环上 SN2 进攻多从直立方向进行；NMR 中直立–直立邻位偶合常数大（8–13 Hz），可用来判断构型。
E2 requires the leaving group and β-H both axial (trans-diaxial); ring SN2 attack often comes axially; NMR axial–axial couplings are large (8–13 Hz), aiding configuration assignment.
年表Timeline
- 1953
巴顿等统一采用 axial/equatorial 术语
Barton and colleagues standardise the terms axial and equatorial