定义Definition
环己烷不是平面的，最稳定的是椅式构象：所有键角接近 109.5°，所有相邻 C–H 键均为交叉式，既无角张力也无扭转张力。其他构象有船式、扭船式和半椅式。
Cyclohexane is not planar; its most stable conformation is the chair, with bond angles near 109.5° and all adjacent C–H bonds staggered — free of angle and torsional strain. Other conformations are the boat, twist-boat and half-chair.
能量与翻环Energies and ring flip
扭船式比椅式高约 23 kJ/mol，船式约 29 kJ/mol（船头船尾氢的“旗杆”作用和重叠），半椅式（翻环过渡态）约 45 kJ/mol。室温下椅式翻环每秒约 10⁵ 次，翻环使直立键与平伏键互换。
The twist-boat is ~23 kJ/mol above the chair, the boat ~29 kJ/mol (flagpole hydrogens and eclipsing) and the half-chair transition state ~45 kJ/mol. Chairs flip ~10⁵ times per second at room temperature, swapping axial and equatorial positions.
取代环己烷Substituted cyclohexanes
取代基处于平伏键更稳定（避免 1,3-双直立作用），能量差即 A 值；叔丁基几乎完全“锁定”在平伏键。顺-1,2-、反-1,3-、顺-1,4- 二取代为一直立一平伏，反-1,2-、顺-1,3-、反-1,4- 可二平伏。
Substituents prefer equatorial positions to avoid 1,3-diaxial interactions (the A-value); tert-butyl essentially locks the ring. cis-1,2, trans-1,3 and cis-1,4 disubstituted rings are axial/equatorial, while trans-1,2, cis-1,3 and trans-1,4 can be diequatorial.
历史History
萨克斯（1890）提出非平面环己烷；莫尔（1918）发展；哈塞尔用电子衍射、巴顿将其用于甾体反应性分析。
Sachse (1890) proposed non-planar cyclohexane, Mohr (1918) developed the idea, Hassel confirmed it by electron diffraction and Barton applied it to steroid reactivity.
年表Timeline
- 1890
萨克斯提出无张力的非平面环己烷
Sachse proposes strain-free non-planar cyclohexane
- 1943
哈塞尔用电子衍射证实椅式
Hassel confirms the chair by electron diffraction
- 1950
巴顿构象分析
Barton’s conformational analysis
相关公式Formulas
例题Worked example
甲基环己烷 A = 7.3 kJ/mol：25 °C 时 K = e^(7300/2478) = 19，平伏键构象占 95%。叔丁基 A ≈ 20 kJ/mol，几乎全部平伏。
Methylcyclohexane A = 7.3 kJ/mol: K = 19, 95 % equatorial at 25 °C. tert-Butyl (A ≈ 20 kJ/mol) is essentially all equatorial.