定义Definition
构象是分子通过单键旋转而产生的原子空间排列；构象之间的转换不需要断裂化学键，通常能垒很低，室温下快速互变。
Conformations are spatial arrangements reached by rotation about single bonds; interconversion breaks no bonds and usually has low barriers, so conformers interconvert rapidly at room temperature.
能量来源Energy contributions
扭转张力（重叠时键电子对排斥，乙烷能垒约 12 kJ/mol）、空间张力（基团靠得太近）、角张力（小环）以及偶极、氢键作用。布居按玻尔兹曼分布。
Torsional strain (eclipsing bonds; ethane barrier ~12 kJ/mol), steric strain, angle strain (small rings) and dipole or hydrogen-bond interactions; populations follow Boltzmann statistics.
构象分析的意义Why it matters
构象决定反应性（E2 需反式共平面、SN2 的位阻）、生物活性（药物与受体结合的活性构象）和蛋白质折叠。巴顿因构象分析获 1969 年诺贝尔化学奖（与哈塞尔共享）。
Conformation governs reactivity (anti-periplanar E2, SN2 sterics), bioactivity (receptor-bound conformations) and protein folding. Barton and Hassel shared the 1969 Nobel Prize for conformational analysis.
与构型的区别Versus configuration
构型异构（对映、非对映、顺反）必须断键才能转换；构象异构只需绕键旋转。旋转受严重阻碍时可分离出阻转异构体。
Configurational isomers interconvert only by breaking bonds; conformers by rotation. Severely hindered rotation gives separable atropisomers.
年表Timeline
- 1936
皮策发现乙烷存在约 12 kJ/mol 的旋转能垒
Pitzer finds ethane’s ~12 kJ/mol rotational barrier
- 1950
巴顿发表甾体构象分析论文
Barton publishes conformational analysis of steroids
- 1969
巴顿与哈塞尔获诺贝尔化学奖
Barton and Hassel receive the Nobel Prize
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.
例题Worked example
N-甲基哌啶类的 N-烷基化：主要构象未必生成主要产物。
N-alkylation of substituted piperidines: the major conformer need not give the major product.