3.6
乙烷和丙烷的构象
在有机分子中，围绕碳-碳单键的自由旋转导致分子能量不同的构象异构体。 由于这种旋转（称为内旋转），乙烷具有两种主要构象——交错构象和重叠构象。
交错构象是一种低能量且更稳定的构象，前碳上的 C-H 键相对于后碳上的 C-H 键呈 60° 二面角，从而减少扭转应变。 在交错乙烷中，一个C-H键的成键分…
从碳-碳键轴方向观察乙烷分子，其C-H基团以60°的二面角间隔排列，这种构象称为乙烷的交叉式构象。
乙烷的交叉式构象能量最低。这是因为C-H键之间的距离最远，使键中电子的立体排斥作用最小，从而稳定了分子。
另一个稳定交错构象的因素是占据的成键分子轨道与未占据的反键分子轨道之间的有利相互作用。
通过固定较近的碳原子并旋转较远的碳原子，可产生无限多种构象。
在 0° 二面角时，两个碳原子上的 C-H 键相互靠近并重叠。这是乙烷的重叠式构象。
由于空间位阻增大且缺乏稳定作用，重叠式乙烷的能量升高了12 kJ/mol，每个重叠的H-H相互作用被赋予4 kJ/mol。
重叠构象与交叉构象之间的能量差称为扭转张力或扭转能垒。
沿碳-碳键将分子从 0º 旋转至 360º，可产生多个简并的交叉式和重叠式构象。
在室温下，乙烷气体样品中约有 99% 的分子处于最低能量的交叉式构象。
分子碰撞所获得的能量用于克服扭转势垒，从而实现内旋转。 因此，该分子在经过高能重叠式构象后，转变为另一种交叉式构象。
下一个烃类——丙烷——也有两种主要的构象异构体：重叠式和交叉式。
重叠式构象具有14 kJ/mol的扭转张力。每一对重叠的氢原子贡献4 kJ/mol，而甲基-氢（CH3-H）的重叠相互作用贡献6 kJ/mol。
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Q1: What is the staggered conformation of ethane?
The staggered conformation of ethane occurs when C-H groups are positioned at 60° dihedral angles, placing hydrogen atoms as far apart as possible. This arrangement minimizes steric repulsion between electron-rich bonds and is stabilized by favorable interactions between bonding and antibonding molecular orbitals, making it the lowest energy and most stable form of ethane.
Q2: Why is the eclipsed conformation of ethane less stable than the staggered form?
In the eclipsed conformation, C-H bonds align at 0° dihedral angles, causing increased steric repulsion and eliminating stabilizing orbital interactions. This results in 12 kJ/mol higher energy than the staggered form, with each H-H eclipsing interaction contributing 4 kJ/mol. This energy difference is called torsional strain or torsional barrier.
Q3: How does internal rotation affect ethane molecules at room temperature?
At room temperature, approximately 99% of ethane molecules exist in the lowest energy staggered conformation. Molecular collisions provide energy to overcome the torsional barrier, allowing molecules to rotate through high-energy eclipsed states and interconvert between different staggered conformations continuously throughout the sample.
Q4: What are the major conformers of propane?
Propane has two major conformers: staggered (low energy, stable) and eclipsed (high energy, unstable). The eclipsed conformer has 14 kJ/mol torsional strain. Each eclipsing H-H pair contributes 4 kJ/mol, while CH3-H eclipsing interactions contribute 6 kJ/mol, making propane's torsional barrier slightly higher than ethane's.
Q5: How does dihedral angle relate to ethane's conformational energy?
Dihedral angle measures the spatial relationship between C-H bonds on adjacent carbons. At 60° angles, ethane adopts the staggered, low-energy conformation. Rotating to 0° creates the eclipsed, high-energy form. Continuous rotation from 0° to 360° generates alternating staggered and eclipsed states with varying energy levels.
Q6: What role do molecular orbitals play in stabilizing staggered ethane?
In staggered ethane, the occupied bonding molecular orbital of one C-H bond interacts favorably with the unoccupied antibonding molecular orbital of another C-H bond. This orbital overlap provides additional stabilization beyond simple steric effects, contributing to the staggered conformation's lower energy and greater stability.
Q7: How does propane's torsional strain compare to ethane's?
Propane's eclipsed conformation has 14 kJ/mol torsional strain compared to ethane's 12 kJ/mol, making propane slightly less stable in its eclipsed form. The difference arises because propane contains CH3-H eclipsing interactions worth 6 kJ/mol each, in addition to H-H interactions, increasing overall strain significantly.