有机化学Chapter 2. Alkanes 烷烃解析.ppt

Torisonal Strain(扭转张力 or 排斥力) Van der Waals Radius (范德华半径与范德华张力) In a molecular crystal, One-half the distance between two atoms of an element that are as close to each other as possible without being formally bonded to each other except for van der Waals forces. 2. Conformational Analysis of Butane Rotation about C2－C3 bond in butane 对位交叉 部分重叠 部分交叉 全重叠 全重叠 对位交叉 Stability: 部分交叉 部分重叠 2.5.2 Conformation of Cyclohexane 1. Cis-trans isomerism in Cyclohexane Ex. 1,2-Dimethylcyclopentane: The rotation is limited by “rigid” ring Stereoisomers: trans- cis- The same constitution, the different spacial arrangement of atoms or atomic groups. Rotation freedom around C－Cσ bond in open-chain alkanes. 　　 trans-1,2- Dimethylcyclopentane cis-1,2- Dimethylcyclopentane The structure of cyclohexane is nonplanar: Chair conformation （椅式构象） Boat conformation （船式构象） 2. Conformation of Cyclohexane Both conformations are interconverted: But the chair conformation is more stable than boat (1nm=1000pm). In chair conformation, all C－H bonds are staggered;but in boat conformation, all C－H bonds are eclipsed. In chair conformation, C1,C3,C5 are co-planar; C2,C4,C6 are co-planar. Two kinds of C－H bonds: 6 C－H bonds are axials; 6 C－H bonds are equatorials. a a a a a a e a e a e a e e a a a e e e e e e e One chair conformation is converted to another chair : As the result of ring inversion, any axial bond becomes equatorial. Conformational mobility of cyclohexane Chair-chair interconversion : Chair Half-chair Twist boat Boat Twist boat Half-chair Chair Energy: Chair Twist boat Boat Half-chair Conformational Analysis of mono-substituted cyclohexanes In methylcyclohexane: Van der waals strain 1,3-diaxial repulsion(排斥) Methyl group: From axial to equatorial 5% 95% 0.01% 99.99% As the group is larger, the conformation of cyclohexane with the group equatorial is more stable than the axial form. 4. Conformational Analysis of