ChapterTwoSaturatedHydrocarbons(15.9)详解.ppt

Structures of Alkanes and Cycloalkanes (9) I. Conformational Analysis of Butane Structures of Alkanes and Cycloalkanes (10) J. Conformations of Cyclhexane（环己烷的构象） Substituted cyclohexanes are the most common cycloalkanes because of their wide occurrence in nature. For example： Cholesterol (胆固醇) has 3 six-membered rings and 1 five-membered ring. Structures of Alkanes and Cycloalkanes (11) Two conformations of cyclohexane Chair conformation and boat conformation are two different conformations of cyclohexane. The most stable nonplanner comformation of cyclohexane is the “chair” comformation. It is free of angle strain. Boat cyclohexane is less stable than chair cyclohexane, having both steric(空间的) strain (张力) and torsional (扭转的)strain. Structures of Alkanes and Cycloalkanes (12) Axial and equatorial bonds in cyclohexane Chair cyclohexane has six axial (轴的)hydrogens that are perpendicular(垂直的) to the ring( parallel to the ring axis) and six equatorial(赤道的) hydrogens that are in the rough plane of the ring(around the ring equator) Axial bonds alternate up-down Equatorial bonds alternate between sides around the ring Structures of Alkanes and Cycloalkanes (13) Conformations of monosubstituted cyclohexanes Different chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions. This inter- conversion of chair conformations, usually referred to as a ring-flip(环翻转) ，is shown below. The energy barrier to this interconversion is only about 45kJ/mol. A ring-flip in chair cyclohexane interconverts axial and equatorial positions Structures of Alkanes and Cycloalkanes (14) Conformations of monosubstituted cyclohexanes Although cyclohexane rings rapidly flip between confor- mations at room temperature, the two conforme