secondary -CH2-CH-CH2-C--CH3 Conformations of Alkanes Cycloalkanes Structural formulas are useful for showing the attachment of atoms, and three-dimensional drawings are useful for showing molecular shapes. Neither of these, however, conveys much information regarding the dynamics of molecular conformations and the role that these play in controlling equilibrium shapes and reactivity of organic molecules. As mentioned previously, there is generally free rotation around carbon-carbon single bonds. At room temperature, this rotation can be quite rapid and can occur with a rate constant of *10 sec4. For ethane, this rotation has only a small intrinsic energy barrier since the van der Waals radius of the hydrogen atoms on the adjacent carbons is sufficiently small so that overlap is minimal. A movie file demonstrating this rotation is shown below (Click on the icon above to view the movie: use the BAcK button to return to this page) This can be contrasted. however, with rotation around the central carbon-carbon bond in butane, shown in the movie panel below, in which two methyl groups clearly overlap during a single rotat ion (the van der Waals radii of the methyl hydrogen atoms clearly overlap)Conformations of Alkanes & Cycloalkanes Structural formulas are useful for showing the attachment of atoms, and three-dimensional drawings are useful for showing molecular shapes. Neither of these, however, conveys much information regarding the dynamics of molecular conformations and the role that these play in controlling equilibrium shapes and reactivity of organic molecules. As mentioned previously, there is generally free rotation around carbon-carbon single bonds. At room temperature, this rotation can be quite rapid and can occur with a rate constant of 108 sec-1. For ethane, this rotation has only a small intrinsic energy barrier since the van der Waals radius of the hydrogen atoms on the adjacent carbons is sufficiently small so that overlap is minimal. A movie file demonstrating this rotation is shown below: (Click on the icon above to view the movie; use the BACK button to return to this page) This can be contrasted, however, with rotation around the central carbon-carbon bond in butane, shown in the movie panel below, in which two methyl groups clearly overlap during a single rotation (the van der Waals radii of the methyl hydrogen atoms clearly overlap)