Butane, bond distances conformations

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

Use SpartanView to step through the sequence of structures showing bond rotation in butane, and identify the gauche and anti conformers. What changes in the C2-C3 bond distance and C-C-C bond angles accompany bond rotation What do you think causes these changes ... 

One-electron oxidation of alkanes leads to a-radical cations (equation 5.55). ° Such ionization removes an electron from an orbital associated with cr bonding among carbon atoms. The radical cation of butane, for example, shows elongation of the C2-C3 bond to a distance of about 2.0 A and a much lower difference in energies of the anti and gauche conformers than is the case with the parent hydrocarbon. Ionization of methane... 

The energy of butane as a function of the dihedral angle about the bond between carbons 2 and 3.The lowest energy conformation occurs when the two methyl groups are the maximum distance apart 9 = 180°). The highest energy conformation occurs when the two methyl groups are eclipsed (9 = 0°). 

Prepare models of the four isomeric butenes, C4H8. Note that the restricted rotation about the double bond is responsible for the cis-trans stereoisomerism. Verify this by observing that breaking the n bond of cw-2-butene allows rotation and thus conversion to rran5 -2-butene. Is any of the four isomeric butenes chiral (nonsuperposable with its mirror image) Indicate pairs of butene isomers that are structural (constitutional) isomers. Indicate pairs that are diastereomers. How does the distance between the Cl and C4 atoms in mw5 -2-butene compare with that of the anti conformation of butane Compare the Cl to C4 distance in cw-2-butene with that in the conformation of butane in which the methyls are eclipsed. 

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