Butane eclipsed conformation

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... 

FIGURE 3 8 Ball and spoke and space filling models of methyl-methyl eclipsed conformation of butane... 

We have seen that Newman projections are a powerfnl way to show the different conformations of a molecule. We mentioned earlier that there are staggered conformations and eclipsed conformations. In fact, there are three staggered and three eclipsed conformations. Let s draw all three staggered conformations of butane. The best way to do this is to keep the back carbon atom motionless (so the fan in the back is not spinning), and let s slowly turn the groups in the front (only the front fan is spinning) ... 

Now let s look at the three eclipsed conformations of butane. Again, let s keep the back where it is, and let s just rotate the front carbon atom with its three groups ... 

The equilibria (relative stabilities) and equilibration (rate of interconversion) of the rotational conformations of ethane and butane were discussed in Section 5-2. If you review this material, it will be clear that forming a ring from a hydrocarbon chain will greatly reduce the number of possible staggered and eclipsed conformations. 

Conformational isomerism around the central bond in butane is more complex because file various staggered and eclipsed conformations are not equivalent as they are in ethane and propane (Figure 6.7). Starting with the eclipsed... 

The activation energy for bond rotation in propane is expected to be somewhat higher than that in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed conformation. This strain is, however, less than the van der Waals strain between the methyl groups of butane, which makes the activation energy for bond rotation less for propane than for butane. 

Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy. 

As in ethane, the eclipsed conformations are not stable since any rotation leads to a more stable conformation. The staggered conformations are stable since they each lie in a potential energy well. The anti-periplanar conformation, with the two methyl groups opposite each other, is the most stable of all. We can therefore think of a butane molecule as rapidly interconverting between synclinal and anti-periplanar conformations, passing quickly through the eclipsed conformations on the way. The eclipsed conformations are energy maxima, and therefore represent the transition states for in ter conversion between conformers. 

When these ideas are extended to butane, there are three retainers about the central C-C bond which need to be considered. Not only are there the extremes of the staggered (1.16) and eclipsed conformations (1.17), in which the methyl group interactions are at a minimum and a maxi-mmn respectively, but there is also a gauche conformation (1.18) which is intermediate between these. 

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