Propene staggered

The conformation of simple alkenes can be considered by beginning with propene. There are two families of conformations available to terminal alkenes eclipsed and bisected conformations, as shown below for propene. The eclipsed conformation is preferred by about 2 kcal/mol and represents a barrier to rotation of the methyl group. A simple way to relate the propene rotational barrier to that of ethane is to regard the tt bond as a banana bond (see p. 7). The bisected conformation of propene is then seen to correspond to the eclipsed conformation of ethane, while the more stable eclipsed conformation corresponds to the staggered conformation of ethane. ... 

In the preferred conformation of propene one of the hydrogens eclipses the double bond. The rotational barrier is 1.98 kcal mol" (Tide and Mann, 1957 Souter and Wood, 1970), and the staggered conformation is con-... 

An ab initio calculation using a STO-3G basis set was carried out on propene in two distinct geometries, eclipsed and staggered. 

Acetaldehyde shows the same conformational preference for the eclipsed over the staggered form as does propene, but the magnitude of the preference is reduced to 1.2 kcal / mol, vs. 2.0 kcal / mol for propene. Provide a rationalization for this observation. 

Figure 14-16 HOMO of propene in two conformations. Methyl group (a) eclipses and (b) staggers the double bond. The energy is lower in (a).

Notice that the HOMO of Fig. 14-16 is qualitatively similar to one of the leg — le" HOMOs of ethane. The QMOT frontier orbital argument for the stability of staggered ethane is basically the same as that for the stability of form (a) in propene. Observe that this MO also resembles the HOMO of 1,3-butadiene, and would lead to the prediction that the trans form of this molecule is more stable than the cis. This is, in fact, observed to be the case. 

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