Propene eclipsed

Fig. 1.29. Interactions between two hydrogen Is orbitals and carbon 2p orbitals stabilize the eclipsed confonnation of propene.

There are two families of conformations available to terminal alkenes. These are the eclipsed and bisected conformations shown below for propene. The eclipsed conformation is more stable by about 2kcal/mol. ... 

The origin of the preference for the eclipsed conformation of propene can be explained in MO terms by focusing attention on the interaction between the double bond and the n component of the orbitals associated with the methyl group. The dominant interaction is a repulsive one between the filled methyl group orbitals and the filled n orbital of the double bond. This repulsive interaction is greater in the bisected conformation than in the eclipsed conformation. ... 

Apply the bent-bond model to the preferred conformations of acetaldehyde and propene. Do bent-bonds maintain or remove eclipsing interactions in the equilibrium structures of the two molecules Formulate a simple rule based on the bent-bond model for predicting conformational preferences in systems containing trigonal atoms. 

As a consequence of the energy decrease in both u and 7r, only the —rr interaction need be considered when dealing with the rotational barrier in CH3CH=0. As in the propene case, the eclipsed conformer will be favored. However, as can be seen from the diagrams below, the eclipsed form will be favored to a lesser extent in acetaldehyde relative to propene. 

Experimentally, acetaldehyde is known to exist in the eclipsed conformation. It has a methyl rotational barrier of 1.16 kcal/mol94-96 as contrasted with a barrier of 2.00 kcal/mol in the case of propene. 

A similar reasoning shows that propene is also more stable in its eclipsed form ... 

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-... 

Figure 6.29 The energy difference between eclipsed and bisected conformers of propene. The sp /p description of double bonds is shown.

According to NBO analysis, hyperconjugative interactions are the main reason for the greater stability of the eclipsed structure of propene (Figure 6.29). The most important hyperconjugative interactions observed between the methyl and vinyl groups are divided into three components the interaction, the cH interaction, and the vicinal interaction between the in-plane orbitals of the methyl group and the o -orbital of the antiperiplanar vinyl C-H bond. 

The relative importance of different eclipsed conformations in substituted propenes can be controlled stereoelectronically by variations in the donor and acceptor properties of aUyUc C-X bonds and the alkene (Figure 6.31). For example, the difference between the two conformations of aUyl fluoride is small (the -0.2-0.8 kcal/mol gas phase preference for the conformation with the C-F eclipsed bond). This is an apparent violation of the main stereoelectronic rule (the best acceptor, p, is orthogonal to the best donor in this... 

Let us now consider which bonding model is more amenable to qualitative predictions of molecular conformation. Specifically, what should be the preferred conformation of propene Walters noted that two conformers of propene (designated as I and II) can be visualized as Newman projections observed by sighting down the C3-C2 bond (Figiue 1.33). In conformer I, a C-H bond eclipses a carbon-carbon double bond, hi conformer II, a C-H bond eclipses a C—H bond. Assuming that there is greater electron density in a double bond than in a C-H single bond, we would expect conformer II to be more stable. Experimentally, however, conformer I was found to be more stable by about 2 kcal/mol. 

The preference for eclipsed conformations around the sp -sp bond of alkenes is general, but the reasons for the preference are not as yet understood. The rotational barriers are relatively low, that for propene being about 2 kcal/mol. Simple substituent effects appear straightforward. Methyl substitution at C(2), as in 2-methyl-1-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-l-butene, the two eclipsed conformations are of approximately equal energy. ... 

A number of aldehydes have been studied by NMR and found to have analogous rotameric compositions. Only when the substituent is exceptionally stericaliy demanding, as in (CH3)3CCH2CHO, does the hydrogen-eclipsed conformation become more stable. The barrier heights are somewhat smaller than for the analogous 1-alkenes. For acetaldehyde, the rotational barrier is 1.1 kcal/mol, versus 2.0 kcal/mol for propene. ... 

The cases of propene and acetaldehyde are interesting and illustrate a way in which overlap populations can provide insight into the rotational process. A positive overlap in MO theory corresponds to a bonding interaction a negative overlap, to a nonbonded repulsion. As was discussed previously, the most stable conformations of acetaldehyde and propene have the methyl C-H bond eclipsed with the carbon-oxygen or carbon-carbon double bond. 

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. 

---