Propene rotational barrier

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. ... 

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. 

Experimentally, it is known that the cis isomer in 1-substituted propenes is more stable and has a lower rotational barrier. Some pertinent data are shown in Tables 13—14. In most cases, the experimental results agree with our predictions. An interesting trend obtains in the alkyl vinyl ether series. Specifically, two types of nonbonded attraction can obtain in these molecules ... 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

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-... 

The first systematic B3LYP DPT investigation into the complete catalytic cycle of 1,3-butadiene hydroformylation using HCo(CO)3 as active catalyst was reported by Huo et al. [78]. It was found that the coordinatimi mode of 1,3-butadiene to HCo (CO)3 is similar to that of propene [55]. The most stable structure has the C=C bond perpendicular to the H-Co bond (10), and butadiene moiety can rotate freely with the rotation barrier of 23.8 kJ/mol in free energy, while the structures with the C=C bond parallel to the H-Co bond facilitate the subsequent insertion reaction leading to Markoviukov alkyl complex (11a) or anti-Markovnikov alkyl complex (11b), as shown in Scheme 14 (the relative free energies are based on HCo(CO)3, butadiene, CO, and H2, scaled at 403 K and 200 atm.). 

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. ... 

Allyl and benzyl radical are substantially stabilized, as anticipated from the resonance structures (see Section 1.3.6). Comparing the BDEs of propene and toluene to an appropriate reference such as ethane suggests resonance stabilization energies of 12.4 and 14.1 kcal / mol, respectively. An alternative way to estimate allyl stabilization is to consider allyl rotation barriers (Eq. 2.12). Rotating a terminal CH2 90° out-of-plane completely destroys allyl resonance, and so the transition state for rotation is a good model for an allylic structure lacking resonance. For allyl radical the rotation barrier has been determined to be 15.7 kcal / mol, in acceptable agreement with the direct thermochemical number. 

Figure 8 Depiction of major factors contributing to propene internal rotation barrier, Cme-Csec bond weakening, designated in red Cme-Hip bond and Csec-H antibond interaction, designated in green. Contour diagrams are as in Figure 5...

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Propene and cis-butene-2 possess the molecular symmetries C2 and C2 v (28, 29), respectively. (Fig. 4 the geometry parameters given are derived from force field calculations (19, 30) and agree well with experiment.) The barriers // observed for the rotation of the methyl groups amount to 1.98 kcal mole-1 for propene (28), and 0.75 kcal mole-1 for cis-butene-2 (29). The torsional force constants K for the methyl groups... 

Ab initio MRCI calculations showed that the barrier from trimethylene to propene is 7.9kcalmol 1 higher than that from trimethylene to cyclopropane.11 Thus, cyclopropane stereomutation may occur through trimethylene as an intermediate (Chart 3). Trimethylene biradical may cyclize by double rotation of the two C C bonds in conrotatory or disrotatory fashion or successive single rotation. The calculations showed that the PES at the... 

To calculate the insertion transition states from each propene adduct, the authors considered the olefin rotation in clockwise and counterclockwise fashion for these two intermediates, as previously described by Carbo et al [115]. For the equatorial-axial adduct, the barrier to propene insertion leading to the linear insertion product was predicted to be 2.8 kcal/mol smaller than the barrier for the insertion reaction leading to the branched product. For the equatorial-equatorial adduct, the barrier for the insertion leading to the branched product was predicted to be 1.4 kcal/mol lower in energy than the barrier for the reaction leading to the linear product. Therefore, it appears that for this type of catalyst there are two separate propene insertion reaction channels, one generating almost exclusively the linear product, and the other producing primarily the branched product. 

Since the p orbitals on carbons 2 and 3 overlap, there is some partial double bond character in the central o bond, helping to keep the structure planar. This overlap means that it is slightly harder to rotate this formal single bond than might be expected—it requires about 30 kj mol"1 to rotate it whereas the barrier in propene is only around 3 kj mol-1. 

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