Allyl cation rotational barrier

Two equivalent structures can be drawn, and the true structure is intermediate between these. The most direct consequence is that the positive charge is located to an equal extent on the two terminal carbon atoms. The electrons are delocalized over the 7T system. A second structural consequence is that the allyl cation adopts a planar geometry, because a planar structure maximizes the overlap of the three p orbitals. As a result, there is an energy barrier to rotation about the carbon-carbon bonds in the allyl cation. This barrier has not been directly measured but estimates based on appropriate thermodynamic cycles or on extrapolation from methyl derivatives are about 25-28 kcal/mol. ... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Scheme 5.2. Rotational Energy Barriers for Allyl Cations (kcal/mol)"...

Some of these modules also show fairly low (17.7 to 21.9 kcal/mol) barriers to rotation about the N=C bond. This can be ascribed to the stabilized allylic cation appearing in the transition state. Although no further splitting occurs above... 

Unfortunately, while it is clear that the allyl cation, radical, and anion all enjoy some degree of resonance stabilization, neither experiment, in the form of measured rotational barriers, nor higher levels of theory support the notion that in all three cases the magnitude is the same (see, for instance, Gobbi and Frenking 1994 Mo el al. 1996). So, what aspects of Hiickel theory render it incapable of accurately distinguishing between these three allyl systems ... 

Allyl cations 80 and 81 have been generated and studied by NMR spectroscopy.237 Although sterically crowded, cation 80 proved to be surprisingly stable up to 80°C. The rotational barrier estimated on the basis of the coalescence temperature of the 13C NMR signals is 16.8 kcal mol-1, in good agreement with the calculated value (MNDO, 16.5 kcal mol-1). In contrast, the rotational barrier of cation 81 was found to be less... 

Alternatively, interconversion between the stereoisomeric allyl cations can take place by capture of a nucleophile at either end, followed by rotation about the more or less normal single bond, and then regeneration of the cation by ionisation. Interconversion between the corresponding anions can take place similarly by cr coordination (771) to a metal at one end or the other. Because of the availability of these pathways, experimental measurements of the barrier to rotation have confirmed that it is less than the very approximate theoretical value of 116 kJ mol-1 (28 kcal mol ). Furthermore, measurements have generally been made on significantly more substituted systems. Such substitution can stabilise the filled, half-filled or empty p orbital, or the double bond, even when these components are no longer conjugated, and so appropriate substituents lower the barrier to rotation. 

C, and the cation 2.111 into the W-shaped cation 2.110 with the same half-life at 35 °C. These correspond to enthalpies of activation of 74 and 101 kJ mol 1 (18 and 24 kcal mol-1), respectively. This measurement only sets lower limits to the rotation barrier of an allyl cation, because it is not known whether rotation takes place in the cations themselves or in the corresponding allyl chlorides with which they could be in equilibrium.141 The barrier in cations is also much affected by solvation and by the degree of substitution at the termini, since the transition structure for rotation draws on such stabilisation more strongly than the delocalised allyl cation does. 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

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