Activation energies rearrangement

Calculating the activation energy (rearranged form of Arrhenius equation) (517) ... 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

It has been possible to measure absolute rates and activation energies for rearrangement of the substituents in a series of 2-substituted 2,2-dimethylethyl radicals. The rates at 25°C and the E for several substituents are indicated below. 

No proton transfers were observed in linear oligomers (catemers) of pyrazoles 8 in the solid, a fact which was understandable because such rearrangements would require a very high activation energy [97JCS(P2)101]. A possible exception to this rule is a catemer 8f, for which slow proton transfer was observed in the solid state [97JCS(P2)1867]. 

The rearrangement has been investigated kinetically. Hawthorne and Strahm found a reaction of the first order at 100°C in diethyl carbitol with k — 5.45 X 10", corresponding to a half-life of 212 min. The activation energy was 28 kcal, the activation entropy —3 1... 

The study by Baechler and coworkers31, cited above, also provided data on the (1,3)-allylic rearrangement in /J-methylallyl phenyl sulfoxide. Using the same approach as was used in reinterpreting the sulfone data, the activation energy is estimated to be 151 kj mol- and AfH°(PhSO) = 45kJmol-1. 

We can determine the activation energy from a series of measurements by plotting the logarithm of the rate constant against the reciprocal temperature, as rearrangement of Eq. (45) sho vs ... 

These reactions have very low activation energies when the intermediate is a free carbene. Intermolecular insertion reactions are inherently nonselective. The course of intramolecular reactions is frequently controlled by the proximity of the reacting groups.113 Carbene intermediates can also be involved in rearrangement reactions. In the sections that follow we also consider a number of rearrangement reactions that probably do not involve carbene intermediates, but lead to transformations that correspond to those of carbenes. 

From the most humble, qualitative resonance depiction given by 5a and 5b to the advanced ab initio calculations of the activation energies of [1,3]-sigmatropic rearrangements of allylboranes21 (Eq. 5),... 

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