Stable carbocations, definition

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

A cationic-cationic domino process of a 3-hydro-xytetrahydro-P-carboline which leads to a stereo-seledive preparation of ( )-ulein was described by Blechert et al.1151 Although the intermediates of this fast proceeding sequence could not be identified definitely, it is acceptable to assume that the process is initiated by the formation of a 2-indolyl carbocation, which cyclizes after tautomerization to a more stable iminium ion. 

The 1,2,3-tri-f-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete ion. ° Quantitative estimation of the stability of the unsubstituted cyclopropenium ion can be made in terms of its pXjj+ value of —7.4, which is intermediate between such highly stabilized ions as triphenylmethyl cation and the M-(4-methoxyphenyl)methyl cation. (See Section 4.4.1 for the definition of pXr+). An HF/6-31G MO calculation on the following isodesmic reaction ... 

In principle, although not always in practice, reaction intermediates may be isolated. They exist at minima on the energy curve and have a definite lifetime, which may be long or short. For example, in the reaction of an excess of bromoethane with methylamine to give the ammonium salt [Et3NMe]+Br (8.6), the intermediates ethylmethylamine and diethylmethylamine are stable compounds, and if the reaction was stopped at an appropriate time, they could be isolated. Other intermediates, such as the carbocation, 8.2, are short lived and unstable and cannot be isolated, although they can sometimes be observed spectroscopically (Figure 8.7). 

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