Carbocations, stability stable solutions

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

One way of determining carbocation stabilities is to measure the amount of energy required to form the carbocation by dissociation of the corresponding alkyl halide, R-X - R+ + X . As shown in Figure 6.10, tertiary alkyl halides dissociate to give carbocations more easily than secondary or primary ones. As a result, trisubstituted carbocations are more stable than disubstituted ones, which are more stable than monosubstituted ones. The data in Figure 6.10 are taken from measurements made in the gas phase, but a similar stability order is found for carbocations in solution. The dissociation enthalpies are much lower in solution because polar solvents can stabilize the ions, but the order of carbocation stability remains the same. 

PATr+ is the pK value for the reaction R+ + 2 H2O ROH + HsO" " and is a measure of the stability of the carbocation. The //r parameter is an early obtainable measurement of the stabihty of a solvent (see p. 371) and approaches pH at low concentrations of acid. In order to obtain pA) +, for a cation R" ", one dissolves the alcohol ROH in an acidic solution of known //r. Then the concentration of R" and ROH are obtained, generally from spectra, and pATr+ is easily calculated. A measure of carbocation stability that applies to less-stable ions is the dissociation energy D(R -H ) for the cleavage reaction R — H R" " -f H , which can be obtained from photoelectron spectroscopy and other measurements. Some values of D(R+ H ) are shown in Table 5.2. Within a given class of ion (primary, secondary, aUylic, aryl, etc.), D(R H ) has been shown to be a linear function of the logarithm of the number of atoms in R, with larger ions being more stable. " ... 

Both the carbanion and carbocations are stable provided they contain [An+ 2) K electrons. For example, cyclopentadienyl anion, cyclopropenium cation, and tropyhum cation exhibit unusual stability. Stable carbanions do, however, exist. In 1984 Ohnstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyllithium, and 12-crown-4 at low temperatures. Addition of n-butyUithium to triphenyhnethane in THF at low temperatures followed by 12-crown-4 resulted in a red solution and the salt complex precipitated at —20°C. The central C-C bond lengths are 145 pm with the phenyl ring propelled at an average angle of 31.2° (Scheme 3.11). 

In El reactions, the rate of the reaction depends mainly on carbocation stability, just as we saw for S).jl reactions. (Remember primary > secondary > tertiary for carbocations, conjugation either with multiple bonds or lone pairs of electrons stabilizes carbocations, and nonplanar carbocations cannot readily be formed). Most of the quantitative data (Table 10.2) for carbocation stability, however, derive from substitution rather than elimination reactions, and even some of these are a little misleading. Rates of substitution of bromomethane and bromoethane tell us little about carbocations these are purely 5 2 reactions in which no carbocation participates. While good gas phase and superacid data do exist, what they can tell us about reactions in solution is limited. That said, the data do tell us clearly that tertiary cations are the most stable and that conjugation stabilizes carbocations. 

Under superacidic, low nucleophilicity so-called stable ion conditions, developing electron-deficient carbocations do not find reactive external nucleophiles to react with thus they stay persistent in solution stabilized by internal neighboring group interactions. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

The stabilities of most other stable carbocations can also be attributed to resonance. Among these are the tropylium, cyclopropenium, and other aromatic cations discussed in Chapter 2. Where resonance stability is completely lacking, as in the phenyl (CeH ) or vinyl cations, the ion, if formed at all, is usually very short lived. Neither the vinyl nor the phenyl cation has as yet been prepared as a stable species in solution. ... 

Various quantitative methods have been developed to express the relative stabilities of carbocations. One of the most common of these, though useful only for relatively stable cations that are formed by ionization of alcohols in acidic solutions, is based on the equation ... 

Sufficient stability of the hydrocarbon ions, as the salt or in the solution, is an obvious prerequisite for these procedures, and, in practice, selecting or designing the stable ions and choosing a proper solvent are tasks of primary importance. As an ordinary stability index for the ions, thermodynamic scales referred to the water molecule, i.e. p CR+ and pKa values, are chosen for the carbocation and carbanion, respectively. 

Carbon atoms in organic molecules are most often neutral. Positively charged carbocations have attracted the interest of synthetic organic chemists, because of their use as intermediates in reactions leading to formation of carbon-carbon bonds. Our work on carbocations has focused on defining the stability of these species as intermediates of solvolysis reactions, through the determination of rate and equilibrium constants for these stepwise reactions (Scheme 1). This has led to the development of experimental methods to characterize these parameters for carbocations that are sufficiently stable to form in aqueous solution. 

In the 40 years since Olah s original publications, an impressive body of work has appeared studying carbocations under what are frequently termed stable ion conditions. Problems such as local overheating and polymerization that were encountered in some of the initial studies were eliminated by improvements introduced by Ahlberg and Ek and Saunders et al. In addition to the solution-phase studies in superacids, Myhre and Yannoni have been able to obtain NMR spectra of carbocations at very low temperatures (down to 5 K) in solid-state matrices of antimony pentafluoride. Sunko et al. employed a similar matrix deposition technique to obtain low-temperature IR spectra. It is probably fair to say that nowadays most common carbocations that one could imagine have been studied. The structures shown below are a hmited set of examples. Included are aromatically stabilized cations, vinyl cations, acylium ions, halonium ions, and dications. There is even a recent report of the very unstable phenyl cation (CellJ)... 

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 N02, 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.137 Many spectral138 measurements have been made on radicals trapped in this manner. Even under these conditions the methyl radical decomposes with a half-life of 10 to 15 min in a methanol lattice at 77 K.139 Since the lifetime of a radical depends not only on its inherent stability, 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. 

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