Carbocations aromatic systems

The incorporation of the alkene product of carbocation deprotonation into an aromatic system results in the expected changes in the absolute rate constant kp and the product rate constant ratio kjkp for reaction of the carbocation. 

Electrophilic and nucleophilic substitution in aromatic systems right angles to that of the ring in the carbocation intermediate ... 

X-ray diffractometry is the most powerful method to determine atomic coordinates of molecules in the solid state. X-ray crystal structure analysis was, however, rarely applied in the early years of development of persistent, long-lived alkyl carbocations and studies were only performed to investigate structures of carbocations of aryl derivatives and aromatic systems.65 This is due to the low thermal stability of alkyl carbocations and to the difficulties in obtaining single crystals of carbocations suitable for analysis. Since then, however, methods and instrumentation have improved significantly and X-ray crystal structure analysis has become a powerful tool to solve structural problems of carbocations.65,66... 

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442... 

Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. 

This carbocation has to have a monocyclic structure and is the simplest potentially degenerate (CH) carbocation. It can either be a degenerate cyclopropenyl cation [339] rearranging as in (221) or a static aromatic 2 t-electron structure [340], i.e. the simplest aromatic system (222). The ion... 

On the basis of the evaluation of the proton affinity (860.6 kJmoP for hexa-methylbenzene and 845.6 kJmoP for tetramethylbenzene 148)), the possibility of obtaining hexamethylbenzene and tetramethylbenzene as carbocations in the pores of a zeolite had been excluded. However, Haw and co-workers 146) recently demonstrated by means of NMR spectroscopy that H-heptamethylbenzene may be formed in the cavities of a H(3 zeolite. H-hexamethylbenzene and H-tetramethyl-benzene ions have been observed in zeolite H(3 by a combination of IR and UV-visible spectroscopies 149,150). DRS UV-Vis- and FTIR spectroscopy proved to be techniques well suited to verify, under reaction conditions, the existence of stable H-hexamethylbenzene and H-tetramethylbenzene in the zeolite. Owing to the symmetry properties of H-hexamethylbenzene and H-tetramethylbenzene, characteristic changes of their vibrational features were observed when the aromatic system was perturbed upon protonation. In the same study it was found that the lower polymethylbenzene homologues, such as 1,3,5-trimethylbenzene (PA = 836.2 kJmol ), did not undergo appreciable protonation in H(3 zeolite. On the basis of these results, a proton affinity limit for hydrocarbons that form stable... 

In simple terms, electrophilic aromatic substitution proceeds in two steps. Initially, the electrophile E adds to a carbon atom of the benzene ring in the same manner in which it would react with an alkene, but here the 7C-electron cloud is disrupted in the process. However, in the second step the resultant carbocation eliminates a proton to regenerate the aromatic system (Scheme 2.1). The combined processes of addition and elimination result in overall substitution. 

The hybridization state of the carbon atom that is attacked changes from sp to sp and the planar aromatic system is destroyed. An unstable carbocation is simultaneously produced and so it is clear that this step is energetically unfavourable. It is therefore the slower step of the sequence. 

Electrophilic attack on benzene and related molecules proceeds by an addition-elimination mechanism. Initial attack generates a carbocationic intermediate from which loss of a proton restores the aromatic system. The carbocation intermediate is stabilized by resonance. 

Most carbocations are high in energy, but some are higher in energy than others. There are four ways that carbocations can be stabilized interaction of the empty C(p) orbital with a nonbonding lone pair, interaction with a 1t bond, interaction with a cr bond (hyperconjugation), and by being part of an aromatic system. Hybridization also affects the stability of carbocations. 

Carbocations in which the empty C(p) orbital is part of an aromatic system (Chapter 1) are considerably stabilized over what one would expect just from resonance stabilization alone. The most important aromatic carbocation is the tropylium (cycloheptatrienylium) ion, which is so stable that one can buy its salts from commercial suppliers. Conversely, those carbocations in which the empty C(p) orbital is part of an antiaromatic system (e.g., cyclopentadienylium) are considerably destabilized. 

The 77 bonds in aromatic compounds are also reactive toward electrophiles, although not nearly so much as alkenes. The aromatic ring attacks an electrophile to give an intermediate carbocation. The carbocation then undergoes fragmenta-tive loss of H+ (sometimes another cation) from the same C to which the electrophile added to re-form the aromatic system and give an overall substitution reaction. Thus, the predominant mechanism of substitution at aromatic rings under acidic conditions is electrophilic addition-elimination, sometimes referred to as SpAr. The reaction of toluene and nitric acid is indicative. 

The reaction occurs via the o-complex, the arenium carbocation, which is relatively unstable and reacts with the hydrogensulfate anion (HS04-) to regenerate the stable aromatic system (six -electrons). In sulfonation, all four steps are reversible consequently, sulfonation, unlike nitration, is a reversible process, so that by heating a sulfonic acid with dilute sulfuric acid the parent compound can be regenerated (desulfonation). 

If a carbocation at the same time is also a Huckeloid (4n+2) n aromatic system, resonance can cause substantial stabilization. There were numerous aromatically stabilized Huckeloid systems128-13 s) generated in superacidic media in recent years and characterized by NMR spectroscopy. Some of the best known examples are the following. 

Even more important is the fact that the formation of the triol carbocations (PAHTC) has not been correctly calculated. Any treatment based on a simple Hiickel-MO or PMO calculations for odd AH ions neglect the effect of the differently charged carbon atoms and hence, must be in error. The ionic charge distributed over the aromatic system affects the electronegativity of carbon atoms in specific ways and this has a profound effect on the 7i-energy. Breakdowns of both the PMO and HMO approximations with ionic reaction intermediates are documented in the work of Dewar and Thompson [36,70], Streitwieser et al [35,71] and Szentpaly [39]. The reactivity patterns with radical and ionic reaction intermediates of PAH are different [34-39]. It has been pointed out by Dewar [36] that the PMO method works better for radical than ions, and adequate modifications of the PMO method have been developed for ionic intermediates [16,38,39]. 

In many ways, the principles of substitution, elimination, and addition converge in aromatic systems in what is genetically called aromatic substitution.256 Addition to electrophilic centers, substitution of carbocations, nucleophilic displacement, and elimination of leaving groups are all mechanistic features of various aromatic substitution reactions. 

This section will expand the theme of reactions between a carbocation and a nucleophilic species such as an alkene or an alcohol. In this section, however, the carbocation will be attacked by an aromatic ring to form a Wheland-type intermediate, which leads to a substituted aromatic system. These are the Friedel-Crafts reactions and they are among the most important reactions in organic chemistry. 

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