Cyclohexadienyl cation from benzene

Figure 12 3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene The first step is rate determining m it benzene reacts with nitro mum ion to give the cyclohexadienyl cation intermediate In the second step the aro maticity of the ring is restored by loss of a proton from the cyclohexadienyl cation... 

Complexation of bromine with iron(III) bromide makes bromine more elec trophilic and it attacks benzene to give a cyclohexadienyl intermediate as shown m step 1 of the mechanism (Figure 12 6) In step 2 as m nitration and sulfonation loss of a proton from the cyclohexadienyl cation is rapid and gives the product of electrophilic aromatic substitution... 

Figure 12 7 illustrates attack on the benzene ring by tert butyl cation (step 1) and subsequent formation of tert butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate (step 2)... 

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

After what we have seen to date, it surely comes as no great surprise to find that the ratio of o- to p-product obtained from substitution of C6H5Y, where Y is o-/p-directing, is seldom, if ever, the statistical ratio of 2 1. There is found to be very close agreement between calculation and n.m.r. data for the distribution of +ve charge—p-> o- m—around the ring in the cyclohexadienyl cation (57), which is the Wheland intermediate for proton exchange in benzene (cf. p. 133) ... 

This ion is less stable than the cyclohexadienyl cation formed during bromination of benzene. (g) Sulfonation of furan takes place at C-2. The cationic intermediate is more stable than the cyclohexadienyl cation formed from benzene because it is stabilized by electron release from oxygen. 

Aromatic compounds similarly undergo photoprotonation, as evidenced by their fluorescence quenching in acidic media, acid-catalyzed ipso photosubstitution of alkoxy benzenes, and photochemical deuterium exchange of aromatic ring protons [100,101]. McClelland has summarized in a recent review the observation of transient spectra from cyclohexadienyl cations upon laser flash photolysis of several aromatic compounds in HFIP [5,98,102-105]. 

In a typical aromatic substitution reaction, the electrophile (means electron loving) accepts the electron pair from the pi system of benzene, resulting in a carbocation. This cation of benzene is called the cyclohexadienyl cation. Then, the cyclohexadienyl cation loses a proton forming the substitution product. 

By stabilizing the cyclohexadienyl cation intermediate, lone-pair donation from fluorine counteracts the inductive effect to the extent that the rate of electrophilic aromatic substitution in fluorobenzene is, in most cases, only slightly less than that of benzene. With the other halogens, lone-pair donation is sufficient to make them ortho, para directors, but is less than that of fluorine. 

Deactivating and ortho, para-directing The halogens are the most prominent members of this class. They withdraw electron density from all the ring positions by an inductive effect, making halobenzenes less reactive than benzene. Lone-pair electron donation stabilizes the cyclohexadienyl cation intermediates for ortho and para substitution more than those for meta substitution. 

Write a structural formula for the most stable cyclohexadienyl cation intermediate formed in each of the following reactions. Is this intermediate more or less stable than the one formed from benzene ... 

Cyclohexadienyl cations are well known as intermediates in the electrophilic substitution of benzene. Indeed benzene is protonated by superacids to give solutions containing the ion, which can be characterized spectroscopically. These ions are, however, far too reactive to be isolated as salts, and as they are generated only in acidic media, their reactions with nucleophiles cannot be studied. In contrast, cyclohexadienyltricarbonyliron hexafluorophosphate is a stable yellow solid which can be stored indefinitely at room temperature and can even be recrystallized from water. This is an especially striking example of the stabilization of a reactive organic species by coordination to a transition element. 

As in the substitution reaction of D for H on the aromatic ring, the general reaction is completed by the removal of a proton from the intermediate cyclohexadienyl cation by a base, regenerating the aromatic benzene ring (Rg. 14.23). 

ANSWER The mechanism for the transformation of benzenesulfonic acid into benzene is the reverse of the mechanism for formation of benzenesulfonic acid from benzene (Figs. 14.25 and 14.26). Protonation gives a cyclohexadienyl cation that can either lose a proton to revert to benzenesulfonic acid, or lose the sulfonic acid group to give benzene and protonated sulfuric acid, which will lose its proton to water. 

---