Carbocation from benzene

Alkenyl halides such as vinyl chloride (H2C=CHC1) do not form carbocations on treatment with aluminum chloride and so cannot be used m Friedel-Crafts reactions Thus the industrial preparation of styrene from benzene and ethylene does not involve vinyl chloride but proceeds by way of ethylbenzene... 

Stabilized carbocations can be generated from allylic and benzylic alcohols by reaction with Sc(03SCF3)3 and results in formation of alkylation products from benzene and activated derivatives.39... 

Any one of these resonance forms is a satisfactory answer to the question. Because of its tertiary carbocation character, this carbocation is more stable than the corresponding intermediate formed from benzene. 

The mechanism for this reaction begins with the generation of a methyl carbocation from methylbromide. The carbocation then reacts with the n electron system of the benzene to form a nonaromatic carbocation that loses a proton to reestablish the aromaticity of the system. 

The generation of carbocations from these sources is well documented (see Section 4.4). The reaction of aromatics with alkenes in the presence of Lewis acid catalysts is the basis for the industrial production of many alkylated aromatic compounds. Styrene, for example, is prepared by dehydrogenation of ethylbenzene, which is made from benzene and ethylene. 

Naphthalene is more reactive than benzene toward electrophilic substitution because the carbocation intermediate is more stable and, therefore, easier to form than the analogous carbocation intermediate formed from benzene. Because of naphthalene s increased reactivity, a Lewis acid is not needed for bromination or chlorination. 

A simple example illustrates the reaction. When benzene reacts with benzyl chloride in the presence of 0.4 equivalents of AICI3, diphenylmethane (45) is isolated in 59% yield. If this reaction proceeds by electrophilic aromatic substitution, then the sp carbon of benzyl chloride is a precursor to a carbocation. To form a carbocation from benzyl chloride, the chlorine atom must react as a Lewis base with AICI3 to form PhCH2 AlCL. Benzene reacts with this carbocation via electrophilic aromatic substitution in the same manner as the reaction with Br in the previous section to form an arenium ion intermediate (see 40) to give 45. 

Topologically, these isomerizations are reminiscent of alkyl shifts, which we studied earlier in connection with carbocation rearrangements (Section 9-3). To generate a carbocation from a benzene derivative, we protonate with acid (Section 15-8, Exercises 15-23 and 15-24). Protonation can occur anywhere on the ring and is reversible. 

Treating 5.5 g of 2-amino-4,5-dimethylthiazole HCl with 0.66 g of solid sodium hydroxide 15 min at 220°C yields 53% of 4.4. 5.5 -tetramethyT 2,2 -dithiazolylamine, whose structure w as proved by identification with the produa obtained from the reaction between dithiobiuret and 3-bromo-2-butanone (467). This result is comparable to the reaction between 2-aminopyridine and its hydrochloride to yield bis(pyridyl-2)amine (468). Gronowitz applied this reaction to 2-aminothiazole, refluxing it with its hydrochloride 4 hr in benzene and obtained the dimeric 2-aminothiazole (236). He proposed a mechanism (Scheme 143) that involves the addition of a proton to the 5-position of the ring to give 234. The carbocation formed then reacts on the 5-position of a second... 

The first step m the reaction of electrophilic reagents with benzene is similar An electrophile accepts an electron pair from the tt system of benzene to form a carbocation... 

A second difference between alkene addition and aromatic substitution occurs after the carbocation intermediate has formed. Instead of adding Br- to give an addition product, the carbocation intermediate loses H+ from the bromine-bearing carbon to give a substitution product. Note that this loss of H+ is similar to what occurs in the second step of an El reaction (Section 11.10). The net effect of reaction of Br2 with benzene is the substitution of H+ by Br+ by the overall mechanism shown in Figure 16.2. 

O An electron pair from the benzene ring attacks the positively polarized bromine, forming a new C-Br bond and leaving a nonaromatic carbocation intermediate. 

Aromatic rings can be nitrated by reaction with a mixture of concentrated nitric and sulfuric acids. The electrophile is the nitronium ion, N02+, which is generated from HNO3 by protonation and loss of water. The nitronium ion reacts with benzene to yield a carbocation intermediate, and loss of H+ from this intermediate gives the neutral substitution product, nitrobenzene (Figure 16.4). 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

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