12.5 Halogenation of Benzene Step 1: Sulfur trioxide attacks benzene in the rate-determining step FIGURE 12.3 The me- chanism of sulfonation of benzene. An electrostatic pc an be viewed on Learning By Modeling Benzene and sulfur trioxide Cyclohexadienyl cation intermediate Step 2: A proton is lost from the sp hybridized carbon of the intermediate to restore the aromaticity of the ring. The species shown that abstracts the proton is a hydrogen sulfate ion formed by ionization of sulfuric acid HOSO,OH y OSO,OH Cyclohexadienyl Hydrogen Benzenesulfonate ion cation intermediate Step 3: A rapid proton transfer from the oxygen of sulfuric acid to the oxygen of benzenesulfonate completes the process. H-OSO,OH + OSO,OH Sulfuric acid Bromine, although it adds rapidly to alkenes, is too weak an electrophile to react at an appreciable rate with benzene. a catalyst that increases the electrophilic properties of bromine must be present. Somehow carpet tacks can do this. How? The active catalyst is not iron itself but iron(iD)bromide, formed by reaction of (Fe Br3)is ron and bromine 2Fe 3B 2Fe Br. Iron(lin bromide is a weak Lewis acid. It combines with bromine to form a Lewis acid Lewis base complex Lewis base Lewis acid Lewis acid-Lewis base Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteBromine, although it adds rapidly to alkenes, is too weak an electrophile to react at an appreciable rate with benzene. A catalyst that increases the electrophilic properties of bromine must be present. Somehow carpet tacks can do this. How? The active catalyst is not iron itself but iron(III) bromide, formed by reaction of iron and bromine. Iron(III) bromide is a weak Lewis acid. It combines with bromine to form a Lewis acid￾Lewis base complex. Br Br Lewis base FeBr3 Lewis acid FeBr3  Br Br Lewis acid-Lewis base complex 3Br2 Bromine 2Fe Iron 2FeBr3 Iron(III) bromide 12.5 Halogenation of Benzene 449 O H Benzene and sulfur trioxide slow Step 1: Sulfur trioxide attacks benzene in the rate-determining step Step 3: A rapid proton transfer from the oxygen of sulfuric acid to the oxygen of benzenesulfonate completes the process. Step 2: A proton is lost from the sp3 hybridized carbon of the intermediate to restore the aromaticity of the ring. The species shown that abstracts the proton is a hydrogen sulfate ion formed by ionization of sulfuric acid. S O H Cyclohexadienyl cation intermediate O O S O O Cyclohexadienyl cation intermediate fast H OSO2OH Hydrogen sulfate ion Benzenesulfonate ion HOSO2OH Sulfuric acid Benzenesulfonate ion H±OSO2OH Sulfuric acid fast OSO2OH Hydrogen sulfate ion Benzenesulfonic acid H O S O O O O O S O O O± S O O S O FIGURE 12.3 The me￾chanism of sulfonation of benzene. An electrostatic po￾tential map of sulfur trioxide can be viewed on Learning By Modeling. Iron(III) bromide (FeBr3) is also called ferric bromide. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website