5ecaeaa5aemaahetrteaphhetaoeaby because the bonds formed are proton los ghehtaeoat6rtoseeaia8aeproces.me [@灯 o o-0. ○w 然圣 品 15-9 Halogenation of Benzene:the Need for a 监 Cdneaegyakohieeshowhatheiectrophic 15-10 Nitration and Sulfonation of Benzene -112 kcal mo Reaction 10.5 kcal mo uorination of benzene is very exothermic (explosive). 9 9 Electrophilic aromatic substitution in benzene proceeds by addition of the electrophile followed by proton loss. Electrophylic aromatic substitution is a two-step process. The cationic intermediate is resonance-stabilized: The initial attack of the electrophile is endothermic because the sp3 carbon generated interrupts the cyclic conjugation. The transition state is not aromatic. The loss of the proton regenerates the sp2 carbon atom and aromaticity is restored. This process is more favored than the nucleophilic trapping by the anion accompanying E+. The overall reaction is exothermic because the bonds formed are stronger than the bonds broken. Halogenation of Benzene: the Need for a Catalyst 15-9 Benzene in normally unreactive to halogens because they are not electronegative enough to disrupt its aromaticity. Halogens can be activated by Lewis acid catalysts, however, such as ferric halides (FeX3) or aluminum halides (AlX3), to become much more powerful electrophiles. This activated bromine complex can attack the benzene molecule, allowing the other bromine atom to depart with the good leaving group FeBr4 - The FeBr4 - next abstracts a proton from the cyclohexadienyl cation intermediate, and in the process regenerates the original FeBr3 catalyst. Bond energy calculations show that the electrophilic bromination of benzene is exothermic: Phenyl-H +112 kcal mol-1 Br-Br +46 kcal mol-1 Phenyl-Br -81 kcal mol-1 H-Br -87.5 kcal mol-1 --------------------------------------------------------- Reaction -10.5 kcal mol-1 Fluorination of benzene is very exothermic (explosive). Chlorination and bromination require an activating catalyst. Iodination is endothermic and does not occur. 15-10 Nitration and Sulfonation of Benzene Benzene is subject to electrophilic attack by the nitronium ion. Benzene can be attacked by concentrated nitric acid in the presence of concentrated sulfuric acid