Reactions of Aromatic Compounds Just like an alkene,benzene has clouds of it electrons above and below its sigma bond framework B-H attack on an electrophile sigma complex substituted 20PronEae,Ino Although the nt electrons are in a stable aromatic system,they are still available for reaction with strong electrophiles This generates a carbocation which is resonance stabilized(but not aromatic) igma comple (arenium ion) This cation is called a sigma complex because the electrophile is joined to the benzene ring through a new sigma bond. The sigma complex(also called an arenium ion)is not aromatic since it contains an sp'carbon (which disrupts the required loop of p orbitals). Chl7 Reactions of Aromatic Compounds (landscape).docx Pagel
Ch17 Reactions of Aromatic Compounds (landscape).docx Page1 Reactions of Aromatic Compounds Just like an alkene, benzene has clouds of electrons above and below its sigma bond framework. Although the electrons are in a stable aromatic system, they are still available for reaction with strong electrophiles. This generates a carbocation which is resonance stabilized (but not aromatic). This cation is called a sigma complex because the electrophile is joined to the benzene ring through a new sigma bond. The sigma complex (also called an arenium ion) is not aromatic since it contains an sp3 carbon (which disrupts the required loop of p orbitals)
The loss of aromaticity required to form the sigma complex explains the highly endothermic nature of the first step. (That is why we require strong electrophiles for reaction). The sigma complex wishes to regain its aromaticity,and it may do so by either by a reversal of the first step(i.e. regenerate the starting material)or by loss of the proton on the spcarbon(leading to a substitution product). H H H base E base-H H H When a reaction proceeds this way,it is electrophilic aromatic substitution. There are a wide variety of electrophiles that can be introduced into a benzene ring in this way,and so electrophilic aromatic substitution is a very important method for the synthesis of substituted aromatic compounds. Chl7 Reactions of Aromatic Compounds (landscape).docx Page2
Ch17 Reactions of Aromatic Compounds (landscape).docx Page2 The loss of aromaticity required to form the sigma complex explains the highly endothermic nature of the first step. (That is why we require strong electrophiles for reaction). The sigma complex wishes to regain its aromaticity, and it may do so by either by a reversal of the first step (i.e. regenerate the starting material) or by loss of the proton on the sp3 carbon (leading to a substitution product). When a reaction proceeds this way, it is electrophilic aromatic substitution. There are a wide variety of electrophiles that can be introduced into a benzene ring in this way, and so electrophilic aromatic substitution is a very important method for the synthesis of substituted aromatic compounds
Bromination of Benzene Bromination follows the same general mechanism for the electrophilic aromatic substitution(EAS). Bromine itself is not electrophilic enough to react with benzene. But the addition of a strong Lewis acid(electron pair acceptor),such as FeBr3,catalyses the reaction,and leads to the substitution product. The bromine molecule reacts with FeBr;by donating a pair of its electrons to the Lewis acid,which creates a more polar Br-Br bond,and thus a more reactive electrophile. Benzene will now attack this electrophile to generate the sigma complex Step /Formation of a stronger electrophile. Br-Bir:+FeBry Br,.FeBr,intermediate (a stronger electrophile than Br) Step 2:Electrophilic attack and formation of the sigma complex. FeBr ma complex Step 3:Loss of a proton gives the products FeBr, bromobenzene Chl7 Reactions of Aromatic Compounds (landscape).docx Page3
Ch17 Reactions of Aromatic Compounds (landscape).docx Page3 Bromination of Benzene Bromination follows the same general mechanism for the electrophilic aromatic substitution (EAS). Bromine itself is not electrophilic enough to react with benzene. But the addition of a strong Lewis acid (electron pair acceptor), such as FeBr3, catalyses the reaction, and leads to the substitution product. The bromine molecule reacts with FeBr3 by donating a pair of its electrons to the Lewis acid, which creates a more polar Br-Br bond, and thus a more reactive electrophile. Benzene will now attack this electrophile to generate the sigma complex
Bromide ion from the FeBr can act as a weak base to remove the proton,thus generating the aromatic product,H- Br,and regenerating the catalyst(FeBr3). The formation of the sigma complex is an endothermic and energetically unfavorable process-it is therefore the rate determining step. The second step is exothermic since it regenerates the aromatic n system. The overall reaction is exothermic by about 11 kcal/mol.(-45kJ/mol) rate-limiting transition state 车1 reactants H +Br2 Br -FeBr4 intermediate products Br+HBr -45 kJ/mol FeBr3 reaction coordinate- Chl7 Reactions of Aromatic Compounds (landscape).docx Page4
Ch17 Reactions of Aromatic Compounds (landscape).docx Page4 Bromide ion from the FeBr4 - can act as a weak base to remove the proton, thus generating the aromatic product, HBr, and regenerating the catalyst (FeBr3). The formation of the sigma complex is an endothermic and energetically unfavorable process - it is therefore the rate determining step. The second step is exothermic since it regenerates the aromatic system. The overall reaction is exothermic by about 11 kcal/mol. (-45kJ/mol)
Comparison with Alkenes Alkenes react spontaneously with bromine to give addition products. E.g. H Br2 Br △H°=-121kJ Br (-29 kcal) This reaction is exothermic by 29kcal/mol. An analogous addition reaction between benzene and bromine would be endothermic by 2kcal. H Br △HP=+8kJ -Br (+2kc) The destruction of the aromatic sextet causes this endothermicity This reaction is not observed under normal reaction conditions. The substitution of bromine for hydrogen is an overall exothermic process,but requires a catalyst to convert the bromine molecule into a more reactive electrophile. Chl7 Reactions of Aromatic Compounds (landscape).docx Page5
Ch17 Reactions of Aromatic Compounds (landscape).docx Page5 Comparison with Alkenes Alkenes react spontaneously with bromine to give addition products. E.g. This reaction is exothermic by 29kcal/mol. An analogous addition reaction between benzene and bromine would be endothermic by 2kcal. The destruction of the aromatic sextet causes this endothermicity. This reaction is not observed under normal reaction conditions. The substitution of bromine for hydrogen is an overall exothermic process, but requires a catalyst to convert the bromine molecule into a more reactive electrophile