[25.2] electrophilic substitution reactions of benzene

• it is a planar ring of 6 carbon atoms • each carbon forms a sigma bond with the 2 adjacent carbon atoms and single hydrogen atom • the remaining 1 electron of each carbon is in a p-orbital that is at right angles to the plane of the ring • adjacent p-orbtials overlap above and below the ring • this overlapping creates a system of π bonds that spreads over the 6 carbons • the 6 electrons in this π bond system are delocaised

• inducing a dipole • attacking the electrophile • attacking the carbocation

the high electron density in the alkene pi bond induces a temporary dipole in the halogen, making the halogen an electrophile

• the pi bond electrons attack the electrophile and form a bond • the bond between halogen atoms break by heterolytic fission, generating a nucleophilic anion and a carbocation

the nucleophile attacks the cation to form the addition product

• due to the stability of the delocalised ring structure, benzene and its derivatives do not readily take part in the typical reactions of alkenes • aromatics usually take part in substitution rather than addition reactions - this allows the product to retain the stability of the benzene ring • in typical reactions, the benzene ring reacts with an electrophile to undergo electrophilic substitution

• benzene will react with an electrophile if the electrophile is generated first. this is called electrophilic substitution • one (or more) of the Hs is replaced by another group • the energetically stable delocalised π-electron ring

C₆H₆ + E⁺ -> C₆H₅E + H⁺

• Br₂ - AlBr₃ • Cl₂ - AlCl₃ • HNO₃ - H₂SO₄, 50 °C • C₂H₅Cl - AlCl₃ • CH₃COCl - AlCl₃

• if the temperature of the reaction rises above 50 °C, further substitution reactions may occur leading to the production of dinitrobenzene • this shows the importance of temperature control in the preparation of organic compounds

• friedel-crafts reaction • increases number of C-C bonds

• the electrophile is attracted to the ring due to he high electrol density caused by delocalised π-electrons • a pair of electrons from the ring are donated to the electrophile to form a covalent bond

• an unstable intermediate forms with an incomplete ring structure and a positive charge > the incomplete ring structure must be over halfway around the ring • the bond witht he hydrogen atom undergoes heterolytic fission to donate electrons to the ring structure

the stable ring structure returns leaving the substituted product and a hydrogen ion

• halogens, haloalkanes and alcyl chlorides do not react with benzene unless a catalyst (a halogen carrier is present) • halogen carriers make the electrophiles • the balogen carrier must also be regenerated at the end (eg. H⁺ + AlCl₄⁻ -> AlCl₃ + HCl)

• AlCl₃ • FeCl₃ • AlBr₃ • FeBr₃

• TNT (trinitrotoluene) or 2,4,6-trinitromethylbenzene, is formed by nitrating methylbenzene (aka toluene) at high temperature, leading the the substitution of three hydrogen atoms • TNT has a fairly low melting point, and is fairly stable to shock and friction, making it safer to handle than other types of explosive