Eduqas Chemistry for A Level Year 2: Student Bk

121 OA1.2 Aromaticity γ -hexachlorocyclohexane, was produced under the trade name of Lindane© and used as an insecticide. The use of Lindane© is now restricted as this toxic material is very persistent in the environment. Benzene generally reacts by substitution reactions where the delocalised system of electrons is retained. The mechanism of electrophilic substitution, such as in the nitration, halogenation and Friedel–Crafts alkylation of benzene, as the characteristic reactions of arenes Benzene has a delocalised ring of electrons above and below the plane of the carbon atoms. This area of high electron density makes it susceptible to attack by an electrophile . A hydrogen atom (as H + ) is replaced by the incoming electrophile. This type of reaction mechanism is called electrophilic substitution. To replace a hydrogen atom by an electrophile X, the stability of the ring needs to be disturbed, giving an unstable intermediate (called a Wheland intermediate), which then loses a hydrogen ion, H + , in a rapid step. + H + X H + X X + intermediate Nitration In a nitration reaction a hydrogen atom is replaced by a nitro group, NO 2 . The electrophile is the nitryl cation (or nitronium ion), NO 2 + , and this is produced by the reaction of concentrated nitric acid and concentrated sulfuric(VI) acids (sometimes called a nitrating mixture). The nitryl cation reacts with benzene giving nitrobenzene, C 6 H 5 NO 2 , as the organic product. HNO 3 + 2H 2 SO 4 NO 2 + + H 3 O + + 2HSO 4 – One mechanism for the nitration is the overall reaction is + HNO 3 + H 2 O H 2 SO 4 50ºC NO 2 + H 2 SO 4 NO 2 H HOSO 2 O NO 2 NO 2 + – + Nitrobenzene is a yellow liquid that is reduced to phenylamine, C 6 H 5 NH 2 , by using tin metal and hydrochloric acid. If the temperature of nitration exceeds 50 ° C then some 1,3-dinitrobenzene is also produced. YOU SHOULD KNOW › › › ››› the mechanism of electrophilic substitution for the nitration, halogenation and Friedel–Crafts alkylation of benzene Key Term An electrophile is an electron- deficient species that can accept a lone pair of electrons. Extra Help In the nitration of benzene, the hydrogensulfate anion acts as a base by removing a proton from the intermediate. It is the lone pair of electrons on an oxygen atom that is the lone pair donor. NO 2 NO 2