Chemistry · Ch 15 — Hydrocarbons
Chemical properties of benzene
Chemical properties of benzene
Aromatic compounds are chemically characterised above all by electrophilic substitution reactions, in which a ring hydrogen is replaced by an electrophile while the aromatic ring itself, with its resonance-stabilised delocalised pi system, is preserved intact -- this preference for substitution over addition is the direct chemical consequence of the aromatic stability discussed in section 15.4.2. Four classic substitution reactions of benzene follow this pattern, each proceeding by first generating a specific electrophile and then having that electrophile attack the ring. Halogenation: chlorine or bromine, with iron (or FeCl3/AlCl3/red phosphorus) as catalyst in the dark, substitutes a ring hydrogen to give chlorobenzene or bromobenzene plus HX; the attacking electrophile is a halonium ion (Cl+ or Br+) generated when the halogen molecule reacts with the catalyst. Iodination is not practically achievable this way because it is a reversible process. Nitration: heating benzene with a mixture of concentrated nitric and concentrated sulphuric acid (the nitrating mixture) at 313-333 K substitutes a ring hydrogen with a nitro group, giving nitrobenzene; the electrophile is the nitronium ion, NO2+. Sulfonation: heating benzene with fuming sulphuric acid (oleum) at 373 K gives benzenesulfonic acid; the electrophile is free sulfur trioxide, SO3. Friedel-Crafts alkylation and acylation: benzene reacts, with anhydrous AlCl3 as catalyst, with an alkyl halide (giving an alkylbenzene, e.g. toluene from methyl chloride, electrophile R+) or with an acyl halide/acid anhydride (giving an acyl benzene, e.g. acetophenone from acetyl chloride or acetic anhydride, electrophile the acylium ion R-C+=O); alkylation is specifically noted as a way of extending a carbon chain onto the outside of the aromatic ring. Beyond this dominant substitution chemistry, benzene CAN be forced into addition reactions under special, more forcing conditions: with Cl2 under UV/sunlight it adds three molecules to give benzene hexachloride; with H2 over heated nickel (453-473 K …
Worked out. Despite normally preferring substitution, benzene CAN be forced to add across its ring under special conditions. With chlorine gas under bright sunlight or UV light, benzene adds three molecules of Cl2 to give benzene hexachloride (C6H6Cl6). With hydrogen gas over heated nickel catalyst at 453-473 K, benzene adds 3 H2 to give cyclohexane (C6H12). With ozone in an inert solvent (CCl4), benzene forms an unstable benzene triozonide, which zinc dust and water then decompose into three molecules of glyoxal (ethanedial, CHO-CHO) pl …
Worked out. In the dark, with iron (or FeCl3, anhydrous AlCl3, or red phosphorus) as catalyst, chlorine substitutes a ring hydrogen of benzene to give chlorobenzene plus HCl; the attacking electrophile is the chloronium ion, Cl+, generated as Cl-Cl reacts with FeCl3 to give Cl+ plus [FeCl4]-. Bromination proceeds the same way with FeBr3, giving bromobenzene and, as electrophile, Br+ generated from Br-Br plus FeBr3. Iodination of benzene is not practically possible because it is a reversible process. With excess chlorine and anhydrous AlCl3, benzene instead gives fully-substitute …
Worked out. Nitration: heating benzene with a mixture of concentrated nitric acid and concentrated sulphuric acid (the nitrating mixture) at about 313-333 K gives nitrobenzene plus water; the attacking electrophile is the nitronium ion, NO2+, generated when HO-NO2 reacts with two molecules of H2SO4 to give 2 HSO4- plus H3O+ plus NO2+. Sulfonation: heating benzene with fuming sulphuric acid (oleum) at 373 K gives benzenesulfonic acid plus water; the electrophile here is free sulfur trioxide, SO3, generated as 2 H2SO4 g …
Worked out. Alkylation: benzene reacts with an alkyl halide (e.g. methyl chloride) in the presence of anhydrous AlCl3 to give an alkylbenzene plus HCl -- methyl chloride gives toluene (methylbenzene); this reaction is used to extend a chain outside the benzene ring, and the attacking electrophile is a carbocation R+, generated as R-Cl plus AlCl3 gives R+ plus AlCl4-. Acylation: benzene reacts with an acyl halide (e.g. acetyl chloride) or an acid anhydride (e.g. acetic anhydride), also with anhydrous AlCl3 and heat, to give the corresponding acyl benzene plus HCl (or the carboxylic acid, from the anhydride) -- both acetyl chloride and acetic anhydride give 1-phenylethanone (acetophenone); the attacking electrophile here is the acylium …
Worked out. Heated in air, benzene burns with a characteristically sooty flame (reflecting its high carbon content) to give carbon dioxide and water: C6H6 + 15/2 O2 gives 6CO2 + 3H2O. The general combustion equation for any hydrocarbon CxHy is CxHy + (x + y/4) O2 gives x CO2 + y/2 H2 …