JEE Chemistry Hydrocarbons Notes Set 07

Aromaticity

Arenes are cyclic, planar, fully conjugated rings that are unusually stable. The general formula is \( \ce{CnH2n-6} \) for benzene and its alkyl derivatives. They undergo substitution rather than addition because substitution keeps the aromatic stabilisation.

1. Huckel's rule: a planar, cyclic, fully conjugated system with (4n + 2) pi electrons (2, 6, 10, 14 ...) is aromatic.
2. A planar cyclic conjugated system with 4n pi electrons (4, 8 ...) is antiaromatic and unusually unstable; cyclobutadiene is an example, and cyclooctatetraene avoids it by becoming tub shaped (non-planar).
3. Non-planar rings are not aromatic even if the electron count fits: [10]annulene twists because its internal hydrogens clash.
4. Ions can be aromatic: the cyclopentadienyl anion (6 electrons), the cyclopropenyl cation (2) and the tropylium (cycloheptatrienyl) cation (6) are aromatic; the cyclopentadienyl cation (4) is antiaromatic.
5. Heterocycles: pyridine (nitrogen lone pair in an sp2 orbital, outside the pi system, so it is basic), pyrrole, furan and thiophene (the heteroatom lone pair is part of the six-electron system).

Structure of Benzene

Kekule proposed a ring of six carbons with alternating single and double bonds that rapidly interchange. The modern picture is a hybrid of the two Kekule forms. All six carbons are sp2, all C-C bonds are equal (139 pm, bond order 1.5, between 154 pm for single and 134 pm for double), all H-C-C and C-C-C angles are 120 degrees, and the six p orbitals overlap sideways to form a continuous pi cloud above and below the ring. There are 12 sigma bonds and 3 pi bonds; benzene is planar, non-polar (zero dipole moment) and one type of carbon is present.

Evidence for the structure: only one monosubstituted product, three disubstituted isomers (ortho, meta and para), no addition of bromine water or Baeyer's reagent, and the heat of hydrogenation is about 150 kJ/mol less than three times that of cyclohexene (the delocalisation energy). Ozonolysis of benzene gives glyoxal, and ozonolysis of o-xylene gives glyoxal, methylglyoxal and dimethylglyoxal in the ratio 3 : 2 : 1, supporting the two Kekule forms.

Sources and Preparation

1. Coal tar distillation: light oil (up to 170 degrees C) gives benzene, toluene and xylenes; middle oil (170 to 230) gives phenol and naphthalene; heavy oil (230 to 270) gives cresols; anthracene oil (270 to 400) gives anthracene; pitch remains.
2. From petroleum by reforming, for example n-hexane to benzene and n-heptane to toluene over Pt or \( \ce{Cr2O3/Al2O3} \).
3. Trimerisation of ethyne in a red-hot iron tube.
4. Decarboxylation of sodium benzoate with soda lime.
5. Reduction of phenol with hot zinc dust, or of benzenediazonium chloride with hypophosphorous acid.
6. Hydrolysis of benzenesulphonic acid with superheated steam.

Electrophilic Aromatic Substitution

The electrophile E+ attacks the pi cloud in a slow step to form a delocalisation-stabilised arenium (sigma complex) cation. A base then removes H+ in a fast step and the ring becomes aromatic again.

1. Nitration has no isotope effect: benzene and hexadeuterobenzene nitrate at the same rate because loss of H+ is not rate determining.
2. Acylation is preferred to alkylation for making straight-chain alkyl derivatives: n-propyl chloride gives isopropylbenzene (cumene) by rearrangement, so n-propylbenzene is made by acylation then Clemmensen reduction, or by the Wurtz-Fittig reaction.
3. Friedel-Crafts reactions fail on rings that carry strongly deactivating groups (\( \ce{-NO2} \), \( \ce{-COR} \), \( \ce{-SO3H} \)) or groups that complex with \( \ce{AlCl3} \) (\( \ce{-NH2} \)). Aryl and vinyl halides cannot act as the alkyl halide.
4. Excess benzene with \( \ce{CH2Cl2} \) and \( \ce{AlCl3} \) gives diphenylmethane; with \( \ce{CHCl3} \) it gives triphenylmethane.
5. Thiophene is sulphonated much more easily than benzene, so commercial benzene is freed of thiophene by shaking with cold conc. \( \ce{H2SO4} \).

Directing Effects

1. Reactivity toward electrophiles falls in the order phenol > toluene > benzene > chlorobenzene > benzoic acid > nitrobenzene.
2. Ortho-para directors push electrons into the ring (+I, hyperconjugation or +M) and stabilise the arenium ion when the attack is at the ortho or para position. Meta directors withdraw electrons and so least destabilise attack at the meta position.
3. Toluene nitration gives o- and p-nitrotoluene; excess nitration finally gives 2,4,6-trinitrotoluene (TNT), a high explosive. Nitration of nitrobenzene gives 1,3-dinitrobenzene.
4. Among xylenes, m-xylene is the most reactive in sulphonation, because both methyl groups activate the same positions.

Addition and Oxidation of Benzene

Toluene and Other Alkylbenzenes

Xylenes oxidise to the dicarboxylic acids: ortho gives phthalic acid, meta gives isophthalic acid and para gives terephthalic acid. Wurtz-Fittig reaction: \( \ce{C6H5Br + CH3Br + 2 Na} \) gives toluene.

Polynuclear Hydrocarbons