Unit 20: Aromatic Hydrocarbons — Long Questions

Step 1 Because one of the S=O bonds is polarised (Sᵟ⁺=Oᵟ⁻), sulphur is the electrophilic centre. A curly arrow is drawn from the delocalised π electron cloud of the ring (at the position para to the CH₃ group) to the S atom, forming a new C-S σ bond; simultaneously a curly arrow is drawn from the polarised S=O π bond onto that oxygen, placing a full negative charge on it. This gives the arenium ion (sigma complex) intermediate: a cyclohexadienyl cation in which the ring carbon bonded to sulphur is now sp³ (bearing both H and the -SO₃⁻ group), the positive charge is delocalised over the remaining five ring carbons, and the sulphur-containing group is an -SO₃⁻ (three oxygens, one bearing the negative charge, no O-H yet). Step 2: A curly arrow is drawn from the C-H bond on that sp³ carbon into the ring, re-forming the aromatic π system and restoring full delocalisation/aromaticity; a second curly arrow shows the departing H⁺ being picked up by the negatively charged oxygen of the -SO₃⁻ group. This regenerates the aromatic ring and converts -SO₃⁻ into -SO₃H, giving D = 4-methylbenzenesulphonic acid (p-toluenesulphonic acid) and releasing H⁺, exactly as shown in the scheme.

Purification (recrystallisation) Dissolve the impure solid in the minimum volume of a hot suitable solvent (e.g. hot water or ethanol) in which 3-nitrobenzoic acid is much more soluble hot than cold. Filter the hot solution (using a pre-heated funnel) to remove any insoluble impurities. Allow the filtrate to cool slowly to room temperature (then in ice) so that crystals of pure 3-nitrobenzoic acid grow out, leaving soluble impurities in solution. Filter off the crystals under reduced pressure (Büchner filtration), wash with a small volume of cold solvent, and dry (e.g. in a desiccator or warm oven below the melting point).

Percentage yield Molar mass of benzoic acid (C₆H₅COOH) = 122 g mol⁻¹, so moles of benzoic acid = 4.97/122 = 0.0407 mol. Since the reaction is 1:1, theoretical moles of 3-nitrobenzoic acid (C₇H₅NO₄, M = 167 g mol⁻¹) = 0.0407 mol, giving a theoretical mass = 0.0407 × 167 = 6.80 g. Percentage yield = (actual mass/theoretical mass) × 100 = (4.85/6.80) × 100 ≈ 71.3%.

Checking purity Determine the melting point of the dried crystals using a melting point apparatus and compare it with the known (literature) melting point of pure 3-nitrobenzoic acid. A pure sample melts sharply over a narrow range (≤1-2 °C) at (or very close to) the literature value; an impure sample melts over a broader range and at a lower temperature than expected. (Thin-layer chromatography showing a single spot, compared against a pure reference, could also be used.)

Alkenes have a localised, electron-rich C=C π bond that is readily attacked by electrophiles. As Cl₂ approaches the double bond, the π electrons induce a temporary dipole in the Cl-Cl bond (Clᵟ⁺-Clᵟ⁻), making Cl₂ itself a sufficiently reactive electrophile; no catalyst is needed, and the alkene undergoes electrophilic addition readily at room temperature, with both chlorine atoms adding across the double bond (addition does not destroy any special stability, since the alkene's π bond is not delocalised).

Benzene, by contrast, has its six π electrons fully delocalised around the ring, giving it a large resonance (delocalisation) stabilisation of about 152 kJ mol⁻¹ and making it far less reactive than an alkene. Chlorine alone is not polarised strongly enough to act as an electrophile towards this very stable, delocalised π system, so a Lewis acid halogen carrier (e.g. AlCl₃ or FeCl₃) is required to coordinate with Cl₂ and generate the much more powerful electrophile Cl⁺. Furthermore, even when it does react, benzene undergoes electrophilic substitution rather than addition: substitution replaces a ring hydrogen and restores the aromatic delocalisation in the product, whereas addition (as happens with alkenes) would permanently destroy that delocalisation/stability. Thus alkenes react with chlorine easily and by addition, while benzene reacts only under catalysis and by substitution, reflecting its much greater thermodynamic stability due to delocalisation.

An electrophile is an electron-deficient species — either a positive ion or a molecule with a region of partial positive charge — that is attracted to, and accepts a pair of electrons from, an electron-rich centre (such as a π bond or a lone pair) to form a new covalent bond. Electrophiles are central to two major classes of organic reaction: electrophilic addition in alkenes, and electrophilic (aromatic) substitution in arenes.

Aliphatic example — electrophilic addition of hydrogen bromide to ethene:
CH₂=CH₂ + HBr → CH₃CH₂Br
Mechanism: the H-Br bond is polar (Hᵟ⁺-Brᵟ⁻). A curly arrow from the π electrons of the C=C double bond points to the δ+ hydrogen, forming a new C-H bond and breaking the H-Br bond heterolytically to release Br⁻; this generates a carbocation intermediate on the other carbon. A curly arrow then shows a lone pair on Br⁻ attacking the positively charged carbon, forming the C-Br bond and giving bromoethane.

Aromatic example — electrophilic substitution (nitration) of benzene:
C₆H₆ + HNO₃ --H₂SO₄--> C₆H₅NO₂ + H₂O
Mechanism: H₂SO₄ protonates HNO₃ and water is lost, generating the electrophile NO₂⁺. The delocalised π electrons of benzene attack NO₂⁺, forming a new C-N σ bond and a positively charged arenium ion (sigma complex) in which aromaticity is temporarily lost. The H⁺ on the substituted carbon is then removed by HSO₄⁻; these electrons move back into the ring, restoring the delocalised aromatic system and regenerating the H₂SO₄ catalyst, giving nitrobenzene.

In both cases the electrophile seeks out an electron-rich π system, but the outcome differs with the substrate's stability: the alkene's localised, non-delocalised π bond is permanently broken by addition, whereas benzene's highly stabilised delocalised π system is only temporarily disrupted, with aromaticity restored by substitution rather than addition.