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Aromatic Chemistry Chapter 25 OCR
22/11/2022
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Benzene I phenols! 25.1 Benzene kekule's model : 2 cyclo-1,3,5- hexa triene. 1) Lack of reactivity of benzene. If benzene had C=C double bonds, it would undergo electrophillic addition reactions and decolourise bromine water.... It doesn't. Intermediate bond lengths: ↳₂ X-ray diffraction showed that all the loonds were of equal length and in between the lengths of a single and. double bond... 3 Hydrogenation enthalpies: 15 Hydrogenation enthalpy of C=C_bond is = Actual is - 208k Imal Benzene is 152kJmolt more stable (How does the delocalised structure geour? - Each carbon 3 out of its 4 uses electrons to bond to two other carbons I one hydrogen atom. Each carbon has an electron in ap-orbital at right angles to the plane of the cartoon atoms and hydrogen atom. P. orbitals overlap above and below the plane of the carbon atom forming a region of elechon density! The overlapping of p. orbitals creates a which are Said to be delocalized. They spread over the six carbon atoms. 25.2 Nitration of benzene Conditions: • 50°C H₂SO4 catalyst (CONC) Step 1 : HNO₂ + H₂SO4 → HSO₂ + NO ₂ + +H₂0 Step 2: 0 Step 3: ● →NO ₂ + HV NO₂ + HSO₁²¯ + H² → H₂ SO4 4 lectrophile NO 2 +H* (Halogenation of Benzene → Benzene is too stable and does not possess sufficient electron density to polarise a bromine molecule Step 1: Br₂ + Fe Br₂ → Fe Bru Step 2: Step 3: 0 →Br+ H., Br Ht + Febru FeBr → + Br 01 + H+ Fe Br₂ + HBr 3 Alky! Acy! Friedel-Crafts Alkylation + Acylation) Step 1: Step 2: Step 3: Step 1: Step...
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Alternative transcript:
2: Step 3: H H-C-CI + AICI ₂ →→CO + AICL₁ H H H₁ CH₂ H Ⓒ H² + AICI →→ AICI₂ + HCI H₂ COCH 3 + CH₂ COCI+ ALC1₂ → + COCH₂ + AICI ₁ H₂₁C=0 **COCH 3 (1) U H² + AICI₁ → AIC1₂ + HCl. + H +H+ Comparing reactivity of alkenes and arenes Alkenes ·localised electrons in π-bond • Sufficient election density to polarise molecule • No halogen camer required Undergoes electrophillic addition I substitution ● . Arenes delocalised ● electrons in A-bond • Insufficient electron density to polarise a bromine molecule Benzene only reacts halogen camer present Undergoes electrophillic substitution. ● Phenol OH Weak acid. Electrophillic Substitutions of он Bromination of phenal: O phenol: → Reacts with bromiine water + 3Bv₂ OH → Decolourises bromine water + white ppt. forms. OH Br 0 Br +ht · Br + 3HBr Ⓒ Nitration of phenol OH 10 + HNO 3 2-nitrophenal OH NO? → Halogen camer →Conc HNO₂+ Conc. H₂584 OH 4-nitrophenal NO₂ Reactivities of benzene and phenol Benzene Phenol • bromine I nitric acid → No halogen camer → dilute ++2₂2₂0 ✓ "The lone pour of elections on the O' in -OH group is donated into π- system, increasing elechon density