Aromatic compounds.docx

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University of South Australia
CHEM 1007

Aromaticity and aromatic compounds: Structure: - Planar with delocalized π electrons - Multiple resonance structuresVery stable - All C-C bond angles are 120° - Each carbon is hybridized - Each carbon has a p orbital perpendicular to the plane of the ring Aromaticity: Rule1: it must have an uninterrupted cyclic cloud of π electrons above and below the plane of the molecule. - π cloud: molecule must be a planar. - Cyclic: the molecule must be cyclic and have double bonds - Uninterrupted: All carbon atoms are carbons. Rule 2: the π cloud must have an odd number of pairs of π electrons. Naming: Mono-substituted: - Ends with –benzene Example: -halogens: Chlorobenzene -alkyl group: Propylbenzene -nitrate group: Nitrobenzene (NO ) 2 - Alternate groups: - Phenol(-OH) - Toluene(-CH ) 3 - Aniline(-NH )2 - Benzaldehyde(-COH) - Benzoic acid(-COOH) Disubstituted benzene: - Ortho-(o): 1, 2 disubstituted - Meta-(m): 1, 3 disubstituted - Para-(p): 1,4 disubstituted More than 2 substituent’s - Substituent’s listed alphabetically - Lowest possible numbers used Reactivity of benzene: Substitution reactions: - Halogenation: Reactants: halogens+ catalyst (FeBr ) 3 Example: - Nitration/ Sulfonation: Reactants: HNO / SO +Catalyst (H SO ) 3 3 2 4 Example: - Fredial-Crafts Alkylation/Acylation: Reactant: alkyl halides/ acyl halides + catalyst (AlCl ) 3 Example: - Oxidation of the alkyl groups: * aromatic ring don’t react with the catalyst. Reactant: alkylbenzene + Catalyst (KMnO +H 4) 2 Example: Effect of the Substituent’s: Formation of the carbocation is the rate determining step and anything that helps or hinders this affects reactivity. - Donating and withdrawing electron inductively through the Ϭ bonds -Donates: Alkyl substituents: the electrons in the Ϭ bond move towards benzene. making it more stable Example: -CH 3 -Withdraw: Substituents is more electronegative than carbon: the electrons in the Ϭ bond move away from benzene making it less stable Example: -NO ;2Cl ;-Br - Donating and withdrawing electrons by resonance through the π bonds -Donate: Substituent is directly attached to the benzene has a lone pair, the lone pair can be delocalized into the ring  A –ve charge makes the ring more reactive (activators)
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