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Chapter 5 Aromatic Compounds

Chapter 5 Aromatic Compounds - Chapter 5 Aromatic Compounds...

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Chapter 5 Aromatic Compounds 5.1 Structure of Benzene: The Kekule Proposal 1. Benzene is relatively unreactive towards most reagents that attack alkenes 2. Benzene reacts with Br 2 in the presence of iron to give substitution product C 6 H 5 Br rather than addition product C 6 H 6 Br 2 . Monobromo substitution product was known and no isomers had been prepared 3. Kekule Proposal: benzene contains a ring of carbon atoms and can be formulated as cyclohexa-1,3,5-triene. This structure would accound for the isolation of only a single monobromo substitution product since all six carbon atoms and six hydrogens in cyclohex-1,3,5-triene are equivalent 4. Benzene shows none of the behaviour characteristic of alkenes. 5. Differences in reactions between alkenes and benzene Alkene Benzene Reacts with KMnO 4 to give 1,2-diols. Hydroxylation No reaction with KMnO 4 Reacts with aqueous acid to give alcohols. Hydrolysis No reaction with aqueous acid React with HCl to give chloroalkanes No reaction with HCl Benzene does not undergo electrophilic addition reactions 6. All carbon-carbon bonds in benzene have the same length intermediate between typical single bond and typical double bond.
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5.2 Structure of Benzene: The Resonance Proposal 1. allylic carbocation can be described as a resonance hybrid of two contributing forms. Neither resonance form is correct by itself; the true structure is somewhere between the two forms 2. Resonance theory says that benzene cant be described satisfactorily by single line- bond structure but instead is a resonance hybrid of the two forms. Benzene does not oscillate back and forth between the two forms; its true structure is somewhere between the two. Each carbon-carbon connection is an average of 1.5 bonds, midway between a single and double bond 3. Benzene is flat symmetrical molecule with the shape of regular hexagon. a. All C-C-C bond angles are 120 degree b. Each carbon atom is sp 2 hybrid c. Each carbon has a p orbital perpendicular to the plane of six membered ring 4. since all six p orbitals are equivalent, it’s impossible to define three localized Π bond in which a given p orbital overlaps only one neighboring p orbital . Rather, each p orbital overlaps equally well with both neighboring p orbitals leading to a structure for benzene in which the Π electrons are shared around the ring in two doughnut shape clouds 5. By resonance theory, benzene is more stable and less reactive than typical alkene because it has more two resonance forms of equal energy 5.3 Naming Aromatic Compounds Table 5.1 (Page 142) Common Names of Some Aromatic Compounds
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1. Monosubstituted benzenes are systematically named in the same manner as other hydrocarbons with -benzene used as the parent name 2. Example C 6 H 5 Br C 6 H 5 C 2 H 5 Bromobenzene Ethylbenzene 3. phenyl is used for –C 6 H 5 unit when benzene is considered as substituents. Benzyl is used for C 6 H 5 CH 2 - 4. Disubstituted are named using the prefix ortho-(o), meta- (m) and para- (p) a.
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