Compounds with 4n T Electrons Are Not Aromatic May be antiaromatic) Planar, cyclic molecules with 4nT electrons are much less stable than expected (anti-aromatic) They will distort out of plane and cyclobutadiene behave like ordinary alkenes 4- and 8-electron compounds are not delocalized(single and double bonds) Cyclobutadiene is so unstable that it cyclooctatetraene dimerizes by a self- Diels-Alder reaction at low termperature Cyclooctatetraene has four double bonds, reacting with Br2, KMnO4, and HCl as if it were four alkenes
Compounds With 4n p Electrons Are Not Aromatic (May be Antiaromatic) • Planar, cyclic molecules with 4 n p electrons are much less stable than expected (anti-aromatic) • They will distort out of plane and behave like ordinary alkenes • 4- and 8-electron compounds are not delocalized (single and double bonds) • Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder reaction at low termperature • Cyclooctatetraene has four double bonds, reacting with Br2 , KMnO4 , and HCl as if it were four alkenes cyclobutadiene cyclooctatetraene
156 Recall: Key Ideas on benzene Unusually stable-heat of hydrogenation 150 kJ/mol less negative than a cyclic triene Planar hexagon: bond angles are 120 carbon-carbon bond lengths 139 pm Undergoes substitution rather than electrophiLIc addition Resonance hybrid with structure between two line -bond structures One more important factor is the number of electrons in the cyclic orbital
Recall: Key Ideas on Benzene • Unusually stable - heat of hydrogenation 150 kJ/mol less negative than a cyclic triene • Planar hexagon: bond angles are 120° , carbon–carbon bond lengths 139 pm • Undergoes substitution rather than electrophilic addition • Resonance hybrid with structure between two line-bond structures • One more important factor is the number of electrons in the cyclic orbital p.156
p. 5. 4 Naming aromatic compounds Many common names(toluene= methylbenzene aniline - aminobenzene Monosubstituted benzenes systematic names as hydrocarbons with-benzene CH Br= bromobenzene CHSNO2- nitrobenzene, and CH CH2 CH2CH3 is propylbenzene B NO CH2CHCH Bromobenzene Nitrobenzene Propylbenzene Thomson. Brooks Col
5.4 Naming Aromatic Compounds • Many common names (toluene = methylbenzene; aniline = aminobenzene) • Monosubstituted benzenes systematic names as hydrocarbons with –benzene C6H5Br = bromobenzene C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is propylbenzene p.177
The Phenyl group When a benzene ring is a substituent the term phenyl is used(for CBH5 You may also see ph, or‘” in place of Ch “Beny(苄卡基) refers to“CHCH2 I CH CHCHCH2CH2, CH2之 234567 A phenyl group 2-Phenylheptane a benzyl group Thomson- Brooks Cole
The Phenyl Group • When a benzene ring is a substituent, the term phenyl is used (for C6H5 ⺷) You may also see “Ph” or “f” in place of “C6H5 ” • “Benzyl” (苄基) refers to “C6H5CH2 ⺷
Disubstituted benzenes Relative positions on a benzene ring ortho-(0, p on adjacent carbons meta=(m, /E separated by one carbon (1,3) para(p, Xi) separated by two carbons Describes reaction patterns("occurs at the para position”)
Disubstituted Benzenes • Relative positions on a benzene ring – ortho- (o,邻) on adjacent carbons (1,2) –meta- (m,间) separated by one carbon (1,3) – para- (p,对) separated by two carbons (1,4) • Describes reaction patterns (“occurs at the para position”)