Benzene. Named benzene by Eilhardt Mitscherlich in < Molecular formula: C 6 H 6. < Representative of the aromatic hydrocarbon family:

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1 Benzene < Discovered in 1825 by Michael Faraday: e called it bicarburet of hydrogen. Named benzene by Eilhardt Mitscherlich in < Molecular formula: C 6 6 < Representative of the aromatic hydrocarbon family: benzene, reduction product of benzoic acid, found in gum benzoin, an aromatic balsam from Styrax benzoin, the Benjamin tree. toluene, distilled from gum tolu, an aromatic balsam from the Tolu tree. < C 6 6 DU = (C) - ½( + X) + ½(N) + 1 = 6 - ½(6) + 1 = 4 < UV/Vis absorption spectrum: 8 max = 254 nm < Shouldn t benzene react like alkenes or alkynes? C 6 6 Br 2 CCl 4, dark no reaction C 6 6 Br 2 cat. FeBr 3 C 6 5 Br + Br C 6 6 xs. 2 cat. Pd no reaction

2 Some Possible Structures for Benzene C 6 6 DU = 4 Dewar benzene: Dewar, James, Proceedings of the Royal Society of Edinburgh 1866/1867, 6, 82. Ladenburg benzene: Ladenburg, Albert, Chemische Berichte 1869, 2, 140. Loschmidt s C 6 = one hexavalent C concept: Loschmidt, Joseph Konstitutions Formeln der organischen Chemie in graphischer Darstellung, Chemische Studien I, Vienna, 1861.

3 Kekulé Benzene C 6 6 DU = 4 I was sitting writing at my textbook but the work did not progress; my thoughts were elsewhere. I turned by chair to the fire and dozed. Again the atoms were gamboling before my eyes...my mental eye...could now distinguish larger structures of manifold conformation: long rows...all twining and twisting in snake-line motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes...i awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis. Let us learn to dream, gentlemen, then perhaps we shall find the truth. But let us beware of publishing our dreams till they have been tested by the waking understanding. (Rapp, J. Kekulé Memorial Lecture, Journal of the Chemical Society 1898, 73, 100.)

4 C 6 6 Benzene X-ray crystallographic studies show that: C all C C bonds are the same length, D C all pccc are equivalent, 120 E (therefore, all carbons are sp 2 -hybridized) C recall: Csp 2 Csp 2 σ bond = 1.46 D Csp 2 =Csp 2 σ + π bond = 1.34 D C so, benzene s carbon-carbon bonds are like the bonds of conjugated alkadienes, with partial double bond character.

5 LCAO diagrams of benzene sigma bond system pi bond system

6 Tremendous resonance stabilization: 1,3,5-cylcohexatriene, a hypothetical molecule, vs. benzene, a real molecule: Pt /C ) ( )) = 36 kcal / mol ) ( )) = 4 kcal / mol E kcal / mol 2 Pt /C 54 2 Pt /C 54 2 Pt /C Pt /C 2 Pt /C 2 Pt /C 2 Pt /C

7 Nomenclature of Arenes name as a substituted benzene: F fluorobenzene (S)-sec-butylbenzene or (S)-(1-methylpropyl)benzene NO 2 nitrobenzene

8 Common Arenes: toluene styrene 1,2-dimethylbenzene ortho-xylene o-xylene O O phenol benzaldehyde 1,3-dimethylbenzene meta-xylene m-xylene O N 2 O aniline benzoic acid 1,4-dimethylbenzene para-xylene p-xylene

9 Three examples of multiply substituted arenes Br 1-bromo-4-methylbenzene 4-bromotoluene p-bromotoluene (ortho, meta, and para are used only for disubstituted benzenes) 2-ethyl-1-isopropyl-4-methylbenzene 3-ethyl-4-isopropyltoluene NO 2 O 2 N NO 2 2,4,6-trinitrotoluene (TNT)

10 Common substituents containing aromatic rings, C 6 5 phenyl O trans-2-phenyl-1-cyclohexanol, C 6 5 C 2 benzyl Br cis-4-benzyl-3-bromo-1-cyclopentene

11 Do all all-sp 2 hybridized ring systems exhibit aromaticity? 1,3-cyclobutadiene benzene (1Z,3Z,5Z,7Z)1,3,5,7-cyclooctaetraene 4 B electrons 6 B electrons 8 B electrons C2-C3 and C4-C1 planar nonplanar, so p orbtials don t overlap all p orbtials overlap p orbtials don t overlap not aromatic aromatic not aromatic

12 Is (1E, 3Z, 5E, 7Z, 9Z)-cyclodecapentaene an aromatic hydrocarbon?

13 Aromaticity of eterocycles N N.. N N.. pyrrole pyridine 6 π electrons in 5 p orbitals 6 π electrons in 6 p orbitals

14 Aromaticity of eterocycles O N O.. O.. N.... O.. furan oxazoline 6 π electrons in 5 p orbitals not aromatic: one of the ring carbons is sp 3 -hybridized

15 Spectroscopy of aromatic compounds IR aromatic overtones ~ 1600 Csp 2 =Csp 2 stretch %Transmittance > 3000 Csp 2 stretch ~ Csp 2 bend Wavenumbers (cm-1)

16 Spectroscopy of aromatic compounds NMR benzylic hydrogens: ~ ppm aromatic hydrogens: ~ ppm

17 Spectroscopy of aromatic compounds NMR C C aromatic C region: ~ ppm C 3 C

18 Spectroscopy of aromatic compounds MS allkylbenzenes often have a prominent m/z = 91, the benzyl or tropilium ion + or +

19 Spectroscopy of aromatic compounds UV-Vis arenes typically have B band λ max in the region nm; occasionally may also see the more intense E band at shorter λ. λ max = 265 nm