CHEM Aromatic Chemistry. LECTURE 1 - Aromaticity

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1 1 CHEM40003 Aromatic Chemistry LECTURE 1 - Aromaticity Alan C. Spivey a.c.spivey@imperial.ac.uk May 2018

2 2 Format and scope of presentation Aromaticity: Historical perspective (Kekulé) Characteristics, NMR ring currents Valence bond & molecular orbital representations (Hückel's rule) Anti-aromaticity Key further reading: Clayden, Greeves & Warren, rganic Chemistry, 2 nd Ed., Chapter 7 conjugation pages aromaticity pages

3 Aromaticity: historical perspective 3 Aromatic aroma natural fragrances e.g. benzaldehyde (peaches) Unusual stability/unreactivity for an unsaturated hydrocarbon KMn 4 C 2 H C 2 H KMn 4 HCl H 3 H cf. HCl H 3 No reaction Cl Late 1800 s: benzene combustion analysis molecular formula C 6 H 6 Ladenburg Dewar Kekulé No-decolourisation of bromine water (i.e. addition of 2 across double bonds) Fe 3 2 C 6 H 5 C 6 H 4 2 = substitution! three isomers!

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5 Kekulé and his dream of snakes... 5 Loschmidt 1861 Kekulé 1861 Kekulé or Loschmidt? It began with a daydream: the 150 th anniversary of the Kekule benzene structure, A. Rocke Angew. Chem. Int Ed. 2015, 54, (DI)

6 Aromaticity: stability and bond lengths 6 Quantification of unusual stability: heats of hydrogenation (calorimetry) see: Vollhardt J. Am. Chem. Soc. 2000, 122, 7819 (DI) X-ray and neutron diffraction data show all bond lengths to be the same 1.39Å cf. ave C-C 1.54Å & ave C=C 1.34Å

7 Aromaticity: NMR ring currents 7 Protons INSIDE ring would experience SHIELDING (B appl -B ind ) i.e. field (small d/ppm) Protons UTSIDE ring experience DESHIELDING (B appl +B ind ) i.e field (large d/ppm) Note that the field is induced as the result of electron movement within orbitals & resulting diamagnetic anisotropy, see ( min)

8 Aromaticity: NMR ring currents 8 Benzene gives a singlet at d 7.27 ppm i.e. deshielding due to ring current (cf. d ppm for typical alkene protons) i.e. singlet because all H s are in identical environment (C 6 symmetry) Higher aromatic systems have stronger ring currents resulting in dramatic shielding/deshielding:

9 Benzene Valence Bond and Molecular rbital Representations of Aromaticity 9 VALENCE BND (VB) THERY resonance hybrids - imaginary structures which differ only in position of electrons (atoms/nuclei do not move) Not all resonance structures contribute equally - real structure is weighted average of resonance structures Review: Hiberty et al. Chem. Rev. 2011, 111, 7557 (DI) VB structure MLECULAR RBITAL (M) THERY Linear Combination of Atomic rbitals (LCA) s-bonding framework formed from sp 2 hybridised carbons leaves a p-orbital on each C atom orthogonal to the ring 6 atomic p-orbitals (As) [LCA maths] 6 Molecular rbitals (Ms): M structure each M capable of containing 2 electrons 6 electrons available to occupy the 6 Ms placed in 3 molecular orbitals of lowest energy: bonding orbitals 3 anti-bonding orbitals remain vacant

10 Benzene - Molecular rbital Description 10 Linear Combination of Atomic rbitals (LCA) 6 2p atomic orbitals give 6 molecular orbitals: degenerate pair of anti-bonding Ms degenerate pair of bonding Ms The +/- signs (and associated blue/orange colour designations) do not represent electrostatic charge, but refer to phase signs in the equations that describe these orbitals. When the phases are the same, the orbitals overlap to generate a common region of like-phase; the orbitals having the greatest like-phase overlap are the lowest in energy (hence, π 1 is the lowest in energy).

11 Musulin-Frost diagrams: M diagrams without the maths 11 Graphical device for constructing M energy diagrams: Frost & Musulin J. Chem. ys. 1953, 21, 572 (DI) & Zimmerman J. Am. Chem. Soc. 1966, 88, 1564 (DI) Draw appropriate regular polygon within a circle (with atoms touching circumference) Ensure one atom is at lowest point ring atom positions represent energy levels Centre of circle is zero energy level (i.e. bonding orbitals below, anti-bonding above)

12 Hückel s rule 12 Empirical rule for aromaticity: Hückel Z. ys. 1931, 70, 204; Review: Berson Angew. Chem. Int. Ed. Engl. 1996, 35, 2750 (DI) For compounds which are planar & have a contiguous, cyclic array of p-orbitals perpendicular to plane of ring: Those with 4n+2 p electrons display special stabilisation: i.e. aromatic Those with 4n p electrons display special instability: i.e. anti-aromatic: In practice, molecules that could be anti-aromatic tend to adopt structures which are nonaromatic typically by twisting to preclude communication between adjacent p-orbitals (e.g. Cyclooctatetraene which adopts a tub shaped conformation) or by adopting hybridisation states that do not present a contiguous cyclic array of p-orbitals (e.g. Cyclopropenyl anion, for which two low energy electronic configurations have been located computationally, neither of which is anti-aromatic).

13 Evidence for anti-aromaticity 13 Deprotonation of cycloprop(en)yl ketones H NaD D 2 D cf. H NaD D 2 4 electrons anti-aromatic D 6000 times slower! Silver assisted solvolysis of cyclopent(adien)yl iodides I Ag + AgI 'very fast' cf. I Ag N REACTIN 4 electrons anti-aromatic