BENZENE AND AROMATICITY

1. BENZENE AND AROMATICITY Chapter 15 Farshid Zand

2. The Term Aromatic • Aromatic used to be used to describe a fragrant substance. • Today the word aromatic refers to benzene and its structural relatives. • Benzaldehyde, toluene, and benzene are all aromatic compounds. Farshid Zand

3. The steroidal hormone estrone, the analgesic morphine, and the tranquilizer diazepam (Valium) are all examples of aromatic compounds. Farshid Zand

4. 15.1 Sources of AromaticHydrocarbons • Hydrocarbons come from two main sources: coal and petroleum. • Fractional distillation of coal yields benzene, toluene, xylene (dimethylbenzene), naphthalene, and a host of other aromatic compounds. • Petroleum consists largely of alkanes with a few aromatic compounds. Farshid Zand

5. 15.2 Naming Aromatic Compounds • Aromatic substances have more nonsystematic names than any other class of organic compounds. • Some widely used names are allowed by IUPAC rules. Methylbenzene is known as toluene, hydroxybenzene as phenol, aminobenzene as aniline. Farshid Zand

6. Common Names of Aromatic Compounds Farshid Zand

7. Monosubstituted Benzenes • Monosubstituted benzenes are systematically named as other hydrocarbons, with –benzene as the parent name. Farshid Zand

8. Alkyl-substituted Benzenes and Phenyl-substituted Alkanes • Alkyl-substituted benzenes are called arenes. If the alkyls substituent is smaller than the ring (six or fewer carbons), the arene is named as an alkyl-substituted benzene. • If the alkyl substituent is larger than the ring (more than six carbons), the compound is named as a phenyl-substituted alkane. • Phenyl is sometimes abbreviated as Ph or Φ. Farshid Zand

9. Phenyl and Benzyl Groups Farshid Zand

10. Disubstituted Benzenes • Disubstituted benzenes use prefixes ortho-(o), meta(m), or para-(p). These prefixes are useful when discussing reactions. Farshid Zand

11. Benzenes with More Than Two Substituents • Benzenes with more than two substituents are named by numbering the positions of each substituent, so that the lowest possible numbers are used. Farshid Zand

12. Structure and Stability of Benzene • Benzene is more stable than typical alkenes. • All C-C-C bond angles are 120 degrees, all six carbon atoms are sp2 hybridized, and each carbons has a p orbital perpendicular to the plane of the six-membered ring. Farshid Zand

13. Comparison of Heats of Hydrogenation This provides a measure of benzene’s unusual stability; its resonance energy is 29.6 kcal mol-1. Farshid Zand

14. Bonds of Benzene Rings • Each π bond overlaps equally well with both neighboring p orbitals; six π electrons are delocalized around the ring. Farshid Zand

15. Planar molecule with the shape of a regular hexagon. All C-C-C bond angles are 120 degrees All six carbons are sp2 hybridized. Each carbon atom has a p orbital perpendicular to the plane of the six-membered ring. All six carbon atoms and six p orbitals in benzene are equivalent. 15.4 Molecular Orbital Description of Benzene Farshid Zand

16. The six pi electrons are completely delocalized around the ring. This is why it is difficult to define when one p orbital overlaps only one neighboring p orbital. Farshid Zand

17. Molecular Orbital of Benzene Farshid Zand

18. Bonding and Anti-bonding combinations • Six benzene molecular orbitals result from the cyclic combination of six p atomic orbitals. • Three low energy molecular orbitals are “bonding” • Three high energy molecular orbitals are “anti-bonding” Farshid Zand

19. Let’s review… • Benzene is a cyclic conjugated molecule • It is unusually stable, with a heat of hydrogenation of -206 kJ/mol • It is planar with the shape of a regular hexagon; bond angles 120 • It undergoes substitution reactions that retain the cyclic conjugation • It is a resonance hybrid Farshid Zand

20. So what makes a molecule aromatic? • It must be cyclic • It must be conjugated • It must be flat so that the p orbital overlap can occur • It must also have 4n + 2 pi electrons… Farshid Zand

21. 15.5 The Huckel 4n + 2 Rule To be aromatic, the molecule must also follow the Huckel 4n + 2 rule: A molecule must have 4n + 2 pi electrons where n is an integer (0, 1, 2, 3, etc…) Farshid Zand

22. The 4n + 2 rule does not apply just to neutral hydrocarbons. For example: the cyclopentadienyl anion and cycloheptatrienyl cation are aromatic Farshid Zand

23. 15.6A Cyclopentadienyl Anion • Cyclopentadiene is not aromatic because it is not conjugated. • The –CH2 carbon in the ring is sp3hybridized preventing cyclic conjugation • Both the radical and carbocation are unstable Farshid Zand

24. 15.6A Cyclopentadienyl Anion Removing one H and no electrons from cyclopentadiene leaves the anion which has six pi electrons (a Huckel number!) This anion now meets all the standards for aromaticity. Farshid Zand

25. 15.6B Cycloheptatrienyl Cation • Likewise, the cycloheptatrienyl cation has six pi electrons • Its radical and anion have seven and eight pi electrons, respectively (not Huckel numbers) Farshid Zand

26. 15.7 Aromatic HeterocyclesPyridine & Pyrrole Heterocycle:a cyclic compound that contains an atom(s) in its ring other than carbon and can be aromatic. The heteroatom is often O or N. Farshid Zand

27. Pyridineis a 6 membered ring like Benzene with 6 π electrons perpendicular to the plane of the ring (5 from the sp2 hybridized C’s and one from the sp2 hybridized N). Nitrogen’s lone pair of electrons are in the plane of the ring acting like the H of the benzene C-H bond. A single electron is in the P orbital perpendicular to the ring resulting in 6 electrons in the cyclic pi system. Pyridine is an aromatic heterocyclic compound, a nitrogen analogue of benzene. Farshid Zand

28. Pyrroleis a five membered ring also with 6 π electrons perpendicular to the plane of the ring. However, 4 are from the sp2 hybridized C’s and two are from the lone pair on the sp2 hybridized N. In Pyrrole, the H of the N-H bond is in the plane of the ring, just like the H’s of the C-H bonds. This puts the two lone pair electrons in the P orbital perpendicular to the ring. Farshid Zand

29. 15.8 Why 4n + 2? 2, 4, 6, 10, 14, 18 π electrons create aromatic stability because of molecular orbitals. Benzene has 6 overlapping atomic p orbitals therefore it also has 6 molecular orbitals. The six molecular orbitals are divided evenly between bonding and nonbonding orbitals. There is one lowest energy orbital, two degenerate (equal energy) pairs of orbitals and one highest energy orbital. Farshid Zand

30. The single lowest energy molecular orbital holds two electrons. • Each degenerate (equal energy) bonding orbital above it holds 2 electrons and since there is a pair of orbitals in each degenerate energy level, each degenerate orbital level (shell) holds 4 electrons. The first degenerate energy level (n=1) holds 4 electrons (2 pairs). • n= # of filled pairs of degenerate orbitals (energy level). • Anything but 4n+2 would leave an partially unfilled • orbital. Farshid Zand

31. Benzene has 6 molecular orbitals: 3 filled bonding orbitals: (bottom=lower energy) 1 filled lowest energy orbital, 2 filled degenerate orbitals (n=1) and 3 empty nonbonding orbitals (top=higher energy). AROMATIC Cyclobutadiene has 4 molecular orbitals: 1 filled lowest energy 2 half filled degenerate orbitals (n=0 because orbitals not filled) 1 empty nonbonding orbital ANTIAROMATIC Cyclooctatetraene has 8 molecular orbitals: 3 filled lowest energy bonding orbitals (n=1) 2 half filled orbitals (only fully filled orbitals in an entire energy level increase n) 3 empty nonbonding orbitals For a total of 8 electrons. NON AROMATIC Farshid Zand

32. 15.9 Polycyclic AromaticCompounds • Huckel Rule applies only to monocyclic compounds but the general concept of aromaticity can also be applied to polycyclic aromatic compounds. • Examples of polycyclic aromatic compounds are: Napthalene, anthracene and phenanthrene Farshid Zand

33. All polycyclic aromatic hydrocarbons can be represented by multiple resonance forms yet the true structure is a hybrid of these resonance forms with delocalization of the pi electrons (see electrostatic potential map on right). Napthalene has 10 pi electrons, 2n + 2, (n=4) and is aromatic. Each ring, if looked at separately, has six pi electrons, but because they share the common central bond, the entire polycyclic molecule has only 10 electrons making it aromatic. Farshid Zand

34. 15.10 Spectroscopy Summary of Spectroscopic Information on Aromatic Compounds Kind of Absorption Spectroscopy Position Interpretation ____________________________________________________________ Infrared cm-1 3030 Aryl C-H Stretch 1500 and 1600 Two absorptions due to ring motions 690-900 Intense C-H out-of-plane bending Ultraviolet (nm) 205 Intense absorption 255-275 Weak Absorption HNMR (δ) 2.3-3.0 Benzylic Protons 6.5-8.0 Aryl Protons CNMR (δ) 110-140 Aromatic Ring Carbons Farshid Zand

35. IR: Aromatic Rings • Low intensity C-H stretching absorption at 3030 cm-1, just to the left of typical saturated C-H band. • Series of C-C (ring bonds) at 1450 to 1600 cm-1. Four absorptions are often observed in a set of 2 pairs. One pair near 1450 cm-1 and one pair near 1600-1650 cm-1. One band of each pair is usually stronger than the other, generally at or near 1500 cm-1 and at 1600 cm-1. • Multiple weak absorptions in the 1660 – 2000 cm-1 region • Strong absorptions in the 690-900 cm-1 range. The exact position of the absorption can identify the substitution pattern of the ring. Farshid Zand

36. UV Spectroscopy UV Spec is applicable only to conjugated pi systems. Aromatic compounds show these characteristic ring absorptions: • a series of bands of fairly intense absorption near 205 nm. • a less intense absorption in the range of 255 to 275 nm. Farshid Zand

37. 1HNMR • Aromatic rings are easily identified by their strongly deshielded hydrogens that absorb between 6.5 - 8.0 δ. • The difference between vinylic protons that absorb between 4.5 - 6.5 and aromatic (aryl) protons is ring current, an induced field created by the delocalized pi electrons circulating around the ring, creating a magnetic field of their own. Aryl protons therefore experience a greater magnetic field than that which is applied and come into resonance at a lower applied field. • Benzyllic protons (H on C next to aromatic ring) absorb downfield from alkane H’s in the region of 2.3 to 3.0δ. Farshid Zand

38. 13CNMR • Aromatic (ring) carbons absorb in the range of 110-140 δ (the same range as alkene C’s). Thus, the presence of 13C absorptions are not conclusive evidence of an aromatic ring. Aromaticity must be confirmed by IR, UV or HNMR. Farshid Zand