UNIT 2 AROMATIC MATERIALS AND THEIR REACTIONS

1. UNIT 2AROMATIC MATERIALS AND THEIR REACTIONS • Structure of aromatics (Kekulé structure) • Properties of aromatics (chemical/physical/spectroscopy) • Aromatic/anti-aromatic/non-aromatic • Huckel’s rule • Electrophilic Aromatic Substitution • Nucleophilic Aromatic Substitution • Reactions on aromatic side chains

2. Aromatic compounds • Benzene in the simplest of the aromatic compounds it was first • isolated in 1825 by Michael Faraday. • Benzene has a carbon to hydrogen ratio of 1:1 and a molecular • formula of C6H6. • Other related compounds with low C:H ratios had a pleasant • smell, so they were classified as aromatic.

3. Aromatic compounds Kekulé proposed a cyclic structure for benzene that consisted of alternating single and double bonds in 1866. • There are a number problems with this structure when we consider the chemistry of benzene. • There is only a single 1,2-dichlorobenzene, not two which is possible with the above structure. • The C-C bond length would be expected to be different, but all of the bonds are the same.

4. Aromatic compounds Benzene is a planar resonance hybrid of the two Kekulé structures. The carbon-carbon bonds are all equivalent with a bond length of 1.397 A. Which is about half way between a C-C single (1.48 A) and double bond (1.34 A). The pi bonds (electrons) are delocalized over the ring. Since the bonds are delocalized over the ring it is common to draw a circle in an aromatic ring. However, we will commonly use the Kekulé structures in drawing reaction mechanisms where we are showing the movement of individual pairs of electrons.

5. Aromatic compounds Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. This results in a continuous cloud of delocalized electrons on both faces of the ring.

6. Aromatic compounds Unusual reactivity of benzene. Benzene is a cyclic conjugated triene. With three double bonds we would expect benzene to undergo the same reactions as alkenes. Alkene + KMnO4 diol (addition)Benzene + KMnO4 no reaction. Alkene + Br2/CCl4  dibromide (addition)Benzene + Br2/CCl4  no reaction.

7. Aromatic compounds Unusual reactivity of benzene. Benzene does not undergo simple addition reactions like alkene, except under extreme conditions. Interestingly, addition of a Lewis acid catalyst to a solution of benzene and bromine does result in a reaction. But the product of this reaction is notthe addition of bromine across a double bond. The product that is obtained is a substitution product where a hydrogen atom has been replaced by a bromine atom.

8. Aromatic compounds Unusual stability of benzene. The extra stability of benzene is due to the “resonance energy” extra stability associated with the resonance/delocalization of the pi electrons. This is illustrated by the heats of hydrogenation shown below.

9. Aromatic compounds Annulenes Annulenes are similar to benzene in that they are cyclic hydrocarbons with an analogous conjugated system of single and double bonds.

10. Aromatic compounds Annulenes

11. Aromatic compounds Annulenes Kekulé-like resonance structures can be drawn for annulenes similar to what we saw with benzene. However, many of these materials do not exhibit unusual stability like benzene. Hückel’s Rule: If the number of pi electrons in the cyclic system is: (4N+2), the system is aromatic; (4N) the system is antiaromatic, where N is an integer. This rule applies to planar cyclic compounds with atoms other than carbon, positive/negative charges and odd number ring sizes.

12. Aromatic compounds MO model for benzene • Six overlapping p orbitals must form six molecular orbitals. • Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no nodes. (bonding orbital) • As energy of MO increases, the number of nodes increases. (antibonding orbital)

13. Aromatic compounds MO model benzene

14. Aromatic compounds Polygon Rule The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom.

15. Aromatic compounds MO model for benzene • The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an extremely stable arrangement.

16. Aromatic compounds MO model cyclobutadiene

17. Following Hund’s rule, two electrons are in separate orbitals. This diradical would be very reactive. Aromatic compounds MO model cyclobutadiene

18. Aromatic compounds Aromatic Requirements • Structure must be cyclic with some conjugated pi bonds. • Each atom in the ring must have an unhybridized p orbital. (no sp3 hybridized atoms) • The p orbitals must overlap continuously around the ring. (Planar structure or close to planar.) • Compound is more stable than its open-chain counterpart. (lower electronic energy)

19. Aromatic compounds Anti- and Nonaromatic • Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart. • Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar.

20. Anti- and Non-aromatic Aromatic compounds Hückel’s Rule: If the number of pi electrons in the cyclic system is: (4N+2), the system is aromatic; (4N) the system is antiaromatic, where N is an integer. This rule applies to planar cyclic compounds with atoms other than carbon, positive/negative charges and odd number ring sizes.

21. Aromatic compounds Cyclopentadienyl anion The anion has a nonbonding pair of electrons in a p orbital, 6 e-’s, aromatic. What about the cyclpentadienyl cation?

22. Aromatic compounds Cyclopentadienyl cation

23. Aromatic compounds Cycloheptatrienyl cation How many pi electrons are in the above ion? How many p orbitals are in the above? Aromatic or antiaromatic? What about the cycloheptantrienyl anion?

24. Aromatic compounds Cyclooctatetraene How many pi electrons are in the above anion?

25. Aromatic compounds Cyclooctatetraene Huckel’s rule does not apply because there is not continuous over lap of p orbitals around the ring.

26. Aromatic compounds + - - See page 722 for additional examples.

27. Aromatic compounds Which of the following are aromatic?

28. Aromatic compounds Which of the following are aromatic, antiaromatic, nonaromatic?

29. Heterocyclic Aromatic compounds Pyridine : Aromatic heterocycle that is a weak base pKb = 8.8, often used in organic synthesis as a base. nitrogen heterocyclics

30. Pyrrole: Weaker base than pyridine because the lone pair of electrons on the nitrogen are delocalized over the ring. Heterocyclic Aromatic compounds nitrogen heterocyclic

31. Aromatic compounds Fused ring aromatics

32. Aromatic compounds Fused ring aromatics As the number of aromatic rings increases, there is a decrease in the resonance energy per ring. As a result these materials can undergo reactions that are more characteristic of nonaromatic polyenes.

33. Allotropes of Carbon Allotropes are pure elemental substances that can exist with different crystalline structures. • Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. • Diamond: a lattice of tetrahedral C’s. • Graphite: layers of fused aromatic rings. • Fullerenes (1985) contain closed structures with alternating 6- and 5- membered rings

34. Diamond • One giant molecule. • Tetrahedral carbons. • Sigma bonds, 1.54 Å. • Electrical insulator.

35. Graphite • Planar layered structure. • Layer of fused benzene rings, bonds: 1.415 Å. • Only van der Waals forces between layers. • Conducts electrical current parallel to layers.

36. New Allotropes of Carbon • Fullerenes: 5- and 6-membered rings arranged to form a “soccer ball” structure. • Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings.

37. Aromatic compounds Nomenclature polynuclear aromatic hydrocarbons naphthalene anthracene phenanthrene

38. Heterocyclic Aromatic compounds Heterocyclic aromatics furan thiophene indole pyridine pyrrole

39. Pyrimidine has two basic nitrogens. Imidazole has one basic nitrogen and one nonbasic. Heterocyclic Aromatic compounds nitrogen heterocyclic

40. Aromatic compounds Common aromatic materials (IUPAC) phenol (benzenol) toluene (methylbenzene) aniline (benzenamine) anisole (methoxybenzene) styrene (vinylbenzene) acetophenone benzaldehyde benzoic acid

41. Aromatic compounds Common aromatic ions cycloheptatrienyl cation tropylium ion cyclopentadienyl anion

42. Aromatic compounds Nomenclature of substituted benzene

43. Aromatic compounds Nomenclature of substituted benzene Number the ring so that you have the smallest possible numbers. Note: you can only use ortho, meta and para on disubstituted benzenes.

44. Aromatic compounds Nomenclature of substituted benzene Name the following benzene derivatives.

45. Aromatic compounds Nomenclature of benzene derivatives • If the benzene ring is attached to a complex structure it can be difficult to name the compound as a benzene derivative. In these case the benzene ring is named as a phenyl substituent. ( in drawings it is common for the phenyl group to be abbreviated as Ph or Φ.

46. An aryl group (Ar) is used to refer to a phenyl substituent that has substituents on the ring. Ar-OH = an aryl alcohol (a phenol) Ar-NH2 = an aryl amine Aromatic compounds Nomenclature of benzene derivatives X = any group other than H and can be ortho, meta or para. Benzyl group

47. Aromatic compounds Physical Properties • Physical properties of benzene and derivatives • Benzene has a boiling point and melting point comparable cyclohexane. b.p. = 80 C for benzene vs. 81 C for cyclohexane m.p. = ~6 C for both benzene and cyclohexane • Substitutions that make stacking of molecules more difficult in the solid state dramatically reduce melting points. m.p. = 6 C m.p. = 95 C

48. Aromatic compounds Physical Properties • Physical properties of benzene and derivatives • Asymmetrical derivatives with polar groups tend to have higher boiling points. (Dipole moment allows for stronger dipole-dipole interactions in liquid state.) • Symmetrical derivatives tend to stack better in the solid state and have higher melting points as a result.  = 0 b.p. = 181 C m.p. = 17 C b.p. = 173 C m.p. = 25 C b.p. = 170 C m.p. = 54 C