ORGANIC CHEMISTRY CHM 207

1. ORGANIC CHEMISTRY CHM 207 CHAPTER 4: AROMATIC COMPOUNDS (BENZENE AND TOLUENE) NOR AKMALAZURA JANI

2. Aromatic compounds • Organic compound that contains a benzene ring in its molecule is known as an aromatic compounds. • Sometimes called arenes. • Molecular formula: C6H6 • Represented as a regular hexagon containing an inscribed circle.

3. Structure of Benzene • Can be represented in two abbreviated ways. • The corner of each hexagon represents a carbon and a hydrogen atom.

4. Kekulé Structure of Benzene Molecular formula is C6H6 All the hydrogen atoms are equivalent Each carbon atom must have four covalent bonds.

5. Resonance Structure • Resonance theory: the structure of benzene is a resonance hybrid structure of two Kekulé cononical forms. • The hybrid structure is often represented by a hexagon containing an inscribed circle.

6. Hexagonal ring – 6 carbon-carbon bonds are equal. • Circle – delocalised electrons of the benzene ring

7. CRITERIA OF AROMATIC COMPOUNDS • Structure must be cyclic, containing some number of conjugated pi bonds. • Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp2 hybridized or occasionally sp hybridized). • The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar (or nearly planar) for effective overlap to occur. • Delocalization of the pi electrons over the ring must lower the electronic energy. * Antiaromatic compound: fulfills the first three criteria, but delocalization of the pi electrons over the ring increase the electronic energy.

8. Huckel’s rule • Used to determine aromaticity for planar, cyclic organic compounds with a continous ring of overlapping p-orbitals. • If the number of pi (π) electrons in the monocyclic system is (4N+2), the system is aromatic. N is 0, 1, 2, 3….. • Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2 is a aromatic. • Systems that have 4, 8, and 12 pi electrons for N = 1, 2, 3 are antiaromatic.

9. Naming Aromatic Compounds

10. A substituted benzene is derived by replacing one or more of benzene’s hydrogen atoms with an atom or group of atoms. A monosubstituted benzene has the formula C6H5G where G is the group that replaces a hydrogen atom. All hydrogens in benzene are equivalent. It does not matter which hydrogen is replaced by G.

11. Monosubstituted Benzenes

12. Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene. nitro group ethyl group • The name is written as one word. nitrobenzene ethylbenzene

13. Certain monosubstituted benzenes have special names. These are parent names for further substituted compounds. hydroxy group methyl group phenol toluene

14. carboxyl group amino group benzoic acid aniline

15. Disubstituted Benzenes

16. Three isomers are possible when two substituents replace hydrogen in a benzene molecule. • The prefixes ortho-, meta- and para- (o-, m- and p-) are used to name these disubstituted benzenes.

17. ortho disubstituted benzene substituents on adjacent carbons ortho-dichlorobenzene(1,2-dichlorobenzene)mp –17.2oC, bp 180.4oC

18. meta disubstituted benzene substituents on adjacent carbons meta-dichlorobenzene(1,3-dichlorobenzene)mp –24.82oC, bp 172oC

19. para disubstituted benzene substituents are on opposite sides of the benzene ring para-dichlorobenzene(1,4-dichlorobenzene)mp 53.1, bp 174.4oC

20. When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound. 3-nitrophenol phenol

21. When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound. 3-nitrotoluene toluene

22. Tri- and Polysubstituted Benzenes

23. When a benzene ring has three or more substituents, the carbon atoms in the ring are numbered. Numbering starts at one of the substituent groups. The numbering direction can be clockwise or counterclockwise. Numbering must be in the direction that gives the substituent groups the lowest numbers.

24. 6-chloro 1-chloro 4-chloro clockwise numbering 6 1 5 4 2 3 1,4,6-trichlorobenzene

25. 2-chloro 1-chloro 4-chloro counterclockwise numbering chlorine substituents have lower numbers 2 1 3 4 6 5 1,2,4-trichlorobenzene

26. When a compound is named as a derivative of the special parent compound, the substituent of the parent compound is considered to be C-1 of the ring.

27. 1 2 6 3 5 4 1 6 2 3 5 4 toluene 2,4,6-trinitrotoluene(TNT)

28. When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative. • Example:

29. Naming compounds that cannot be easily named as benzene derivatives • Benzene named as a substituent on a molecule with another functional group as its root by the prefix phenyl. diphenylmethane 4-phenyl-2-pentene

30. The phenyl group, C6H5-

31. If the hydrocarbon chain contains more than three carbon atoms, phenyl is used as part of the name. • Examples:

32. PHYSICAL PROPERTIES OF BENZENE AND ITS DERIVATIVES • Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, and pack better into crystals and have higher melting points. • Density: - Slightly dense than non-aromatic analogues, but still less dense than water. - halogenated benzenes are denser than water. • Insoluble in water • Boiling points depends on the dipole moments of compounds.

33. REACTION OF BENZENE • ELECTROPHILIC SUBSTITUTION REACTIONS OF BENZENE • stability of π-electron system is lost when benzene undergoes addition reactions. • benzene and its derivatives undergo substitution reaction rather than addition reactions. • product of substitution reactions: aromatic compounds and not saturated compounds.

34. Mechanism of electrophilic substitution of benzene

35. ELECTROPHILIC SUBSTITUTION REACTIONS

36. ELECTROPHILIC SUBSTITUTION REACTIONS

37. Reagents, electrophiles and catalysts in electrophilic substitution reactions

38. HALOGENATION OF BENZENE

39. MECHANISM: BROMINATION OF BENZENE

40. MECHANISM: NITRATION OF BENZENE

41. MECHANISM: FRIEDEL-CRAFTS ALKYLATION

42. MECHANISM: FRIEDEL-CRAFTS ACYLATION

43. Ortho-Para and Meta Directing Substituents • When substituted benzenes undergo further substituents, the substituent group present in the benzene derivative will influence electrophilic substitution in 2 ways which are: i) Reactivity ii)Orientation

44. EFFECTS OF SUBSTITUENTS ON THE REACTIVITY OF ELECTROPHILIC AROMATIC SUBSTITUTION • Substituent group present in the benzene ring can influence the rate of reaction of further substitutions. • Electron-donating groups make the ring more reactive (called activating groups) thus influence the reaction become faster. • Electron-withdrawing groups make the ring less reactive (called deactivating groups) thus influence the reaction become slower.

45. EFFECTS OF SUBSTITUENTS ON THE ORIENTATION OF ELECTROPHILIC AROMATIC SUBSTITUTION • A substituents group already in the ring influences the position of further electrophilic substitution whether at ortho, meta or para position. • Ortho-para directors: the groups that tend to direct electrophilic substitution to the C2 and C4 positions. • Meta directors: the groups that tend to direct electrophilic substitution to the C3 position.

46. Effetcs of substituent groups on the benzene ring

49. REACTIONS OF BENZENE DERIVATIVES • Alkylbenzene such as toluene (methylbenzene) resembles benzene in many of its chemical properties. • It is preferable to use toluene because it is less toxic. • The methyl group activates the benzene nucleus. • Toluene reacts faster than benzene in all electrophilic substitutions.

50. Reactions of toluene Reactions of the methyl group Reactions of the benzene ring Substitution -halogenation Oxidation • Electrophilic • substitutions • - Halogenation • - Nitration • Friedel-Crafts reactions • Sulfonation Addition reaction -hydrogenation