PPT 102 ORGANIC CHEMISTRY 1 SEM 1 (2012/2013)

1. PPT 102ORGANIC CHEMISTRY 1 SEM 1 (2012/2013) Aromatic Compounds Dr. Hayder Kh. Q. Ali hayderali@unimap.edu.my

2. Course Outline • Aromatic Compound • Aromatic Heterocyclic Compound • Antiaromatic • Nomenclature • Electrophilic aromatic substitution • Nitration of benzene • Friedel Craft Acylation • Friedel Craft Alkylation

3. Criteria for Aromaticity • A compound must have an uninterrupted cyclic cloud of p electrons above and below the plane of the molecule: • - For the p cloud to be cyclic, the molecule must be cyclic • For the p cloud to be uninterrupted, every atom in the ring must have a p orbital • For the p cloud to form, each p orbital must overlap with the p orbitals on either side of it, therefore the molecule must be planar 2. The p cloud must contain an odd number of pairs of p electrons, or 4n + 2 (n = 0, 1, 2 …) total electrons.

4. Criteria for Aromaticity Each carbon of benzene has a p orbitals The overlap of the p orbitals form a cloud of π electrons above and below the plane The eleectrostatic potential map of benzene

5. Needs 4n + 2 (n = 0, 1, 2, 3…) electrons to fill orbitals Hückel’s Rule For a planar, cyclic compound in order to be aromatic, its uninterrupted p cloud must contain (4n + 2) p electrons, where n is any whole number. According to Huckel’s rule the aromatic compounds must have 2 (n=0), 6(n=1), 10(n=2), 14(n=3) and so on π electrons.

6. Heteroatom donates one electron Heteroatom donates two electrons Aromatic Heterocyclic Compounds A heterocyclic compound is a cyclic compounds in which one or more of the ring atom is an atom other than carbon. The atom that is not carbon is called a heteroatom. The most common hetereatoms are N, O, and S The heteroatom donates either one or two electrons to the  system

7. Examples of Heterocyclic Aromatic Compounds

8. Antiaromaticity A compound is classified as being antiaromatic if it fulfills the first criterion for aromatic but does not fulfill the second. A compound is antiaromatic if it is a planar, cyclic, continuous loop of p orbitals with an even number of pairs of p electrons: Antiaromatic compounds are highly unstable, but the nonplanar versions are stable

9. [1] Aromatic: A cyclic, planar, completely conjugated compound with 4n + 2 π electrons. [2] Antiaromatic: A cyclic, planar, completely conjugated compound with 4n π electrons. [3] Not aromatic: A compound that lacks one (or more) of the four requirements (nonaromatic) to be aromatic or antiaromatic.

10. Nomenclature of Monosubstituted Benzenes Some are named by attaching “benzene” after the name of the substituent:

11. Some have to be memorized:

12. A benzene substituent is called phenyl. A benzene substitutuent with a methylene group is called benzyl.

13. Electrophilic Aromatic Substitution Reactions

14. INTRODUCTION • Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions.  • Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete π electron delocalization (resonance). • Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved.  •  For example, benzene reacts with bromine to form bromobenzene.

15. REACTIONS ; • NITRATION OF BENZENE • FRIEDEL – CRAFTS ACYLATION • FRIEDEL – CRAFTS ALKYLATION

17. An electrophile, E+, is an electron poor species that will react with an electron rich species. • Aromatic because the reaction is characteristic of aromatic systems. • A substitution implies that a group is replaced (usually H).

18. Electrophilic Aromatic Substitution

20. Benzene reacts with the electrophile (E+ ) forming a carbocation intermediate. A base in the reaction mixture (Y-) pulls off a proton from the carbocation intermediate, and the electrons that held the proton move into the ring to reestablish its aromaticity.

21. NITRATION OF BENZENE

22. EQUATION OF NITRATION OF BENZENE

23. FORMATION OF ELECTROPHILE Nitronium ion • Nitronium ion is formed by reaction between the nitric acid and sulphuric acid. • Sulphuric acid protonates nitric acid. • Protonated nitric acid loses water to form nitronium ion. • Nitronium ion is the electrophile required for nitration.

24. MECHANISM OF BENZENE NITRATION • The electrophile, nitronium ion attaches to the ring. • As the NO2+ ion approaches the delocalised electrons in the benzene, those electrons are strongly attracted towards the positive charge. • Two electrons from the benzene ring are used to form a new bond with the NO2+ ion. • The delocalisation is partly broken, and in the process the ring gains a positive charge. • A base, HSO4- in the reaction mixture remove the proton from the carbon that form the bond with the electrophile. • The remove hydrogen from the ring form back sulphuric acid. • The catalyst has therefore been regenerated.

25. EXAMPLE: The nitration of methylbenzene (toluene) 2-nitromethylbenzene And 4-nitromethylbenzene

26. If the reaction temperature is above 55oC, further • nitration will occur which lead to the formation of • 2,4,6-trinitromethylbenzene.

27. question Why toluene is more reactive than benzene?

28. ANSWER • Methylbenzene (toluene) is formed when a methyl group is attached to the benzene ring. • The methyl group is an electron donating group, thus, it destabilizes the benzene ring by increasing the electron density of the benzene ring. • This allow the electrophilic aromatic substitution to take place easier. • The electrophile can attached to the benzene ring more readily.

29. THE FRIEDEL-CRAFTS ACYLATION OF BENZENE

30. Friedel-Crafts Acylation • Friedel-Crafts acylation forms a new C-C bond between a benzene ring and an acyl group.

31. Friedel-Crafts Acylation • The electrophile is an acylium ion.

32. Friedel-Crafts Acylation • An acylium ion is represented as a resonance hybrid of two major contributing structures. • Friedel-Crafts acylations are free of major limitation of Friedel-Crafts alkylations; acylium ions do not rearrange, do not polyacylate (why?), do not rearrange.

33. FRIEDEL-CRAFTS ALKYLATION

34. friedel-craftsalkylation • Reaction develop by Charles FriedelandJames Mason Crafts

35. introduction Friedel- Crafts alkylation involves alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst,

36. Friedel-Crafts alkylation substitutes an alkyl group for a hydrogen R = alkyl group

37. GENERATION OF THE ELECTROPHILE • Formation of carbocation from the reaction of an alkyl halide (R-Cl) with (AlCl3). • Alkyl chloride, alkyl bromide and alkyl iodide can also be used • Vinyl halide and aryl halidecannotbe use, because their carbocation are too unstable to be formed.

38. Rearrangment of carbocation 1) 1,2-hydride shift • The reactive electrophile, the carbocation is prone to rearrangement to a more stable carbocation which will then undergo the alkylation reaction.

39. Continue 2) 1,2-methyl shift

40. THE MECHANISM: • The attack of nucleophile: • Theπ electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C+. • This step destroys the aromaticity giving the cyclohexadienylcationintermediate

41. Continue… • 2) Protonation and regeneration : • Removal of the proton from the sp3 C bearing the alkyl- group. • Reforms the C=C and the aromatic system, • Generating HCl and regenerating the active catalyst.

42. Biological application A Friedel-Crafts alkylation reaction involve in one of the step to in the biosynthesis of Vitamin KH2, the coenzyme required to form blood clots.

44. questions  1)The strength of the polarization of the activated complex depends on the alkyl residue as well as on the ... Please make a choice. a. Lewis acid. b. Lewis base. c. Conjugate base

45. Continue… 2) What is the major product from the reaction ? ? a. b.

46. THE END