Aromatic hydrocarbons

1. Aromatic hydrocarbons By: Hamada Abulkhair Ph.D

2. Teaching team • Magda Abdel-Aziz “Associate Prof” • Hamada Abulkhair “Associate Prof”

3. Teaching team • Aya El-Matary “Assistant Lecturer” • Mohamed Shehda “Demonstrator” • HebaFarid “Demonstrator” • Mohamed Zidan “Demonstrator” • Samia Salah “Demonstrator” • Alaa Salah “Demonstrator”

4. Faculty Vision Faculty of Pharmacy, Horus University, aims to achieve excellence in Pharmacy through innovative education, contemporary practice, and valuable scientific contributions.

5. Faculty Mission Faculty of Pharmacy, Horus University, provides an exemplary pharmacy education, to prepare graduates capable of providing high quality pharmacy practice and health care to the community.

6. Course contents “PC-204” • Aromatic hydrocarbons • Aryl halides • Phenols & Aromatic alcohols • Nitro-compounds, Amines & Diazonium salts • Sulphonic acids • Carbonyl containing compounds

7. Importance of the course By: Hamada Abulkhair Ph. D.

8. Course Learning Objectives By the end of this course you should be able to: • Differentiate between aliphatic and aromatic compounds • Understand the major functional groups of organic compounds • Conduct structure-chemical reactivity relationships • Identify how could common functional group converted to the others • Manage and control the orientation of electrophilic aromatic substitution reactions By: Hamada Abulkhair Ph. D.

9. Grading Policy

10. Attendance يجب على الطالب المواظبة على حضور الدروس النظرية والتطبيقية والعملية ويجب ألا تقل نسبة الحضور عن 75٪ من مجموع ساعات تدريس المقرر. اذا تجاوزت نسبة الغياب 25% دون عذر يقبله مجلس القسم و لجنة شئون التعليم والطلاب ومجلس الكلية يكون لمجلس الكلية الحق فى حرمان الطالب من دخول الامتحان النهائي.

11. Introduction • Benzene is the parent of all aromatic compounds. • The study of aromatic compounds began with the discovery in 1825 of benzene by the English chemist Michael Faraday. • In 1834 a German chemist synthesized benzene by heating benzoic acid with calcium oxide. • C6H5CO2H + CaO -------heat------> C6H6 + CaCO3 By: Hamada Abulkhair Ph. D

12. Introduction • To be classified as aliphatic the chemical behavior of the compound reacts like an alkane, an alkene, an alkyne, or one of their derivatives. • To be classified as aromatic the compound had a low ratio of hydrogen-to-carbon and that it was “fragrant”. By: Hamada Abulkhair Ph. D

13. The Kekulé Structure for Benzene • August Kekulé proposed the first definite structure for benzene which is still used today. • Kekulé suggested that the carbon atoms of benzene are in a ring, that they are bonded to each other by alternating single and double bonds, and that one hydrogen atom is attached to each carbon atom. • This structure satisfied the requirements of the structural theory that carbon atoms form four bonds and that all the hydrogen atoms of benzene are equivalent and monovalent. By: Hamada Abulkhair Ph. D

14. Structure of Benzene • Benzene is a ring of six carbon atoms, each bonded to one hydrogen atom. • All the carbon–carbon bonds are the same length, (1.4 Å) and all the bond angles are exactly 120°. • Each carbon atom has an unhybridized p orbital perpendicular to the plane of the ring, and six electrons occupy this circle of p orbitals. By: Hamada Abulkhair Ph. D.

15. A clock with no numbers fell to the ground. • What time is it. • There are exactly 3 false statements in this list. • 3:24 • 10:18 • 10:48 • 11:38 By: Hamada Abulkhair Ph. D.

16. Reactions of Benzene • Benzene is highly unsaturated (C6H6), and expected to react like unsaturated compounds through addition. • Benzene expected to react like an alkene by decolorizing bromine through addition. They expected that it would change the color of aqueous potassium permanganate by oxidation, that it would add hydrogen rapidly in the presence of a metal catalyst, and that it would add water in the presence of strong acids through hydrolysis. Benzene does none of these By: Hamada Abulkhair Ph. D

17. By: Hamada Abulkhair Ph. D

18. Reactions of Benzene • Benzene does react with bromine but only in the presence of a Lewis acid catalyst such as ferric bromide. However, it reacts not by addition but by substitution. By: Hamada Abulkhair Ph. D

19. Aromaticity • A compound to be called aromatic, it has to react by substitution rather than addition even if it was highly unsaturated. • Not all of cyclic compounds with alternating single and double bonds are aromatic. • A compound like Cyclooctatetraene found not at all like benzene. • Cyclooctatetraene reacts with bromine by addition, it adds hydrogen readily, it is oxidized by solutions of potassium permanganate, and thus it is clearly not aromatic. By: Hamada Abulkhair Ph. D

20. Aromaticity • A compound to be aromatic it must be: • Cyclic • Planar • All atoms in the ring must be sp2 hybridized • Posses 4n+2 p electrons ( n = 0, 1, 2, 3, and so on (i.e., rings containing 2, 6, 10, 14, . . etc., p electrons) obey Huckel’s rule By: Hamada Abulkhair Ph. D

21. Hückel’s Rule: The 4n + 2 p Electron Rule • Planar monocyclic rings with 2, 6, 10, 14, ..., delocalized electrons should be aromatic By: Hamada Abulkhair Ph. D

22. Polycyclic aromatic hydrocarbons(Benzenoid aromatic compound) By: Hamada Abulkhair Ph. D

23. Heterocyclic aromatic compounds By: Hamada Abulkhair Ph. D

24. Considering aromaticity, a compound can be classified in one of three ways: • Aromatic, A cyclic, planar, completely conjugated compound with 4n + 2  electrons. • Antiaromatic, A cyclic, planar, completely conjugated compound with 4n  electrons. • Non aromatic, A compound that lacks one (or more) of the following requirements for aromaticity: being cyclic, planar, and completely conjugated.

25. Aromaticity • Example 1: Benzene • Cyclic  • Planar  • Conjugated  • 6  Electrons 

26. Aromaticity Other Examples? Cyclic  Planar  Conjugated  6 Electrons X Cyclic  Planar  Conjugated  6 Electrons X

28. Non benzenoid aromatics There are conjugated cyclic systems (monocyclic or polycyclic) having (4n+2) -electrons and not benzenoid compounds. They may be neutral molecules, carbocations or carbanions.

29. Annulenes Annulenes are conjugated cyclopolyalkenes. The ring size is indicated by a number between brackets, [n]annulenes. Annulenes obeying the 4n+2 rule and having a planar carbon skeleton are aromtic., e.g. [14], [18], and [22]annulenes , where n =3, 4, 5, respectively. The [12], [16], and [24]annulene , on the other hand, are not aromatic [14]anulene [16]annulene [18]annulene (aromatic) (not aromatic) (aromatic)

30. Different [10] annulenes have been synthesized but none is aromatic because their rings are not planar, e.g.

31. Heterocyclic rings The heteroatom (O. N. S) in the case of the 5-membered rings provides two electrons for the aromatic sextet, while in the case of the 6-membered ring (pyridine) only one p-electron is provided from the heteroatom for the aromatic sextet. pyrrole thiophene furan pyridine

32. Furan and Pyrrole • The oxygen atom of furan is sp2 hybridized. • One unshared electron pairs of electrons on oxygen is a part of the aromatic sextet.

33. Pyridine • The nitrogen atom of pyridine is sp2 hybridized. • The unshared pair of electrons is not a part of the six pi electrons of the aromatic system (the aromatic sextet).

34. Aromatic ions Cyclopropenium ion The free cyclopropenyl cation is unstable, but 1,2,3-triphenyl-cyclopropenyl cation is prepared.

35. Tropylium ion (cycloheptatrienyl cation) Cycloheptatriene has six electrons, however, it is non-aromatic, because they are not delocalized over the whole ring due to the presence of the sp3-hybridized CH2 group with no available p-orbital. When cycloheptatriene loses a hydride ion, it converted to the cycloheptatrienyl (or tropylium) cation, which is very stable.

36. Cyclopentadienyl anion The anion obeys Hckel's rule 4n + 2 = 6 electrons, n = 1.

37. N.B.: Cyclic conjugated systems which have only 4n electrons do not fit the Hückel’s rule. They are said to be anti-aromatics, and in some cases they are found to be less stable than their acyclic analogues, e.g.