What are arenes?

1. What are arenes? Arenes are aromatic hydrocarbons that contain one or more benzene rings. The name aromatic means strong-smelling substances found in nature. For example,vanillin(right) isthe fragrant compound found invanilla plants. However, this was WRONG. Many compounds containing benzene rings have no odour, but the name stays with us, like a bad smell…..lol  https://www.youtube.com/watch?v=xD7Z7SHix-w

2. L.O. Arenes or Aromatic compounds- understand and draw the bonding in benzene- understand using thermochemical evidence, the additional stability conferred by the structure- understand and be able to explain why the structure causes benzene to undergo electrophilic reactions- understand the mechanism of nitration and the importance of nitration in synthesis - understand Friedel-Crafts acylation and its importance is synthesis

3. Kekulé structure of benzene Kekulé proposed a cyclic structure consisting of alternating single and double bonds. Kekulé’s claimed that his inspiration came from a dream of a snake eating its own tail. There were a number of problems with Kekulé’s structure

4. Having 3 double bonds means benzene should readily undergo electrophilic addition reactions Problems with the Kekulé structure However, unlike alkenes, benzene (left) does not decolourize bromine water.

5. 2nd Problem with the Kekulé structure Benzene bonds are ALL equal to 139 nm. See in the chart below, if Kekule’ structure was right, the hexagon would be irregular with alternating shorter and longer sides; but all bionds equal 139nm Benzene’s bond length = 1.39 Å is intermediate between the carbon—carbon single bond (1.53 Å) and C=C = 1.34 Å So there are NO double or single bonds but DELOCALIZED pi bonding around the rIng

6. Thermodynamic Stability of Arenes When cyclohexene has H2 added 120kJ is energy C6H10(l) + H2(g) ——> C6H12(l) + 120kJ Where does this heat energy come from? When the reaction happens, bonds are broken (C=C and H-H) and this costs energy. Other bonds have to be made with the carbons and hydrogens and this releases energy. You learned this in Grade 11

7. THERMODYNAMIC EVIDENCE FOR STABILITY C6H10(l) + H2(g) ——> C6H12(l) + 120kJ C6H6(l) + 3H2(g) ——> C6H12(l) + 360 kJ Theoretically, if benzene had 3 C=C bonds it SHOULD release 360kJ. Actual benzene only releases only 208kJ per mole So we had to put in MORE energy in to break benzene; more than was released when the 3H2 formed across it. Basically, 152 KJ shows the extra energy you need to break benzenes structure MORE STABLE THAN EXPECTED by 152 kJ mol-1 Theoretical - 360 kJ mol-1 (3 x -120) 2 3 Experimental - 208 kJ mol-1 - 120 kJ mol-1

8. Benzene Delocalization • The Six Pi Electrons are more Loosely Held the compared to the sigma Bonds • 6 Electrons are Delocalized around this FLAT planar sp2 structure.

9. HYBRIDISATION OF ORBITALS - REVIEW Alternatively, only three orbitals (an s and two p’s) HYBRIDISE to give three new orbitals. The remaining 2p orbital is unchanged. sp2HYBRIDISATION 2s22p2 2s12p3 3 x sp2 2p UNHYBRIDISED HYBRIDISE PROMOTE

10. What sp hybridizations are used to form the indicated bonds? Csp2-Csp2 Csp2-Csp3 Csp2-Hs Csp2-Csp2 Csp2-Csp2

11. STRUCTURE OF BENZENE ANIMATION

12. ACID - Inductive Effects for Benzene Alcohols are VERY weak acids. The pKa of water is 15.74 and methanol is 15.54 However phenol is a much better acid than methanol, why? Answer: phenol in which the enhanced acidity is generally attributed to stabilization of the phenoxide ion by resonance delocalization. What pattern do we see with the addition of X here? X = -H -Cl -Br -NO2 pKa. ~ 10 9.4 9.3 7.2 Here we see that as highly electronegative atoms are joined to the aromatic help to withdraw electrons from the phenoxide ion making hydrogen better able to leave or donate

13. BASE - Is there a pattern here? The nitrogen electronegative the delocalised electrons will withdrawn Into the ring as with the acids on previous slide, the ammonia Molecule will have a more Negative charge.

14. Exam points • Benzene is a flat molecule with 6 carbons bonded in a Planar ring • Each carbon is covalently joined to two other carbons and one hydrogen. A total of three covalent bonds • The remaining outer electron of each carbon is shared with the other carbons in the ring. The six electrons are delocalised around the ring system, giving stability • All bond lengths are the same.

15. Naming Aromatic Compounds • C6H5NO2 C6H5Br • One common name you MUST know is toluene = methylbenzene Monosubstituted benzenes C6H6 is benezene C6H5 is a MONOSUBSTITUTEDBENZENE • C6H5NO2 CH3 Toluene

16. Funny Phenyl Fellas When the benzene ring is a substituent of a parent chain, it is referred to as a phenyl group.  4-phenylheptane

17. Disubstituted Benzenes Positions ortho- (o) (1,2) meta- (m) (1,3) para- (p) (1,4) ortho-dibromobenzene or o-dibromobenzene or 1,2-dibromobenzene meta-dibromobenzene or m-dibromobenzene or 1,3-dibromobenzene para-dibromobenzene or p-dibromobenzene or 1,4-dibromobenzene

18. Which structure matches the given name? o-dichlorobenzene

19. What is the correct name for this compound? A) 3-Nitrotoluene B) 4-Nitromethylbenzene C) p-Nitrotoluene D) (4,1)-Methylnitrobenzene C) p-Nitrotoluene or 4-methyl-1-nitrobenzene

20. Benzenes 3 or more Substituents The benzyl group, another common substituent that contains a benzene ring, differs from a phenyl group. • Choose numbers to get lowest possible values • List substituents alphabetically with hyphenated numbers 1-hydroxy-2,4,6- trinitrobenzene 1-methyl-2,4,6- trinitrobenzene \

21. What is the correct name for this compound? • 4-bromo-m-xylene. • B) 1-bromo-2,4-dimethylbenzene. • C) p-bromo-m-methyltoluene. • D) o-bromo-m-methyltoluene. B) 1-bromo-2,4-dimethylbenzene.

22. 1,3-dimethylbenzene (m-xylene) 2-phenylbutane 4-methyl-1-chlorobenzene 2-hydroxybenzoic acid 3,5-dinitrobenzoic acid

23. What is the correct name? • 1-fluoro-3-isopropyl-5-ethylbenzene • 1-ethyl-3-isopropyl-5-fluorobenzene • 1-ethyl-3-fluoro-5-isopropylbenzene • 1-isopropyl-3-fluoro-5-ethylbenzene C) 1-ethyl-3-fluoro-5-isopropylbenzene

24. Give the IUPAC name for each compound, Ph = phenyl m-butylphenol Or 3-butyl-1-hydroxybenzene 2-bromo-5-chlorotoluene Or 2-bromo-5-chloromethylbenzene PhCH(CH3)2 Isopropylbenzene or 2-phenylpropane

25. 4-chloro-1,2-diethylbenzene

26. REACTIONS OFAROMATICS Combustion of arenes Arenes burn in air to give characteristically sooty flames.

27. NITRATION – ELECTROPHILIC SUBSTITUTION(same as acylation) + [H2SO4 ] + H2O [HNO3] + HSO4- + NO2 + NO2 NO2 Electrophile formation(or Super electrophile) nitronium ion Reaction of electrophile with aromatic compound Reflux at below 500C or many nitrations occur NO2 H + + + H The H+ ion reacts with the HSO4- to reform the catalyst H2SO4 Uses is to nitrate more and make explosives.

28. FRIEDEL-CRAFTS ACYLATION – ELECTROPHILIC SUBSTITUTION Electrophile formation O AlCl3 + RCOCl AlCl4 + – C R + acylium ion C6H6 + CH3COCl ———> C6H5COCH3 + HCl phenylethanone 

29. FRIEDEL-CRAFTS ACYLATION WARNING! - Classic question to test this, is This a Friedel-Crafts acylation? + HCl The H replaced is on N and not on the benzene ring. Friedel-Crafts acylation substitutes DIRECTLY onto the ring! So, what is it? Answer: Addition - Elimination

30. SUMMARY OF REACTIONS OF AROMATIC COMPOUNDS

31. USES OF AROMATIC COMPOUNDS EXPLOSIVES DYES PHARMACEUTICALS